# Altering the speed of light

Doc Al
Mentor
I understand, basically the two clocks are out of sync because of simultaneity, the earth and the marker both start their clocks at the same tie, but because the rocket is in motion, the clock start is NOT simoutaneous according to the rockets referance frame, maing the clocks out of sync. Right?
You got it.

Why is it that light goes faster with the rotation of earth, and light goes slower against the rotation of earth?

h8ter said:
Why is it that light goes faster with the rotation of earth, and light goes slower against the rotation of earth?
Light is a constant speed no matter in which direction you point it at (when in the same medium). Ive never heard of light being faster in ine direction. Do you have a source?

pervect
Staff Emeritus
h8ter said:
Why is it that light goes faster with the rotation of earth, and light goes slower against the rotation of earth?
If you synchronize your clocks in the manner proposed by Einstein, the speed of light is independent of direction. So it's not really true that light goes faster or slower from east/west or from west/east.

But also note that is impossible to synchronize all clocks on a rotating planet or disk according to Einstein's "clock in the middle" method. See for instance

http://www.smcm.edu/nsm/physics/SMP03S/KeatingB.doc.pdf [Broken]

This has been the source of a *lot* of confusion, as a brief websearch will reveal :-(.

There is a common synchronization method widely used, for instance in the definition of TAI time, that does allow all the clocks on a rotating object to be synchronized. This is not Einstein's method, of course, since we've already mentioned that it's impossible to do this via Einstein's method. One first thinks, perhaps, of emitting a signal from the center of the earth and synchronizing outwards - when the obvious practical difficulties with this method raise their head, one thinks of synchronizing only along north-south lines to master clocks at one or both of the poles.

Before one gets too excited about the existence of such alternate synchronization methods, one should realize that Einstein's method, and only Einstein's method, allows one to keep Newtonian momentum p=mv or relativistic momentum p=mv/sqrt(1-(v/c)^2) as a conserved quantity.

So before one gets too excited by various commonly-found-on-the-web-but-ill-informed claims about the "one-way" speed of light being different from e-w or w-e, remember to check whether or not, by the definitions used, that a body of mass m moving east at a velocity v will come to a stop when colliding with a body moving west at a velocity v, according to the particular coordinate scheme used to measure velocity. It can, in fact, be shown that only an isotropic synchronization scheme such as Einstein's will conserve momentum. I'll mention that the isotropy requirement comes from Noether's theorem, but I think it would be getting to technical and wandering too far afield to go into any more detail at this point.

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So, how do you explain the sagnac effect? It shows how rotation alters the speed of light. Well, how energy fields alters the speed of light. Although the MM experiment "disproved" the aether, this was only done by disproving it through Maxwell's equations. So, is there any explanation without throwing in Maxwell's equations (since they only deal with detectors and emitters AT REST...not moving). Please try not to be bias when explaining this. I want a good explanation of it.

pervect
Staff Emeritus
h8ter said:
So, how do you explain the sagnac effect? It shows how rotation alters the speed of light.
Did you read the reference I posted?

I'm looking at the section right now which starts out as

The sagnac effect, and indeed all of the "pardoxes" described in this paper, can be explained by the impossibility of synchronizing all clocks on the disk such that all observers will agree that they are in fact synchronized.
I would guess you probably didn't read it, especially from the rapidity of your response :tongue2:

The paper really is worth reading IMO - it's quite good, and goes into a lot more detail than I can. It talks in detail about what happens when various observers make a "round trip" around the disk, and compare their clocks, for instance.

Another quote caught my eye, the concluding quote from the URL I cited previously (and repeated below):

http://www.smcm.edu/nsm/physics/SMP03S/KeatingB.doc.pdf [Broken]

In the words of Rizzi and Tartaglia [reference omitted], "... a rotating disk does not admit a well-defined "proper frame"; rather, it should be regarded as a class of infinite number of local proper frames, considered in different points and different times, and glued together according to some convention.

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Haha, I couldn't read it because work didnt have acrobat reader (I'm not supposed to download programs to computer). I'll read it in a minute or tomorrow and post a response. I do know that the sagnac effect is accepted by relativists. I read that somewhere. That is very contradictive. Thanx for the link--again. I'll have a response ASAP after I read it.

$$d_{rest} = d_{moving} \sqrt{1-v^{2} / c^{2}}$$

if you use the actual numbers does it still work?
44000lightyears = d_{moving} \sqrt{1-269813212m/s^2/299792458m/s^2}

44000 = d_{moving} \sqrt{1-0.809999999}

44000 = d_{moving} \sqrt{0.190000001}

44000 = d_{moving}0.435889896

44000/0.435889896=d_{moving}

100942.923=d_{moving}

i had to have done something wrong here.

bino said:
$$d_{rest} = d_{moving} \sqrt{1-v^{2} / c^{2}}$$

if you use the actual numbers does it still work?
44000lightyears = d_{moving} \sqrt{1-269813212m/s^2/299792458m/s^2}

44000 = d_{moving} \sqrt{1-0.809999999}

44000 = d_{moving} \sqrt{0.190000001}

44000 = d_{moving}0.435889896

44000/0.435889896=d_{moving}

100942.923=d_{moving}

i had to have done something wrong here.
its a colt easier if you take your speed and convert it into a decimal of the speed of light. (ie: 0.8c, 0.5c etc..). Lets say you have 2,000,000m/s. To convert it, you would do the following:
$$\frac{2,000,000m/s}{300,000,000m/s/c} = v$$
$$v = 0.0066c$$

use this method, its a lot easier to work with, and units cancel out a lot easier.

