Relativity, speed of light and stuff...

[ricmat]

Hi guys!

this is my first post here, and I would like to ask a few questions concerning relativity, speed of light and concerning stuff.
I know that a lot has already been talked around here about this areas, but I have not found one thread that puts this issue in the way I would like to put it.

First, I would like to make just a few points about my background:
-I am not a physic, nor I have some background in physics, apart from school teaching, obviously
-I've read some books, threads, posts and articles about physics, but that is pretty much my background!
-So, please, if you mind to answer this thread be critic. If I'll say something very stupid, please state so! just let me know. I really love to learn and discuss this topics.





So, my issues:

My very first doubt urged when I read about einstein. I am a strong fan of Newton, but I have to admit I do not "like" the way Einstein aproaches all concepts in his theories.

My point is just a simple one: I do not understand why do physics consider light so much special! Why is its speed the limit!

I have read a lot about this, and how the very nature of light (not a wave, but a wave-particle) is so important to define speed of light as the limit. or even as it is calculated from the relations within universe laws.

My point is just this one - if we are sublight beings (we work tops at light speed minus chemicals relations in our neurons), if all our creations are sublight (a computer cant work at light speed) ... ... ...

how can we measure light speed if we work, tops, at sublight? how can we say that there is nothing that can travel faster than speed of light if we cant "see" at over speed of light?


this is - if we had no eyes, and only ears, wouldnt we think that sound speed would be the greatest speed ever? wouldnt Einstein put "c" as the speed of sound?

of course we never experienced for sure faster than light experiences. but is that possible? we have no machines working at that speed! Is there a flaw in my reasoning?

and i even cant understand that argument people usually say, that if one travels that speed of light it would almost like we do something before it occurs - how is that???

yes, we would do something and then travel faster than the speed of light, but that's not time travel. It is just like making a sound and then travelling at mach 3, only multiplied many times, but not even close to "change" time line

This is not so obvious for me, and I have never read anything that answered this question clearly.


I have also read something stating that it actually makes sense once we are the ones to "analyse" all this phenomena, and it makes sense to the actual observer (us, humans) but for me physics is the study of nature rules, and not nature rules as seen by humans. At least, that's the way I see it!


Hope to discuss this further with you!


Cheers,
ricmat

[berkeman]

This article should answer most of your questions, including how the speed of light is measured, and the issues concerning FTL.

http://en.wikipedia.org/wiki/Speed_of_light


.

[JesseM]

The "specialness" of the speed of light basically has to do with the way the laws of physics look in different coordinate systems. If you know something about Newtonian physics you may be familiar with the idea of inertial coordinate systems, where different inertial (non-accelerating) observers can construct coordinate systems to assign position and time coordinates to different events, and each observer's own rest frame is the one where his own position coordinate isn't changing with time. In Newtonian coordinate systems, if you have two observers in motion relative to one another, then naturally they will get different values for the speed of some object in their own coordinate systems. This means that in Newtonian coordinate systems, if Maxwell's equations of electromagnetism work in one observer's coordinate system they can't work in the coordinate system of a different observer moving relative to the first, since Maxwell's equations predict that the speed of light is c, and if they were c in the coordinate system of the first observer and the second observer was moving at speed v relative to the first, then the second observer would measure the light to be moving at v+c or v-c. So Einstein's work was in part an attempt to construct a different set of coordinate systems than the Newtonian ones, such that Maxwell's laws could work correctly in every observer's coordinate system. The set of coordinate systems he found are related to one another by a set of equations called the "Lorentz transformation", which is different from the "Galilei transformation" which relates the coordinates of different inertial observers in Newtonian physics. Einstein also made the postulate that if we use such coordinates, all the fundamental laws of physics (not just Maxwell's laws) will turn out to obey the same equations in all these coordinate systems, a property known as "Lorentz-invariance". This postulate has shown to be correct as more fundamental laws continued to be discovered (asking why all the laws of physics obey this symmetry is not really something that physicists can answer, their job is only to discover what the laws of nature are like, not why the laws have the particular form they do and not some other). You could indeed construct a different set of coordinate systems where the speed of sound waves (or some other object) was the same in each coordinate system, the difference is that the known laws of physics would not show the same sort of symmetry with respect to these coordinate systems.

[rbj]

it's not just visible light.
nor is it just electromagnetic interaction (of which light is).

all fundamental interactions (caused by a generating agent) have their effect (on the responding agent) delayed by a time (as observed by a third party equal distant from the two agents) that is proportional to the distance between the cause and effect agent. whether the two of us are holding charges that are attracted (by the E&M action) to each other or the two of us are much bigger and holding planets that are attracted (by gravitation) to each other, if i wave my charge (or planet) around, your charge (or planet) will be disturbed accordingly. the time between my agitation and the disturbance you detect will be longer (as observed by the third party) if the distance between you and me is longer. that implies a speed of interaction. that speed is the same whether it's E&M (light), gravity, or the nuclear forces. and there aren't any other interactions that we know about. it's a property of space and time (and the relationship betwixt the two), not specifically of light, nuclear forces, or gravitational attraction.

[DaleSpam]

if we had no eyes, and only ears, wouldnt we think that sound speed would be the greatest speed ever? wouldnt Einstein put "c" as the speed of sound?Hi ricmat, welcome to PF

I have to admit that I didn't understand special relativity for several years using exactly this same line of reasoning. The key difference between sound and light is that sound requires a medium in which to propagate and light does not.

The speed of sound is only constant wrt its medium, and can be measured faster or slower depending on the speed of the medium wrt the measuring apparatus. We can detect the motion of the medium with many other techniques (e.g. finger in the wind) and accurately determine what the speed of sound should be in our reference frame.

By contrast, since there is no medium for light, the speed of light is never measured faster or slower than c regardless of the speed of the measuring apparatus. We cannot in any way detect the motion of empty space (this is essentially what M&M were trying to do), and so the speed of light is always c in any reference frame.

I hope this helps, because I understand your reasoning since I used it too for many years.

[atyy]

Although we are sublight beings, we can measure the speed of light by making it go over a very great distance, so that the time interval measured is very long.

But this is not so relevant. The constancy of the speed of light actually comes from the Newtonian idea that if you are in a car, the cars going in the same direction will seem to move more slowly, and cars going in the opposite direction will seem to approach more quickly than if you were standing by the road.

In the Michelson-Morley experiment, the car is the earth going round the sun. At different times of the year, the earth is going in different directions, and so if light is moving with respect to some external medium, it should change its speed according to the time of the year. The change should be around 15 m/s, which perhaps you will more readily believe sublight beings can detect?

[ricmat]

Thank you guys for your quick answers.

a lot of food for thought to digest, and little time to do so! so sorry for not being able to address all points, but i'll do soon.


just a quick answer to DaleSpam:

Hi ricmat, welcome to PF

I have to admit that I didn't understand special relativity for several years using exactly this same line of reasoning. The key difference between sound and light is that sound requires a medium in which to propagate and light does not.

The speed of sound is only constant wrt its medium, and can be measured faster or slower depending on the speed of the medium wrt the measuring apparatus. We can detect the motion of the medium with many other techniques (e.g. finger in the wind) and accurately determine what the speed of sound should be in our reference frame.

By contrast, since there is no medium for light, the speed of light is never measured faster or slower than c regardless of the speed of the measuring apparatus. We cannot in any way detect the motion of empty space (this is essentially what M&M were trying to do), and so the speed of light is always c in any reference frame.

I hope this helps, because I understand your reasoning since I used it too for many years.



so great i find a person who has already thought the same way. I just have one question, as I seem to find a flaw in your reasoning: light does not travel the same way in each mean! as we can find in the link above mentioned:

"Light traveling through a medium such as air (for example, this laser) travels slower than light through a vacuum."


my question is - yes, sound do not travels in vacuum, but they are different kind of waves! one is mechanic the other one is electromagnetic, so that maybe the reason!
I would just like to recall that before einstein we thought that there was no vacuum, and light traveled through ether... i have a phd friend in cosmology, who told me very recently that this very same thesis as come up more and more in present days again... it seem that there is no consensus among today scientist that there is no "something" like ether as opposite to vacuum....

[granpa]

The speed of sound is only constant wrt its medium, and can be measured faster or slower depending on the speed of the medium wrt the measuring apparatus. We can detect the motion of the medium with many other techniques (e.g. finger in the wind) and accurately determine what the speed of sound should be in our reference frame.
.

thats perfectly true, but if you became length contracted, time dilated, and experienced loss of simultaneity as you approached mach one then you would not be able to detect any change in the speed of sound.

[FeynmanMH42]

I just have one question, as I seem to find a flaw in your reasoning: light does not travel the same way in each mean! as we can find in the link above mentioned:

"Light traveling through a medium such as air (for example, this laser) travels slower than light through a vacuum."


my question is - yes, sound do not travels in vacuum, but they are different kind of waves! one is mechanic the other one is electromagnetic, so that maybe the reason!

Ahh, but the speed of light in a medium is irrelevant. There's no law saying that the speed of light in glass is a universal constant - in fact it's perfectly okay to travel faster than the speed of light passing through a medium, and this can produce Cherenkov radiation. (http://en.wikipedia.org/wiki/Cherenkov_radiation)
Special relativity is based upon the speed of light in a vaccuum only. There is something special about this speed BECAUSE then light is travelling only through a vaccuum... get me?

[DaleSpam]

I seem to find a flaw in your reasoning: light does not travel the same way in each mean! as we can find in the link above mentionedThe constant, c, refers to the speed of light exclusively in vacuum, which is the same for all inertial observers. I agree with FeynmanMH42's comments above.

