Why Doesn't Relativity Apply to Light?

[AnssiH]

well there is velocity "addition", it just that this:

\frac{v_1 + v_2}{1 + \frac{v_1 v_2}{c^2}}

is how you add it.

Whoops, let me rephrase myself; There is no velocity addition to the motion of light.

That assertion was a response to Line's original question; "I'm trying to understand if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?"

i don't quite get that. if c is not isotropic, then at least one set of laws of nature (Maxwell's Equations) are not the same (and least not quantitatively) for every observer. in contrast, for sound, there is a quantitative difference in the physical equations describing sound propagation between one frame that is wind-free and another that moves through the air at a known velocity.

That's valid if you consider the ether theory, but if you consider emitter theory (which is how I interpreted Line's velocity addition question), then there is no ether to reveal any universal motion against light. In velocity addition scheme Maxwell's laws would be found to be just what they are within any single system (like in experiments on earth). The refutations against emitter theory are different than those against ether theory.

Yet another twist to the tale comes from considerations about just what must be the relationship between matter and space in any scheme (I referred to "naive idea of space" earlier). There is no empirical data to give us any metaphysical "border" between an atom and space, and it seems quite useful to consider atoms in an extended sense, or that the "environment" between pieces of matter is a manifestation of the given pieces of matter themseves. I.e. that there is no such thing as empty space, but information simply moves in matter (in its extended sense).

This makes the borderline between different objects quite fuzzy, but so it appears to be in any case. This could provide some openings to consider how gravity exists and how QM phenomena exists (=light can know something about the state of the atom it is heading to). And yes, it does bring the tale very close to GR, only GR builds this picture in spacetime instead of just space (which may or may not be the correct way).

Well, the point I want to make is that relativity too, was laid onto quite simple assumptions about what is space, and AFAIK no one really investigated the possibility that matter is "extended". Hmmm, but this hasn't got that much to to do with Line's question anymore, so Line, don't confuse your head with such ideas before you get relativity. :)

 

[AnssiH]

OK so light doesn't move? SO in other words it's us who are moving at the speed of light?

Well sort of; some people like to think of it as if we are moving through spacetime at the speed of light. At rest we move through time dimension and objects that are moving in our frame use some of that speed to move in space dimensions also.

But while it may seem like a fun idea, I would not encourage that sort of interpretation. It doesn't really give you the whole picture (you have to understand what it means to change direction in space), and any idea of "motion" inside static spacetime is rather unelegant, to the point of being invalid as an ontological interpretation (while as a tool to understand real phenomena it may be just fine).

So, if you kind of get how relativity of simultaneity could offer a way to constant C, I think you are ready to look at what it means to change your direction (velocity) in spacetime. You can undestand this too without any math (contrary to the popular belief :)

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

Look at the animated spacetime diagram. This does not express space, but one dimension of space (horizontal) and one dimension of "time" (vertical). The dots are not objects but events that happen in one particular location in space at one particlar moment of time.

The lower diagonal lines represent the motion of information from spacetime reaching you (bottom lines). I.e. the events on that line are what you "see" at one instant.

If you imagine a straight horizontal line in the center, that would represent the "now-moment", or at least what you would assume to exist at one particular moment.

Notice how, when you change direction (accelerate to left or right in space), the whole spacetime diagram is scaled (in 45 degree angle) instead of just tilted. In velocity addition you would simply tilt the diagram, but in Lorentz-transformation you scale it.

Notice also how in some cases some events move from below that horizontal line (past) back to above the line (future), and notice how even then, you can never see this (at the diagonal lines)

And that's what it means to change direction in the case that simultaneity is relative to different directions of motion.

 

[Line]

SO not only do we not know if light is particle or wave but photons don't have mass? Is this like electrons hwhich have mass but it's too small to matter or it has no mass whatsoever?

And light has no frame for itself? DO we have the right frame? Cause from what I understand we are spinning around the Milkyway Galaxy at the speed of light. And who knows how fast the universe is moving. So just the galaxy moving at the speed of light means that light really moves at twice C.

 

[Line]

Farsight is not far off the mark, it is much clearer to look at light as it exists in static sense in spacetime if you really want to understand relativity.

It is not like relativity is based on the idea that "nothing moves faster than light". This speed limit is just what naturally follows from the two postulates. It is rather topsy-turvy way to explain relativity to someone as if it is the assumption about the speed limit that causes relativistic effects.

