Is the Constancy of Light in Vacuum a Proof or a Postulate?

[Boorglar]

Einstein postulated that the speed of light in vacuum is constant and is the same for all observers. It this related to the fact that in Maxwell's equations for electromagnetic waves in a vacuum,

c = \frac {1} {\sqrt{\mu_0 \epsilon_0}}?

The electric and magnetic constants, which are properties of free space, should indeed remain constant no matter how you are observing.

In this case, why do we say that he "postulated", when in fact it must be true by Maxwell's equations? Shouldn't we say that he actually prove it?EDIT:

Actually now that I think about it, its the other way around. I'm confused, now, because since no matter at what speed you move, you obviously will measure the same electric and magnetic constants, and so light ought to have the same speed. But as you move faster, your speed is added to that of the light and so you should see light moving faster. Since we know this is false, it would follow that the e/m constants are not, in fact, constants?
Can someone clarify that to me?

 

[Integral]

Yes, that is precisly where the postulate originates, and why AE was able to make that postulate and no one raised an eyebrow.

That result of Maxwells was the initiator of the Great Schism of physics which lasted from the publishing of Maxwells work till AE's 1905 paper on relativity. This used to be a topic taught in elementary school, guess not anymore.

 

[Bill_K]

Einstein postulated that Maxwell's Equations remain unmodified regardless of the velocity of the rest frame. Also that the group of linear transformations that leave Maxwell's Equations unchanged also leave the rest of physics unchanged, including mechanics.

Yes, that is precisly where the postulate originates, and why AE was able to make that postulate and no one raised an eyebrow.

Many eyebrows were raised.

 

[TSny]

It could be that the motivation for the light speed postulate was that Einstein felt that Maxwell's equations should be valid in all inertial frames. However, in the first 1905 paper, he took the light speed postulate as an independent postulate. The other postulate was of course the relativity postulate which implied that the true laws of electromagnetism would have to be the same in all inertial frames.

In the paper, he did not postulate that Maxwell's equations satisfy the relativity postulate. Rather, he first derived the Lorentz transformations from the light postulate. Then he spent a good deal of the paper proving (not postulating) that Maxwell's equations do indeed take on the same form in all inertial frames if the electric and magnetic fields transform in certain ways.

 

[TSny]

My previous remark was based on memory and is not correct. After digging out the paper I see that after deriving the Lorentz transformations from the light postulate, Einstein goes on to show that if you assume (postulate) that the Maxwell equations have the same form in all inertial frames, then the electric and magnetic fields must transform in certain ways. Overall, he's proving the consistency of Maxwell's equations with the postulates of relativity. And this only takes up 2 or 3 pages of the paper.

Nevertheless, I believe that in the paper Einstein was taking the light postulate as an independent postulate on which to base the theory of relativity.

 

[cosmik debris]

To be picky Einstein's postulate was that, the speed of light was independent of the speed of the source. They amount to the same thing in the end I guess.

 

[Histspec]

In this case, why do we say that he "postulated", when in fact it must be true by Maxwell's equations? Shouldn't we say that he actually prove it?

Note that when you talk about "Maxwell's equations" of that time, then you are actually talking about Maxwell's theory as formulated by Lorentz in 1895. Here, only in one "aether" frame is the speed of light constant in all directions and independent of the speed of the source. Einstein took this as a postulate, generalized it to all inertial frames, and abandoned the aether. Einstein wrote in 1912 ("Relativity and Gravitation. Reply to a remark of M. Abraham", in: The Collected Papers of Albert Einstein, Volume 4: The Swiss Years: Writings, 1912–1914:)

It is impossible to base a theory of the transformation laws of space and time on the principle of relativity alone. As we know, this is connected with the relativity of the concepts of "simultaneity" and "shape of moving bodies." To fill this gap, I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz’s theory of the stationary luminiferous ether, and which, like the principle of relativity, contains a physical assumption that seemed to be justified only by the relevant experiments (experiments by Fizeau, Rowland, etc.)

and in 1928 Einstein said about Lorentz: (in: A. Pais, Subtle is the Lord, p. 169):

The enormous significance of his work consisted therein, that it forms the basis for the theory of atoms and for the general and special theories of relativity. The special theory was a more detailed exposé of those concepts which are found in Lorentz's research of 1895.

Here is Lorentz's 1895 paper that Einstein referred to:
http://en.wikisource.org/wiki/Attempt_of_a_Theory_of_Electrical_and_Optical_Phenomena_in_Moving_Bodies

 

[lugita15]

There's one thing that should be noted. It is indeed true that special relativity, in particular the second postulate, follows from the assumption that Maxwell's equations are true in all inertial reference frame. However, this assumption would have seemed absurd to the physicists of the nineteenth century, including Maxwell himself. When Maxwell completed his four equations, he realized that they lead to the implication that electromagnetic waves (light) propagate at a speed c. Now Maxwell was a firm believer in Newtonian mechanics, so he reasoned that since his equations were clearly not invariant under Galilean transformations, by the Principle of Relativity they could not be "real" laws of physics, i.e. they could only be true in one reference frame. And he assumed that this was the rest frame of the aether. In any other frame, he assumed that the correct equations describing the electromagnetic field would not be his original equations, but rather equations that were slightly different because they would include terms dependent on the velocity of the aether with respect to that frame. These are the so-called modified Maxwell equations (formulated by Hertz), and they include aether velocity terms added to the Ampere-Maxwell Law and Faraday's Law. According to these modified equations, the speed of light in a frame moving with respect to the aether is either c+v or c-v depending in which way the light is headed, where v is the speed of the aether with respect to your frame. This is exactly analogus to sound waves: the sound wave equation is only exactly true in the rest frame of the air, and in moving frames it must be replaced by another equation that includes a term dependent on the speed of the air.