[DaleSpam]

thats perfectly true, but if you became length contracted, time dilated, and experienced loss of simultaneity as you approached mach one then you would not be able to detect any change in the speed of sound.Yes, and if such things happened then the invariant speed would be mach one, light would travel at the speed of sound, and tornados would be relativistic phenomena.

[granpa]

Yes, and if such things happened then the invariant speed would be mach one, light would travel at the speed of sound, and tornados would be relativistic phenomena.

of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.

[DaleSpam]

nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.Of course not. But think about it. Why not?

[granpa]

I imagine that you mean that we cant stick our hand out the window and feel the breeze of the aether going by. but particles are waves too. If we are also waves in the aether then we should hardly expect to be able to do so.

[DaleSpam]

Precisely, there are many physical experiments that we can do to measure the velocity of air. There are no experiments that we can do to measure the velocity of the vacuum.

[rbj]

of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.

i might claim that, given the premise (which we don't believe).

[Doc Al]

but nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.
The speed of sound is not the same for all observers.

[granpa]

The speed of sound is not the same for all observers.

you seem to have missed the 'would' in my post.

read post 8

[Doc Al]

you seem to have missed the 'would' in my post.

read post 8
Oops. My bad.

[granpa]

i might claim that, given the premise.

An engineer, a mathematician, and a physicist were travelling through Scotland when they saw a black sheep through the window of the train.
"Aha," says the engineer, "I see that Scottish sheep are black."
"no," says the mathematician, "You mean that some Scottish sheep are black."
unable to agree, they asked the physicist to decide which was the correct statement.
The physicist thought for a moment then said "there is one sheep in Scotland and it is black on one side".

[rbj]

of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.

i might claim that, given the premise (which we don't believe).

An engineer, a mathematician, and a physicist were travelling through Scotland when they saw a black sheep through the window of the train.
"Aha," says the engineer, "I see that Scottish sheep are black."
"no," says the mathematician, "You mean that some Scottish sheep are black."
unable to agree, they asked the physicist to decide which was the correct statement.
The physicist thought for a moment then said "there is one sheep in Scotland and it is black on one side".

... if you became length contracted, time dilated, and experienced loss of simultaneity as you approached mach one then you would not be able to detect any change in the speed of sound.

i dunno gramps, the observer approaching c (from the perspective of some other observer) does not, from his/her own POV, become length contracted, time dilated, etc. life is perfectly normal for him or her yet he/she still observes no variance in c. if we measured the speed of sound (whatever that could be if it wasn't the compressions and rarefractions of air) to be invariant no matter which direction or how fast we were moving (w.r.t. something, perhaps the ground), then we can conclude that either the medium that sound propagates in is smart enough to move along with us at whatever velocity we move, or there isn't such a medium.

[granpa]

i dunno gramps, the observer approaching c (from the perspective of some other observer) does not, from his/her own POV, become length contracted, time dilated, etc. life is perfectly normal for him or her

so? whats your point?

if we measured the speed of sound (whatever that could be if it wasn't the compressions and rarefractions of air) to be invariant no matter which direction or how fast we were moving (w.r.t. something, perhaps the ground), then we can conclude that either the medium that sound propagates in is smart enough to move along with us at whatever velocity we move, or there isn't such a medium.

the premise of this whole hypothetical argument was a person becoming length contracted, time dilated, and experiencing loss of simultaneity as they approach mach one. this would cause the speed of sound to be the same for all observers and it certainly wouldnt cause the medium of sound to magically disappear.

[granpa]

of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesnt have a medium.

i might claim that, given the premise (which we don't believe).

An engineer, a mathematician, and a physicist were travelling through Scotland when they saw a black sheep through the window of the train.
"Aha," says the engineer, "I see that Scottish sheep are black."
"No," says the mathematician, "You mean that some Scottish sheep are black."
Unable to agree, they asked the physicist to decide which was the correct statement.
The physicist thought for a moment then said "there is one sheep in Scotland and it is black on one side".

I messed it up. My memory isn't what it should be.

An engineer, a mathematician, and a physicist were travelling through Scotland when they saw a flock of black sheep through the window of the train.
The engineer says "From this observation we can deduce that Scottish sheep are black."
"no," says the mathematician, "We can only deduce that some Scottish sheep are black."
Unable to agree, they asked the physicist to decide which was the correct statement.
The physicist thought for a moment then said "there is one flock of sheep in Scotland and they are black on one side".[/QUOTE]

[Primordial]

The constant, c, refers to the speed of light exclusively in vacuum, which is the same for all inertial observers. I agree with FeynmanMH42's comments above. Is this true , where the electromagnetic interaction is exiting the event horizon of a black hole, or is the event horizon considered to be something other than vacuum?

[JesseM]

Is this true , where the electromagnetic interaction is exiting the event horizon of a black hole, or is the event horizon considered to be something other than vacuum?
When physicists say the speed of light in a vacuum is always c, they are talking either about an inertial coordinate system in the flat spacetime of special relativity, or a "locally inertial" coordinate system of a freefalling observer in the curved spacetime of general relativity ('locally' because the coordinate system only covers a very small region in the neighborhood of the observer, small enough that the curvature of spacetime is negligible). A freefalling observer passing by a light beam leaving a black hole will still measure the speed of that beam to be c in his own local neighborhood, even if he crosses paths with the light beam at the same moment he reaches the event horizon (from his perspective at that moment, the event horizon also seems to be moving outward at exactly c). On the other hand, in the Schwarzschild coordinate system which is often used to describe the entire black hole, light may move at different speeds at different distances from the black hole, but that's OK because this is not an inertial coordinate system (even in flat SR spacetime you can use non-inertial coordinate systems where the speed of light may be different at different points in space).

[rbj]

the premise of this whole hypothetical argument was a person becoming length contracted, time dilated, and experiencing loss of simultaneity as they approach mach one. this would cause the speed of sound to be the same for all observers and it certainly wouldnt cause the medium of sound to magically disappear.

just that it would have no measureable properties.

if the aether exists but has no measureable or perceivable properties whatsoever, what difference does it make if it exists or not? it may as well be pixie-dust or The Force or whatever it is in anyone's religion. if some hypothesized something is utterly unmeasureable, that's a pretty good indicator that it's non-existant.

that's the point. (that you should instead be mindful about it magically appearing rather than it magically disappearing. it never appeared in the first place.)

[rbj]

oh, and the other point is that the time dilation and length contraction are the effect and the invariancy of c is the cause.

[JesseM]

just that it would have no measureable properties.

if the aether exists but has no measureable or perceivable properties whatsoever, what difference does it make if it exists or not? it may as well be pixie-dust or The Force or whatever it is in anyone's religion? if some hypothesized something is utterly unmeasureable, that's a pretty good indicator that it's non-existant.
But in the thought-experiment where the speed of sound is the ultimate limit, air would still have measurable properties--it would be made of molecules, for example, and so any given region of air would have a specific average rest frame. If you had two air-filled ships passing by one another, and in each ship the air was at rest with respect to its own ship, it would be true that if people on board each ship clapped at the moment their positions lined up, then each would measure the sound wave on the other ship to move at the same speed as their own sound wave in spite of the fact that the two sound waves would be traveling through regions of air with different rest frames. Of course this wouldn't be true in our universe because clocks don't approach a rate of zero ticking as you approach the speed of sound, and rulers don't approach being compressed to zero length as you approach the speed of sound. But as a thought-experiment I don't see anything inherently impossible about a universe where the above was true.

[JesseM]

oh, and the other point is that the time dilation and length contraction are the effect and the invariancy of c is the cause.
Only if you choose to derive length contraction and time dilation from the two postulates that Einstein used. But nothing would stop you from taking length contraction and time dilation as postulates, and deriving the fact that anything moving at c in one inertial frame will be measured to move at c in all other inertial frames; there aren't really any fundamental physical concerns that force you to conclude which of these is a postulate and which is a conclusion, it's more of an aesthetic choice (sort of like how there are multiple different sets of theorems you might use to constitute the axioms of geometry or arithmetic, which would be equivalent in the sense that they wouldn't give different answers to whether or not a given theorem is true or false).

[rbj]

Precisely, there are many physical experiments that we can do to measure the velocity of air. There are no experiments that we can do to measure the velocity of the vacuum.

that (the meaninglessness of a moving vacuum) is why, i believe, Einstein, though he was likely knowledgable of the Michaelson-Morley experiment and null result, was never surprized by that null result. when asked about some of the properties of nature, he was quoted as questioning if God had any other choice (in how nature had come out). not implying that Einstein was a theist in the traditional sense, he said he "believe[d] in Spinoza's God (http://en.wikipedia.org/wiki/Baruch_Spinoza#Philosophy), who reveals Himself in the lawful harmony of the world,..."

[rbj]

Only if you choose to derive length contraction and time dilation from the two postulates that Einstein used. But nothing would stop you from taking length contraction and time dilation as postulates, ...

Occam's razor.

[rbj]

But in the thought-experiment where the speed of sound is the ultimate limit, air would still have measurable properties--it would be made of molecules, for example, and so any given region of air would have a specific average rest frame. If you had two air-filled ships passing by one another, and in each ship the air was at rest with respect to its own ship,

again, Occam's razor. sure, somehow the aether which really exists is smart enough to move around the Sun along with the Earth because it knows that Michaelson and Morley are set out to measure our speed through it. the aether sticks to the planet's surface no matter what time of day or what season of the year. so that's why the experiment had a null result.

it would be true that if people on board each ship clapped at the moment their positions lined up, then each would measure the sound wave on the other ship to move at the same speed as their own sound wave in spite of the fact that the two sound waves would be traveling through regions of air with different rest frames. Of course this wouldn't be true in our universe because clocks don't approach a rate of zero ticking as you approach the speed of sound, and rulers don't approach being compressed to zero length as you approach the speed of sound. But as a thought-experiment I don't see anything inherently impossible about a universe where the above was true.

but silly and complicated explanations of observed phenomena are deprecated in favor of concise explanations. sure, you can say that it's because of the length contraction and time dilation that we measure c to be invariant, but you offer no mechanism for why such length contraction and time dilation would happen in the first place.

