question on the speed of light

[gib65]

Who first discovered c? Wasn't it Maxwell? Perhaps he wasn't the first to measure the speed of light (but I think he was), but I'm pretty sure he was the first to discover that c was a constant.

 

What I'm actually wondering - my real question - is this: does the conclusion that c is constant - at least during Maxwell's time - necessarily imply that it is such irrespective of the observer (as entailed by relativity)?

 

I ask this because it seems to me, based on my understand of the history of physics, that there was a contending theory to that of relativity which didn't require c to have its value irrespective of the observer. I'm talking, of course, of the ether theory.

 

If you assume the ether is always fixed, then light can propagate through it at the constant c without requiring that every observer, regardless of his speed, measure it to be c relative to himself. So c would be a constant and you wouldn't need relativity.

 

Further, it would explain why Michelson and Morley conducted their experiment, expecting as they did to find differences in the measured speed of light depending on the orientation of their table.

 

I'm just wondering where Einstein got his inspiration from. I'm pretty sure he rejected the ether theory (and wasn't being too controversial in doing so since many were abandoning it at the time), and so he probably was lead to special relativity as the only conceivable alternative.

 

Anyone know if I'm right about any of this?

[Martin]

Who first discovered c? Wasn't it Maxwell? Perhaps he wasn't the first to measure the speed of light (but I think he was), ...

 

Ole Roemer, around 1675.

 

Young Dane, working at the Paris Observatory. He timed the orbits of Jupiter satellites, Io in particular if I remember right. He found that Io eclipses happen earlier when the earth is closer to Jupiter and later when the earth is farther from Jupiter.

He estimated the speed of light and came within 10 percent or so.

 

Before Roemer nobody was sure light even had a finite speed. Galileo suspected it might have a finite speed and tried to measure it, but was unsuccessful.

 

Ole is short for Olaus. Probably Wikipedia has an article about Roemer.

 

At the time the speed was measured, it would have been natural to assume that the speed was a universal constant in Newton's absolute space. The speed at which Newton's particles of light traveled. I doubt anyone raised any questions about relativity or ether at that point in history.

Edited May 4, 2009 by Martin

[gib65]

At the time the speed was measured, it would have been natural to assume that the speed was a universal constant in Newton's absolute space. The speed at which Newton's particles of light traveled. I doubt anyone raised any questions about relativity or ether at that point in history.

 

Right, because it was a corpuscular theory. As soon as you switch to a wave theory, you need a medium. You need an ether.

[Sisyphus]

A constant C doesn't necessitate the principle of relativity, no. It could indeed by constant only with respect to an aether medium, or with respect to its source. There were at one time theories holding both views. The Michelson-Morley experiments, though not conclusive in themselves, were AFAIK the first to suggest something like relativity. Though even then there were alternative explanations put forth, like "aether-dragging" around massive bodies and so forth. More and more precise experiments continued to suggest a principle of relativity, however, which is why Einstein had something to try (and, it turns out, largely succeed) to make sense of. Even then, it wasn't fully accepted until significantly after that. It was controversial enough that Einstein's Nobel Prize in 1921 makes no mention of relativity, but only his discovery of the photoelectric effect.

[Severian]

If I remember correctly, in his Principia, Newton first postulates a constant speed of light and then later sets it to infinity (by observation).

[gib65]

A constant C doesn't necessitate the principle of relativity, no. It could indeed by constant only with respect to an aether medium, or with respect to its source. There were at one time theories holding both views. The Michelson-Morley experiments, though not conclusive in themselves, were AFAIK the first to suggest something like relativity. Though even then there were alternative explanations put forth, like "aether-dragging" around massive bodies and so forth. More and more precise experiments continued to suggest a principle of relativity, however, which is why Einstein had something to try (and, it turns out, largely succeed) to make sense of. Even then, it wasn't fully accepted until significantly after that. It was controversial enough that Einstein's Nobel Prize in 1921 makes no mention of relativity, but only his discovery of the photoelectric effect.