This was the context in which the Michelson-Morley experiment was done: they believed that the speed of light would not be c as predicted by the original Maxwell equations, but rather would be c+v or c-v as predicted by the modified equations, and they hoped to find that value v. But instead, of course, they found that the speed of light seemed to be the same in all inertial reference frames, and thus Maxwell's equations true in all frames. (In stark contrast to the wave equation for sound.) But as mentioned above, Maxwell's equations are not Galilean-invariant, so this posed a perceived threat to the Principle of Relativity. Lorentz's solution to this problem was to say that objects moving with respect to the aether experienced length contraction and time dilation (due to electromagnetic effects), which made the speed of light appear to be constant even though it "really" wasn't. He assumed that if we had "accurate" rulers and clocks, which did not experience these effects, then they would show that the speed of light IS c+v or c-v in frames moving with respect to the aether, and thus the modified Maxwell equations are what are "really" true in all frames, even Maxwell's original equations appear to be true.

Now although Lorentz believed that the "errors" caused by length contraction and time dilation prevented accurate measurement of the speed of light and thus the speed of the aether, he still held out hope that the speed of the aether could in principle be determined. Perhaps if A and B were moving at different speeds with respect to the aether, A could measure how much B's ruler contracted, and B could measure how much A's ruler contracted, and by comparing results they could somehow deduce their speeds relative to the aether. But then Poincare demonstrated that the Lorentz transformations relate not only the aether frame to a moving frame, but also related moving frames to each other. (Specifically, he showed that the composition of LT's is an LT, and the inverse of an LT is an LT.) However, Poincare just viewed this as a curious phenomenon. It took Einstein to realize the real significance of all this: that the invariance of Maxwell's equations was fully compatible with the Principle of Relativity, and that it was our traditional notions of space and time that needed rethinking.

 

[Nugatory]

Lugita's comments about the historical perspective are spot-on, and it's easy to lose sight of just how strange and perplexing the electrodynamics of moving bodies once seemed.

To the modern ear, the second postulate sounds redundant ("Postulate 2: We really mean what we're saying in postulate 1" would be a fair paraphrase). It didn't sound that way at the turn of the last century.

 

[PhilDSP]

If the answer is "yes" then it can only apply in a vacuum. I believe Maxwell was fully cognizant that the speed of light is highly variable when charges are interspersed in the space that light travels but it will take a bit of sorting through his material to find an appropriate quote.

His contemporary, Helmholtz, had already worked out the specifics of EM dispersion and how the speed of EM propagation is effected in a most sophisticated manner. All of that really is quite apart from whether there is or isn't an aether and whether a theoretical aether does or doesn't move with respect to a particle. Both Hertz and Lorentz benefited from Helmholtz' work.

The presence of charges and current density causes the constants \epsilon_0 and \mu_0 to become the variables \epsilon and \mu.

 

[harrylin]

Einstein postulated that the speed of light in vacuum is constant and is the same for all observers.

Not exactly: you refer here to two different postulates. However, getting back to your title: Yes, Einstein's postulate for the speed of light (see posts #6 and #7) was a consequence of Maxwell's theory, which had been firmly established by then. The issue was how to combine it with the relativity postulate, which seemed to be incompatible with it. Einstein explained that succinctly as follows:

"We will raise this conjecture (the purport of which will hereafter be called the “Principle of Relativity”) to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell's theory for stationary bodies." (emphasis mine).
- http://www.fourmilab.ch/etexts/einstein/specrel/

[..] I'm confused, now, because since no matter at what speed you move, you obviously will measure the same electric and magnetic constants, and so light ought to have the same speed. But as you move faster, your speed is added to that of the light and so you should see light moving faster. Since we know this is false, it would follow that the e/m constants are not, in fact, constants?
Can someone clarify that to me?

That's exactly why Einstein wrote "apparently irreconcilable" - that was the main riddle that the new theory was meant to solve (and it did)! Lorentz in 1904 (with one or two glitches) and next Einstein in 1905 (very smoothly) showed how it is possible that one can observe the same natural phenomena even if the speed of light in vacuum is, as Maxwell's theory has it, independent of the motion of the source. The essential change was that Newton's theory had to be modified, so that the measurement of more physical quantities than before had to be made "relative" - in particular measures of distance and time.

 

[harrylin]

[..] Now although Lorentz believed that the "errors" caused by length contraction and time dilation prevented accurate measurement of the speed of light and thus the speed of the aether, he still held out hope that the speed of the aether could in principle be determined. Perhaps if A and B were moving at different speeds with respect to the aether, A could measure how much B's ruler contracted, and B could measure how much A's ruler contracted, and by comparing results they could somehow deduce their speeds relative to the aether. [..]