[granpa]

If you had two air-filled ships passing by one another, and in each ship the air was at rest with respect to its own ship, it would be true that if people on board each ship clapped at the moment their positions lined up, then each would measure the sound wave on the other ship to move at the same speed as their own sound wave .

I dont agree with that.

[JesseM]

I dont agree with that.
I think you missed the point, I was speaking in the context of your own thought-experiment where, if mach 1 is represented by the symbol s, then moving clocks slow down by a factor of \sqrt{1 - v^2/s^2} and moving rulers shrink by the same factor. This would apply to things like the distance between air molecules in the direction of motion too, I'm assuming that all the laws of physics are invariant under the equivalent of a Lorentz Transformation with c replaced by s. In this case it would certainly be true that sound waves on the two ships would stay in line with one another as they moved.

[JesseM]

Occam's razor.
I'd say Occam's razor only applies to empirically different theories, not to different sets of logically equivalent axioms to be used in some formal proof.

[JesseM]

again, Occam's razor. sure, somehow the aether which really exists is smart enough to move around the Sun along with the Earth because it knows that Michaelson and Morley are set out to measure our speed through it. the aether sticks to the planet's surface no matter what time of day or what season of the year. so that's why the experiment had a null result.
Since when were we discussing the aether? Of course I don't believe in such a thing. I was discussing granpa's thought-experiment where length contraction and time dilation depend on the speed of sound, and I understood the point of this thought-experiment to be a pedagogical point about why in the real universe the speed of light is "special" in the way the speed of sound is not (essentially because the laws of physics are Lorentz-symmetric and the Lorentz transformation has c as its speed constant...if the laws of physics were symmetric under a transform that had the speed of sound as its speed constant, then the speed of sound would be 'special' in the same way the speed of light is in our universe, but they don't so it isn't).
sure, you can say that it's because of the length contraction and time dilation that we measure c to be invariant, but you offer no mechanism for why such length contraction and time dilation would happen in the first place.
No theory of physics offers a "mechanism" to explain why the fundamental laws of physics take the form they do. The goal of physics is just to discover what the fundamental equations are, not "why" they are.

[granpa]

I think you missed the point, I was speaking in the context of your own thought-experiment where, if mach 1 is represented by the symbol s, then moving clocks slow down by a factor of \sqrt{1 - v^2/s^2} and moving rulers shrink by the same factor. This would apply to things like the distance between air molecules in the direction of motion too, I'm assuming that all the laws of physics are invariant under the equivalent of a Lorentz Transformation with c replaced by s. In this case it would certainly be true that sound waves on the two ships would stay in line with one another as they moved.


ok. I guess you are right but what on earth is your point? why would you want to move the air with the planes? its completely unnecessary. it adds nothing and distracts from the point I was making.

[granpa]

No theory of physics offers a "mechanism" to explain why the fundamental laws of physics take the form they do. The goal of physics is just to discover what the fundamental equations are not "why" they are.

excellent point.

[JesseM]

ok. I guess you are right but what on earth is your point? why would you want to move the air with the planes? its completely unnecessary. it adds nothing and distracts from the point I was making.
What was the point you were making? As I said to rbj, I interpreted your point like this:
I understood the point of this thought-experiment to be a pedagogical point about why in the real universe the speed of light is "special" in the way the speed of sound is not (essentially because the laws of physics are Lorentz-symmetric and the Lorentz Transformation has c as its speed constant...if the laws of physics were symmetric under a transform that had the speed of sound as its speed constant, then the speed of sound would be 'special' in the same way the speed of light is in our universe, but they don't so it isn't).
So, showing that the speed of sound would remain constant in a universe with such a symmetry even if the medium it was traveling in moved at different speeds is quite relevant to this point, which is that it is symmetries in the laws of physics that determine what speed is special (and if these symmetries have picked out a particular speed as special then other issues, like whether or not some wave moving at that speed has a medium or not, become irrelevant, since the speed will be invariant regardless).

[granpa]

So, showing that the speed of sound would remain constant in a universe with such a symmetry even if the medium it was traveling in moved at different speeds is quite relevant to this point


NO. the point isnt that the speed of sound would be constant when the medium was moving. the point is that the speed of sound would be constant even when the observer was moving. why would the aether move?

[rbj]

I'd say Occam's razor only applies to empirically different theories, not to different sets of logically equivalent axioms to be used in some formal proof.

i'd say that Occam's razor applies to "explanation[s] of any phenomenon [and prefers those that] make as few assumptions as possible, eliminating those [assumptions] that make no difference in the observable predictions of the explanatory hypothesis or theory."

you have one explanation that the laws of physics are identical for any inertial observer, independent of how these observers are passing through the vacuum (or that the concept of a moving vacuum, a moving nothing is meaningless). from that single postulate, many relativistic consequences can be predicted to be observed.

or you can set up a world of make-believe where time dilation and length contraction occur to fast moving objects for no apparent reason. it's just magic. and then a consequence of that time dilation and length contraction (which is inexplicable) is that the speed of the electromagnetic interaction (as well as gravity and nuclear interactions) are measured to be constant. Occam's razor perfectly applies to these two explanations.

[rbj]

No theory of physics offers a "mechanism" to explain why the fundamental laws of physics take the form they do. The goal of physics is just to discover what the fundamental equations are, not "why" they are.

excellent point.

to each his own, i guess.

i think the goal of physics is to discover what the fundamental causes and interactions are. that means discovering why some (less fundamental) phenomena occur as a consequence of more fundamental phenomena. John Baez wrote this in Wikipedia regarding fundamental physical constants:

The list of fundamental physical constants increases when experiments measure new relationships between physical phenomena. The list decreases when physical theory advances and shows how some previously fundamental constant can be computed in terms of others.

i don't think that i'm extrapolating too far to say that physical theory advances when it shows how some previously fundamental phenomenon can be derived or predicted in terms of other fundamental phenomena, thereby removing the first phenomenon from the list of fundamental interactions. that's what holy grails (GUTs) are supposed to be about.

the invariancy of the laws of physics (for inertial observers) is the fundamental principle, resulting in the constancy of c, and time dilation and length contraction are consequences of that. these are not fundamentally equivalent.

[atyy]

but silly and complicated explanations of observed phenomena are deprecated in favor of concise explanations. sure, you can say that it's because of the length contraction and time dilation that we measure c to be invariant, but you offer no mechanism for why such length contraction and time dilation would happen in the first place.

How about thinking about it this way:

1) We get Maxwell's equations from experiments done with currents, magnets, metal plates, metal wires, iron filings etc.

2) Maxwell's equations describe how the electromagnetic field changes when you move charges, magnets etc. These changes in the electromagnetic field provide a mechanism for length contraction, which can actually break things:

http://en.wikipedia.org/wiki/Bell%27s_spaceship_paradox

A good and entertaining reference is John Bell's "Speakable and Unspeakable in Quantum Mechanics".

[Al68]

No theory of physics offers a "mechanism" to explain why the fundamental laws of physics take the form they do. The goal of physics is just to discover what the fundamental equations are, not "why" they are.

I hate to disagree, but science is (from American Heritage Dictionary) the observation, identification, description, experimental investigation, and theoretical explanation of phenomena. I would say that length contraction and time dilation are "phenomena".

Of course the last part of that definition is not emphasized the way it used to be in science.

Al

[atyy]

BTW, the whole discussion about the speed of sound being the ultimately limit, and sound still having a medium. Wouldn't that seriously change our formulation of the laws of physics? Under the present formulation of special relativity, anything travelling with constant speed in any special relativistic inertial reference frame must have zero rest mass. So if experiments suggested that the thing with constant speed in any reference frame had mass, we might have to rethink how mass-energy transforms in a frame change. Or would we have to ditch the Principle of Special Relativity?

Actually, is this discussion equivalent to: What would the consequences be for our formulation of the laws of physics if the photon were measured to have mass?

I guess the photon would be demoted to a neutrino, and the Principle of Special Relativity would remain intact?

[granpa]

I think you mean 'anything moving at the speed of light must have zero rest mass'. is that right?

bear in mind that space itself expanded much faster than light shortly after the big bang.

[JesseM]

i think the goal of physics is to discover what the fundamental causes and interactions are. that means discovering why some (less fundamental) phenomena occur as a consequence of more fundamental phenomena.
Well, I agree in general, but you weren't talking about deriving some higher-level laws from more fundamental laws, instead you were just talking about different sets of logically equivalent sets of axioms for deriving precisely the same general laws of SR.
the invariancy of the laws of physics (for inertial observers) is the fundamental principle, resulting in the constancy of c, and time dilation and length contraction are consequences of that. these are not fundamentally equivalent.
But you were talking about different sets of axioms which could be used to derive Lorentz-symmetry, which presumably is what you mean by "the invariancy of the laws of physics (for inertial observers)". But a key point here is that this description is overly vague, since without some additional assumptions it could also describe Galilei symmetry in Newtonian physics. To derive Lorentz-symmetry, you can start from the axiom that all fundamental laws of physics are the same in every inertial frame, plus the axiom that the speed of light is the same in every inertial frame; or you can start from the axiom that all the fundamental laws of physics are the same in every inertial frame, plus the axiom that in each frame rulers moving at v are measured to shrink by \sqrt{1 - v^2/c^2} and that in each frame clocks moving at v are measured to have the time between ticks lengthened by 1 / \sqrt{1 - v^2/c^2}. These two possible sets of axioms are completely equivalent in terms of their physical implications.