Like I said, it is imperative to understand how relativity of simultaneity can replace Newtonian velocity addition. This is the key to grasping the idea, and everything else (like the speed limit) will follow. This is the unintuitive part of the theory, and this is exactly what makes it tick. It is rather abhorrent to reply to Line's question with "Because the formula to calculate motion is this and that". Having any arbitrary formula doesn't change reality. It is the formula that follows from the assumption about relativity of simultaneity!

So, Line, let's try a simple thought experiment for a fit.

I assume you are very much familiar with Newtonian relativity of motion.

Let's consider a lab frame where you are standing 10 light seconds away from a pole (You and the pole are at rest in lab frame).

Let's say the speed of light is mere 1 m/s. (the pole is 10 meters away from you)

Whenever you receive a light pulse from the pole, we assume the light started its journey 10 seconds ago. And indeed, we can verify this by placing a clock at the pole to register the moment the light departs. If the clock by the pole shows "0 seconds" when the light departs, an identical synchronized clock by your position shows "10 seconds" when the light is received.

So far all fine and well, but what if we suppose there is another observer (B), only he is moving towards the pole at 5 m/s. Let's consider a light pulse that you both receive at the very moment you pass each others. How could it be that the light was approaching him also at 10 m/s instead of 15 m/s? How to "lower" the speed of light from 15 m/s to 10 m/s?

Well, looking at the situation from B's inertial frame, the moment the light departed is not known. We get to the intuitive 15 m/s only by assuming the light departed at such and such moment. If we can assume the light departed much earlier, we can lower the "required" speed to 10 m/s. (Note how in B's frame the light moves much longer distance than 10 meters; after all, it is the pole that is moving towards B, and the light must have started its journey much earlier than when the distance between was reduced to 10 meters)

If there are clocks at rest in B's inertial frame that are registering where the light pulse was at different moments, the clocks would show - according to relativity - that the pulse was on its way much longer than 10 seconds.

This assumption about relativity of simultaneity (=notion of simultaneity is different in each inertial frame) also means that at the moment you two are passing each others, the clock at the pole has different reading in Bs inertial frame from that of yours. In your "now-moment" the clock reads 10 seconds (albeit you cannot see this yet), and in his "now-moment" it reads more than 10 seconds (obviously he cannot see it either).

It also immediately follows, that if you consider there to really exist any "now-moment", then you must also accept that if the observer now stops, the clock in his own "reality" snaps backwards in time to 10 seconds (beyond his observations of course). Hence the idea about static spacetime.

And finally, when you assume this sort of reality, from the point of view of that light pulse, time did not move at all during its "journey". When it departed, sure, the clock at the pole showed 0 seconds, but the clock in your wrist was already showing exactly 10 seconds and observer B was also already next to you. So in a very real sense, relativity says that the light did not move at all (hence what Farsight said). Its whole path merely "exists" in different inertial frame from yours, and in its own frame its whole motion exists "simultaneously". Only in our frame we find it in one place at one instant and in a new place at another instant.

Well, I hope this helps little bit. It may take a moment to really wrestle the idea in. If you have access to any 3D-modeling software, it may help to build spacetime diagrams which you can then simply scale to perform Lorentz-transformation (to get from one inertial frame to another properly). I'll explain you how if you want to.

-Anssi

I would just sya B's equipment is broken. Is this real and proven? Is there any use for this?

 

[Aether]

Is this real and proven? Is there any use for this?

This is a mixture of mathematical and physical concepts. The two "postulates" are non-physical assumptions (e.g., they are not real, and can not be proven by experiment) that make all the rest which is real and proven by experiment easier to handle mathematically...sort of like arbitrarily placing a set of x,y,z axes anywhere you please in that they are non-physical assumptions that help you to put numbers to things that are physical.

 

[AnssiH]

SO not only do we not know if light is particle or wave but photons don't have mass? Is this like electrons hwhich have mass but it's too small to matter or it has no mass whatsoever?

In the mathematical descriptions light has got no mass, but it causes a momentum on any material it hits. Ontological descriptions can vary. For example, you can consider inertia to be an emergent phenomenon of some sort, and electromagnetism to be just one component of that.

Also it is not necessarily correct to say "we don't know if light is a particle or a wave". Rather we should say it is neither, but it does exhibit some properties of both. Namely, it appears to move as a particle but all its possible trajectories seem to interfere with each others. I think any reasonable ontological interpretation of the math of QED should probably let go of any ideas about photons with identity. (Although alternatively, a spacetime interpretation of QM effects could say something about why the wave/particle duality might exist; https://www.physicsforums.com/showthread.php?t=130623 )

In any case, it is not necessary to understand these properties of light to get the idea behind relativity.