Actually, such effects were his explanation in 1904 of why experiments like MMX could bring nothing at all - up to any precision. Perhaps we should explain how light from a spherical light source is measured as propagating at c in all directions according to relatively moving inertial reference systems. Boorglar, is that the issue that you have?

 

[grav-universe]

In addition to the excellent post by lugita15, the aberration of stars was the first important observation. It showed that the speed of light was the same regardless of the motion of the source, the stars in this case.

 

[grav-universe]

Quote of nonsense post that refutes relativity deleted.[/color]

Yes, but the M-M experiment showed that light pulses will travel identical perpendicular distances and back in the same time, even when the apparatus is turned in any direction, so the two way time of propagation of the light in any direction away and back to a single clock is always the same, and from this it is then possible to synchronize clocks within the observing frame such that they measure the same one way speed of light in any direction using the Einstein simultaneity convention. But if that is the result of an experiment performed within our own frame, then since there is nothing special about our own (inertial) frame, for instance we cannot consider that only our frame just happens to be always at rest with the field, then it must be true for all inertial frames, so all inertial frames must measure a constant speed of light in any direction when stationary clocks within those frames are synchronized according to the Einstein simultaneity convention.

 

[lugita15]

However, getting back to your title: Yes, Einstein's postulate for the speed of light (see posts #6 and #7) was a consequence of Maxwell's theory, which had been firmly established by then.

I would word things slightly differently. The invariance of Maxwell's equations implies the invariance of the speed of light, but in the nineteenth century no one believed in the invariance of Maxwell's equations, precisely because it DID imply the invariance of the speed of light, and they believed that contradicted the principle of Relativity, and so they concluded that Maxwell's equations only held in one reference frame. That is why the null result of the Michelson-Morley experiment was so surpising (except for the believers in the aether drag hypothesis and the contraction hypothesis, who of course would have their own problems later on).

 

[lugita15]

Actually, such effects were his explanation in 1904 of why experiments like MMX could bring nothing at all - up to any precision.

My understanding is that Lorentz didn't realize the symmetry of the Lorentz transformations until Poincare pointed it out, and that Poincare didn't realize the significance of that symmetry until Einstein explained it. So I'm guessing that by 1904 Poincare had already done his work, and Lorentz was building off of it. But I think in Lorentz's original aether theory, he still believed there was possibility that some combination of experiments could somehow find the aether frame. Do I have that right?

 

[DrGreg]

I can't claim to be an expert in the history of relativity, but everything lugita15 has said so far agrees with my understanding.

I would add (I don't think anyone's explicitly said this yet) that Lorentz came up with the Lorentz transform (between the aether frame and any other inertial frame) as exactly what was needed to make Maxwell's equations valid in all inertial frames.

 

[ardenmann0]

I can't claim to be an expert in the history of relativity, but everything lugita15 has said so far agrees with my understanding.

I would add (I don't think anyone's explicitly said this yet) that Lorentz came up with the Lorentz transform (between the aether frame and any other inertial frame) as exactly what was needed to make Maxwell's equations valid in all inertial frames.

I would add that Lorentz came up with the right equations, Lorentz transform. But his interpretation of the contraction is very different from that of Einstein's.

 

[pervect]

My impression of the times is that Michelson was motivated by wanting to replace the physical meter standard with an interferometric one, based on the wavelength of some particular light source.

(my source for this impresion isn't based on much reading, just the wiki article on the history of the meter: http://en.wikipedia.org/w/index.php?title=Metre&oldid=502620767)

In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of length

This goal - replacing the physical meter with one based on the wavelength of light - wasn't actually accomplished until the 1960's, however.

To make progress towards this goal, Michelson needed to pin down experimental sources of error. One source of expected error was "ether wind". He , and everyone else, was rather surprised to find out that the expected effect didn't exist.

 

[harrylin]

I would word things slightly differently. The invariance of Maxwell's equations implies the invariance of the speed of light, but in the nineteenth century no one believed in the invariance of Maxwell's equations, precisely because it DID imply the invariance of the speed of light, and they believed that contradicted the principle of Relativity, and so they concluded that Maxwell's equations only held in one reference frame.[..].

Maxwell's equations did not imply the invariance of the speed of light - but yes, I should have written that Maxwell's theory for stationary systems was firmly established (which is of course what Einstein meant with "stationary bodies"; at the time he evidently did not distinguish bodies from systems).

My understanding is that Lorentz didn't realize the symmetry of the Lorentz transformations until Poincare pointed it out [..] I think in Lorentz's original aether theory, he still believed there was possibility that some combination of experiments could somehow find the aether frame. Do I have that right?

I think that the part that I retained here is correct. However, such a discussion is a bit besides the topic.

PS. Strange: the OP did not react on our clarifications... I won't comment anymore if he/she isn't really interested in his/her topic!

 

[PhilDSP]

... but in the nineteenth century no one believed in the invariance of Maxwell's equations, precisely because it DID imply the invariance of the speed of light ...