[atyy]

I think you mean 'anything moving at the speed of light must have zero rest mass'. is that right?

bear in mind that space itself expanded much faster than light shortly after the big bang.

No, I meant what I wrote. I don't think the Principle of Special Relativity and the Lorentz transformations would work with 2 invariant speeds. At least one of them must give. I'm only thinking within SR, no GR, inflation etc.

[JesseM]

I hate to disagree, but science is (from American Heritage Dictionary) the observation, identification, description, experimental investigation, and theoretical explanation of phenomena. I would say that length contraction and time dilation are "phenomena".

Of course the last part of that definition is not emphasized the way it used to be in science.

Al
I don't think it's a good idea to approach questions in philosophy of science by appealing to dictionary definitions. The definition is good enough to cover most situations in science, where you're explaining some high-level laws governing a system by appealing to more fundamental laws which govern the basic parts of that system (reductionism); but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws?

Here is Feynman writing about this topic in The Character of Physical Law, using gravitation as an example:
On the other hand, take Newton's law for gravitation, which has the aspects I discussed last time. I gave you the equation:

F=Gmm'/r^2

just to impress you with the speed with which mathematical symbols can convey information. I said that the force was proportional to the product of the masses of two objects, and inversely as the square of the distance between them, and also that bodies react to forces by changing their speeds, or changing their motions, in the direction of the force by amounts proportional to the force and inversely proportional to their masses. Those are words all right, and I did not necessarily have to write the equation. Nevertheless it is kind of mathematical, and we wonder how this can be a fundamental law. What does the planet do? Does it look at the sun, see how far away it is, and decide to calculate on its internal adding machine the inverse of the square of the distance, which tells it how much to move? This is certainly no explanation of the machinery of gravitation! You might want to look further, and various people have tried to look further. Newton was originally asked about his theory--'But it doesn't mean anything--it doesn't tell us anything'. He said, 'It tells you how it moves. That should be enough. I have told you how it moves, not why.' But people are often unsatisfied without a mechanism, and I would like to describe one theory which has been invented, among others, of the type you migh want. This theory suggests that this effect is the result of large numbers of actions, which would explain why it is mathematical.

Suppose that in the world everywhere there are a lot of particles, flying through us at very high speed. They come equally in all directions--just shooting by--and once in a while they hit us in a bombardment. We, and the sun, are practically transparent for them, practically but not completely, and some of them hit. ... If the sun were not there, particles would be bombarding the earth from all sides, giving little impuleses by the rattle, bang, bang of the few that hit. This will not shake the earth in any particular direction, because there are as many coming from one side as from the other, from top as from bottom. However, when the sun is there the particles which are coming from that direction are partially absorbed by the sun, because some of them hit the sun and do not go through. Therefore the number coming from the sun's direction towards the earth is less than the number coming from the other sides, because they meet an obstacle, the sun. It is easy to see that the farther the sun is away, of all the possible directions in which particles can come, a smaller proportion of the particles are being taken out. The sun will appear smaller--in fact inversely as the square of the distance. Therefore there will be an impulse on the earth towards the sun that varies inversely as the square of the distance. And this will be the result of a large number of very simple operations, just hits, one after the other, from all directions. Therefore the strangeness of the mathematical relation will be very much reduced, because the fundamental operation is much simpler than calculating the inverse of the square of the distance. This design, with the particles bouncing, does the calculation.

The only trouble with this scheme is that it does not work, for other reasons. Every theory that you make up has to be analysed against all possible consequences, to see if it predicts anything else. And this does predict something else. If the earth is moving, more particles will hit it from in front than from behind. (If you are running in the rain, more rain hits you in the front of the face than in the back of the head, because you are running into the rain.) So, if the earth is moving it is running into the particles coming towards it and away from the ones that are chasing it from behind. So more particles will hit it from the front than from the back, and there will be a force opposing any motion. This force would slow the earth up in its orbit, and it certainly would not have lasted the three of four billion years (at least) that it has been going around the sun. So that is the end of that theory. 'Well,' you say, 'it was a good one, and I got rid of the mathematics for a while. Maybe I could invent a better one.' Maybe you can, because nobody knows the ultimate. But up to today, from the time of Newton, no one has invented another theoretical description of the mathematical machinery behind this law which does not either say the same thing over again, or make the mathematics harder, or predict some wrong phenomena. So there is no model of the theory of gravity today, other than the mathematical form.

If this were the only law of this character it would be interesting and rather annoying. But what turns out to be true is that the more we investigate, the more laws we find, and the deeper we penetrate nature, the more this disease persists. Every one of our laws is a purely mathematical statement in rather complex and abstruse mathematics.

...[A] question is whether, when trying to guess new laws, we should use seat-of-the-pants feelings and philosophical principles--'I don't like the minimum principle', or 'I do like the minimum principle', 'I don't like action at a distance', or 'I do like action at a distance'. To what extent do models help? It is interesting that very often models do help, and most physics teachers try to teach how to use models and to get a good physical feel for how things are going to work out. But it always turns out that the greatest discoveries abstract away from the model and the model never does any good. Maxwell's discovery of electrodynamics was made with a lot of imaginary wheels and idlers in space. But when you get rid of all the idlers and things in space the thing is O.K. Dirac discovered the correct laws for relativity quantum mechanics simply by guessing the equation. The method of guessing the equation seems to be a pretty effective way of guessing new laws. This shows again that mathematics is a deep way of expressing nature, and any attempt to express nature in philosophical principles, or in seat-of-the-pants mechanical feelings, is not an efficient way.

It always bothers me that, according to the laws as we understand them today, it takes a computing machine an infinite number of logical operations to figure out what goes on in no matter how tiny a region of space, and no matter how tiny a region of time. How can all that be going on in that tiny space? Why should it take an infinite amount of logic to figure out what one tiny piece of space/time is going to do? So I have often made the hypothesis that ultimately physics will not require a mathematical statement, that in the end the machinery will be revealed, and the laws will turn out to be simple, like the chequer board with all its apparent complexities. But this speculation is of the same nature as those other people make--'I like it', 'I don't like it',--and it is not good to be too prejudiced about these things.

[Fredrik]

I hate to disagree, but science is (from American Heritage Dictionary) the observation, identification, description, experimental investigation, and theoretical explanation of phenomena. I would say that length contraction and time dilation are "phenomena".

I agree with both of you, even though you say you disagree with him. I agree with you because if we can't say e.g. that GR explains Newton's law of gravity, then we might as well remove the word "explain" from the English language since there are no better explanations than that. However, I also agree with Jesse, because he just said that science doesn't explain the fundamental laws. Newton's law of gravity isn't fundamental in GR, but Einstein's equation is. So what he said is consistent with my view, which is that GR explains Newton's law of gravity but not Einstein's equation, just like Newton's theory of gravity explains the elliptical orbits and falling apples but not the inverse square law.

Edit: Lol, you guys wrote a whole bunch of posts while I wrote this.

[JesseM]

BTW, the whole discussion about the speed of sound being the ultimately limit, and sound still having a medium. Wouldn't that seriously change our formulation of the laws of physics?
Wouldn't what change our formulation, exactly? The thought-experiment as I understood it was if the laws of physics transformed according to an altered transformation where the speed of light c was replaced by the speed of sound s, but sound waves were still understood as waves in a physical medium made of discrete particles. Of course this would mean very different laws of physics from the real universe, that's why it's a thought experiment! I suppose in order for waves in the air to move at the speed of sound, the molecules of air would individually have to move this fast (or they'd have to be the equivalent of tachyons, moving even faster than sound), so they'd have to have zero rest mass (or imaginary rest mass as with tachyons).
Under the present formulation of special relativity, anything travelling with constant speed in any special relativistic inertial reference frame must have zero rest mass. So if experiments suggested that the thing with constant speed in any reference frame had mass, we might have to rethink how mass-energy transforms in a frame change. Or would we have to ditch the Principle of Special Relativity?
I've forgotten precisely what additional assumptions beyond Lorentz-invariance are used to derive the relation E^2 = m^2*c^4 + p^2*c^2 (which naturally implies that if a particle is moving at the speed of light, the only way it can avoid having infinite energy is if it has 0 rest mass m, since p = mv/\sqrt{1 - v^2/c^2} which will approach infinity in the limit as v approaches c unless m is zero). I think you might need to assume conservation of energy and momentum to derive it but I'm not sure. Whatever additional assumptions you need are pretty basic, I think.

[granpa]

No, I meant what I wrote. I don't think the Principle of Special Relativity and the Lorentz transformations would work with 2 invariant speeds. At least one of them must give. I'm only thinking within SR, no GR, inflation etc.

my bad. you meant 'anything moving at that speed which is constant for all observers'.

still not sure what you mean about photon mass. are you talking about air molecules that would have to move at least as fast as the sound wave. I'm not sure that holds for a solid medium though (waves can move pretty fast through a spring). but even if it did it is a fact that space itself seems to be able to move faster than light.

[rbj]

But you were talking about different sets of axioms which could be used to derive Lorentz-symmetry, which presumably is what you mean by "the invariancy of the laws of physics (for inertial observers)". But a key point here is that this description is overly vague, since without some additional assumptions it could also describe Galilei symmetry in Newtonian physics. To derive Lorentz-symmetry, you can start from the axiom that all fundamental laws of physics are the same in every inertial frame, plus the axiom that the speed of light is the same in every inertial frame;...

i've been in this argument before. in some other thread, i was saying (and i still maintain) that the 2nd postulate of SR is unnecessary or superfluous when you have the first. the second postulate (the constancy of c) is a consequence of the first (that the laws of physics remain invariant for every inertial frame of reference). by "laws of physics", i mean not only the functional form of the laws, but also that the parameters (like c, G, \hbar, and \epsilon_0) in those laws remain invariant. two different sets of Maxwell's equations, identical in every respect except for the permittivity parameter, are not identical laws of physics. "identical", in the strong sense of the word, means not only qualitatively the same, but also quantitatively the same.

the "plus" is semantically not necessary.

time dilation and length contraction are a consequence of the fact that every inertial observer observe identical laws of nature in observed phenomena which means they observe identical speeds of propagation of the E&M interaction as well as all other "instantaneous" interactions (gravitation and nuclear).