And light has no frame for itself? DO we have the right frame? Cause from what I understand we are spinning around the Milkyway Galaxy at the speed of light. And who knows how fast the universe is moving. So just the galaxy moving at the speed of light means that light really moves at twice C.

Well, the thing that Newtonian relativity shows already is that it is nonsensical to invoke the idea of absolute motion. Motion, as a semantical concept, can only make sense if it is expressed as relative between two objects.

Physicists got, again, hung up on the idea of absolute motion when they imagined ether. It would have been pretty odd if space was like a giant backdrop with identity to its locations. And it would have been pretty odd if space was behaving like matter does. There's no reason to assume that. "Empty space" is just one big unnecessary metaphysical entity if you think of reality that way. Better let it go and look at objects as stable constructions that can have motion in relation to each others but not in relation to any backdrop. This assumption does not yet lead to relativity of time, mind you.

(It is not given that the backdrop doesn't exist, but there is no indication of such a thing, and consequently, if you assume it does exist, then your descriptions of the laws of nature and explanations of various phenomena become very complex)

I would just sya B's equipment is broken. Is this real and proven? Is there any use for this?

Well, it is not directly proven. In fact, the logic doesn't allow for direct observation of relativity of simultaneity. It just allows for observation of time dilation. There are time dilation observations that agree with the predictions of relativity, but it is possible to construct models that make the same predictions with absolute simultaneity (they may not be mathematically as elegant, but one can argue that they are ontologically more elegant).

So, we have observed, for example, that objects in inertial acceleration suffer time dilation, and we have some indications that objects in motion relative to us, will suffer time dilation in our frame (cosmic particles with very short life times seem to live longer than expected, which can be explained by time dilation).

Anyway, I would say it is required to learn the ins and outs of the logic behind relativity, before you can hope to see what elements of it are not necessarily set in stone, and how various experiments have been interpreted according to relativity, and what needs to be interpreted differently in other schemes. Any "other scheme" probably must also say something about inertia and gravity (and their equivalence), and it certainly helps if it can also say something about QM phenomena.

 

[Aether]

Thank-you, AnssiH. That's a fair summary.

Any "other scheme" probably must also say something about inertia and gravity (and their equivalence), and it certainly helps if it can also say something about QM phenomena.

http://www.arxiv.org/hep-th/0501149" a nice paper that was published in a special edition of Annalen der Physik on the 100th anniversary of Einstein's paper on special relativity: C.D. Froggatt et al., Derivation of Poincare Invariance from general quantum field theory, Annalen der Physik, Volume 14, Issue 1-3 , Pages 115 - 147 (2005); hep-th/0501149.

In the present work we shall present a model which has an absolute time without either Lorentz invariance or rotational invariance at the outset. Nevertheless, under very general assumptions, it essentially leads to the familiar Poincare invariant quantum electrodynamics.

 

[gonegahgah]

If, then, you are meaning to ask why light behaves like light -- well, why is anything the way it is? It just is. And that's not a question for science.

What a great answer! Please beam me back to the dark ages.

Can someone give me a down to Earth explanation why this works?

Hi Line, I think I may be able to help.

Although my answer doesn't use terms like 'frame' I will still explain in normal language what a 'frame' is.

A frame is like a picture that you are in. Unlike a normal picture the frame can have things moving in it around you. The frame itself does not move. For the frame the second thing that does not move is the observer (you or the detector). This is because the observer defines the frame. If the observer changes speed or direction relative to the other things in the picture then the frame remains with them. In essense, for a frame, the observer never changes speeds, only the other things in the frame.

So the key thing to remember is that the observer defines the frame. If another observer is moving around in your frame then for them they have their own stationary frame around themselves and it is in fact you moving in their frame; not them.

Does that explain frames for you? Not that it matters but it is nice for you to understand what it means and not have it adding confusion.

Understanding that each observer observes things relative to themselves is one of the founding principles of SR.

Other people here have been kind enough to explain what this means but I will explain it differently for them and you.

Things are all relative to themselves (which establishes the fixed frame around themself of course, but...) so that when anything accelerates it is accelerating relative to you - even if you think you are doing the accelerating.

According to SR only light can move at the speed of light relative to you; nothing else can be moving at the speed of light relative to you (or in your frame). And also nothing can be moving at the speed of light - except light - relative to all other observer's around you (relative to their fixed unmoving frame around themself).

What does this mean? Well easy example. Let's say observer B is moving away from you at half the speed of light. Let's jump to observer B. For observer B it is actually you moving away from them at half the speed of light (in their frame).