Doc Al
Mentor
length contraction

bino said:
$$d_{rest} = d_{moving} \sqrt{1-v^{2} / c^{2}}$$

if you use the actual numbers does it still work?
....
i had to have done something wrong here.
You need to interpret the length contraction equation properly. I prefer to write it like this:
$$L = L_0 \sqrt{1 - v^2/c^2}$$
Where $L_0$ is the proper length of the object (measured in its rest frame) and $L$ is the length of the object when measured from the frame in which it moves with speed $v$.

It's unreal how that paper is biased to the support of SR!! Do you have any evidence that disregards SR? That's what I'm truly looking for. SR has not been proven, so it is a possibility that the Sagnac effect is present. I was talking to my physics teacher today about the Lorentz Transform, and he said he doesn't think that is a good explanation for what is really happening. Sometimes he talks crazy though. :rofl:

pervect
Staff Emeritus
If I had any reproducible evidence that disproved SR, I'd be standing in line to get my Nobel prize

One might guess from the fact that SR is a well accepted theory that very well-known results like the Sagnac effect don't falisfy it. (I've never quite understood why this wasn't obvious to people - I've got a somewhat off-topic guess that it may have to do with the prevalence of lying in American society today, but that's just a wild-assed-guess.)

I've also never quite understood why people make up their minds in advance that SR must be wrong, and then go looking for anything that might support their preconceived ideas. :grumpy:

SR is really a well tested theory, and it has been for probably the last 50 years or so.

if i were moving at .90c would the speed of light look slower or would it still be moving as fast as if i were standing still?

bino said:
if i were moving at .90c would the speed of light look slower or would it still be moving as fast as if i were standing still?
the speed of light would still look like normal. It would still move at a speed of c.

h8ter said:
It's unreal how that paper is biased to the support of SR!! Do you have any evidence that disregards SR? That's what I'm truly looking for. SR has not been proven, so it is a possibility that the Sagnac effect is present. I was talking to my physics teacher today about the Lorentz Transform, and he said he doesn't think that is a good explanation for what is really happening. Sometimes he talks crazy though. :rofl:
SR has been one of the most remarkably accurate theories known to man. It the only theory that works as wee as it does, and giver results that are as accurate as they are.

eventhough im going almost the same speed?

yes thats the thing about light, its speed is always c, no matter in waht frame youre in. Even if yuo send two beams of light at each other, the speed at which they approach eachoter is not c+c, it is just c.

so then if we were going the speed of light, light would still be going a lot faster than us?

Doc Al
Mentor
bino said:
so then if we were going the speed of light, light would still be going a lot faster than us?
As has been mentioned several times in this (seemingly endless) thread, one cannot go at the speed of light. So, let's say you were in your rocket going at 0.99c with respect to the earth. Someone (anyone!) fires off a beam of light. Observers on both rocket and earth will measure that beam as traveling at speed c with respect to themselves. The logical consequences of this fact lead to all the SR "effects" that we've been discussing throughout this thread: length contraction, time dilation, and simultaneity.

So, to answer your question directly. Everyone will always measure light as moving at speed c with respect to themselves. So, if that's what you mean by "going a lot faster", then yes. But another observer, watching you go by at say 0.99c, also measures that light as going at speed c with respect to him, so as far as he is concerned the light is only going 0.01c faster than you.

thats interesting.

so you find the length of a moving object by the time*speed?

Why is it that when objects only loose length when traveling at relativistic speeds? Well, it is noticeable at relativistic speeds. What actually defines the magnitude of relativistic speed? When do you start applying the Lorentz Transform and why?

In addition to the statement about length contration, I would like to say that length is not PHYSICALLY lost. In one reference frame the length of an object at relativistic speeds is contracted, while relative to that object it is the same. It is not physically lost, because it is contracted and proper in two reference frames. This is contradictory. Nothing can loose length and keep it at the same time. Just my two coins going in.

h8ter said:
Why is it that when objects only loose length when traveling at relativistic speeds? Well, it is noticeable at relativistic speeds. What actually defines the magnitude of relativistic speed? When do you start applying the Lorentz Transform and why?
Since everyday speeds are nothing compared to the speed of light, the length contraction/time dilation/mass increase are neglected. When moving at relativistic speeds (close to the speed of light) these effects are very noticable. For example, mass is a little more than 2 times greater at .9c than the rest mass. Which shows that you have to get very close to the speed of light for the mass to start sky rocketing.

In addition to the statement about length contration, I would like to say that length is not PHYSICALLY lost. In one reference frame the length of an object at relativistic speeds is contracted, while relative to that object it is the same. It is not physically lost, because it is contracted and proper in two reference frames. This is contradictory. Nothing can loose length and keep it at the same time. Just my two coins going in.
Correct, nothing is physically happening to it, there is no force making it contract. It simply IS shorter at a certain velocity.

Doc Al
Mentor
bino said:
so you find the length of a moving object by the time*speed?
As was discussed several posts ago, one way of measuring the length of a moving object is to time how long it takes to pass you, then multiply that time by its speed.