[Fredrik]

i've been in this argument before. in some other thread, i was saying (and i still maintain) that the 2nd postulate of SR is unnecessary or superfluous when you have the first. the second postulate (the constancy of c) is a consequence of the first (that the laws of physics remain invariant for every inertial frame of reference).

I don't think that's exactly right. You can't derive the second from the first. What I've been saying in other threads (and still maintain) is that Einstein's "postulates" are ill-defined because they use the term "inertial frame" without a definition, and that any definition of "inertial frame" that's appropriate for SR must include both of Einstein's "postulates" in some form.

Einstein's "postulates" shouldn't be treated as axioms. They are just a list of properties that he wanted the theory he was trying to find to have.

[atyy]

still not sure what you mean about photon mass.

I've forgotten precisely what additional assumptions beyond Lorentz-invariance are used to derive the relation E^2 = m^2*c^4 + p^2*c^2 (which naturally implies that if a particle is moving at the speed of light, the only way it can avoid having infinite energy is if it has 0 rest mass m

Hi granpa, JesseM's quote is what I'm talking about. I believe if you have the Principle of Relativity (existence of a class of reference frames moving with constant velocity relative to each other in which the laws of physics all look the same), and you also have an velocity vi that is invariant in all the frames, from those 2 assumptions you can derive the Lorentz transformations, with vi replacing the usual speed of light c. With some additional assumptions, which JesseM and I have both forgotten, we can derive E=mvi2, where m is the relativistic mass. From which we see that a thing moving at the vi must be massless. So if sound were to be a thing that travelled at vi, and it also had mass, then presumably at least one of the assumptions in getting to E=mvi2 must be wrong.

Edit:
it seem that there is no consensus among today scientist that there is no "something" like ether as opposite to vacuum....
I'm having second thoughts that a massive medium for the thing that travels at the invariant velocity causes difficulties. I'm not really sure, so I'm just going to state a bunch of stuff and let someone correct it. The dispersion relation for a phonon is like a photon. So maybe even though phonons are made from a medium, they can be considered massless. And maybe photons, by analogy to phonons, can be considered to be made from a medium. I wonder if ricmat is thinking about a model like this: http://arxiv.org/abs/cond-mat/0210040

[atyy]

two different sets of Maxwell's equations, identical in every respect except for the permittivity parameter, are not identical laws of physics. "identical", in the strong sense of the word, means not only qualitatively the same, but also quantitatively the same.

the "plus" is semantically not necessary.

I agree, but I think you are counting Maxwell's equations as a zeroth postulate, whereas JesseM doesn't have this zeroth postulate and puts the constancy of the speed of light as a second postulate. So the number of postulates is still the same, ie.

Maxwell's equations + Principle of Relativity = Principle of Relativity + constancy of speed of light

[DaleSpam]

NO. the point isnt that the speed of sound would be constant when the medium was moving. the point is that the speed of sound would be constant even when the observer was moving. "Six of one, half-dozen of the other"

[granpa]

So if sound were to be a thing that travelled at vi, and it also had mass, then presumably at least one of the assumptions in getting to E=mvi2 must be wrong.


it isnt clear to me that the particles in a solid which is transmitting a sound would necessarily be moving at the speed of sound. one can increase the speed of sound simply by increasing the stiffness of the material and decrease the motion of the particles by simply decreasing the amplitude of the sound. or at least, I guess you can. I'm not an expert on sound. or anything else for that matter.

[Al68]

I don't think it's a good idea to approach questions in philosophy of science by appealing to dictionary definitions. The definition is good enough to cover most situations in science, where you're explaining some high-level laws governing a system by appealing to more fundamental laws which govern the basic parts of that system (reductionism); but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws?

Here is Feynman writing about this topic in The Character of Physical Law, using gravitation as an example:

Well, if we did have a good "theoretical explanation", it would belong in a physics textbook, not a literature textbook. So I would consider it physics. The fact that we don't have such a theory doesn't mean that such a theory is not within the scope of physics.

Al

[JesseM]

Well, if we did have a good "theoretical explanation", it would belong in a physics textbook, not a literature textbook. So I would consider it physics. The fact that we don't have such a theory doesn't mean that such a theory is not within the scope of physics.
I repeat my question: but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws? I can't imagine what a theoretical explanation of fundamental laws would even look like (aside from boiling them down to some minimal set of axioms), so it's not clear what you're imagining here, but it sounds like you're talking about something totally unprecedented in the history of science.

[atyy]

it isnt clear to me that the particles in a solid which is transmitting a sound would necessarily be moving at the speed of sound. one can increase the speed of sound simply by increasing the stiffness of the material and decrease the motion of the particles by simply decreasing the amplitude of the sound. or at least, I guess you can. I'm not an expert on sound. or anything else for that matter.

Yes, hence the second thoughts in my above post. So I guess the question can be split in 2:
1) What transformations are consistent with 2 invariant speeds (speed of light and something else). I suppose this us related to doubly special relativity.
2) Can light (and gravity) be usefully modelled as a medium? For light, it appears the answer is yes. For gravity, the answer is unknown, but there are several intriguing leads (http://arxiv.org/abs/0712.0427)

[granpa]

2 invariant speeds?

it gets confusing since we are talking about sound but what we are really talking about is light. I never meant to say anything implying 2 invariant speeds.

[atyy]

Well, if we did have a good "theoretical explanation", it would belong in a physics textbook, not a literature textbook. So I would consider it physics. The fact that we don't have such a theory doesn't mean that such a theory is not within the scope of physics.

Al

I repeat my question: but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws? I can't imagine what a theoretical explanation of fundamental laws would even look like (aside from boiling them down to some minimal set of axioms), so it's not clear what you're imagining here, but it sounds like you're talking about something totally unprecedented in the history of science.

There's an interesting discussion by Wen in his Quantum Field Theory book (OUP 2004, p 12):
Chinese philosophers theorized that the division could be continued indefinitely, and hence that there were no elementary particles. Greek philosophers assumed that the division could not be continued indefinitely ... Those ultimate particles were called "atomos".

He quotes the Dao De Jing (p11): The Dao that can be stated cannot be eternal Dao. The Name that can be named cannot be eternal Name. The Nameless is the origin of the universe. The Named is the mother of all matter.

Which he mischievously translates as (footnote, p11): The physical theory that can be formulated cannot be the final ultimate theory. The classification that can be implemented cannot classify everything. The unformulable ultimate theory does exist and governs the creation of the universe. The formulated theories describe the matter we see everyday.

Preface (pviii): we still know so little about the richness of nature. However, instead of being disappointed, I hope the readers are excited by our incomplete understanding. ... The human imagination is also boundless. ..... I wonder which will come out as a 'winner', the richness of nature or the boundlessness of the human imagination.

[atyy]

2 invariant speeds?

it gets confusing since we are talking about sound but what we are really talking about is light. I never meant to say anything implying 2 invariant speeds.

Ah, I see, the discussion was just on the second point then. Another interesting quote from Wen's QFT book, this particular one is quite uncontroversial, but he has nice imagery:
Our vacuum is more like an ocean which is not empty. Light and fermions are collective excitations that correspond to certain patterns of 'water' motion.

[Fredrik]

I repeat my question: but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws? I can't imagine what a theoretical explanation of fundamental laws would even look like (aside from boiling them down to some minimal set of axioms), so it's not clear what you're imagining here, but it sounds like you're talking about something totally unprecedented in the history of science.
I don't know what his answer is, but my answer would be that a deeper, more fundamental theory, can be considered a theoretical explanation of the fundamental laws in your theory. For example, general relativity is a theoretical explanation of Newton's law of gravity (the inverse square law). Newton's theory of gravity can't explain the inverse square law because it's a fundamental law of the theory, but GR can explain it because it's just one of many results that can be derived from the fundamental laws of that theory.

[JesseM]

I don't know what his answer is, but my answer would be that a deeper, more fundamental theory, can be considered a theoretical explanation of the fundamental laws in your theory. For example, general relativity is a theoretical explanation of Newton's law of gravity (the inverse square law). Newton's theory of gravity can't explain the inverse square law because it's a fundamental law of the theory, but GR can explain it because it's just one of many results that can be derived from the fundamental laws of that theory.
My question was, 'but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws?' If a given theory turns out to be an approximation of some more fundamental theory, like Newtonian gravity is understood as an approximation of GR, that shows that the first theory (Newtonian gravity) wasn't really one of the "most fundamental laws". Of course I'm assuming here that there are some final, most fundamental laws out there waiting to be discovered; as atyy brought up, it's conceivable that it's just wheels within wheels forever, that every particle is really a composite entity made up of even smaller particles, etc.

[MeJennifer]

INewton's theory of gravity can't explain the inverse square law because it's a fundamental law of the theory, but GR can explain it because it's just one of many results that can be derived from the fundamental laws of that theory.
I think you are mistaken. In fact if you look how Einstein derived GR you will see he simply included the Newtonian limit as a given. Technically GR is simply Newtonian gravity plus relativistic effects. GR does not explain anything, it is simply a more accurate theory.

If you think I am mistaken, please demonstrate how it explains the inverse square law. Or an even simpler question: How does GR get to the Newtonian limit.

