Length contraction (no calculations)

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Discussion Overview

The discussion revolves around the concept of length contraction in the context of special relativity, particularly as it pertains to light traveling vast distances, such as those in astronomical contexts. Participants explore the implications of length contraction for light and the appropriate use of related formulas, including the Lorentz transformations.

Discussion Character

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

Main Points Raised

  • Some participants question whether length contraction applies to light traveling at the speed of light, noting that using the formula leads to an undefined situation (zero in the denominator).
  • Others argue that length contraction is not applicable to massless particles like photons, emphasizing that it only applies to massive particles traveling below the speed of light.
  • One participant suggests using Lorentz transformations instead of length contraction formulas, stating that the latter is only valid for simultaneous events on separate worldlines.
  • There is a suggestion that discussing the distance a photon "sees" is meaningless within the framework of relativity.
  • Some participants express confusion about the concepts and formulas involved, indicating a need for clearer explanations.

Areas of Agreement / Disagreement

Participants generally agree that length contraction does not apply to light, but there is disagreement on the implications of this and the appropriate mathematical framework to use. The discussion remains unresolved regarding the best approach to understanding these concepts.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about simultaneity and the application of formulas in different frames of reference. The complexity of the topic and the varying levels of understanding among participants contribute to the ongoing debate.

mousemouse123
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i am wondering when i read about planets or solar systems that are like 1million lightyears away, those that take into account length contraction, because if the light is traveling at the speed of light then the distance it has to go will contract.

also another (possibly) stupid question: formula for length contract is like

L=Lo/sqrt of 1-v^2/c^2

light travels at the speed of light so it will be L=Lo/0 which is impossible(zero in denominator)... does that mean length can't contract when it comes to light?
 
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I recommend avoiding the use of the length contraction and time dilation formulas and focusing on the Lorentz transforms instead.

The length contraction formula is only valid for simultaneous pairs of events on two separate worldlines where the pairs are colocated in one of the frame. Here you do not have two separate worldlines and even if you did there is no frame where they would be colocated.

The length contraction formula simply does not apply, but the Lorentz transform does. There is no discernable advantage to the shorter formulas.
 
mousemouse123 said:
i am wondering when i read about planets or solar systems that are like 1million lightyears away, those that take into account length contraction, because if the light is traveling at the speed of light then the distance it has to go will contract.

also another (possibly) stupid question: formula for length contract is like

L=Lo/sqrt of 1-v^2/c^2

light travels at the speed of light so it will be L=Lo/0 which is impossible(zero in denominator)... does that mean length can't contract when it comes to light?

It is best to use the Lorentz transformations always until you have had practice in using them, otherwise you end up using the contraction and dialtion formulas inappropriately.

As it happens, the Lorentz contraction formula works in this case for a a frame that approaches the speed of light, since the speed of light in every frame must be c! A frame for light is therefore meaningless.

So yes, as the frame approaches the speed of light, the distance the Earth has to travel to get to the origin in that frame approaches 0.
 
mousemouse123 said:
light travels at the speed of light so it will be L=Lo/0 which is impossible(zero in denominator)... does that mean length can't contract when it comes to light?

It simply doesn't make sense to ask what distance the photon sees as the distance between 2 points. Length contraction only applies to massive particles that exclusively travel below the speed of light. It's a simple tenet of special relativity.
 
DaleSpam said:
I recommend avoiding the use of the length contraction and time dilation formulas and focusing on the Lorentz transforms instead.

The length contraction formula is only valid for simultaneous pairs of events on two separate worldlines where the pairs are colocated in one of the frame. Here you do not have two separate worldlines and even if you did there is no frame where they would be colocated.

The length contraction formula simply does not apply, but the Lorentz transform does. There is no discernable advantage to the shorter formulas.


LOL i don't know what the lorentz transformation formula is. (i hate physics now too confusing)
 
mousemouse123 said:
LOL i don't know what the lorentz transformation formula is. (i hate physics now too confusing)

Don't worry about it. I believe DS's explanation was too complex of an explanation for a simple question. All that really matters is the fact that it's meaningless to talk about what length a photon measures in relativity.
 
mousemouse123 said:
i am wondering when i read about planets or solar systems that are like 1million lightyears away, those that take into account length contraction, because if the light is traveling at the speed of light then the distance it has to go will contract.

also another (possibly) stupid question: formula for length contract is like

L=Lo/sqrt of 1-v^2/c^2

light travels at the speed of light so it will be L=Lo/0 which is impossible(zero in denominator)... does that mean length can't contract when it comes to light?
You seem to be misunderstanding "length contraction". That has nothing to do with the speed of light moving bewtween two places. The distance, as observed from one of the places would have to do with the speed of each place relative to the other or, as observed from a third place, the speed of the two places relative to the third.
 

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