The Speed of Light: Relative to Us?

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Discussion Overview

The discussion revolves around the concept of the speed of light and its implications in the context of reference frames, particularly whether a photon can serve as a valid frame of reference. Participants explore the nature of speed, time dilation, and length contraction as described by Special Relativity, while questioning the validity of certain assumptions and calculations related to relative velocities.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants argue that a photon cannot have a valid reference frame, as time and space do not exist at the speed of light.
  • Others propose that the universe moves relative to a photon at the speed of light, questioning the implications of this perspective.
  • One participant challenges the notion of speeds exceeding the speed of light, asserting that relativistic velocity addition must be used to calculate relative speeds correctly.
  • Concerns are raised about the measurement of speed and the implications of two objects moving away from each other at high speeds, suggesting that conventional addition of velocities does not apply.
  • Some participants express skepticism about the foundational assumptions of the speed of light as a constant, suggesting it may be an arbitrary number.
  • Questions are posed regarding how two objects can measure their relative speed if they cannot communicate or detect each other due to their high velocities.

Areas of Agreement / Disagreement

Participants generally disagree on the validity of using a photon as a reference frame and the implications of relativistic velocity addition. There is no consensus on the foundational assumptions regarding the speed of light and its measurement.

Contextual Notes

Limitations include unresolved mathematical steps in the application of relativistic velocity addition and the dependence on definitions of speed and reference frames. The discussion also highlights the complexity of measuring relative speeds in scenarios where communication is not possible.

  • #31
Try to think of space and time as being stretched between two mass possessing objects in motion. Light ignores it, other than doppler shifting. So, the two spaceships stretch the medium between them, but, all light does is get a bit redder.
 
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  • #32
Relativity is hard to conceptualize. Ed, the rope you imagined tying to the two spaceships also gets stretched in the process. Any time you stretch time, you also stretch space. There is no way to separate the two. There are no absolute reference frames. That is the fundamental concept of relativity. It works and has survived every test ever devised to prove it wrong.
 
  • #33
UglyEd said:
Is this question basically that 2 objects can't separate from each other at 400,000 k/s? What if you had 2 rockets that you have tested and know go 200,000k/s and tied a rope to them that was 400,000km long then set the rockets back to back and simultaniously sent then out opposite directions in 1sec would they would pull the rope tight?
Yes, they would. And an observer on Earth would see the ships separating at 400,000 km/sec. But that does not mean the ships are traveling 400,000 k/sec with respect to each other.
 
  • #34
So if there are three stars in a line, A-B-C, separated by 0.8 lightyears each way from B, and the spaceships leave B... one heads for A, one heads for C, each traveling at 0.8C...

How long does it take for each to reach its destination?
How far did each travel?
What is that time multiplied by that distance?
 
  • #35
Adam said:
So if there are three stars in a line, A-B-C, separated by 0.8 lightyears each way from B, and the spaceships leave B... one heads for A, one heads for C, each traveling at 0.8C...
Once again, we have to be clear as to what frame is claiming that they are separated by 0.8 lightyears. I will assume that those distances are as measured in the rest frame of the stars. And that the speed of the spaceships is with respect to that frame.

Recall that different frames will observe different lengths and times.

How long does it take for each to reach its destination?
According to the star frame: t = D/v = 0.8/0.8 = 1 year.
According to the ship frame: t' = D'/v = (0.48)/0.8 = 0.6 year.
How far did each travel?
According to the star frame: D = 0.8 lightyears.
According to the ship frame: D' = D \sqrt{(1-v^2/c^2)} = D (0.6) = 0.48 lightyears.
What is that time multiplied by that distance?
Why would you do that? Perhaps you mean: what is the distance divided by the time? Answer: the speed, which equals 0.8c. :smile:
 
  • #36
Adam said:
How long does it take for each to reach its destination?
t = \frac{d}{V}

t = \frac{d \sqrt{1 - v^{2}/c^{2}}}{0.8c}

t = \frac{0.8ly \sqrt{1 - (0.8c)^{2}/c^{2}}}{0.8c}

t = 0.6y

Adam said:
How far did each travel?

d_o = d\sqrt{1 - v^{2}/c^{2}}

d_o = 0.8ly \sqrt{1 - (0.8c)^{2}/c^{2}}

d_o = 0.48ly

Adam said:
What is that time multiplied by that distance?

I see no point to multiplying time and distance.
 
  • #37
damn, Doc Al keeps beating me to answers, BLAST!
 
  • #38
I'm just trying to get my head around the maths. I'm crap at maths. I want to see all the bits and pieces.
 
  • #39
look, you first of all photon is nothing as you are thinking. you are treating a photon as a particle to explain for which higher dimensions of dimensions of universe are required.
 
  • #40
Adam said:
I'm just trying to get my head around the maths. I'm crap at maths. I want to see all the bits and pieces.

you don't understand what I did in my higher post?
 
  • #41
Anyone know the link for the David Hoggs SR Book?

This might help him out.

It sure has shed some light on me.

I'll post it later if I find it.
 
  • #42
A journalist once asked Sir Arthur Eddington... I hear that only three people on Earth understand relativity... After repeating the question, Eddington replied, hmm, I am still tying to figure out who the third person is.. a physics joke, but a good one.
 
  • #43
Nenad said:
you don't understand what I did in my higher post?

I never studied maths in high school. I quit school early to join the military. I basically have to play catch-up with everyone else.
 
  • #44
  • #45
Chronos said:
A journalist once asked Sir Arthur Eddington... I hear that only three people on Earth understand relativity... After repeating the question, Eddington replied, hmm, I am still tying to figure out who the third person is.. a physics joke, but a good one.

they were actually talking about General Relativity. The joke is irrelivant on a Special relativity thread, which is quite easy to understand.
 

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