Now to help explain things better, let's say that on the opposite side of observer B from you is another observer C. Observer C is moving away from observer B at half the speed of light in the opposite direction (in observer B's frame).

So on one side of observer B is you moving away at half the speed of light and on their opposite side is observer C moving away at half the speed of light. According to SR this is perfectly allowable as neither you nor observer C are moving at the speed of light relative to observer B in their frame. Cool.

Does this mean that observer C is moving away from you at the speed of light? No according to SR because nothing except light can move away or towards you at the speed of light except light. Instead for you (in your frame) observer C will only be moving away at a calculatable greater amount than half the speed of light (obviously it is greater than half because observer B is moving away from you at half light speed and C is further on than B and moving away).

So for you observer C will achieve less distance for the same amount of time than observer C will achieve for observer B.
Let's explain. Observer B will always find itself midway between you and observer C; at the same time observer C will never be twice as far away from you as observer B.

This paradox is supposed to be fixed by aspects of time dilation. So although for observer B he will get the same time report from you as he gets from observer C; for you you will not get the same time report from observer C that you get from observer B if they were both able to report to you at the same time.

So you can see that in B's frame he has you and observer C moving away from each other at the speed of light but not from him. But in your frame you do not see the same thing. In the same amount of time that B sees you and C moving apart at light speed, neither you nor observer C see the same thing in your frames. Instead you will measure C moving slower than twice the speed of B and observer C will measure you moving at less than twice the speed of B; and as such less than the speed of light in your and C's frames.

Now I'm going to spoil things after what has hopefully been the most helpful explanation here. This is because personally I believe this whole theory of SR is the Theory of BS. However, I hope that I have been able to make this more understandable for you.

 

[Doc Al]

In Newtonian/Galilian Relativity, the speed of light c measured in a moving
frame is viewed by a rest frame as c+v. This is simple arithmetic
logic.

In Special Relativity, the speed of light c measured in a moving frame
is viewed by a rest frame as c. What happened to the V then between
the moving frame and the stationary frame? Good question. What happened is that the SR rest observer rescaled the c+ v to c.
Example: 3+1 = 4 is now 3.25/1.25 = 3. Problem is the original
velocity between the frames used the same set of rulers and clocks
for viewing each other. For no logical reason, SR proposed a
new set of rulers and clocks to re-view the same events in the moving frame so that the speed of light would become c. What it ends up
saying is that 3= 3 and 4 = 3.

Not quite. Everyone uses their own, identically constructed, rulers and clocks. True, under SR, rulers and clocks in one moving frame are observed to "shrink" and operate slowly according to measurements made using "stationary" rulers and clocks. But no one is saying that 4 = 3; arithmetic still works just fine.

 

[AnssiH]

Things are all relative to themselves (which establishes the fixed frame around themself of course, but...) so that when anything accelerates it is accelerating relative to you - even if you think you are doing the accelerating.

I want to clarify, to avoid confusion, that acceleration is not relative but absolute. The rate of acceleration is not absolute, but when something accelerates, it really does accelerate (obviously; he also feels the acceleration). Other objects in his frame are still in rest.

I also want to stress that "nothing moves faster than the speed of light" is not a fundamental postulate from which all the other effects spring. Nothing moves faster than light is an effect which springs from the relativity of simultaneity, which springs from the assumption that C is isotropic.

Now I'm going to spoil things after what has hopefully been the most helpful explanation here. This is because personally I believe this whole theory of SR is the Theory of BS.

Yeah well, my hopes are not particularly high either.

 

[russ_watters]

What a great answer! Please beam me back to the dark ages.

Learn to understand the difference between science and philosophy before throwing around one-liners like that. All "why" lines of questioning eventually end up in the realm of philosophy and it is quite common for people just learning about Relativity to understand the concept but not accept it because they don't like the "why". That's what it appeared to me that the OP was after.

 

[gonegahgah]

Russ,

I don't understand your reply. From your post "It just is" you seem to be the one pushing philosophy and not science. You seem to be adherring to the idea of science as a religion of faith rather than as a tool towards ultimately explaining the mechanism.

In this respect I think you have attempted to be obstructive and the least helpful to the original poster in this thread.

 

[gonegahgah]

Just to help clarify things further...

Let's say that you and another spaceship leave the Earth side by side. You are both traveling at one tenth the speed of light away from Earth.
You are designated spaceship A and the other spaceship is shaceship B.

You decide to accelerate away from spaceship B until they are falling behind you at one tenth the speed of light. Soon spaceship B notices and accelerates until he catches up to you.

You repeat this again and again spaceship B is falling behind you at one tenth the speed of light. Again he notices and catches up to you.

