Relativity, speed of light and stuff

In summary, the conversation discusses the concept of the speed of light and how it is considered the limit in physics. The speaker questions why light is seen as so special and how it is measured. They also question the possibility of travel faster than light and how it relates to the laws of physics. The concept of coordinate systems and their role in understanding the speed of light is also mentioned.
  • #1
ricmat
2
0
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 can't 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 can't "see" at over speed of light?


this is - if we had no eyes, and only ears, wouldn't we think that sound speed would be the greatest speed ever? wouldn't 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 can't 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 traveling 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
 
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  • #3
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.
 
  • #4
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.
 
  • #5
ricmat said:
if we had no eyes, and only ears, wouldn't we think that sound speed would be the greatest speed ever? wouldn't 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.
 
  • #6
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?
 
  • #7
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:

DaleSpam said:
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...
 
  • #8
DaleSpam said:
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.
 
  • #9
ricmat said:
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 http://en.wikipedia.org/wiki/Cherenkov_radiation" [Broken]
Special relativity is based upon the speed of light in a vacuum only. There is something special about this speed BECAUSE then light is traveling only through a vaccuum... get me?
 
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  • #10
ricmat said:
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
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.
 
  • #11
granpa said:
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.
 
  • #12
DaleSpam said:
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 doesn't have a medium.
 
  • #13
granpa said:
nobody would be claiming that because its velocity is the same for all observers that sound doesn't have a medium.
Of course not. But think about it. Why not?
 
  • #14
I imagine that you mean that we can't 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.
 
  • #15
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.
 
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  • #16
granpa said:
of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesn't have a medium.

i might claim that, given the premise (which we don't believe).
 
  • #17
granpa said:
but nobody would be claiming that because its velocity is the same for all observers that sound doesn't have a medium.
The speed of sound is not the same for all observers.
 
  • #18
Doc Al said:
The speed of sound is not the same for all observers.

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

read post 8
 
  • #19
granpa said:
you seem to have missed the 'would' in my post.

read post 8
Oops. My bad.
 
  • #20
rbj said:
i might claim that, given the premise.

An engineer, a mathematician, and a physicist were traveling 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".
 
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  • #21
granpa said:
of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesn't have a medium.

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

granpa said:
An engineer, a mathematician, and a physicist were traveling 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".

granpa said:
... 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 don't know 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.
 
  • #22
rbj said:
i don't know 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? what's your point?

rbj said:
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 wouldn't cause the medium of sound to magically disappear.
 
  • #23
granpa said:
rbj said:
granpa said:
of course. but nobody would be claiming that because its velocity is the same for all observers that sound doesn't have a medium.

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

An engineer, a mathematician, and a physicist were traveling 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 traveling 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]
 
  • #24
DaleSpam said:
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?
 
  • #25
Primordial said:
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).
 
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  • #26
granpa said:
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 wouldn't 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.)
 
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  • #27
oh, and the other point is that the time dilation and length contraction are the effect and the invariancy of c is the cause.
 
  • #28
rbj said:
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.
 
  • #29
rbj said:
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).
 
  • #30
DaleSpam said:
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 knowledgeable 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, who reveals Himself in the lawful harmony of the world,..."
 
  • #31
JesseM said:
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.
 
  • #32
JesseM said:
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.
 
  • #33
JesseM said:
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 don't agree with that.
 
  • #34
granpa said:
I don't 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 [tex]\sqrt{1 - v^2/s^2}[/tex] 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.
 
  • #35
rbj said:
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.
 

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