[rbj]

I think you are mistaken. In fact if you look how Einstein derived GR you will see he simply included the Newtonian limit as a given. Technically GR is simply Newtonian gravity plus relativistic effects. GR does not explain anything, it is simply a more accurate theory.

If you think I am mistaken, please demonstrate how it explains the inverse square law. Or an even simpler question: How does GR get to the Newtonian limit.

does John Baez do that here (http://www.math.ucr.edu/home/baez/einstein/node6a.html)?

or maybe Sean Carroll does that here (http://preposterousuniverse.com/grnotes/grtinypdf.pdf) or here (http://preposterousuniverse.com/grnotes/four.ps)?

i think they can derive the inverse-square relationship (or maybe it's a 1/r relationship for potential energy) for the flat space-time limit. the constant of proportionality in the Einstein equation (8 \pi G) does come about to be compatible with Newtonian gravitation.

i can't actually do the math myself (i am ashamed to confess i never figured out tensors), but it appears that this is what they do.

[kev]

I tend to agree with Jennifer here. I can not see how a physicist isolated in a small spacestation that had never experienced gravity or even heard of it, would conclude from a knowledge of Special Relativity alone, that two particles would have to move towards each other, let alone that they accelerate towards each other with an acceleration inversely proportional to the distance separating them.

As far as I can tell General Relativity started with a knowledge that we experience "Newtonian gravity" and extrapolated or reverse engineered that knowledge to more extreme conditions than we normally experience. It is hardly surprising that Newtonian gravity is recovered from GR in the weak field limit because GR started with that assumption. lease do not get me wrong here. I am not saying there is anything wrong with GR, I am just saying that it does not fundementally explain or predict gravity and just provides a pretty good mathematical description of what we observe.

Put it another way. In multiverse theories where there are any number of possible universes each with their own laws of nature, would a universe that obeys the laws of Special Relativity have to have an inverse square law of gravity in the weak field limit or come to that, any gravity at all?

[JesseM]

I tend to agree with Jennifer here. I can not see how a physicist isolated in a small spacestation that had never experienced gravity or even heard of it, would conclude from a knowledge of Special Relativity alone, that two particles would have to move towards each other, let alone that they accelerate towards each other with an acceleration inversely proportional to the distance separating them.
When did Fredrik say anything like that? He didn't say you could discover the inverse-square law from pure thought, he just said that if you already know the equations of GR you can get the inverse-square law as a derived consequence. Of course you could say the same thing about the equations of Newtonian gravity in some sense, so I'm not sure this is a totally clear distinction, but at least in Newtonian gravity it's obvious from the fundamental equations whereas in GR it's not.
As far as I can tell General Relativity started with a knowledge that we experience "Newtonian gravity" and extrapolated or reverse engineered that knowledge to more extreme conditions than we normally experience.
I don't know whether or not that's true of Einstein's original derivation as a historical matter, but it is at least true that GR can be derived from assumptions that have nothing to do with Newtonian gravity--on this page (http://www.physics.ucdavis.edu/Text/Carlip.html) Steve Carlip writes:
If you want to derive the Einstein field equations from scratch, you can do so without making very many assumptions. You must assume that

1. the geometry of spacetime is dynamical;
2. there are no extra fixed, nondynamical "background structures" that influence the geometry;
3. special relativity becomes a good approximation when gravitational fields are weak;
4. the field equations can be derived from a Lagrangian, or an action principle; and
5. the field equations involve no more than second derivatives; that is, they determine "accelerations" rather than requiring accelerations as initial data.

These assumptions lead almost uniquely to a set of field equations with two undetermined constants. One of these is Newton's constant, which determines the strength of the gravitatonal interaction. The other is the cosmological constant, Lambda.
I think it is also true that you can come up with theories that are identical to Newtonian gravity in every respect except for the fact that the strength of the force is inversely proportional to some other real power like r^2.05, whereas in GR you don't have this sort of wiggle room, trying to make it no longer obey an inverse-square law would give a very different theory (presumably it would require violating one of Carlip's basic assumptions above).

[granpa]

the electric field follows an inverse square law because space is 3 dimensional. aether theory explains this very well.

[Fredrik]

My question was, 'but when you reach the level of the most fundamental laws, what exactly would it mean to have a "theoretical explanation" of these laws?'

To me, the idea of a law being fundamental or not only makes sense within the framework of a specific theory. What you call "fundamental laws" seems to be what I would describe as "a final theory". If such a theory is found, it won't be possible to explain its postulates. This isn't a very deep statement. It's just the definition of what I mean by "final".

When did Fredrik say anything like that? He didn't say you could discover the inverse-square law from pure thought, he just said that if you already know the equations of GR you can get the inverse-square law as a derived consequence.

Thanks. If I hadn't been asleep I would have said something very similar.

To Kev and MeJennifer, I would like to add a couple of things:

I don't consider the way Einstein discovered SR and GR to be "derivations" of those theories. In both cases he wrote down a somewhat ill-defined list of properties that he wanted the theory to have, and then searched for a theory that had those properties. The reason why I can't consider this method a "derivation" is that the "list of properties" was ill-defined to begin with, and later made well-defined by the theory that was found. (E.g. we need Minkowski space to properly define the inertial frames in which the speed of light is supposed to be a constant).

I understand that your opinion is that the fact that GR was found by looking only for theories that could reproduce the Newtonian limit means that GR can't be said to explain the inverse square law. That is a valid opinion (about the meaning of the word "explain") but I don't agree with it. There is no deeper form of understanding than having a theory that agrees with experiment, so if derivation from a theory that agrees with experiment can't be considered an explanation, nothing can. It makes no difference to me (at all) how the theory was found. All that matters to me is what range of phenomena it's capable of describing and how well it agrees with experiment.

[rbj]

the electric field follows an inverse square law because space is 3 dimensional.

if you bring into this an additional concept of flux (http://en.wikipedia.org/wiki/Flux#Flux_definition_and_theorems_2), which is conserved and makes Gauss's Law possible. the concept of conserved flux seems natural and satisfying, but it wouldn't have to necessarily be the case. the inverse-square law of gravitation would require the same hypothesis; a gravitational flux emitted by quantities of mass, unless, like we're discussing here, the Newtonian inverse-square law is derived from some other more fundamental principle (like GR).

now, inverse-square laws regarding radiant intensity (E&M or acoustic) do necessarily follow from a combination hypotheses of conservation of energy and 3-dim space (both reasonable). the radiant energy (or power) comprises a natural form of "flux", which is conserved.

BTW, it is because of this concept of flux in inverse-square laws that make me wish that Planck units had originally normalized 4 \pi G and \epsilon_0 rather than normalizing G and 4 \pi \epsilon_0 as was done. i believe these rationalized Planck units are a little more natural (yielding simpler field equations) than the existing definitions. with any extraneous constants removed from the field equations, i think that might lead to insight to what might be behind such. we know that Nature isn't really performing a multiplication in her head to convert a particle wave frequency to its energy. that multiplication is necessary only because of the anthropocentric units we arbitrarily chose to use. and Nature doesn't give a rat's as$ what units humans (or some alien race) chose to use.

aether theory explains this very well.

i don't see a hypothetical aether having anything to do with the inverse-square relationship.

As far as I can tell General Relativity started with a knowledge that we experience "Newtonian gravity" and extrapolated or reverse engineered that knowledge to more extreme conditions than we normally experience. It is hardly surprising that Newtonian gravity is recovered from GR in the weak field limit because GR started with that assumption.

it's not surprising because of the correspondence principle (http://en.wikipedia.org/wiki/Correspondence_principle#Other_Scientific_Theories). any newer, more advanced, theory must degenerate to the old theory in the context where the old theory was known to be valid. even though Einstein knew that his new GR theory would need to do that, i don't think that Newtonian gravity was where he started and extrapolated from. i think it was those classic elevator and spaceship thought experiments (http://en.wikipedia.org/wiki/Introduction_to_general_relativity#Gravity_and_acceleration).

When did Fredrik say anything like that? He didn't say you could discover the inverse-square law from pure thought, he just said that if you already know the equations of GR you can get the inverse-square law as a derived consequence.

I don't know whether or not that's true of Einstein's original derivation as a historical matter, but it is at least true that GR can be derived from assumptions that have nothing to do with Newtonian gravity--

but, because of a concept of flux (which can be cooked up from pure thought) and knowledge of the mathematical fact that a sphere in 3-dimensional space has a surface area of 4 \pi r^2 can lead one to predict or hypothesize an inverse-square law for some quantity. doesn't mean, of course, that the hypothesis need not be tested in reality.

i think that Einstein first, from pure thought experiments with just a few really reasonable postulates (like the laws of physics are invariant for every inertial observer and that a free-falling observer cannot differentiate his or her state from being inertial - the equivalence principle), came up with SR, and with a little mathematical help from folks like Mercel Grossman, the GR. there is no evidence that Einstein ever drew on or referred to the Michaelson-Morley experiment and the null result, and i am convinced that it made little difference to him ("as if God had any choice in the matter"). assuming he knew of the experiment and result, Einstein was likely utterly not surprized. it's amazing what you can cook up from a very few extremely reasonable postulates, thought experiments, and math (all from pure thought). that is, if your brain is the size of a small planet and you have truly historical levels of insight. such persons are rare in history.