Or rather in the nineteenth century anyone who properly understood the Maxwell equations realized they applied to stationary charges only and therefore could not be invariant when applied to a situation with moving charges. Maxwell himself developed a preliminary augmentation of the equations to accommodate moving charges (with no reference to an "aether") but apparently didn't concentrate on further developments. Helmholtz, Heaviside, Hertz, Ritz and Cohn all developed their own variants of the Maxwell equations for moving charges. Each of them are form invariant.

 

[bcrowell]

There are really two questions here. (1) How did Einstein originally present his theory in relation to the experimental and theoretical work that came before? (2) In Einstein's young brain, what thought process led him to come up with his theory?

If the OP has in mind #1, then the (oversimplified) answer is yes, and the complete answer can be determined with certainty by reading Einstein's 1905 paper.

If #2, then it's more complicated, and nobody will ever really know the answer for sure: http://arxiv.org/abs/0908.1545

 

[Histspec]

Now Maxwell was a firm believer in Newtonian mechanics, so he reasoned that since his equations were clearly not invariant under Galilean transformations, by the Principle of Relativity they could not be "real" laws of physics, i.e. they could only be true in one reference frame. And he assumed that this was the rest frame of the aether.

Maybe it should be stated that the term "aether frame" wasn't well defined at this time period:

1) Fresnel in 1818 proposed an aether almost completely at rest, as necessary to explain the aberration of light. Here, light is a transverse wave in the aether. However, he had do include "partial aether dragging" to explain the negative outcome of the Arago experiment. So the speed of light is constant in vacuum, but variable within matter in accordance with Fresnel's dragging coefficient (later confirmed in the Fizeau experiment in 1851).

2) Stokes (1844) proposed that the aether is completely dragged by Earth. He had to invent certain auxiliary hypothesis to derive Fresnel's dragging coefficient and the aberration of light (which wasn't very convincing).

3) Maxwell (1865) derived his equations (actually twenty of them). But he made no comment on the state of motion of the luminiferous aether in matter or its vicinity. Later (shortly before his death) he argued that if there is an aether wind, then it is of second order in v/c when two-way measurements were made. (See Maxwell's own Ether entry in the encyclopedia britannica, section "Relative motion of the aether", 1878).
Encyclopædia Britannica, Ninth Edition/Ether.

4) Michelson (1881) and Michelson&Morly (1887) followed Maxwell's suggestion and executed their famous second order experiment - no aether wind was found. A contradicting situation arose: The Fizeau experiment and the aberration of light "proved" an almost resting aether, while the Michelson-Morley experiment seemed to imply complete aether dragging.

5) Heaviside and Hertz (1890) brought Maxwell's (now four) equations into their modern form - but Hertz believed in complete aether dragging (even though Hertz knew that this was at variance with the Fizeau experiment).

6) Lorentz was the first (1892 and 1895) to argue that the aether is completely motionless and thus totally unaffected by the motion of matter. Now we can talk about one (and only one) "aether frame", in which the speed of light is the same in all directions. To avoid "aether wind" effects, he (together with Larmor and Poincaré) had to include "local time" and "length contraction", and later the complete Lorentz transformation (1904).

7) Einstein used the Maxwell-Lorentz theory of 1895, in which the speed of light was constant in only one frame. Per the relativity principle, he argued that this constancy and Maxwell's equations must apply to all reference frames, none of them should be called "aether" any more. Special relativity was born.

 

[Boorglar]

Wow I did not expect to get so many answers! I was away and didn't read this thread for a while. Unfortunately I am not really knowledgeable on the history and the mathematics behind Einstein's idea and the Lorentz transformations.

Thanks to lugita15 for the first post with the historical facts. You mentioned that in order to reconcile electromagnetism with the relativity principle, Lorentz had to conclude that length and time could contract.

But how exactly were the time/space dilation effects proved so as to keep electromagnetism consistent with relativity? I have seen the weird-looking formulas involving square roots of 1 - (v/c)^2 but I've never understood how they came up with precisely this expression...

Also, how do you make something "invariant" with respect to something else?
I mean, answers mention a lot the "invariance" of the equations, but I don't really get what it means. How did they translate the fact that the equations should work in all reference frames mathematically?

 

[PhilDSP]

In not so technical terms the invariance of the Maxwell equations effectively means that they provide proper solutions for any conceivable situation. Since they don't provide valid solutions for moving charges in Euclidean space (outside of the original inertial frame using a Galilean transformation), they are broken (i.e. not invariant).

Crudely speaking Fitzgerald, Larmor and Lorentz conceived of a game-changing means of allowing the equations to work for moving charges by manipulating time and space variables (which don't directly appear in the equations but are required for interpreting solutions). Normally, when solving differential equations, the relationships between time and space are frozen during the iteration of determining a solution. Poincare investigated the meaning of manipulating the relationships and worked out a lot of details and principles on which SR is based but may have fell short of being able to predict an actual experimental observation. Einstein very definitely declared that there is legitimacy in basing the interpretation of solutions on the translation of time and space variables and further tidied up some loose ends to bring forth the core of SR.

 

[harrylin]

Maybe it should be stated that the term "aether frame" wasn't well defined at this time period:

1) Fresnel in 1818 proposed an aether almost completely at rest, as necessary to explain the aberration of light. Here, light is a transverse wave in the aether. However, he had do include "partial aether dragging" to explain the negative outcome of the Arago experiment. So the speed of light is constant in vacuum, but variable within matter in accordance with Fresnel's dragging coefficient (later confirmed in the Fizeau experiment in 1851).

