# Invariance of the speed of light

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geordief    53

That's fine, you are receiving many replies and it's not always clear which are real answers.

If you like, I can scan and send you an old paper in which this was done.

Thanks,that would be good of you.

My learning technique ,such as it is is to identify my mistaken preconceptions and reorient myself from there forwards.

Hopefully I do not overly contaminate the threads with my initial misconceptions and can learn to a degree from those of you who who have put in the work down the years and are able to speak with some authority in particular areas.

Edited by geordief

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How so? You were talking about light clocks. Nobody actually uses a light clock to measure time. To claim we have no other clocks is ludicrous.

Not at all. You are not responding to what I wrote. You're responding to a cut down version. If you look at MY post, it's made perfectly clear that I'm saying that we have no clocks that are not affected by the time dilation of SR.

In the case of a sound clock, you have access to an external clock not affected by the motion through the medium.

In the case of measuring the speed of light, there is no clock unaffected by time dilation.

So here's a question for you :

IF there was such a thing as a clock, that was not affected by time dilation, would all observers using it measure the same figure for the speed of light?

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Delta1212    894

Not at all. You are not responding to what I wrote. You're responding to a cut down version. If you look at MY post, it's made perfectly clear that I'm saying that we have no clocks that are not affected by the time dilation of SR.

In the case of a sound clock, you have access to an external clock not affected by the motion through the medium.

In the case of measuring the speed of light, there is no clock unaffected by time dilation.

So here's a question for you :

IF there was such a thing as a clock, that was not affected by time dilation, would all observers using it measure the same figure for the speed of light?

Clearly not because it would mean that relativity was wrong.

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geordief    53

Geordief you may remember an old saw about two short planks (not referring to you of course)

Anyway you might like to consider / be intrigued by this article on the subject of Maxwell's equations, the invariance of c and two short planks.

That is quite a minefield for me. I thought (without entitlement or authority) that electro-magnetic theory was more or less "done and dusted" .

Yes ,it does seem slightly farcical that the motion of two wedges can be modeled with mathematics that are identical (?) to that employed in Maxwell's equations .

I am surprised though that there is hardly any online discussion as to this interpretation. This must be in part that it has no practical consequence.

I am sorry that I cannot join that discussion as all I am able to do is appreciate the outline I hope I have drawn fairly correctly . My calculus is fairly rusty and I am taking on trust (I am 100% confident they are right mind you) those wedge equations and their "mirroring" of the Maxwell equations .

Still, thank you for posting that. It was entertaining and a little mind opening.

EDIT: Catt's position on invariance of c seems to me to be that it is not integral to Maxwell's equations. But he does describe M's equations as "hogwash" outside of the E/M proportionality and the actual value of c

Edited by geordief

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Strange    2490

That brings back memories. Ivor Catt was the first crackpot I encountered, through his regular articles in Wireless World. He made some interesting points and then went beyond what was reasonable.

In one article, he even tried throw doubt on the existence of Turing ("I worked at Bletchley Park and never met him ...")

Edited by Strange

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geordief    53

That brings back memories. Ivor Catt was the first crackpot I encountered, through his regular articles in Wireless World. He made some interesting points and then went beyond what was reasonable.

In one article, he even tried throw doubt on the existence of Turing ("I worked at Bletchley Park and never met him ...")

He certainly has a name that would adorn a crackpot

Here this seems to have a lot on that (and other) subjects

http://www.ivorcatt.co.uk/899.htm

Edited by geordief

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swansont    6213

Not at all. You are not responding to what I wrote. You're responding to a cut down version. If you look at MY post, it's made perfectly clear that I'm saying that we have no clocks that are not affected by the time dilation of SR.

Not perfectly clear. Not at all.

So here's a question for you :

IF there was such a thing as a clock, that was not affected by time dilation, would all observers using it measure the same figure for the speed of light?

Hypotheticals that violate the laws of physics generally don't have valid answers.

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mistermack.

I think that there are 2 classes of clock -- inertial, & other.

Other would include pendulums & hour-glasses. These would read zero when in free-fall, & that-there zero would not be affected by time dilation.

And, an inertial clock would tell me my age exact at all times, if i wore it on my wrist (& didn't do any hand waving). Time dilation wouldn't affect it from my point of view -- ie time dilation would affect me & clock equally. After all, there is no such thing as time, or, putting it another way, time is relative, which we all know i guess.

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Janus    687

You've asserted that they would tick at different rates, but you haven't given any evidence for that. It might be right, but it also might not.

All it takes to show this is a little math. For sound pules for a clock aligned perpendicular to the relative motion to the medium The light has to travel along two diagonals each of length

$\sqrt{L^2+ (vt)^2}$

Where L is the distance between source and reflector, v is the velocity of the clock relative to the medium and t is the time.

If the speed of sound in the medium is s, then we know that the length of each diagonal is equal to st. thus

$st= \sqrt{L^2+ (vt)^2}$

$s^2t^2= L^2+ v^2t^2$

$s^2t^2= L^2+ v^2t^2$

$s^2t^2- v^2t^2= L^2$

$t^2(s^2- v^2)= L^2$

$t^2= \frac{L^2}{s^2- v^2}$

$t= \frac{L}{\sqrt{s^2- v^2}}$

This for one diagonal so the round trip would be

$T= \frac{2L}{\sqrt{s^2- v^2}}$

For a sound clock aligned with the motion the sound would be in one direction with respect to the motion relative to the medium on one leg and the in the opposite direction in the other.