[JesseM]

there is no evidence that Einstein ever drew on or referred to the Michaelson-Morley experiment and the null result, and i am convinced that it made little difference to him ("as if God had any choice in the matter").
Well, in his original 1905 paper (http://www.fourmilab.ch/etexts/einstein/specrel/www/), in his first paragraph he discussed some theoretical reasons to suspect that electromagnetism doesn't have a preferred frame, but then in his second paragraph he said:
Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the "light medium,'' suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.
And the Einstein quote you're referring to is "What really interests me is whether God had any choice in the creation of the world"--this was not a positive assertion that he was confident God had no choice about special relativity as you made it sound, it seems like more of a general philosophical question about the laws of physics as a whole (if it's an accurate quote at all, there are a lot of fake Einstein quotes that have circulated around, you can really only trust the ones attributed to some published source).

[rbj]

thanks for the reference to the original 1905 paper. i stand corrected about that. he clearly indicates he knew of the MM experiment and result (but he should have cited it).

And the Einstein quote you're referring to is "What really interests me is whether God had any choice in the creation of the world"--this was not a positive assertion that he was confident God had no choice about special relativity as you made it sound, it seems like more of a general philosophical question about the laws of physics as a whole.

i disagree with you about his position about this (and i assume the quote is for real). i really think that Einstein is questioning whether the form of reality could possibly be different. of the fundamental (dimensionless) constants that go into the description of reality, that's different, but the functional form, i think that Einstein was wondering, even challenging, if they could possibly be different.

[JesseM]

disagree with you about his position about this (and i assume the quote is for real). i really think that Einstein is questioning whether the form of reality could possibly be different. of the fundamental (dimensionless) constants that go into the description of reality, that's different, but the functional form, i think that Einstein was wondering, even challenging, if they could possibly be different.
How is that disagreeing with me, though? That's just what I said, it was a philosophical question about the laws of physics as a whole.

edit: also, note that the quote is listed in the "misattributed" section of this page (http://www.billionquotes.com/index.php/Albert_Einstein#Misattributed). On the other hand, this page (http://74.125.95.104/search?q=cache:LEWS3b78ngYJ:www.aip.org/history/einstein/essay-einsteins-third-paradise.htm+%22interests+me%22+ernst+straus+einstein&hl=en&ct=clnk&cd=1&gl=us) claims that he said it to his assistant, Ernst Straus, but doesn't give a reference.

[granpa]

if light is thought of as a KIND OF sound wave in the aether then I believe it follows naturally that electric fields must follow an inverse square law. there is no difference between the electric field in a light wave and the electric field from an electron.

[DaleSpam]

A wave which propagates through a medium has a propagation velocity that depends on the medium and is relative to that medium. If, by some coincidence, the propagation velocity of a wave in some medium were equal to the invariant speed then all observers would measure the propagation velocity to be the invariant speed regardless of what they measure the velocity of the medium to be.

However, a wave that does require a medium must propagate at the invariant speed. Since light does not require a medium it propagates at the invariant speed, which is how we originally discovered the invariant speed and its implications for the geometry of spacetime.

Since the aether is otherwise undetectable, and since it would be an enormous coincidence if the propagation of light through the aether were equal to the invariant speed, and since the speed of light is more simply explained by assuming it does not require a medium, what is the value of the concept of aether?

[granpa]

your second paragraph is unclear.

it might be an enormous coincidence or it might indicate the existence of an underlying symmetry that we havent been smart enough to figure out yet.

why should light alone of all known waves not require a medium? it is much simpler to just take its wave nature as evidence of the existence of such a medium. in any event, relativity doesnt entirely eliminate the aether. it just renames it 'space'. according to relativity even empty space has properties.

[DaleSpam]

If a wave does not propagate in a medium then what other speed could it possibly propagate at besides the invariant speed?

[granpa]

zero

[DaleSpam]

Then it wouldn't be a wave

[granpa]

exactly

[atyy]

Since the aether is otherwise undetectable, and since it would be an enormous coincidence if the propagation of light through the aether were equal to the invariant speed, and since the speed of light is more simply explained by assuming it does not require a medium, what is the value of the concept of aether?

Not much within SR (only pedagogically for explaining the significance of the Michelson-Morley null result). But apparently it's useful within condensed matter physics (artificial "light" and "electrons"), and the interplay between condensed matter theories, quantum field theories and the search for a quantum theory of gravity. In these theories, the "aether" is typically not so much embedded in space, but spacetime and matter emerge from the "aether".

[atyy]

I understand that your opinion is that the fact that GR was found by looking only for theories that could reproduce the Newtonian limit means that GR can't be said to explain the inverse square law. That is a valid opinion (about the meaning of the word "explain") but I don't agree with it. There is no deeper form of understanding than having a theory that agrees with experiment, so if derivation from a theory that agrees with experiment can't be considered an explanation, nothing can. It makes no difference to me (at all) how the theory was found. All that matters to me is what range of phenomena it's capable of describing and how well it agrees with experiment.

Would it help to say that the inverse square law cannot be derived from GR, only an approximate inverse square law. So GR explains why we were deceived that it is an inverse square law? (If the inverse square law were exactly derivable, we wouldn't have perihelion precession)

[DaleSpam]

granpa, you are not making any sense

[granpa]

it does not follow that a wave without a medium would travel at c. it follows that a wave without a medium would not travel at all and if it did it wouldnt be a wave.

[DaleSpam]

granpa, first, even a propagation velocity of 0 wouldn't work, because 0 in one frame is non-zero in another frame. Second, what you are really saying is that it is not possible for any wave to propagate without a medium, do you have any logical reason to think that?

None of the four fundamental forces have a medium in which they propagate.

[granpa]

none of the forces have a medium? what do you think the aether is?

do you have any reason to think that a wave can propagate without a medium? I've never seen one do so.

[DaleSpam]

Yes, I have a reason to think that a wave can propagate without a medium, and if you have never seen one do so then you must be blind.

\nabla \cdot \mathbf{E} = 0

\nabla \cdot \mathbf{B} = 0

\nabla \times \mathbf{E} = - \frac{\partial\mathbf{B}} {\partial t}

\nabla \times \mathbf{B} = \mu_0\varepsilon_0 \ \ \frac{\partial \mathbf{E}} {\partial t}

So according to you, all four fundamental forces require a medium in which to propagate, they all share the same medium, it is completely undetectable, it just happens that all four forces have the same propagation speed in this medium, and that propagation speed also happens to be the invariant speed. :rolleyes:

Have you even one piece of evidence to support this rather long list of coincidences?

[granpa]

you're using light as an example of a wave without a medium to support your belief that light can move without a medium?

no I dont know that they all share the same medium (it would obviously be simpler if they did not). and you dont know that they all propagate at the same speed.

[JesseM]

you're using light as an example of a wave without a medium to support your belief that light can move without a medium?
Do you think there is something inherently impossible about the idea of an electromagnetic field which fills all of space, and which assigns electrical and magnetic force vectors to each point in space? If not, then you should have no additional problem with the idea that this field obeys Maxwell's equations, which means that waves in the magnitude of the force vectors will propogate at c.

[granpa]

whats the difference between the electric field in a light wave and the electric field from a charge? if aether explains one then it explains the other. I cant imagine why you would think otherwise.

[DaleSpam]

you're using light as an example of a wave without a medium to support your belief that light can move without a medium?No, I am using Maxwell's equations as a justification of why I believe that electromagnetic waves can propagate without a medium. According to the equations all that is needed are the fields themselves, no medium is required.

You still have yet to offer any evidence supporting your position.

[granpa]

maxwells equations describe light, they dont explain it.

[DaleSpam]

Of course not. But they describe it as a wave that does not require a medium in which to propagate.

Still waiting on any evidence of the aether ...

[granpa]

you're just being argumentative now. go argue with someone else.

since, moreover, my own points are being ignored I see no reason to continue.

[JesseM]

whats the difference between the electric field in a light wave and the electric field from a charge? if aether explains one then it explains the other. I cant imagine why you would think otherwise.
What are you talking about? Of course there is no difference, the point is that there is nothing inherently contradictory about the idea that there is only the electromagnetic field operating in both cases, no additional aether needed to explain either one. Do you think there is something contradictory about this idea of space being filled by an electromagnetic field which obeys Maxwell's equations?

[atyy]

Do you think there is something inherently impossible about the idea of an electromagnetic field which fills all of space, and which assigns electrical and magnetic force vectors to each point in space? If not, then you should have no additional problem with the idea that this field obeys Maxwell's equations, which means that waves in the magnitude of the force vectors will propogate at c.

If the electric field fills all space, that is coming very close to the aether isn't it? I kinda of do away with the aether in classical field theory by thinking that light can move into a region where it previously wasn't.

The idea of an electric field filling all space comes very close to quantum field theory, in which light and electrons are excitations of the photon and electron fields which pervade all space, and are very much like the aether in that sense. The fields are more primary than the excitations, because there are (physically meaningful) excitations of the fields which don't really correspond to photons or electrons.

[DaleSpam]

you're just being argumentative now. go argue with someone else.Calling me names doesn't help your position. A scientific theory must be backed up with evidence, and after more than 100 years of looking there is still no direct evidence of the luminiferous aether.

[JesseM]

If the electric field fills all space, that is coming very close to the aether isn't it?
Not really, because it doesn't have a rest frame of its own--I would say that's the defining difference between aether theories and field theories.

[granpa]

when a changed particle moves at relativistic speed its field becomes compressed. how does the field far from the particle know how much to compress itself if it doesnt have a velocity of its own?

the wave nature of light is the evidence for an aether. a medium is intrinsic to the definition of a wave. no wave has ever been empirically observed to not have a medium.