You do this again and again. In total you do this eleven times. Each time spaceship B finds himself falling behind you at one tenth the speed of light. This works under SR because spaceship B is moving relative to your frame of reference.

So you've done this eleven times and you think to yourself "well I've accelerated away to be moving away from spaceship B at one tenth the speed of light eleven times so I should be now moving away from Earth at over the speed of light". However you turn around and to your surprise the Earth is still only falling behind you and spaceship B at less than the speed of light.

I hope this clarifies the Theory of (B)SR for you further Line.

 

[JesseM]

Just to help clarify things further...

Let's say that you and another spaceship leave the Earth side by side. You are both traveling at one tenth the speed of light away from Earth.
You are designated spaceship A and the other spaceship is shaceship B.

You decide to accelerate away from spaceship B until they are falling behind you at one tenth the speed of light. Soon spaceship B notices and accelerates until he catches up to you.

You repeat this again and again spaceship B is falling behind you at one tenth the speed of light. Again he notices and catches up to you.

You do this again and again. In total you do this eleven times. Each time spaceship B finds himself falling behind you at one tenth the speed of light. This works under SR because spaceship B is moving relative to your frame of reference.

So you've done this eleven times and you think to yourself "well I've accelerated away to be moving away from spaceship B at one tenth the speed of light eleven times so I should be now moving away from Earth at over the speed of light". However you turn around and to your surprise the Earth is still only falling behind you and spaceship B at less than the speed of light.

I hope this clarifies the Theory of (B)SR for you further Line.

You forgot to include the fact that each time you change speeds, you make use of a different set of clocks and rulers to measure your speed relative to the other ship, and to the Earth--a set which is at rest relative to you at your current velocity, and where all the clocks in the set have been synchronized using the assumption that light moves at the same speed in all directions in the set's own rest frame. Time dilation and lorentz contraction will mean that each set measures every other set to have its rulers shrunk and its clocks slowed down, and the synchronization rule means that each set will see the clocks of every other set to be out-of-sync. Given this, it should not be so surprising that the velocities won't add the same way they do in Newtonian physics, where rulers and clocks don't change depending on their speed, and you can synchronize clocks in a way that won't cause disagreements between reference frames (just set bring them to a common location and synchronize them there, then since there's no time dilation you can be sure they'll stay synchronized when you move them apart).

There is nothing magical about this. In fact, even if we lived in a purely Newtonian world with no time dilation or length contraction, and there was only one frame where light traveled at the same speed in all directions (the rest frame of the ether, say), if we had an observer who set up rulers and clocks in this frame, and for any other observer moving at velocity v relative to him we gave the moving observer phony gag rulers shrunk by \sqrt{1 - v^2/c^2} and phony gag clocks whose ticks were extended by 1/\sqrt{1 - v^2/c^2}, and instructed them to synchronize their clocks by making the (false, in this universe) assumption that light traveled at the same speed in all directions in their own rest frame, then the measurements made on the distorted rulers and clocks would mirror those made on normal rulers and clocks moving at different velocities in relativity. For example, all observers would measure light to have the same coordinate velocity, and if the observer with the normal rulers and clocks measured a missile to be moving at 0.8c, then an observer moving at 0.6c in the opposite direction in the normal observer's frame would measure the missile to be moving at (0.8c + 0.6c)/(1 + 0.8*0.6) = about 0.946c on his system of artificially-shrunk rulers and artificially-slowed-down (and desynchronized) clocks.

 

[Doc Al]

Just to help clarify things further...

Let's say that you and another spaceship leave the Earth side by side. You are both traveling at one tenth the speed of light away from Earth.
You are designated spaceship A and the other spaceship is shaceship B.

You decide to accelerate away from spaceship B until they are falling behind you at one tenth the speed of light. Soon spaceship B notices and accelerates until he catches up to you.

At which time both spaceships watch the Earth recede at a speed of less than two tenths the speed of light. Being savvy space travelers, this hardly surprises them since they understand relativity.

You repeat this again and again spaceship B is falling behind you at one tenth the speed of light. Again he notices and catches up to you.

You do this again and again. In total you do this eleven times. Each time spaceship B finds himself falling behind you at one tenth the speed of light. This works under SR because spaceship B is moving relative to your frame of reference.

So you've done this eleven times and you think to yourself "well I've accelerated away to be moving away from spaceship B at one tenth the speed of light eleven times so I should be now moving away from Earth at over the speed of light". However you turn around and to your surprise the Earth is still only falling behind you and spaceship B at less than the speed of light.

