Principle of Relativity and Constancy of Light

**Gear300** · Jun8-09, 05:09 PM

I was reading on a conceptual situation, in which the propagation of light was analyzed from two different reference frames. One was from a stationary reference point, whereas the other was from a reference point that was moving with a velocity v in the direction of the propagation of light (v<<c). The velocity of light with respect to the stationary reference body would be c, whereas the velocity of light with respect to the moving reference body would be c-v. This presented a contradiction. Now the contradiction that was being referred to was not a contradiction of the principle of relativity in itself, but a contradiction between the principle of relativity and the constancy of light, right? If so, then what made physicists so sure that light had to retain a constant velocity c in all reference frames?

**ZikZak** · Jun8-09, 06:03 PM

Originally Posted by Gear300

If so, then what made physicists so sure that light had to retain a constant velocity c in all reference frames?

The fact that when your experiment is performed, c-v is not the result. The result is always c.

**diazona** · Jun8-09, 07:14 PM

For what it's worth, the constancy of the speed of light is a consequence of the principle of relativity. So it would be a contradiction of the principle with itself, if the speed of light were not constant.

**Gear300** · Jun8-09, 07:56 PM

Originally Posted by ZikZak

The fact that when your experiment is performed, c-v is not the result. The result is always c.

How were they able to test that?

Originally Posted by diazona

For what it's worth, the constancy of the speed of light is a consequence of the principle of relativity. So it would be a contradiction of the principle with itself, if the speed of light were not constant.

How is it a consequence from the principle of relativity? Before the special theory of relativity, the two were separate concepts that had a contradiction (from what I know). The contradiction was eliminated once it was realized that time was not simultaneous in relative/different frames.

**diazona** · Jun8-09, 08:10 PM

The principle of relativity states that physical laws are the same in all inertial reference frames. This includes Maxwell's equations, which determine the speed of light. So therefore, the speed of light is the same in all inertial reference frames.

As for testing the constancy of the speed of light: probably the most famous way is the Michelson-Morley experiment.

**Gear300** · Jun8-09, 08:22 PM

Originally Posted by diazona

The principle of relativity states that physical laws are the same in all inertial reference frames. This includes Maxwell's equations, which determine the speed of light. So therefore, the speed of light is the same in all inertial reference frames.

As for testing the constancy of the speed of light: probably the most famous way is the Michelson-Morley experiment.

I see. Thanks for the help.

**jtbell** · Jun8-09, 10:29 PM

Originally Posted by Gear300

How were they able to test that?

See the FAQ, Experimental Basis of Special Relativity, in particular the experiments listed in section 3 that have to do with the speed of light. The Michelson-Morley experiment is one of them, but many better experiments have been performed since then.

**Canticle** · Jun12-09, 11:09 AM

but you shouldn't be fooled into thinking that solves the basic light paradox.

Yes, whatever the velocity/vector of the propagator or reciever they measure light at 'c'.

The history of light measurement is long and interesting. The 'fact' came first, the variable reference frame 'explanation' followed naturally from that, but if it seems counter intuitive to you it's because it is. QFT just makes it worse by insisting on that pesky 3rd reference frame. You can study 4 vectors and dilation as long as you like but the paradox remains.

There may be a rush of complacent comments from some now saying that's nonsense, but none of them will be able to actually prove it.

dx · Jun12-09, 11:15 AM

Originally Posted by Canticle

You can study 4 vectors and dilation as long as you like but the paradox remains.

What paradox?

**Canticle** · Jun12-09, 11:35 AM

The basic light paradox in SR, that's well explained but never actually resolved unless we deny the quantum field (as many still do).

i.e. (for all -as AE said we 'should be able to explain physics to a barmaid'!); If you're flying through space at 0.5c towards a stationary light source, the light not only travels across space to you at 'c' (in the reference frame of space) it's also travelling at 'c' in your frame when you measure it.

Equivalence and SR never would work with a 3rd reference frame, as AE himself said.
But it's no big deal. Only one of the many paradoxes we can't spend our lives worrying too much about!

**Naty1** · Jun12-09, 11:37 AM

If so, then what made physicists so sure that light had to retain a constant velocity c in all reference frames?