 

So what was the principle on which Einstein believed light travelled at c irrespective of the observer? Was it just common knowledge at the time? I ask because they say he never mentioned the Michelson-Morley experiment in his proposal of SR, and that to me is the prime source of such knowledge.

[NowThatWeKnow]

So what was the principle on which Einstein believed light travelled at c irrespective of the observer? ...

"Irrespective of the observer"?? All observers will see light at c irrespective of the source. Or am I brain dead tonight?

[swansont]

"Irrespective of the observer"?? All observers will see light at c irrespective of the source. Or am I brain dead tonight?

 

That was not the view pre-relativity.

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So what was the principle on which Einstein believed light travelled at c irrespective of the observer? Was it just common knowledge at the time? I ask because they say he never mentioned the Michelson-Morley experiment in his proposal of SR, and that to me is the prime source of such knowledge.

 

It's a consequence of Maxwell's equations, once you realize that light is an EM wave. Einstein applied the concept to kinematics — his paper was call On the Electrodynamics of Moving Bodies (well, technically it was the German equivalent of that, Zur Elektrodynamik bewegter Körper)

[gib65]

"Irrespective of the observer"?? All observers will see light at c irrespective of the source. Or am I brain dead tonight?

 

Yes, that's what I meant.

 

his paper was call On the Electrodynamics of Moving Bodies (well, technically it was the German equivalent of that, Zur Elektrodynamik bewegter Körper)

 

I appreciate the English translation.

 

So Maxwell's equations do imply a constant c regardless of the speed of the one measuring it? Is this consistent with Michelson and Morley's expectations (that they could measure the speed of light differently depending on how they oriented their table)?

[swansont]

 

So Maxwell's equations do imply a constant c regardless of the speed of the one measuring it? Is this consistent with Michelson and Morley's expectations (that they could measure the speed of light differently depending on how they oriented their table)?

 

No, it's not. But it wasn't a known contradiction until scientists realized that light was an electromagnetic wave, and that this had implications on kinematics.

[gib65]

No, it's not. But it wasn't a known contradiction until scientists realized that light was an electromagnetic wave.

 

When did they find this out? I thought Maxwell discovered this, and didn't he die before Michelson and Morley conducted their experiment?

[swansont]

Maxwell proposed it, but I think it wasn't fully accepted until Hertz produced and analyzed radio waves in the late 1880's.

[D H]

A constant C doesn't necessitate the principle of relativity, no.

It does, however, necessitate something mathematically indistinguishable from relativity. Lorentz and Poincare attempted to keep classical physics alive with what is now called Lorentz ether theory. LET is completely indistinguishable from special relativity in terms of predicted outcomes, but is very distinct from special relativity in terms of axioms. Length contraction and time dilation are axiomatic in LET, as is an ether that defines an absolute reference frame. Making length contraction and time dilation axiomatic is incredibly ad hoc. The big problem with the theory was the axiomatic absolute reference frame. There is no way to detect it. None.

 

The Michelson-Morley experiments, though not conclusive in themselves, were AFAIK the first to suggest something like relativity.

The MM experiments did nothing of the sort. Maxwell's equations predicted the constancy of c. Michelson and Morley were diehard classical Newtonian physicists. They knew Maxwell was wrong, and set out to prove this. The MM experiment was a failure, probably the most important failed experiment in all of physics. They, and others, continued to attempt to truly prove Maxwell wrong after the failure of the MM experiment.

 

Even then, it wasn't fully accepted until significantly after that. It was controversial enough that Einstein's Nobel Prize in 1921 makes no mention of relativity, but only his discovery of the photoelectric effect.

Actually, special relativity was accepted amazingly quickly. By the time he published his paper, experimentalists had shown rather conclusively that the null results from the MM experiment were correct (the universe is not Newtonian) and had shown that Maxwell was right (light is but a small part of the electromagnetic spectrum). Einstein was a nobody in 1905, but Lorentz and Poincare were the heavy hitters of the time. The reason Einstein was not noted for special relativity in his Nobel prize was a problem of attribution much more than a problem of acceptance.