Perhaps "you", having been smuggled aboard from your outpost on some planet still struggling in its pre-relativistic dark age, are surprised at how velocities add. Not so the others.

I hope this clarifies the Theory of (B)SR for you further Line.

The only thing this clarifies is your complete lack of understanding of SR and how velocities really add up.

 

[russ_watters]

In this respect I think you have attempted to be obstructive and the least helpful to the original poster in this thread.

This thread has done just fine without me and will again - I'm out. If you want to discuss this further, we can do so over PM.

 

[gonegahgah]

Hi Doc

Nothing I have said contradicts SR in my explanations of it so please don't be embarrassing to yourself in providing non-rebuttals.

Hi Jesse

I haven't forgotten. However for each person on their spaceship their clock seems to be running normally to themselves and their ruler seems to still be the same length. No matter how fast we travel relative to anything else, we will still measure our elbow to hand to be about 30cm as ours is the preferred frame.

So with the same clock and ruler we will be able to increase our speed relative to spaceship B to being one tenth the speed of light faster no matter how many times spaceship B then catches up again. We can keep doing this again and again. But according to SR this will not result in an equal or greater than speed of light relative to the Earth for our ships. This is as you say for our clocks and rulers relative to the Earth nor for the Earth's clocks or rulers relative to us.

This is what SR says.

 

[JesseM]

Hi Jesse

I haven't forgotten. However for each person on their spaceship their clock seems to be running normally to themselves and their ruler seems to still be the same length. No matter how fast we travel relative to anything else, we will still measure our elbow to hand to be about 30cm as ours is the preferred frame.

Sure, but it's really better to think in terms of using a series of different grids of rulers and clocks, since "inertial frames" are supposed to be coordinate systems that move inertially throughout space and time. Each time you change velocities, it's true that once you return to moving inertially, your ruler will measure the same length as a ruler which has been moving at that velocity for all time, and your clock will tick at the same speed as a clock which has been moving at that speed for all time, so perhaps this distinction isn't so important. Then again, you do have to worry about simulataneity--if you synchronize your clocks using Einstein's light-signal method while moving at one velocity, then accelerate to a new velocity, your clocks will probably no longer be in sync as measured by a set of clocks which have always been moving at your current velocity and which were also synchronized using Einstein's light-signal method (I say 'probably' because it would depend on the details of how different parts of your ship were moving relative to each other during the acceleration itself). So at the very least, you do need to resynchronize your clocks each time you change velocities.

So with the same clock and ruler we will be able to increase our speed relative to spaceship B to being one tenth the speed of light faster no matter how many times spaceship B then catches up again. We can keep doing this again and again. But according to SR this will not result in an equal or greater than speed of light relative to the Earth for our ships.

But in the Earth's frame, your rulers keep shrinking and the clocks keep slowing down each time you accelerate to a new velocity, and each time you resynchronize your clocks after attaining a new velocity they get progressively more out-of-sync...so why should an Earth-observer be surprised to find that your increase in speed relative to the Earth grows smaller and smaller with each jump, even though by your own measurements each jump increases your speed by 0.1c relative to your previous speed?

To bring things back to the Newtonian example, suppose the phony gag rulers were built to actually shrink or expand depending on their speed relative to the observer with the normal rulers and clocks, and suppose the phony gag clocks were built to slow down or speed up depending on their speed relative to him. Also suppose that each time the observer with the gag rulers and clocks changes velocities, he resynchronizes his clocks using the assumption that light travels at the same speed in all directions in his current rest frame (which, again, would actually be a false assumption in this Newtonian world--light would actually only travel at the same speed in all directions in the frame of the observer with the normal rulers and clocks, who we can assume is in the rest frame of the ether). Now, do you agree that in this purely Newtonian situation, we'd get exactly the same results as in the relativistic situation with unmodified rulers and clocks? In other words, do you agree that if the second observer always increments his speed by 0.1c as measured on his own (distorted) rulers and clocks, then the Earth-observer with the normal rulers and clocks will measure him to increase his speed by smaller and smaller increments each time, never reaching the speed of light?

 

[Doc Al]

Nothing I have said contradicts SR in my explanations of it so please don't be embarrassing to yourself in providing non-rebuttals.

If that were true, what is the point of your posts?

 

[pess5]

OK,

So I'm new to your forum here, and trying to figure out how to use this interface. I see gonegahgah seems unconvinced that Einstein got it right. I have found this to generally arise from lack of understanding of some of the finer points of the theory, shortsightness, or the adherence to absolute simultaneity. A see at least a few folks here are up on Relativity, so that's very good.

