Calculating Relativity with Different Speeds: Vacuum or Medium?

  • Context: Graduate 
  • Thread starter Thread starter User11037
  • Start date Start date
  • Tags Tags
    Relativity
Click For Summary

Discussion Overview

The discussion revolves around the calculation of relativistic effects in relation to the speed of light, specifically whether these calculations should reference the speed of light in a vacuum or the speed of light in a medium, such as water. Participants explore the implications of these speeds on relativistic properties and causality.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions whether the Lorentz factor should be calculated using the speed of light in a vacuum or in a medium like water.
  • Another participant asserts that relativistic properties are based on the speed of light in a vacuum, explaining that light's behavior in a medium involves scattering and absorption, which complicates its speed.
  • It is noted that while electrons cannot exceed the speed of light in a vacuum, they can exceed the speed of light in water, as observed in specific experimental conditions.
  • Concerns are raised about the implications of electrons moving faster than light in a medium, suggesting potential violations of causality and mass-energy equivalence.
  • One participant emphasizes that causality is related to the maximum signal speed, which is the speed of light in a vacuum, while interactions in a medium affect the observed speeds of particles.

Areas of Agreement / Disagreement

Participants generally agree that the speed of light in a vacuum is the basis for relativistic calculations, but there is disagreement regarding the implications of particles moving faster than light in a medium and its effects on causality.

Contextual Notes

Some assumptions about the nature of light in different media and the conditions under which particles can exceed light speed in a medium remain unresolved. The discussion does not reach a consensus on the implications of these observations for causality.

User11037
Messages
8
Reaction score
0
Hi, this is probably a silly question but I can't seem to find the answer anywhere:

are the effects of relativity calculated with reference to the speed of light in a vacuum or the speed of light in the medium in which you are traveling? For instance, if traveling in water, would the Lorentz factor be calculated based on c ≈ 3e8 or c ≈ 2.2 e8?
 
Physics news on Phys.org
Speed of light in a vacuum. The physics of light traveling through a medium are very different; the light is being continuously scattered, absorbed, re-emitted by the atoms of the medium. You could imagine the light being absorbed by an atom, then being re-emitted after a delay, traveling at c through the vacuum between atoms, then being absorbed and delayed again.

It's possible for a particle to travel through a medium at a speed greater than the speed of light in that medium; but not faster than c.

In fact, it would be more accurate to say that "c is the speed of a massless particle in a vacuum and light, being massless, travels at that speed in a vacuum" than "c is the speed of light". We don't, but that's for historical reasons - when relativity was being developed we didn't know about the other things that propagate with speed c.
 
  • Like
Likes   Reactions: 1 person
As a result of the relativistic properties being based on the speed of light in vacuum, while it is not possible for an electron to move faster than the speed of light in vacuum, it is possible for an electron to move faster than the speed of light in water. That has been observed in water cooled nuclear reactors.
 
HallsofIvy said:
As a result of the relativistic properties being based on the speed of light in vacuum, while it is not possible for an electron to move faster than the speed of light in vacuum, it is possible for an electron to move faster than the speed of light in water. That has been observed in water cooled nuclear reactors.

How is that possible? Seems to violate causality, and I guess mass/energy equivalence since the electron has a mass.

Is it better said as An electron can move faster than a photon through a medium under xyz conditions; both speeds are less than c*.

*speed of light in a vacuum
 
Well, yes, the whole point is that the speed of light in water is less than the speed of light in vacuum. The whole point is that relativity uses the speed of light in vacuum and an electron can move faster than the speed of light in water. Look at https://en.wikipedia.org/wiki/Cherenkov_radiation
 
nitsuj said:
How is that possible? Seems to violate causality, ...

It seems to me that Causality is related to the maximum signal speed.
The "speed of light in vacuum" happens to equal that maximum signal speed.

The speed of light and of other particles in a medium is more about their interactions with the medium... with causality only playing a role in setting an upper speed limit.
 

Similar threads

  • · Replies 93 ·
4
Replies
93
Views
6K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 25 ·
Replies
25
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 18 ·
Replies
18
Views
2K
  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 14 ·
Replies
14
Views
4K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 13 ·
Replies
13
Views
1K
  • · Replies 6 ·
Replies
6
Views
2K