[JesseM]

when a changed particle moves at relativistic speed its field becomes compressed. how does the field far from the particle know how much to compress itself if it doesnt have a velocity of its own?
I couldn't tell you about the details, but presumably it follows from Maxwell's equations. And Maxwell's equations are local ones (stated in differential form (http://www.physicsforums.com/showthread.php?t=217033)), so the vector at each point only has to "know" how vectors infinitesimally close to it are behaving...I assume you could approximate the field's behavior to arbitrary accuracy using a cellular automaton (http://mathworld.wolfram.com/CellularAutomaton.html) style computer simulation where the electromagnetic field vectors for each cell update themselves every time-increment based only on the field vectors of neighboring cells.
the wave nature of light is the evidence for an aether.
Field theories predict waves too, so no, it isn't "evidence".
a medium is intrinsic to the definition of a wave.
No it isn't. If you think it is, please give your "definition of a wave".
no wave has ever been empirically observed to not have a medium.
You're just begging the question here, as we have empirically observed electromagnetic waves, and there is no evidence whatsoever that they have a medium...what else would it mean to "observe" a wave "to not have a medium", if not just a failure to observe any evidence that it does have a medium, and a theory that adequately explains the wave without the need to invoke a medium?

[DaleSpam]

the wave nature of light is the evidence for an aether.
When asked to offer evidence that light propagates through a medium the best that you have been able to come up with is to point to a few other kinds of waves and essentially say "sound waves and ocean waves are waves in mediums therefore all waves must have mediums". This is certainly not direct evidence for the aether. At best it is an assumption that was reasonable a century ago.

Your logic is "hamburgers and patty melts are sandwiches with beef therefore all sandwiches must have beef". Then, when confronted with a BLT, you simply assert the existance of some beef that miraculously has no taste, no calories, no texture, no weight etc. When challenged to produce evidence of the beef you simply assert that a BLT is a sandwich and therefore there must be beef even if we cannot otherwise detect it.

Do you not see how embarassingly ridiculous your logic is? As JesseM mentioned, you are assuming your conclusion, and going to absurd lengths to do so.

[granpa]

you can hardly use the idea that light has no medium as evidence that light has no medium.

it is you who are using circular logic.

[JesseM]

you can hardly use the idea that light has no medium as evidence that light has no medium.
You can use the fact that we have a self-consistent theory of electromagnetism which gives correct predictions about all empirical observations related to light, and which does not involve a medium, as evidence that there is no physical motivation for postulating that there should be a medium, although of course we can never "prove" there isn't. Similarly, the fact that we have a self-consistent theory of evolution which adequately explains observations and which does not require an intelligent designer is evidence that there is no scientific need to postulate such a designer.
it is you who are using circular logic.
You're the one who made the assertion "no wave has ever been empirically observed to not have a medium." Please answer my question about what you think it would mean for a wave to have been "empirically observed not to have a medium"; if you can't think of any (hypothetical) observations that would qualify, then your argument is nonsensical.

[DaleSpam]

I have used Maxwell's equations and a century of failed experiments designed to detect it as evidence that it doesn't exist.

Where's the beef, granpa?

[granpa]

You can use the fact that we have a self-consistent theory of electromagnetism which gives correct predictions about all empirical observations related to light, and which does not involve a medium, as evidence that there is no physical motivation for postulating that there should be a medium, although of course we can never "prove" there isn't. Similarly, the fact that we have a self-consistent theory of evolution which adequately explains observations and which does not require an intelligent designer is evidence that there is no scientific need to postulate such a designer.

You're the one who made the assertion "no wave has ever been empirically observed to not have a medium." Please answer my question about what you think it would mean for a wave to have been "empirically observed not to have a medium"; if you can't think of any (hypothetical) observations that would qualify, then your argument is nonsensical.

theory is consistent with aether as well.

it wouldnt mean anything because its meaningless. a wave by definition requires a medium. I stand behind what I said. you cant use the idea that light has no medium as evidence that light has no medium. its that simple.

[JesseM]

theory is consistent with aether as well.
Unlike electromagnetic theory, aether theory postulates that there are facts about the physical world which are impossible to determine experimentally, in this case the rest frame of the aether. I suppose you could take any successful theory and add to it the idea of invisible ghostly dragons which are impossible to detect with any physical instrument, and then say the evidence is "consistent with a theory involving invisible ghostly dragons", but the dragons would obviously be superfluous to the theory, and the same is true of the aether.
it wouldnt mean anything because its meaningless. a wave by definition requires a medium. I stand behind what I said.
I specifically asked you for a definition of a wave--if you can't provide one your statement is meaningless. I would say that in physics a wave is a spatial/temporal variation in any physical quantity (whether density in a medium or electromagnetic field strength in the electromagnetic field) which obeys a wave equation (http://mathworld.wolfram.com/WaveEquation.html). If you think a wave "by definition" must be a variation in the density of a medium, can you point to any reputable mainstream sources which use such a definition, or is it one you just made up yourself?
you cant use the idea that light has no medium as evidence that light has no medium.
Not the mere idea, no, but you can use the fact that all observational evidence agrees with a theory which does not involve a medium as evidence that there is no physical need for a medium. Just like you can use the fact that all the evidence is consistent with Darwin's theory of evolution as evidence that there is no scientific need for an intelligent designer. Your argument is analogous to a creationist saying "you can't use the idea that evolution happened with no intelligent designer as evidence that there is no intelligent designer". Of course he'd be right that you can't use the mere idea as evidence, but you can use the fact that this theory has enjoyed great success in the empirical realm of making predictions and explaining observations as evidence that there is no need to postulate such a designer. If you disagree with this analogy, please explain how you think your argument is different from that of my hypothetical creationist above.

[granpa]

well it sounds like you're not really eliminating the aether. your just depriving it of its material nature. that I could not and would not argue against (though I dont think we really know).

[George Jones]

well it sounds like you're not really eliminating the aether. your just depriving it of its material nature. that I could not and would not argue against (though I dont think we really know).

The concept of aether is well outside of mainstream physics as it is now practiced, and, as such, its discussion violates the Physics Forums posting guildelines

http://www.physicsforums.com/showthread.php?t=5374
Overly Speculative Posts: One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Posts deleted under this rule will be accompanied by a private message from a Staff member, and, if appropriate, an invitation to resubmit the post in accordance with our Independent Research Guidelines. Poorly formulated personal theories, unfounded challenges of mainstream science, and overt crackpottery will not be tolerated anywhere on the site.

[DaleSpam]

a wave by definition requires a medium. I stand behind what I said. Then your proper course of argument is to claim that light is not a wave, by definition, since it does not have a medium. You cannot define something into existence.

Definition: Christmas is the day that Santa Claus delivers presents to children all around the world
Observation: December 25 is Christmas
Conclusion: Santa Claus exists

Aren't you a little old for such logic, granpa?

It is fine for you to use a medium as part of the definition of a wave, but then, in order to call something a wave, you must independently demonstrate that it has a medium. Otherwise you could define anything into existance simply by judicious choice of definition, as shown above.

[Vanadium 50]

how does the field far from the particle know how much to compress itself if it doesnt have a velocity of its own?

How does a thermos know to keep something hot hot and something cold cold?

[Vanadium 50]

theory is consistent with aether as well.

If so it's a rather bizarre aether. It's absolutely undetectable, and has properties such that they are consistent with no aether.

[rbj]

theory is consistent with aether as well.

no theory with any observational support.

all sorts of people, past and present, have theories that are consisntant with magic.

a wave by definition requires a medium.

no, waves do not, by definition, require a medium. some waves are a displacement quantity regarding some medium (like sound or waves on a string). but disturbances in interaction from the motion of charge (E&M) or sub-atomic particles (nuclear) or mass (gravity) do not propagate in a medium. the reach of those interactions are across a vacuum, nothing but space. and the speed of propagation of such waves are all the same c which is a consequence of the nature of space and time and not a property of which of these interactions are causing it. all of those interaction are virtually instantaneous, until distances get large enough that those interactions with sufficient effect at those distances (E&M and gravity) gets large enough that some 3rd party can see that the time elapsed between cause and effect of the interaction is not zero.

I stand behind what I said. you can't use the idea that light has no medium as evidence that light has no medium. its that simple.

you can stand behind what you said, but you'll be shot anyway. no one (except maybe you) are using circular logic. we're saying that light (the E&M interaction) has no medium because no properties of such a hypothetical medium have ever been measured, and they should have been measured with the Earth passing through this medium at a speed exceeding 105 km/hr at least some time of the year.

it's simple. and you're simply mistaken.

[rbj]

you're just being argumentative now. go argue with someone else.

pot calling the kettle black?

since, moreover, my own points are being ignored

no, they're being examined (the opposite of ignored) and rejected.

I see no reason to continue.

that would be welcome.

[Primordial]

How does a thermos know to keep something hot hot and something cold cold?
Entropy.

[DrGreg]

i've been in this argument before. in some other thread, i was saying (and i still maintain) that the 2nd postulate of SR is unnecessary or superfluous when you have the first. the second postulate (the constancy of c) is a consequence of the first (that the laws of physics remain invariant for every inertial frame of reference). I almost agree, but the 2nd postulate does contain within it two assumptions that don't automatically follow from the 1st postulate.

1. The speed of light is finite, not infinite.
2. The speed of light is independent of the object that emitted it (which is another way of saying that one photon cannot overtake another following the same spatial path).

Once you have those properties, the fact the speed has to have the same value in every inertial frame follows from the first postulate (otherwise you could identify one frame from another).

For a published paper expressing this view and giving some historical context, see "Two myths about special relativity" (http://www.uio.no/studier/emner/matnat/fys/FYS-MEK1110/v06/MythsSpecRelativAJP193.pdf), Ralph Baierlein, American Journal of Physics 74, Issue 3, March 2006, pp. 193-195 (http://link.aip.org/link/?AJPIAS/74/193/1), section III.

[DaleSpam]

the 2nd postulate does contain within it two assumptions that don't automatically follow from the 1st postulate.

1. The speed of light is finite, not infinite.
2. The speed of light is independent of the object that emitte