For the leg traveling in the direction of the motion, the time of travel would be

$t1 = \frac{L}{s-v}$

and for the other leg it would be

$t2 = \frac{L}{s+v}$

Thus the total round trip would take

$T =\frac{L}{s-v}+\frac{L}{s+v}$

$T= \frac{2Ls}{s^2-c^2}$

Which gives a totally different answer. for example if v= 0.5s, and L = 1, then you get an answer of 2.309 for the first light clock and 2.6667 for the second light clock.

As far as the second goes, an observer travelling with the clock could only note that it ticks at different rates, if he had a different clock to compare with it.

When it comes to measuring the speed of light, we have no other clocks. Time dilation occurs for everything.

which belies the argument that a sound clock and light clock behave the same. You can't have both sound clocks which operate at one speed and light clocks which operate at another behaving identically, because time dilation can only be related to one speed

Which igoes back to my original point. All observers measure the same value for the speed of light, because they use clocks that slow, with their relation to the speed of light.

Except that just relying on clocks running slow can't fully account for the invariant speed of light. If you just rely on the clocks running slow, you run into the same problem as you do above with the sound clock. A light clock aligned with the motion would tick a different rate. The reason it doesn't is due to length contraction. The light clock aligned with the motion is also length contracted which changes L such that the round trip time is the same as the perpendicular clock.

Now you might try to argue that it is the same movement through a medium that contracts the length as slows the clock, but this is just an ad hoc solution. You are just assuming that movement through an aether slows clocks and shortens objects by just the correct amount to cause one to measure the speed of light as being invariant. There's no reason given for this to be true other than it is needed to be true in order to maintain the concept of an aether.

Edited by swansont

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Janus, thanks for that. I found it very interesting. Forty five years ago, I would have sat down and dashed off something similar in no time, but no way could I today, I avoid maths like the plague these days.

Just a few comments. Firstly, I'm not actually arguing for an aether. Back in the thread, I just made a comment that you could do the same for sound waves, (or something like that) and the discussion just rolled on from there.

Secondly, you say that a light clock would also run at different speeds, for different angles, if it were not for length contraction.

So it's length contraction that makes the difference between the light example and the sound example.

Well, the answer might be that the sound clock we are talking about is not a true sound clock. Yes, the pings are the result of sound waves travelling.

But the rest of the clock is made of material OTHER than sound waves.

If the clock was ENTIRELY made of sound waves (however that might happen) then perhaps you would get a similar length contraction to the light clock.

So in comparing the two clocks, we're not actually comparing like for like.

For the last bit, whether it's movement through nothing, or movement through a medium, the result is the same.

It's the combination of time dilation and length contraction that make the result the same for all observers.

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swansont    6213

Well, the answer might be that the sound clock we are talking about is not a true sound clock. Yes, the pings are the result of sound waves travelling.

But the rest of the clock is made of material OTHER than sound waves.

If the clock was ENTIRELY made of sound waves (however that might happen) then perhaps you would get a similar length contraction to the light clock.

So in comparing the two clocks, we're not actually comparing like for like.

There is no basis in theory to propose that sound undergoes length contraction, and

A sound clock made up completely of sound is a ridiculous suggestion. And it's not like for like, since a light clock is not comprised completely of light. You have a source, a mirror, and a detector.

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Having looked at length contraction, there appears to be no reason why it should not occur in this hypothetical sound-clock scenario, IF you actually were able to use sound clocks.

The length contraction arises as a result of time dilation, so observers using sound clocks in different reference frames will measure different lengths for the same object.

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geordief    53

Having looked at length contraction, there appears to be no reason why it should not occur in this hypothetical sound-clock scenario, IF you actually were able to use sound clocks.

The length contraction arises as a result of time dilation, so observers using sound clocks in different reference frames will measure different lengths for the same object.

Would not a hypothetical "sound clock" rely for its functioning on the em connections between processes? So a "sound clock" would be a derivative of an "em clock" and not a "sound clock" at all.

I have though long and hard(if feebly and ineffectively probably) on this in the past and came to that conclusion eventually.

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Would not a hypothetical "sound clock" rely for its functioning on the em connections between processes? So a "sound clock" would be a derivative of an "em clock" and not a "sound clock" at all.

I have though long and hard(if feebly and ineffectively probably) on this in the past and came to that conclusion eventually.

Yes, I'm sure that's right. I don't think it affects the topic particularly though.

We don't know that EM is the most fundamental level of matter or energy. An EM clock might be a derivative of some even more fundamental processes that we are nowhere near discovering. But it all works at our present level of knowledge.

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imatfaal    2478

Yes, I'm sure that's right. I don't think it affects the topic particularly though.

We don't know that EM is the most fundamental level of matter or energy. An EM clock might be a derivative of some even more fundamental processes that we are nowhere near discovering. But it all works at our present level of knowledge.

At high energy scales we know that the forces unify - we have known about the electroweak for a long time now. If you need to know more about that topic ask elsewhere. The invariance of speed of light is a phenomenon at our everyday energy levels and it is an Electromagnetic effect - the invariance of the EM wave pops out of the Maxwell equations and is axiomatic to the most successful theories of physics

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swansont    6213

Having looked at length contraction, there appears to be no reason why it should not occur in this hypothetical sound-clock scenario, IF you actually were able to use sound clocks.

The length contraction arises as a result of time dilation, so observers using sound clocks in different reference frames will measure different lengths for the same object.

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