2) Stokes (1844) proposed that the aether is completely dragged by Earth. He had to invent certain auxiliary hypothesis to derive Fresnel's dragging coefficient and the aberration of light (which wasn't very convincing).

3) Maxwell (1865) derived his equations (actually twenty of them). But he made no comment on the state of motion of the luminiferous aether in matter or its vicinity. Later (shortly before his death) he argued that if there is an aether wind, then it is of second order in v/c when two-way measurements were made. (See Maxwell's own Ether entry in the encyclopedia britannica, section "Relative motion of the aether", 1878).
Encyclopædia Britannica, Ninth Edition/Ether.

4) Michelson (1881) and Michelson&Morly (1887) followed Maxwell's suggestion and executed their famous second order experiment - no aether wind was found. A contradicting situation arose: The Fizeau experiment and the aberration of light "proved" an almost resting aether, while the Michelson-Morley experiment seemed to imply complete aether dragging.

5) Heaviside and Hertz (1890) brought Maxwell's (now four) equations into their modern form - but Hertz believed in complete aether dragging (even though Hertz knew that this was at variance with the Fizeau experiment).

6) Lorentz was the first (1892 and 1895) to argue that the aether is completely motionless and thus totally unaffected by the motion of matter. Now we can talk about one (and only one) "aether frame", in which the speed of light is the same in all directions. To avoid "aether wind" effects, he (together with Larmor and Poincaré) had to include "local time" and "length contraction", and later the complete Lorentz transformation (1904).

7) Einstein used the Maxwell-Lorentz theory of 1895, in which the speed of light was constant in only one frame. Per the relativity principle, he argued that this constancy and Maxwell's equations must apply to all reference frames, none of them should be called "aether" any more. Special relativity was born.

Very nice summary!

 

[harrylin]

[..] But how exactly were the time/space dilation effects proved so as to keep electromagnetism consistent with relativity? I have seen the weird-looking formulas involving square roots of 1 - (v/c)^2 but I've never understood how they came up with precisely this expression...

It cannot be proved in a strong sense; instead a theory is a creative solution or invention that is meant to explain observations. However, in this case there is not much room for alternative solutions, so to that extent it can be considered "proved".
Do you understand why according to Lorentz and Einstein a "moving" interferometer such as that of Michelson and Morley must "be" contracted in length by that factor? If not, that's a good point to start with. Next there is a similar example with a "light clock". You can "google" for those - and you may find hits of this forum.

Also, how do you make something "invariant" with respect to something else?
I mean, answers mention a lot the "invariance" of the equations, but I don't really get what it means. How did they translate the fact that the equations should work in all reference frames mathematically?

That's exactly what is meant with "invariant"! The speed of a light ray or wave is invariant in the sense that it is "observed" as going at c by means of any inertial reference frame that has been set up in a certain way. And to understand how it works, it may be good to start with the things that I mentioned just here above.

 

[Ibix]

Also, how do you make something "invariant" with respect to something else?
I mean, answers mention a lot the "invariance" of the equations, but I don't really get what it means. How did they translate the fact that the equations should work in all reference frames mathematically?

Following Newton:

You are standing in a stationary train which is on a track parallel to the x-axis and I am on the platform outside. We are each holding a ball, and we drop them simultaneously at time t=0. The initial positions of the balls are (x_0,y_0) (there is a z-coordinate perpendicular to the track which would have different values, but I am ignoring here because it doesn't matter). After time t, the positions are both (x_0,y_0-gt^2/2).

Now, we repeat the experiment but with your train passing through the station at speed v in the x direction. We drop the balls at time t=0, as we pass one another. My ball still follows the same path: (x_0,y_0-gt^2/2). But your ball obviously won't. The principle of relativity states, though, that you are entitled to consider yourself at rest, so you must see the same path you did when you were at rest. If you are to see (x_0,y_0-gt^2/2), but from my perspective you are moving at speed v in the x direction then the path I see must be (x_0+vt,y_0-gt^2/2). You can make the exact same argument (except that to you, I am moving at -v): your ball follows (x_0,y_0-gt^2/2); I am entitled to believe I am stationary so my ball must (to you) follow (x_0-vt,y_0-gt^2/2) so that I can continue to believe that.

That is invariance; the equations are the same for you and for me. Nothing different happens just because we are moving with respect to one another. The maths I use to describe me are the same you use to describe you; the maths I use to describe you are the same as the maths you use to describe me. I leveraged that observation to derive something useful: if I want to know the position of a ball according to somebody else, all I need to do is take my observations and add the current position ((vt,0) in the example above) of the other person.

What we are discussing on this thread is a fifty-year investigation that lead to the conclusion that our watches won't agree (although, again, my complaint about your watch will be the same as your complaint about mine). Since I have assumed above that we will always agree on the t coordinate, what I have written is subtly wrong. This would be obvious to everybody if we typically moved at 10% of lightspeed, or thereabouts, but we move a lot slower than that so we never notice. Maxwell's equations, describing things that do move at light speed, noticed.

Does that kind of make sense?

