Can the Speed of Light Be Changed and What Does It Mean for Space Exploration?

[russ_watters]

yes i know this but how was this equation formed? i know that it is some how derived from time dilation. right? don't they do something with the end points.

You could also measure the time it takes to pass you and divide that into the speed.

 

[quantumdude]

You could also measure the time it takes to pass you and divide that into the speed.

You'd multiply the time by the speed.

Always check your units!

 

[Nenad]

Nonsense. You seem to think that the rocket observer is somehow the one who is really moving. Not so. SR "time dilation" works for both frames.

I understand that both observers see the other as the one in motion, and the effects of SR taking effect on the other, and not themselves, but one of them is obviously wrong, and once the two observers join the same frame of reference, then the wrong observer sees that he is wrong.

 

[russ_watters]

You'd multiply the time by the speed.

Always check your units!

Uh, yeah - what he said. Hey, why am I not working right now?!

 

[Doc Al]

Both frames equally valid

I understand that both observers see the other as the one in motion, and the effects of SR taking effect on the other, and not themselves, but one of them is obviously wrong, and once the two observers join the same frame of reference, then the wrong observer sees that he is wrong.

Sorry, Nenad, that's not how SR works. Both observers make measurements with respect to their own inertial frames. Both frames are equally "correct". One is not "obviously wrong". That statement makes me think that you really do view one frame as being "right" and the other "wrong"--that's not how SR works. In fact it misses the entire point of relativity.

 

[Nenad]

ok, this is the way I was taught, that the fame in motion is the frame in which canges occur such as time dilation, mass dialtion and length contraction. I was also taught that in a case with two abservers, one of them will be wrong in his assumptions that he is in motion and not the other. So youre telling me that both observers are right, and they both end up agreeing in the end with nobody changing their mind. Hmm, this makes me think. But how would the time dilation work out for both observers?(twin paradox case) They will both assume that they are aging much more than the other observer, (as a result of their assumptions that the moving frames time is slowing down)?? Just help me with this example, I will get the rest from books.

 

[Nenad]

Simultaneity of space-separated events is frame-dependent.

Let's work out the details. Let's put a clock on that marker, synchronized with the Earth clock. That's no problem, since they are in the same frame. Let's assume that according to the Earth frame the rocket passes the marker exactly when the clocks read zero. Will the rocket observer agree? No! To the rocket observer the Earth and marker clocks are wildly out of synch! To the rocket observer, the clock (calendar, I guess) on Earth reads 43,560 yrs when the rocket passes the marker. Since the rocket arrives when the Earth clock reads 44,444 yrs, the rocket observer would say that Methuselah is only 884 earth-years old. This makes sense, since the rocket sees the Earth clocks as running slow. Of course, the Earth frame will drag out a 44,444 year old geezer and say that he was born exactly the moment that Methuselah was. Of course the rocket guys would laugh--they know that this Earth man was born way before they came anywhere near that marker, long before Methuselah was born.

I hope this makes sense to you. (And that I explained it correctly.)

Doc Al, I think I got it. Since the rocket observer is in motion, simoutanaety in a rest frame is not simoutaneous to him. The he passes the marker, the signal from the marker would read 0 time has passed and it is just starting to count, but since the signal from Earth comes later than the signal form the marker (simoutanious signals in a rest frame), the ship sees that the signal from Earth reads that 43,560 yrs has passed, making him think that the Earth time is not syncronized with the marker time. Is this right? If its not, please explain to me a little more about the case youve set up, its very interesting.

 

[Doc Al]

ok, this is the way I was taught, that the fame in motion is the frame in which canges occur such as time dilation, mass dialtion and length contraction.

But the frames are in relative motion. Relativity teaches that either frame is perfectly entitled to view itself as being at rest.

I was also taught that in a case with two abservers, one of them will be wrong in his assumptions that he is in motion and not the other.

This is a serious misconception. Any inertial frame is just as good as another for the purpose of making observations. Relativity tells us that certain quantities (length, time, clock synchronization) are frame dependent. The Lorentz Transformations describe how the space-time coordinates of an event (and thus measurements of length and time) transform between frames.

So youre telling me that both observers are right, and they both end up agreeing in the end with nobody changing their mind.

Yes, both observers are "right" in the sense that their clocks and measuring sticks really do work according to SR. Whether they have to change their minds or not depends on their level of sophistication: Observers who understand SR expect measurements to be different in different frames.

Hmm, this makes me think. But how would the time dilation work out for both observers?(twin paradox case) They will both assume that they are aging much more than the other observer, (as a result of their assumptions that the moving frames time is slowing down)?? Just help me with this example, I will get the rest from books.

Let's get this clear first: between inertial frames in relative motion, time dilation (and length contraction, and clock synchronization effects) work exactly the same for both observers.

The twin "paradox" -- which, by the way, has nothing to do with the example we've been discussing -- involves a little more analysis. I won't attempt it here; pick up any book on relativity. (David Hogg's book discusses it.) I'll just make a few comments that might help. The twins begin in one frame, but one remains in that frame while the other accelerates then rejoins that frame. That accelerating twin ends up traveling a different (curved) path through space-time--one that always has a shorter proper time.

Since one twin accelerates, he does not remain in a single inertial frame. So, you cannot simply apply SR from that twin's frame as if it did. But the view of the stay-at-home twin is always from a single inertial frame, so SR works just fine for him. He sees the other twin's clock as being slow. You can certainly analyze the twin paradox from both twin's viewpoints even within SR--but that involves the accelerating twin changing inertial frames at some point during his journey. But rest assured, a complete account of both twins unambiguously shows that they both agree that the accelerated twin ends up younger. So, it's not that one is wrong and the other right: they are both right, and they both agree.

 

[Doc Al]

Doc Al, I think I got it. Since the rocket observer is in motion, simoutanaety in a rest frame is not simoutaneous to him.

Now you're talking sense... finally!

The he passes the marker, the signal from the marker would read 0 time has passed and it is just starting to count, but since the signal from Earth comes later than the signal form the marker (simoutanious signals in a rest frame), the ship sees that the signal from Earth reads that 43,560 yrs has passed, making him think that the Earth time is not syncronized with the marker time. Is this right? If its not, please explain to me a little more about the case youve set up, its very interesting.

It's "kind of" right. Let me rephrase it. First, as you point out, simultaneity is frame dependent. SR predicts that the two clocks (marker clock and Earth clock) that are in synch according to marker-earth frame will not be in synch according to the rocket frame. (As I showed earlier, according to the rocket frame when the marker clock reads 0 the Earth clock reads 43,560 yrs.)

Just for fun, let's say that the rocket observer has a super telescope that allows him to see the Earth clock. When the rocket passes the marker (just when the marker clock reads 0) the rocket observer looks through the telescope. After accounting for the time of travel of the light (and time dilation) he will deduce that the Earth clock must be reading 43,560 yrs at the moment he passes the marker. It all fits together.

 

[Nenad]

Just for fun, let's say that the rocket observer has a super telescope that allows him to see the Earth clock. When the rocket passes the marker (just when the marker clock reads 0) the rocket observer looks through the telescope. After accounting for the time of travel of the light (and time dilation) he will deduce that the Earth clock must be reading 43,560 yrs at the moment he passes the marker. It all fits together.

Im kind of getting it, but the one think I do not understand is why the two clocks would be out of sync. They are in the same frame, and if the moving frame has this 'super telescope', why would the time passed on Earth be different than time passed on the marker. Both the marker and Earth are in the same frame of reference. If you could just help me out here that would be great.

 

[The Bob]

Im kind of getting it, but the one think I do not understand is why the two clocks would be out of sync. They are in the same frame, and if the moving frame has this 'super telescope', why would the time passed on Earth be different than time passed on the marker. Both the marker and Earth are in the same frame of reference. If you could just help me out here that would be great.

If I am correct, the objects are in the same frame of reference but in different states: stationary or moving. Therefore it is like a frame in a frame and so the time changes with it.

The Bob (2004 ©)

 

[Chronos]

Incorrect. Each frame in background independent. Simultaneity does not exist in either reference frame.

 

[The Bob]

Incorrect. Each frame in background independent. Simultaneity does not exist in either reference frame.

Ok, sorry.

The Bob (2004 ©)

 

[Doc Al]

Im kind of getting it, but the one think I do not understand is why the two clocks would be out of sync.

Whether two clocks in the same frame are synchronized or not is not an intrinsic property of the clocks--it depends on what frame is observing those clocks. This is the meaning of the "relativity of simultaneity". The marker clock and the Earth clock are in the same frame, and they are synchronized according to the earth-marker frame. But the rocket observer will observe them to be out of synch according to the rocket frame.

They are in the same frame, and if the moving frame has this 'super telescope', why would the time passed on Earth be different than time passed on the marker.

Because the rocket observer wants to know what time is it on Earth right now--the moment he passes the marker. But "right now" means different things to each observer. To the Earth frame "right now" means when both clocks read 0. But the rocket frame sees the Earth clock being way out of synch with the marker clock. And, since the Earth clock is moving, the rocket observer sees that it operates slowly compared to his own clock. And don't forget that the rocket observer measures his distance to the Earth to be shorter that what the Earth frame measures it to be.

To make sense out of these things, one cannot consider length contraction or time dilation as separate, independent effects. One must consider all the relativistic effects--length contraction, time dilation, simultaneity--working in concert. To consider one effect while ignoring the others will result in logical contradiction.

 

[Nenad]

Whether two clocks in the same frame are synchronized or not is not an intrinsic property of the clocks--it depends on what frame is observing those clocks. This is the meaning of the "relativity of simultaneity". The marker clock and the Earth clock are in the same frame, and they are synchronized according to the earth-marker frame. But the rocket observer will observe them to be out of synch according to the rocket frame.

Because the rocket observer wants to know what time is it on Earth right now--the moment he passes the marker. But "right now" means different things to each observer. To the Earth frame "right now" means when both clocks read 0. But the rocket frame sees the Earth clock being way out of synch with the marker clock. And, since the Earth clock is moving, the rocket observer sees that it operates slowly compared to his own clock. And don't forget that the rocket observer measures his distance to the Earth to be shorter that what the Earth frame measures it to be.

To make sense out of these things, one cannot consider length contraction or time dilation as separate, independent effects. One must consider all the relativistic effects--length contraction, time dilation, simultaneity--working in concert. To consider one effect while ignoring the others will result in logical contradiction.

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?

 

[Doc Al]

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.

 

[h8ter]

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

 

[Nenad]

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). I've never heard of light being faster in ine direction. Do you have a source?

 

[pervect]

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

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.

 

[h8ter]

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]

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

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

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.

 

[h8ter]

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.

 

[bino]

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

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.

 

[Nenad]

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

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..). Let's 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]

length contraction

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

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 $v$.

 

[h8ter]

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.

 

[pervect]

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.

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

 

[bino]

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?

 

[Nenad]

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.

 

[Nenad]

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.

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.