How does one add a quote into the respone? I see the checkbox here in the QUICK REPLY window, but it does not allow me to check it. When I go GO ADVANCED, I see nothing regarding unfortunately QUOTE.

thanx

 

[JesseM]

How does one add a quote into the respone? I see the checkbox here in the QUICK REPLY window, but it does not allow me to check it. When I go GO ADVANCED, I see nothing regarding unfortunately QUOTE.

Just press the little "quote" button at the bottom right of the post you want to quote from. Then if you want to break up the comment into multiple quotes, use the {QUOTE}...(quoted text goes here)...{/QUOTE} tags for each chunk of text, but with square brackets instead of the curly brackets I used there. There's a guide to all the tags you can use https://www.physicsforums.com/misc.php?do=bbcode .

 

[jtbell]

Note that if you position your mouse over the "quote" button without clicking the mouse, a little text-box pops up with the words "Reply with Quote". At least my browser (Firefox) does this. If you see any other buttons or widgets whose purpose you don't know, this might help.

 

[gonegahgah]

Hi Doc

My entire life is spent just to get on your goat. Sorry about that.

Hi Jesse

But in the Earth's frame, your rulers keep shrinking and the clocks keep slowing down each time you accelerate to a new velocity, and each time you resynchronize your clocks after attaining a new velocity they get progressively more out-of-sync...so why should an Earth-observer be surprised to find that your increase in speed relative to the Earth grows smaller and smaller with each jump, even though by your own measurements each jump increases your speed by 0.1c relative to your previous speed?

Yours is a good explanation too which is afterall what we are looking to do for Line the original poster. My disagreement with the idea is incidental. The surprise for today's scientist's would be if it didn't happen the way they expect. So if instead the rocket suddenly disappeared - instead of the rocket's rate of acceleration slowing down as it approached light speed relative to us on Earth - there would be a lot of head scratching going on.

I think we can all agree that if you have a ruler on any ship or vehicle and no matter what its speed then if you mark off a centimetre while you are traveling away from the Earth with the pencil moving away from the Earth that that centimetre will actually transcribe more than an actual centimetre in Earth's frame. Of course, because the spaceship is moving while you are drawing the line; so the pencil travels more distance than one centimetre away from the Earth while it appears on your spaceship that it is traveling only one centimetre. This is true but linear ie the faster the spaceship is is proportional to the distance covered by the pencil relative to Earth. SR is not linear though and the rate - as you say - decreases exponentially as the speed of light is approached.

To bring things back to the Newtonian example, suppose the phony gag rulers were built to actually shrink or expand depending on their speed relative to the observer with the normal rulers and clocks, and suppose the phony gag clocks were built to slow down or speed up depending on their speed relative to him. Also suppose that each time the observer with the gag rulers and clocks changes velocities, he resynchronizes his clocks using the assumption that light travels at the same speed in all directions in his current rest frame (which, again, would actually be a false assumption in this Newtonian world--light would actually only travel at the same speed in all directions in the frame of the observer with the normal rulers and clocks, who we can assume is in the rest frame of the ether). Now, do you agree that in this purely Newtonian situation, we'd get exactly the same results as in the relativistic situation with unmodified rulers and clocks?

I'll have to wrap my head around this a bit more. I'll get back to you on this later.

In the meantime I just want to add some more interesting aspects of SR.

Let's again say we have three observers: observer A stays on Earth, observer B shoots off in one direction from Earth at half light speed, observer C shoots off in the other direction from Earth at half light speed.

B and C watch their clocks and wait for 1 hour to pass and report this to observer A. A gets their reports at exactly the same time.

At the same B and C report to each other but B gets C's report later (much later) than the hour and the same vice versa. So even though A gets their reports at the same time B & C get each other's reports at different times. (Assuming an instanteous means of communication).

Also here is another interesting thing about SR.

If a spaceship speeds away from Earth at 0.9c then the people on the spaceship will appear to slow down to us on the Earth. They will age slower.

The interesting thing though is that to the people on the spaceship the Earth will actually be receeding at 0.9c so that in this relative world and according to the rules of equivalence of SR then to the people on the spaceship it will appear as if the people on the Earth are slowing down. So the people on the Earth will appear to age slower to the people on the rocket.

So both will see the other aging more slowly and see themselves aging normally. When they get back together who will actually have aged more?

 

[AnssiH]

So both will see the other aging more slowly and see themselves aging normally. When they get back together who will actually have aged more?