 

[Boorglar]

Yes this makes perfect sense. But then what bothers me is that when the speed of the train approaches c, then you can no longer add v*t in the equation, and I don't know how to reason it out.

Say you are in a spaceship moving with speed v. Next to you comes a ray of light in vacuum with speed c. Now normal intuition would suggest that you will see the ray racing against you at speed c - v. Relativity theory says this is impossible.

So somehow, we need to change another parameter so that you end up seeing light moving with speed c no matter how fast you move. Suppose you and the light ray started at x = 0 in Jupiter. On the clock in your spaceship, you measure t seconds, and find that you are at position v*t next to planet Mars, and light at position x, next to Earth. Then the speed of light as measured by you is c = (x - vt)/t.

From an observer at rest in Jupiter, when you reach Mars, light is at position c*t'. (t' is another time parameter). Now I think that when you reach Mars. both you and the observer see the light ray next to Earth, at position x, am I right? So I can say that x = c*t'.
So c = (c*t' - vt)/t = c*(t'/t) - v so t' = t * (v+c)/c = t*(1+v/c).

This would mean that the observer at rest has lived 1 + v/c times longer than you (from your perspective) when you reached Mars (time dilation). But this is not the true factor which is used in relativity theory, so I get confused even more...

 

[lugita15]

But how exactly were the time/space dilation effects proved so as to keep electromagnetism consistent with relativity? I have seen the weird-looking formulas involving square roots of 1 - (v/c)^2 but I've never understood how they came up with precisely this expression...

See the attached excerpt from the Feynman Lectures on Physics, specifically the last two pages.

 

[Darwin123]

Actually now that I think about it, its the other way around. I'm confused, now, because since no matter at what speed you move, you obviously will measure the same electric and magnetic constants, and so light ought to have the same speed. But as you move faster, your speed is added to that of the light and so you should see light moving faster. Since we know this is false, it would follow that the e/m constants are not, in fact, constants?
Can someone clarify that to me?

If Newton's Laws as written in Principia were precisely true, applying to both ordinary matter and this hypothetical ether scientists believed in, the electric and magnetic constants could be constant only for an observer in the inertial frame where the ether is stationary. There would be a one specific frame where Maxwell's equations could be written in the simplest form. It didn't matter whether or not there really is an ether. By the logical argument that you just gave, Maxwell's equations could be written in a simple form in only one frame. I will call this hypothetical unique frame the "ether frame", even though there may be no ether. If Newton's Laws are precisely valid, the constants are not invariant because the change for observers other than the ether frame.
This also assumes that the measuring instruments are unaffected by the motion relative to the ether frame. This si consistent with Principia. However, Maxwell's equations may not be consistent with Principia. More on that later.
Maxwell's equations would not be valid as currently written in any other frame except one. An experimenter moving at a nonzero constant velocity with respect to that one frame would find different electric and magnetic constants from those of that one "proper frame". In fact, he would probably have to replace those constant numbers by constant matrices.
There was no experiment that showed this wouldn't be true. However, what bothered Einstein is the idea that the "constants" would be different for two observers moving at a constant velocity with respect to each other.
To visualize the problem, imaging that you moving at the speed of light with reference to the "ether frame". Suppose you hold up a mirror that relative to your eyes is in the direction of motion. You couldn't see your reflection! You also probably drop dead since your body has evolved for the electromagnetic constants being what they are on earth.
So this bothered Einstein. However, there is another reason to doubt the Principia. The reason is that the forces that hold bodies together are largely electromagnetic. Therefore, their dimensions and even their movements have to be affected by this speed relative to the ether!
Imagine that extended bodies are held together by a combination of electromagnetic and nonelectromagnetic forces. Our bodies contain a number of electrically charged particles. Using the laws of electromagnetic forces, it is easy to see that when in motion one particle will apply a force to the other particle. You can use the Law of Biot and Savart, the Lortentz force law, and anything else you want. Motion with respect to the ether frame will cause a stress on the extent body that will affect both its dimensions and its motion. For instance, an electrical coil moving with respect to the ether frame should experience a torque. Furthermore, a material that is not birefringen should show birefringence if it is in motion relative to the ether frame.
Efforts were made to measure this stress. There were two scientists, Raleight and Brace, that tried to measure the force on an electrical coil caused by the Earth's motion. This was an experiment not as well known as the Michaelson-Morley experiment. They had a null result that was just as important.
Experimental results indicated that Principia couldn't hold for electrical measurements.The motion of an electrical coil doesn't cause a torque on itself! A nonbirefringent material is nonbirefringent at all speeds!
I got the references to this from a paper by Lorentz. However, I have never found a copy of these experiments. They are just as important as the Michaelson Morley experiment. Yet, I can't find a copy. Maybe you will have better luck.
Null result on measuring the birefringence of an isotropic material in motion.
1) Rayleigh, Phil. Mag. 6(4), 678 (1902).
2) Brace, Phil. Mag. 6(7), 317 (1904)
Null result on measuring the torque on an electrical coil:
3) Troughton and Noble, Roy. Soc. Trans. A 202, 165 (1903).
Experiments 1-3 showed clearly that Principia was faulty.
The problem comes about because measuring instruments are extended bodies! Suppose and observer calibrates his measuring instruments, both rulers and clocks, in the ether frame. Some other observer moving at a constant velocity relative to the first observer grabs those measuring instruments, and accelerates them to his velocity. The internal forces will distort those measuring instruments!
He may try to calibrate them by a series of timing experiments. However, most timing experiments require synchronization of clocks using electrically charged particles. Thus, it appears impossible to calibrate measuring instruments that contain electrically charged particles or electromagnetic radiation.This includes every measuring instrument ever invented, then and now! The presence of nonelectromagnetic forces doesn't change things, because some distortion remains as long as any electromagnetic force is used.
Furthermore, there is a contradiction if both Principia and Maxwell's equations are used. The third law of motion is written in present tense. So Principia implies that the action and the reaction have to occur simultaneously. Maxwell's equations implies a delay in electromagnetic forces consistent with the speed of light. So they both can't be true under all conditions. Either Principia or Maxwell's equations or both are a little bit wrong.
H. A. Lorentz did a complete analysis on how the electromagnetic stress of motion would effect rulers and clocks. It turned out that the rulers and clocks would behave just as Einstein described in special relativity.
However, I will just write down the names of these articles where H. A. Lorentz put down his theory of electrons.
4) "Electromagnetic phenomena in a system moving with any velocity smaller than that of light," by Hendrik Lorentz. Proceedings of the Royal Netherlands Academy of Arts and Sciences, 809-831 (1904).
5) "The Theory of Electrons" by Hendrik Lorentz (1915).

