Thread: Postulates of Special Relativity

1. The first postulate of special relativity is easily digestible. The problem arises when it comes to the second one, at least for me. It states that the velocity of light is constant in any inertial frame of reference. This follows that if A and B are two reference frames and B is moving relative to A with a velocity c, then A will observe light traveling at par with B, which predicts that the velocity of light relative to B is zero. But according to the postulate, this cannot be the case. The relative velocity of light with respect to B is also c, which means that light is traveling at a velocity 2c with respect to A. This result contradicts our assumption that the relative velocity of light is c with respect to A. Then, how is it possible that this postulate is true ? If we assume that this is possible, then how would it explain this seeming contradiction ? Saying that it is an empirical fact does not answer me. I want the theoretical explanation. Please note here that light, as an electromagnetic wave, can never have some velocity other than c because then light would cease to be an electromagnetic wave (I don't know why). Although I find no reasons to deny the observation of light with a velocity other than c, that does not mean light, in reality, travels at some other velocity. After all, this is relativity. However, this postulate denies the observation of light with a velocity other than c. I cannot digest this. Please help.

2.

3. Your example is a bit hard to follow, as you don't mention how the two reference frames are moving relative to each other (i.e. toward, away, etc.)

Picture two light bulbs, A and B, traveling toward each other. Classically speaking, B's light would appear to be traveling 2 times faster to A.

Well Einstein decided this couldn't be true, and light always travels the same speed. In order for this to happen, something must change, and we see that time and mass will change. If I'm standing still, and you pass me at speeds near the speed of light, time for you will be slow relative to my time. You also gain mass.

I don't know a whole lot about relativity, but to answer your question, in order to keep the speed of light constant, time and mass change. Hope this helps and I hope I am correct in my statements

4. Originally Posted by J Rahman
It states that the velocity of light is constant in any inertial frame of reference. This follows that if A and B are two reference frames and B is moving relative to A with a velocity c, then A will observe light traveling at par with B, which predicts that the velocity of light relative to B is zero.
Thats true, the velocity of light relative to B appears to be zero, but for B it is not.

But according to the postulate, this cannot be the case. The relative velocity of light with respect to B is also c, which means that light is traveling at a velocity 2c with respect to A.
This is incorrect, for B the light is moving past at 1c ( x ) This is on par with the second postulate of special relativity, that all reference frames relative to themselves see light pass at the speed of light.

This result contradicts our assumption that the relative velocity of light is c with respect to A.
Only relative to A relative to B, A sees the same velocity and B does not, it sees it differently. These two totally areas of space do not have the same inertial reference frames because relativity only operates for one reference frame in one frame of space. It is potentially limitless in how large that space is, or how far away it is.

Its easy to confuse this matter using Newtonian mechanics as he himself believed that time was the same across the universe, and we know it isn't.

If B by some wide shot miracle would see A's perspective to their own frame of reference, yes light would be passing at 2c, but only (and listen carefully)

B would see light passing A (itself) at 2c relative to B, relative to itself. Now not only does B now have to miraculously observe itself FASTER THAN LIGHT, it also has to take B's reference frame as truth, and even though it is travelling with itself, it still has to take its own reference frame, it can't take another and profess that as truth.

Granted by some wide margain of uncertainty it may be possible, but our science does not have the scope for omincient transmission of relativity across dimensions in space, we have to take our own reference frame.

Then, how is it possible that this postulate is true ?
Because the way you read it was biased, and not understood (easy to understand )

If we assume that this is possible, then how would it explain this seeming contradiction ? Saying that it is an empirical fact does not answer me.
There is no contradiction.

Please note here that light, as an electromagnetic wave, can never have some velocity other than c because then light would cease to be an electromagnetic wave (I don't know why).
No, light can have different values for c. In Chernkhov radiation it can move through atoms at speeds greater than the speed of light. In a Casimir vaccum it can do the same. In a black hole it slows down relatively (theory). It doesn't change the fact that its an electromagnetic wave or particle. Technically its a guage boson, a mediator for the electromagnetic force.

It can change into different particles given different interactions with different particles.

Although I find no reasons to deny the observation of light with a velocity other than c, that does not mean light, in reality, travels at some other velocity.
.

In Minkowski space, it always travels at 1c: ( x )

It might travel at different velocities in different areas of the spacetime continuum. We don't know.

After all, this is relativity. However, this postulate denies the observation of light with a velocity other than c.
Again, because of misdirection and not undestanding the postulate correctly. Which again is very understandable.

No need to digest it because it isn't true. Nothing needed to help you with.

Special Relativity:

1: The laws of physics are the same for all in ordinary Minkowski space.
2: The speed of light is the same in any given reference frame in a vaccuum.

Light will never be faster or slower in any reference frame relative to oneself. EVER, unless the first postulate of special relativity is violated.

Picture two light bulbs, A and B, traveling toward each other. Classically speaking, B's light would appear to be traveling 2 times faster to A.

Well Einstein decided this couldn't be true, and light always travels the same speed.
This isn't testable, it is for one side of the equation (time) but not definitively proven. We will only be able to fully prove this when we are travelling ourselves at 1c or close to 1c and we see the light at 2c. If the photon adds more energy to the particles it hits than it would at 1c, we can relatively say that it was travelling at 2c relative to us relative to the photon.

If it does give extra energy and still causes temporal alterations then there will be a new relativity created to explain other relativistic effects we have no measurement for yet.