Whichever is the one who actually "turned around", i.e. actually accelerated towards the other one (who continued in inertial frame). If both start accelerating towards each others and meet "halfway", then the aging is symmetrical... Surely you knew this, just had to test? :)

Anyway, I think people should talk more about how things look like in SR when we imagine an instantaneous communication (like you do), because it underlines the real point and at the same time the ontological weirdness of SR. Too many people think Einstein merely talks about information delay... :I

 

[pervect]

The problem is that the defintion of "instantaneous communication" depends on the frame of the observer. One persons instantaneous coummunication is seen as communication with the past, or the future, by a different observer. Therfore I don't think it is a good idea to talk about relativity and "instantaneous communication" - the concept is frame dependent, and causes much more confusion than illumination.

 

[AnssiH]

I think it is a good idea just for the reasons you mention. It gives people a clearer picture of reality as described by SR. If people understood better what sort of reality SR describes, there would be much better dialogue about the ontology of SR and especially the ontology of time.

EDIT: Let's still specify that when I say "imagine an instantaneous communication", I do not mean to imagine what would happen if people could communicate instantaneously in this or that inertial frame, but to imagine what the world "really" is like around them at a single instant and what happens to this "instant" when they change direction. This just because it seems so difficult to really drive it home that relativity of simultaneity is not about the order in which different events are observed in different locations in space, which is the way even many documentaries have interpreted Einstein's thought experiments about the train and embankment... 95% of the dialogue around relativity is still about twin paradox or some variation of it, and the problem is always that relativity of simultaneity has not been properly grasped, or even introduced... :I

 

[pervect]

Keep me on the "totally disagree" list, please.

ps - if your'e interested primarly in philsophy, you might want to check out http://philsci-archive.pitt.edu/archive/00000638/00/kant,_goedel_and_relativity.PDF

which should at least serve as an exxample that it's quite possible to philosophize about time at quite great length without introducing the results of impossible experiments.

 

[JesseM]

EDIT: Let's still specify that when I say "imagine an instantaneous communication", I do not mean to imagine what would happen if people could communicate instantaneously in this or that inertial frame, but to imagine what the world "really" is like around them at a single instant and what happens to this "instant" when they change direction.

As an ontological view, I think it makes little or no sense to say that the set of simultaneous events in an observer's instantaneous inertial rest frame represents what the world is "really" like around them. (How can you use the word 'really' if it's observer-dependent? And even if you allow reality to be observer-dependent in this way, why privilege inertial reference frames when they can only be defined locally in general relativity anyway, and GR says that the laws of physics will work the same way in any coordinate system?) The only views I've ever heard either philosophers or physicists advocating are

1) block time--the fundamental reality is 4D spacetime and the worldlines in it, all talk of simultaneity is specific to different coordinate systems placed on this spacetime, with no more ontological significance than the orientation of your spatial coordinate axes (most physicists who adopt any sort of philosophical view about relativity tend to adopt this one, I think)

2) time "really passes" relative to each observer, in the sense that each observer can say that anything in their past light cone is set and anything outside it (future light cone or 'elsewhere') is not (the physicist George F. R. Ellis seems to advocate a view like this http://spacetimecenter.info/4DBook/SpaceTime-Ellis.pdf )

3) time "really passes" in the sense that there is some "metaphysically preferred reference frame" that represents the true present, even though no empirical experiment will distinguish this frame as physically preferred in any way

 

[pervect]

It's also possible to say that the notion of simultaneity is, in Kant's terminology, "transcendental" - basically, the product of human consciousness.

Note that there have been some interesting experiments done in how the brain synthesizes the order in which events occurs from nerve signals that arrive with different propagation delays. The human perception of simultaneous events can be "tricked".

I *think* that this is the position (that simultaneity is transcendental) is the paper I quoted earlier takes, I must admit I haven't read it all that carefully.

This position (roughly speaking, that "Now" is an illusion) may be a bit too extreme, though. Certainly simultaneity has some practical use in defining momentum in an inertial frame. It's also clear that simultaneity depends on the frame of reference in this context.

I'm sure that there has been a lot written about the philospohy of time in relativity, and I offer the paper I cited and its reference list as an example. I don't personally have all that deep an interest in philosphy, though. I would expect that the best place to find people interested in philosophy would be the philosophy forum - I don't know for sure how many people there are interested in the philosophy of relativity though.

 

[MeJennifer]

It's also possible to say that the notion of simultaneity is, in Kant's terminology, "transcendental" - basically, the product of human consciousness.

Not only that but many seem to have a desire to "explain" everything in the universe in terms of some sort of plane of simultaneity, by back calculating the relativistic and Doppler effects instead of taking these things at face value. Apparently in the belief that that approach matches reality better.