There are copies of 4 and 5 free on the Internet. Someone on this forum listed them.
Lorentz only varied from Principia in one regard. He assumed that the forces holding things together have a minimum delay given by the speed of light. It is this delay that makes the measuring instruments act in a "relativistic" way.

In any case, your conundrum is resolved when you consider the stress in a measuring instrument caused by the motion of the measuring instrument. The atomic components of clocks and rulers interact with each other to distort the measurements in a way consistent with relativity.
If all forces are Lorentz invariant, then the changes in the dynamics of the particles that comprise a measuring instrument causes the measurements to determine the same electromagnetic constants in any frame. Furthermore, the speed of light delay in the forces makes it impossible to determine which of the frames is an ether frame.

 

[Ibix]

'Simultaineous' is a hairy concept in relativity. No, the Jupiter and ship observers do not agree that light reaches Earth at the same time that the ship reaches Mars. That's why your maths is wrong.

The usual way to show this is a train of length 2L passing through a station. You are on the train in the exact middle; I am on the platform again. At the instant we pass, lightning strikes the front and back of the train. I see the light from that at time L/c and am able to calculate backwards and determine that the flashes occurred simultaneously. I can also see that you, moving at speed v, saw the flash from the front at time L/(v+c) and the one from the back at time L/(c-v). Not simultaneous. The same thing would occur with the sound, of course, but the special thing about light is that c is constant in all frames. You may consider yourself at rest. Since the flashes occurred equal distances away but you did not see them at the same time, they must not have been simultaneous in your frame.

I am not sure off the top of my head if you can derive the Lorentz transforms directly from this argument - possibly. I also cannot view Lugita's attachment on this phone, but I guess that will explain it fully.

 

[harrylin]

Yes this makes perfect sense. But then what bothers me is that when the speed of the train approaches c, then you can no longer add v*t in the equation, and I don't know how to reason it out.

You can (and must!) if you use the reference frame in which the train is moving; there is nothing to "reason out".

Say you are in a spaceship moving with speed v. Next to you comes a ray of light in vacuum with speed c. Now normal intuition would suggest that you will see the ray racing against you at speed c - v. Relativity theory says this is impossible.

That's a common confusion which stems from the fact that in Newtonian mechanics it's harmless to confound a switch of reference system (= a transformation) with a difference of velocities (= a subtraction). This is because the so-called "Galilean transformation" is numerically identical to calculating a velocity difference. In SR that is not the case and so you have to distinguish those.

Thus we have to rephrase your question (which immediately answers it!) as either:

Say you are in a spaceship moving with speed v relative to Earth, but consider yourself in rest - you set up a "ship reference system" S'. Next to you comes a ray of light in vacuum with speed c according to S'. Now normal intuition would suggest that you will see the ray racing against you at speed c, as Relativity theory also says.

or:

Say you are in a spaceship moving with speed v according to reference system S. Next to you comes a ray of light in vacuum with speed c according to S. Now normal intuition would suggest that you will see the ray racing against you at speed c - v. Relativity theory says this is what you will "observe" if you stick to using reference system S.

PS the topic "constant speed of light" is in discussion in a parallel thread, see my last posting there:
https://www.physicsforums.com/showthread.php?p=4000881

 

[Boorglar]

well this gets pretty hard, I guess... I suppose I'll have to study more electricity and special relativity to understand all that stuff better :P For the moment it's beyond my head, without a solid background.

Thanks for the replies, though! They are very informative.

 

[Devils]

But then Poincare demonstrated that the Lorentz transformations relate not only the aether frame to a moving frame, but also related moving frames to each other. (Specifically, he showed that the composition of LT's is an LT, and the inverse of an LT is an LT.)

So, as a newbie, is this saying Lorenz transforms form a group? With the operator being Lorenz composition? The corollary would be any Lorenz transform must have c as the speed limit.