As noted above, nobody could ever find a different speed, despite the theory of an "ether".....everytime scientists measured the speed of light they got the same "c"....

So Einstein somehow had the inuition that if light speed was actually constant in all reference frames, then length (distance) and time must be the variables....how crazy is that!! Once again "consensus science" has been totally wrong...for probably well over 2,000 years on this one!!!!

If that "relativity" wasn't enough to confuse everyone, he later figured out gravitational potential also causes time to vary!!!....So both speed and gravity affect the relative passage of time.....which in my mind strongly suggests a "unification" we have not yet stumbed upon.....

and if that isn't "relative" or "uncertain" enough, then along comes quantum mechanics, which Einstein helped found, and it was SO CRAZY Einstein never did accept it's additional degrees of variability/uncertainty....position/momentum uncertainty, wave/particle duality, etc, etc

It's a pretty strange world from our perspective, and yet nature seems to have no problem with it!!!!

dx · Jun12-09, 11:39 AM

Originally Posted by Canticle

i.e. (for all -as AE said we 'should be able to explain physics to a barmaid'!); If you're flying through space at 0.5c towards a stationary light source, the light not only travels across space to you at 'c' (in the reference frame of space) it's also travelling at 'c' in your frame when you measure it.

And why do you think this is a paradox? (I'm assuming that you mean 'the reference frame of the light source' when you say 'reference frame of space')

**Naty1** · Jun12-09, 11:41 AM

Only one of the many paradoxes we can't spend our lives worrying too much about!

I disagree: I believe such paradoxes invariably point to a partial lack of understanding on our part. They are not as obvious as a single experimental result which can sink an entire theory, yet in my opinion such paradoxes point to opportunities....maybe akin to "coincidences"....another frequent indicator we have missed something....

**Canticle** · Jun12-09, 12:44 PM

Yes DX, but if the light source is moving as well it's still the 'reference frame of space', i.e. a 3rd frame, and the results are still the same; = 'c'.

Yes I think it's a paradox, and it's generally included with all the others, (similar to the twins paradox which can't handle non inertial frames), as it's completely counter intuitive and we can't prove how it works with present physics.

And Naty1 I agree with you. The paradoxes simply demonstrate our lack of understanding. But most human animals have to get on with life and can't worry too much about them. The problem is; any problems we can't solve we put to the back of our minds - and quickly forget they exist, or many even pretend they ARE solved! (self delusion). There's probably too much of that in physics. But yes I saw them as aopportunities too, - for enlightenment. For those of us who care. The problem is we can't solve them as it would make us cranks! Shame.

Are you still there Gear300? Still interested?

dx · Jun12-09, 01:30 PM

Canticle,

There is no such thing as 'the reference frame of space'. Clearly, you have absolutely no knowledge of even basic physics, yet somehow you feel comforable using words like 'quantum field'. You clearly also misunderstand the meaning of the word 'paradox'. 'paradox' is not the same as 'counterintutive'. Also, nature does not have to comform to your intuition. The twin paradox is also not actually a paradox, but an 'apparent paradox'. There can be no paradox in special relativity, because it's consistency can be demonstrated. Note that this does not mean that SR is true beyond doubt, all it means is that it is consistent as a mathematical structure.

**Max™** · Jun14-09, 05:34 AM

You are measuring the distance the light travels in what you claim as a second (or whatever period) in your frame of reference.

If you're in motion, you are moving through time differently than a resting observer.

It isn't exact math as I don't feel like digging out the formulas, but for an example.

You're moving 150,000 kps, you measure a beam of light for 1 second.

Your measurement shows it travels away from you at 300,000 kps, same as a resting observer.

Then basic math pops up in your head and you think "wait, if it is going away from me that fast, shouldn't that mean it covered 450,000 kilometers during my measurement?"

Oh dear, wouldn't that mean it went at 450,000 kps?

How do you explain this?

Easily, if you understand SR. You didn't measure the distance the beam traveled in a second, you actually measured for a second and a half, you were simply moving through time slowly enough that 1 second of your subjective time was 1.5 seconds for the beam of light.

It isn't a paradox, it's just time dilation.

Note: that math is not exact, technically there should be a slightly different correction in your observed (subjective) time due to your velocity and whatnot as I recall. I just used big round numbers to make it easier to see what happens.