Special Relativity: Light's Relative Speed Explained

  • Context: Undergrad 
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Discussion Overview

The discussion revolves around the principles of special relativity, particularly the constancy of the speed of light and its implications for observers in different reference frames. Participants explore the foundational aspects of special relativity, including its relationship with Maxwell's equations and common misconceptions regarding light's speed.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that light always moves at speed c in any reference frame, regardless of the observer's motion.
  • Others express uncertainty about specific conditions under which light's speed is perceived, seeking clarification on the implications of special relativity.
  • One participant emphasizes that Maxwell's equations establish the constancy of light's speed, which Einstein later postulated as a foundational aspect of special relativity.
  • There is a discussion about the difference between derivation and postulation, with some participants noting that special relativity does not provide a reason for the constancy of c but rather takes it as a starting point.
  • A mathematical expression for the addition of velocities in special relativity is presented, illustrating that the sum of velocities involving light remains c.
  • Some participants challenge each other's interpretations and assumptions about special relativity, indicating a need for clearer conceptual understanding.

Areas of Agreement / Disagreement

Participants generally agree on the principle that light's speed is constant in all inertial frames, but there are competing views regarding the implications and interpretations of this principle, as well as the relationship between special relativity and Maxwell's equations. The discussion remains unresolved on certain conceptual clarifications and misconceptions.

Contextual Notes

Some limitations in understanding arise from differing interpretations of special relativity and its foundational principles, as well as the relationship between theoretical postulates and empirical derivations.

Poy
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I think this is the right place to put this question
Special relativity says light always seems to move at c. But wouldn't it (relative to you) be moving faster than c if its moving away from you and you are moving away from it? I might misundestdn something here, but if someone could clearify for me, thanks.
 
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Originally posted by Poy
Special relativity says light always seems to move at c. But wouldn't it (relative to you) be moving faster than c if its moving away from you and you are moving away from it?
No. Light moves at speed c in any reference frame. No matter how fast you move, you'll measure any light passing by to have speed c. It's a feature of the "spacetime" structure of the universe.
 
I know how that ussually works, but I wasn't sure in this one condition... thanks.
 
Originally posted by Poy
Special relativity says light always seems to move at c.
You may want to reconsider your conceptualization. SR says that light literally moves at c for every observer, not just appearing to do so.
 
If you want to be nit picky it is Maxwell's Equations which state that the speed of light is constant to all observers. Einstein was able to state a constant c as a postulate, because it was derived as such by Maxwell in 1867. The Constancy of c was a major dilemma for Physicists in the last half of the 1800s.
 
Originally posted by Integral
If you want to be nit picky ...
I wasn't trying to be "nit picky." There is a lot of misconception about what SR says concerning the nature of space-time, i.e., "time dilation is a consequence of the delay of light due to its finite speed," being such a misconception. From the wording, I thought that Poy might have such a misconception. Sorry for being presumptuous.




Originally posted by Integral
... it is Maxwell's Equations ...
I'll be nit picky only to point out that a statement by a set of equations does not exclude a statement by SR.
 
There really should be a sticky thread about this as variations of this one seems to be the most oft-asked question:

In special relativity velocities ([itex]u[/itex] and [itex]v[/itex])are summed:

[tex]w = \frac{u + v}{1 + \frac{uv}{c^2}}[/tex]

take the case where [itex]u = c[/itex]

[tex]w = \frac{c + v}{1 + \frac{v}{c}} = \frac{c(c + v)}{c + v} = c[/tex]

So the sum of the two velocites when one is equal to [itex]c[/itex] is always equal to [itex]c[/itex].
 
Originally posted by turin

I'll be nit picky only to point out that a statement by a set of equations does not exclude a statement by SR.

There is a big difference between a derivation from basics and a postulate. Maxwell dervives, Einstein postulates. He was able to do that based on Maxwell's derivation.

Special Relativity does not contain a reason for the constancy of c, it is the starting point. From there SR explores the results of a constant c.
 
Originally posted by Integral
There is a big difference between a derivation from basics and a postulate. Maxwell dervives, Einstein postulates. He was able to do that based on Maxwell's derivation.

Special Relativity does not contain a reason for the constancy of c, it is the starting point. From there SR explores the results of a constant c.
So what? I didn't claim a derivation of any sort. SR says that the speed of light is literally the same in every inertial frame of reference. Do you disagree with this?

I thought that Poy misunderstood this statement, based on his wording. I didn't want to make an issue of whether or not SR is true, and why.
 
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