 

[harrylin]

So, as a newbie, is this saying Lorenz transforms form a group? [..]

I did not understand the part that I left out, but yes, Poincare (who was first of all a mathematician) emphasized that these transformations form a group.
- https://en.wikisource.org/wiki/On_the_Dynamics_of_the_Electron_%28June%29

 

[Devils]

"It is, in fact, possible to derive the Lorentz transformations from the principle of relativity alone and obtain the constancy of the speed of light as a consequence."
http://en.wikipedia.org/wiki/Principle_of_relativity

This is interesting. Can somebody point me to a proof?

 

[bcrowell]

"It is, in fact, possible to derive the Lorentz transformations from the principle of relativity alone and obtain the constancy of the speed of light as a consequence."
http://en.wikipedia.org/wiki/Principle_of_relativity

This is interesting. Can somebody point me to a proof?

The statement in WP is footnoted to a 2004 paper by Friedman, which I don't have access to. But I think they are probably referring to an argument that, in various forms, dates back to 1911:

W.v.Ignatowsky, Phys. Zeits. 11 (1911) 972
Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51
Morin, Introduction to Classical Mechanics, Cambridge, 1st ed., 2008, Appendix I
Palash B. Pal, "Nothing but Relativity," http://arxiv.org/abs/physics/0302045v1
http://www.lightandmatter.com/html_books/0sn/ch07/ch07.html (my own presentation)

 

[Devils]

OK I can see that there has to be a universal speed limit, but why does this have to be the speed of light (or other electromagnetic waves)? What implies that light must travel at a constant speed anyway (waves in water can have various speeds)?

 

[bcrowell]

OK I can see that there has to be a universal speed limit, but why does this have to be the speed of light (or other electromagnetic waves)? What implies that light must travel at a constant speed anyway (waves in water can have various speeds)?

Water waves have a speed relative to the water. The speed of light in a vacuum can't be relative to anything, because there isn't any medium for it to be relative to.

 

[harrylin]

"It is, in fact, possible to derive the Lorentz transformations from the principle of relativity alone and obtain the constancy of the speed of light as a consequence."
http://en.wikipedia.org/wiki/Principle_of_relativity

This is interesting. Can somebody point me to a proof?

Instead I can point to a counter claim in that same encyclopedia:
http://en.wikipedia.org/wiki/Histor...rentz_transformation_without_second_postulate
In fact, that should be obvious: Classical mechanics has the PoR but with the Galilean transformations.

 

[harrylin]

OK I can see that there has to be a universal speed limit, but why does this have to be the speed of light (or other electromagnetic waves)? What implies that light must travel at a constant speed anyway (waves in water can have various speeds)?

That was based on observation combined with Maxwell's theory which models light as a wave with constant speed, similar to the speed of sound in a homogeneous medium.

 

[Devils]

Water waves have a speed relative to the water. The speed of light in a vacuum can't be relative to anything, because there isn't any medium for it to be relative to.

Instead I can point to a counter claim in that same encyclopedia:
http://en.wikipedia.org/wiki/Histor...rentz_transformation_without_second_postulate
In fact, that should be obvious: Classical mechanics has the PoR but with the Galilean transformations.

So the implication here is that if we have 'something' that doesn't travel relative to anything, this 'something' has to travel at the 'universal speed limit' (this is probably provable). Light in a vacuum is an example of this 'something'; are there any others (gravitons?)

Also the speed of light in a vacuum is an axiom, rather than being able to be derivable; even Pauli thought so.

 

[harrylin]

So the implication here is that if we have 'something' that doesn't travel relative to anything [..]

Instead, and sticking with the topic, the model that is used is that of Maxwell, according to which light propagates at speed c relative to a "stationary" frame, or "space":

"light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body"
"Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body."
- Einstein 1905

"We [..] assume that the clocks can be adjusted in such a way that the propagation velocity of every light ray in vacuum - measured by means of these clocks - becomes everywhere equal to a universal constant c, provided that the coordinate system is not accelerated.
- Einstein 1907

Einstein explained it as follows in 1907:

" It is by no means self-evident that the assumption made here, which we will call the "principle of the constancy of the velocity of light", is actually realized in nature, but - at least for a coordinate system in a certain state of motion - it is made plausible by the confirmation of the Lorentz theory [1895], which is based on the assumption of an ether that is absolutely at rest, through experiment". [footnote refers to Fizeau's experiment]

And with the PoR this model can be used for any inertial frame:
"the phenomena of electrodynamics as well as of mechanics possesses no properties corresponding to the idea of absolute rest"
- Einstein 1905

 

[bcrowell]

Also the speed of light in a vacuum is an axiom, rather than being able to be derivable; even Pauli thought so.

Whether it's a postulate or a theorem depends on what system of axioms you pick. We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=534862

 

[Devils]

(Sorry to derail this thread).

So to summarise
- we start with a Euclidean space
- we introduce a "speed limit", which is light, which leads to hyperbolic functions (and hyperbolic identities), which leads to the Lorenz transform
- normally distance = speed * time, but the speed limit distorts this as well, to dilate time

So special relativity is a dressed-up version of hyperbolic algebra. I think it really is that simple, I don't know why people want to make it so complicated.