Travelling greater than speed of light

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Discussion Overview

The discussion revolves around the concept of particles potentially traveling faster than the speed of light in a medium with a high refractive index, specifically examining the implications for time travel and relativistic effects. Participants explore theoretical scenarios, observations from different frames of reference, and the nature of speed limits in relativity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that if particles can travel faster than the speed of light in a medium (c/n), it raises questions about observations from stationary and particle frames of reference, and whether this could imply time travel.
  • Another participant references Cherenkov radiation but does not directly address the time travel question posed.
  • A participant asserts that the ultimate speed limit in relativity is the speed of light in a vacuum (c), implying that traveling faster than light in a medium does not equate to time travel.
  • One participant reiterates the time travel question, proposing that crossing the speed of light in the surrounding medium (c/n) might allow for future travel relative to observers within that medium.
  • Another participant challenges the notion that time travel could occur simply by exceeding the speed of light in a medium, emphasizing that the fundamental limit remains c.

Areas of Agreement / Disagreement

Participants express differing views on whether exceeding the speed of light in a medium could enable time travel. Some argue that it does not, while others propose that it might, indicating a lack of consensus on the topic.

Contextual Notes

The discussion involves assumptions about the nature of speed limits in relativity and the implications of traveling faster than light in various contexts, which remain unresolved.

ABHISEK DATTA
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It is known that in a medium of refractive index 'n' , the speed of light becomes c/n. So if we can make a medium of high refractive index, then the speed of light will be substantially less in that medium. Then it can be possible to accelerate particles inside that medium at speeds greater than the speed of light in that medium i.e. 'c/n'. Then what will be our observation from an stationary frame of reference ? What will be the observation from the particle's frame of reference ? Will the particle travel in time as it is crossing the speed of light in its surrounding medium ?

Also one problem in reaching the speed of light 'c' in vacuum was that - as the speed of a particle approaches 'c' its relativistic mass increases and at v=c, its mass becomes infinite. But in the case as described above the particle is able to cross the speed of light in that medium (of refractive index 'n') without reaching 'c' , and thus its mass does not become infinite.

So is time travel really possible for that particle in that medium of refractive index 'n' ?
 
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ABHISEK DATTA said:
So is time travel really possible for that particle in that medium of refractive index 'n' ?
No. Realize that the limiting speed in relativity is c = 3 x 108 m/s, which happens to be the speed of light in a vacuum, not the speed of light in some medium.
 
yeah, i know about Cerenkov radiation, but that doesn't quite answer my question
 
And, yeah its true that the limiting speed in relativity is 3x10^8 m/s. But my main question is whether the particle will be able to experience time travel or not ? You can assume that the medium of refractive index 'n' is the world for that particle, and in its world the particle is traveling faster than light.
For time travel, i.e. to travel to the future, i think its not necessary to cross 'c', but to cross the speed of light in your surroundings, even if it is c/n. By traveling greater than c/n, the particle with will travel to the future with respect to the observer inside that medium.
 
ABHISEK DATTA said:
For time travel, i.e. to travel to the future, i think its not necessary to cross 'c', but to cross the speed of light in your surroundings, even if it is c/n. By traveling greater than c/n, the particle with will travel to the future with respect to the observer inside that medium.
Well, you're wrong. Time travel (and it's associated paradoxes) would happen if particles could go faster than 3 x 108 m/s, not simply 'faster than light'.
 

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