[Sisyphus]

It does, however, necessitate something mathematically indistinguishable from relativity. Lorentz and Poincare attempted to keep classical physics alive with what is now called Lorentz ether theory. LET is completely indistinguishable from special relativity in terms of predicted outcomes, but is very distinct from special relativity in terms of axioms. Length contraction and time dilation are axiomatic in LET, as is an ether that defines an absolute reference frame. Making length contraction and time dilation axiomatic is incredibly ad hoc. The big problem with the theory was the axiomatic absolute reference frame. There is no way to detect it. None.

 

I don't understand. Without further experiment, how would you know that C is constant relative to to the observer, as opposed to relative to some aether medium, the source of emission, or the nearest big mass?

 

The MM experiments did nothing of the sort. Maxwell's equations predicted the constancy of c. Michelson and Morley were diehard classical Newtonian physicists. They knew Maxwell was wrong, and set out to prove this. The MM experiment was a failure, probably the most important failed experiment in all of physics. They, and others, continued to attempt to truly prove Maxwell wrong after the failure of the MM experiment.

 

I understand that isn't what they were trying to do. You're saying they should have known their expectations were wrong? What's the significance of MM, then? I was under the impression that it was the first real experimental evidence that C is observer-dependent. Am I wrong?

 

Actually, special relativity was accepted amazingly quickly. By the time he published his paper, experimentalists had shown rather conclusively that the null results from the MM experiment were correct (the universe is not Newtonian) and had shown that Maxwell was right (light is but a small part of the electromagnetic spectrum). Einstein was a nobody in 1905, but Lorentz and Poincare were the heavy hitters of the time. The reason Einstein was not noted for special relativity in his Nobel prize was a problem of attribution much more than a problem of acceptance.

 

If you say so.

[D H]

I don't understand. Without further experiment, how would you know that C is constant relative to to the observer, as opposed to relative to some aether medium, the source of emission, or the nearest big mass?

You missed the point. The relativity theory developed mainly by Lorentz and Poincare did have a preferred reference frame, an aether frame. This was the frame in which light properly traveled. By making length contraction and time dilation axiomatic, light appears to have the same velocity in all frames. This theory was mathematically sound and agreed with observation.

 

In order to experimentally distinguish between special relativity from Lorentz ether theory the two theories must at some point predict different outcomes for some experiment. This is an impossibility: The two theories are mathematically equivalent. Special relativity says there is no preferred reference frame. LET says there is, but you can never find it.

 

While the two theories are mathematically identical, they are very different conceptually. One has seemingly ad-hoc and rather complex axioms (length contraction and time dilation) and predicates a metaphysical device as a basis. The other has a simple, aesthetically pleasing axiom (the laws of physics are the same in all inertial frames) and another simple axiom that is dictated by reality (the speed of light is the same for all inertial observers). Even though LET preceded special relativity, the physics world has almost unanimously settled on special relativity as the better explanation. Occam's scalpel most certainly applies here.

 

What's the significance of MM, then? I was under the impression that it was the first real experimental evidence that C is observer-dependent. Am I wrong?

That's right. However, that is how we look at things in hindsight. At the time, many viewed it is as evidence of an experiment badly done. More experimentation was needed (and a lot more experimentation was done).

 

 

If you say so.

 

http://www.mathpages.com/rr/s8-08/8-08.htm

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

[gib65]

Occam's scalpel most certainly applies here.

 

I find that interesting. Although Occam certainly would have identified SR as the simpler solution, it turns out to be the more difficult to grasp. Brings into question what constitutes 'simple'.

[Sisyphus]

I find that interesting. Although Occam certainly would have identified SR as the simpler solution, it turns out to be the more difficult to grasp. Brings into question what constitutes 'simple'.

 

I'm thinking it's not inherently harder to grasp. If we all grew up on a group of independently moving asteroids instead of all together on this seemingly solid planet, I think we'd probably find the idea of "real" motion as distinct from relative motion totally bizarre.