5. Originally Posted by J Rahman
if A and B are two reference frames and B is moving relative to A with a velocity c, then A will observe light traveling at par with B, which predicts that the velocity of light relative to B is zero.
No. For one thing, one relativistic effect is that an object in another inertial frame of reference will never appear to excede or reach c, regardless of their relative velocities as observed by a "stationary" observer between them.

http://www.xs4all.nl/~johanw/PhysFAQ.../velocity.html

Originally Posted by J Rahman
But according to the postulate, this cannot be the case. The relative velocity of light with respect to B is also c, which means that light is traveling at a velocity 2c with respect to A.
If we accept that the light leaves A at c, and arrives at B at c, this would make it appear that the light has accelerated to 2c since leaving A. However, the postulate must stand. And the way we allow for this dilemma is with time dilation - although relative to A the light travels at c, from B it also appears to travel at c because B's time is dilated relative to A - although you might say that the light is travelling slower, B's time is passing slower such that the light always appears to be travelling at c.

6. Originally Posted by Quantime

Picture two light bulbs, A and B, traveling toward each other. Classically speaking, B's light would appear to be traveling 2 times faster to A.

Well Einstein decided this couldn't be true, and light always travels the same speed.
This isn't testable, it is for one side of the equation (time) but not definitively proven. We will only be able to fully prove this when we are travelling ourselves at 1c or close to 1c and we see the light at 2c. If the photon adds more energy to the particles it hits than it would at 1c, we can relatively say that it was travelling at 2c relative to us relative to the photon.

If it does give extra energy and still causes temporal alterations then there will be a new relativity created to explain other relativistic effects we have no measurement for yet.
Regardless of our speed, we will never see light at 2c (postulate 2). As we near c, time will dilate (this has been tested). I'm not quite sure what "the photon" is. In the context I think you're speaking of the supposed photon traveling at 2c? Maybe you could be a little less vague in your explanation?

7. the inertial reference frame at c is not a valid reference frame in relativity.

8. Originally Posted by Arcane_Mathematician
the inertial reference frame at c is not a valid reference frame in relativity.
Frames near c are very valid.

9. Originally Posted by JEllington
Regardless of our speed, we will never see light at 2c (postulate 2). As we near c, time will dilate (this has been tested). I'm not quite sure what "the photon" is. In the context I think you're speaking of the supposed photon traveling at 2c? Maybe you could be a little less vague in your explanation?
I am aware time dilation has been tested. There are three reference frames:

1: Us at 1c
2: Light travelling forwards relative to us
3: Planet approaching us relative to our motion at 1c approaching at 1c

The photon is not travelling at 2c. I cannot explain my postulate without you being able to understand what I have already written. Re-read it again and if you are still interested, reply. What was vauge to you?

10. Originally Posted by Quantime

This isn't testable, it is for one side of the equation (time) but not definitively proven. We will only be able to fully prove this when we are travelling ourselves at 1c or close to 1c and we see the light at 2c. If the photon adds more energy to the particles it hits than it would at 1c, we can relatively say that it was travelling at 2c relative to us relative to the photon.

If it does give extra energy and still causes temporal alterations then there will be a new relativity created to explain other relativistic effects we have no measurement for yet.
We will not see light at 2c, light remains constant relative to all observers so how would we see the light at 2c? And what photon are you speaking of? Are you saying that because we are traveling at near light speeds, our light (photon) will have a higher energy and therefore hit particles with a greater energy than normal and thus we can say, from the point of view of the particles that were hit, that these photons were traveling at 2c?

If this is what you were saying I'm under the impression that it is incorrect.

Maybe someone else understands what you are saying and can translate this?

11. What we would see, is light coming from 2 different directions at c. We could say that, within our own inertial frame, one photon is moving at 2c relative to the other. We are not violating any physical laws by observing this, because each photon we observe only as moving at c relative to us.

From the inertial frame of one of the photons, we are moving towards it at c, and so, weirdly, is the other photon;

This is probably one reason why, as someone said earlier, c is not usually regarded as a valid frame of reference. Another would be that time and length do not exist in this frame of reference.

12. Originally Posted by J Rahman
The first postulate of special relativity is easily digestible. The problem arises when it comes to the second one, at least for me. It states that the velocity of light is constant in any inertial frame of reference. This follows that if A and B are two reference frames and B is moving relative to A with a velocity c, then A will observe light traveling at par with B, which predicts that the velocity of light relative to B is zero. But according to the postulate, this cannot be the case. The relative velocity of light with respect to B is also c, which means that light is traveling at a velocity 2c with respect to A. This result contradicts our assumption that the relative velocity of light is c with respect to A. Then, how is it possible that this postulate is true ? If we assume that this is possible, then how would it explain this seeming contradiction ? Saying that it is an empirical fact does not answer me. I want the theoretical explanation. Please note here that light, as an electromagnetic wave, can never have some velocity other than c because then light would cease to be an electromagnetic wave (I don't know why). Although I find no reasons to deny the observation of light with a velocity other than c, that does not mean light, in reality, travels at some other velocity. After all, this is relativity. However, this postulate denies the observation of light with a velocity other than c. I cannot digest this. Please help.
First, it is not allowable for any inertial reference frame to travel at c with respect to another one.

Second, the postulate is qute simple. No matter what your reference frame the speed of light (of a photon) with respect to any reference frame is c.

You need to read a good book on special relativity, or read the sticky thread in this forum.

A good book on special relativity is Introduction to Special Relativity by Wolfgang Rindler.

You are trying to apply the Galiean transformation, and the whole point of special relativity is that the Galilean transformation is wrong. It is a good approximation for speeds much less than that of light, but it is ultimately wrong.

 Bookmarks
Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement