Is the Speed of Light Truly Unattainable in Relativity?

[DonB]

Has your original question been answered?

Thanks for asking. I would have to answer your question is both yes and no. The "yes" part is that, I'm told, that light has not ref. frame, and thus my movement relative to it and can not be determined. The "no" part is this: at any micro-fraction of a nano second the distance between the beam and me is a definite, measurable distance. Thus, at both T1 and T2 (Time 1 and Time 2) that distance can be measured, and with delta D factored with delta T, a speed can be determined at which, relative to the beam, I am moving (or, conversely, it is moving relative to me). Maybe this is something that I just have to accept for now.

 

[PAllen]

Thanks for asking. I would have to answer your question is both yes and no. The "yes" part is that, I'm told, that light has not ref. frame, and thus my movement relative to it and can not be determined. The "no" part is this: at any micro-fraction of a nano second the distance between the beam and me is a definite, measurable distance. Thus, at both T1 and T2 (Time 1 and Time 2) that distance can be measured, and with delta D factored with delta T, a speed can be determined at which, relative to the beam, I am moving (or, conversely, it is moving relative to me). Maybe this is something that I just have to accept for now.

But these are your distances and times, and lead to you computing the light moves at c. You need to ask about the light's measurements of distance and time, which you've been told are undefinable. In the limit as v->c, you get 0/0.

 

[ghwellsjr]

Thanks for asking. I would have to answer your question is both yes and no. The "yes" part is that, I'm told, that light has not ref. frame, and thus my movement relative to it and can not be determined. The "no" part is this: at any micro-fraction of a nano second the distance between the beam and me is a definite, measurable distance. Thus, at both T1 and T2 (Time 1 and Time 2) that distance can be measured, and with delta D factored with delta T, a speed can be determined at which, relative to the beam, I am moving (or, conversely, it is moving relative to me). Maybe this is something that I just have to accept for now.

If you're going back to my description of how to measure the speed of a light beam from post #42 and you are concerned about the time it takes for the shutter/detector to transmit the start (at T1) and stop (at T2) signals to the timer, as long as it is the same for both times, then it won't have any bearing on delta T, the difference beween between T2 and T1.

If you're not going back to post #42, then I have no idea what you're talking about so could you elaborate?

 

[DonB]

But these are your distances and times, and lead to you computing the light moves at c. You need to ask about the light's measurements of distance and time, which you've been told are undefinable. In the limit as v->c, you get 0/0.

So, you're saying that due to the warp in time/space from the light's traveling at c, the distance from me to the light beam is different from my perspective than it is from the light's perspective? Guess I hadn't grasped that part of relativity before.

I don't know if you've noticed, but I ask a question about this 0/0 (based upon DH's FAQ article) in a separate thread.

 

[DonB]

If you're not going back to post #42, then I have no idea what you're talking about so could you elaborate?

It seems that PAllen identified my problem area (post #52).

 

[kevinfrankly]

I'm a relative newbie to relativity (no pun intended -- I know you've heard that one too many times), as well as to this forum, so forgive me if this is a dumb question...

As I understand it, there is a significant percentage of those that believe that people, spaceships, etc. will never travel at the speed of light (c), right?

If that is so, where is my logic (below) faulty:

1. For any two given things (A and B), the speed of A relative to B will always be the same as the speed of B relative to A.

2. Relativity insists that light always travels at c relative to all things.

Thus, if light is moving at c relative to me, then am I not moving at c relative to that particular beam of light (...and, in fact, all beams of light at any given moment in time)? And thus, the speed of light is not only something that is not unattainable, but has rather never been unattained?

What am I missing?

CMIIW
isnt one of einstein postulate said that the speed of light is constant independant of the observer, or in other words the speed of light is just the same for any observer.

 

[Dale]

1. For any two given things (A and B), the speed of A relative to B will always be the same as the speed of B relative to A.

This is because the inverse of a boost is a boost of the same speed (and opposite direction).

2. Relativity insists that light always travels at c relative to all things.

Thus, if light is moving at c relative to me, then am I not moving at c relative to that particular beam of light

There is no boost to c.

Another way to think of it is that there is a symmetry between two massive observers. There is no sense in which one is "at rest" that doesn't apply equally well to the other, i.e. each can be boosted into a frame where the spacelike components of its four-velocity are 0. There is no such symmetry between a massive observer and light, the massive observer is timelike and the light is lightlike. Light cannot be boosted into a frame where the spacelike components are non-zero.

 

[DonB]

But these are your distances and times, and lead to you computing the light moves at c. You need to ask about the light's measurements of distance and time, which you've been told are undefinable. In the limit as v->c, you get 0/0.

After sleeping on this, I suppose that ultimately what you are saying is that Statement 1 of my original post in this thread (1. For any two given things (A and B), the speed of A relative to B will always be the same as the speed of B relative to A) -- a statement that I think I picked up for Einstein's own material -- is not always true. Specifically, for currently attainable speeds it is true, but as an object approaches the speed of light this becomes less and less true. Would that be a fair conclusion..., and thus point out the flaw in the original logic?

 

[PAllen]

After sleeping on this, I suppose that ultimately what you are saying is that Statement 1 of my original post in this thread (1. For any two given things (A and B), the speed of A relative to B will always be the same as the speed of B relative to A) -- a statement that I think I picked up for Einstein's own material -- is not always true. Specifically, for currently attainable speeds it is true, but as an object approaches the speed of light this becomes less and less true. Would that be a fair conclusion..., and thus point out the flaw in the original logic?

No, it is exactly true for any speed less than c. It is meaningless for c. The relativity statement you are mis-quoting was between material bodies which always move less than c relative to each other. You have the following properties, actually:

1) If B is moving relative to A at less than c, then A's motion relative to B is the same (in the opposite direction).

2) If B is moving at c relative to A, it is moving at c relative to all matter, and there is no such thing as motion relative to B. Further, B has no rest mass and no rest frame.