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I read that nothing with mass can travel at the speed of light. Therefor one can not ask what happens when an observer in a car moves with the speed of light. But, is it possible for a frame of reference to move with the speed of light? Is there something preventing that?

Qmechanic
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Muhammed Roshan
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  • For perspective, I strongly recommend reading this answer: https://physics.stackexchange.com/a/591377 – Chiral Anomaly Jun 21 '21 at 02:20
  • Frames do not "move", whatever that might mean. – WillO Jun 21 '21 at 03:33
  • @WillO why not? I’ve seen many instances where the velocity of frame of reference is taken into calculations and all – Muhammed Roshan Jun 21 '21 at 05:32
  • Possible duplicates: https://physics.stackexchange.com/q/16018/2451 and links therein. – Qmechanic Jun 21 '21 at 05:39
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    here is the definition of frame of reference: "The reference frames used in dynamics are known as coordinate systems with axes (lines) emanating from a point known as the origin. " https://www.britannica.com/science/reference-frame . If your point is not attached to a mass, but is a mathematical coordinate system, you can do anything you want with it (science fiction). If it is attached to a mass, special relativity does not allow velocities of c for massive ones. – anna v Jun 21 '21 at 05:45
  • The linked posts answer a different question about rest frames. A reference frame is a mathematical construct and does not have to be the rest frame of any physical object. So yes, it is possible to explore the properties of a reference frame that travels (relative at another r.f.) at the speed of light (the time dimension collapses) or even faster than the speed of light (see https://en.wikipedia.org/wiki/Tachyon). I am voting to reopen. – gandalf61 Jun 21 '21 at 05:53
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    You can pick any frame you like. However, if you want to use the frame to model physical situations, you should pick a frame that keeps the calculations simple. If you pick a frame moving at C or higher then your physics will become very challenging! I agree with gandalf6 that this is not a complete duplicate. – Marco Ocram Jun 21 '21 at 05:54
  • Thanks @gandalf61. You cleared my doubt. But I have one more doubt. In Lorentz transformation, if the velocity of a frame of reference is c or greater than c, it’s gamma becomes infinite and complex respectively right? So, can I say that in the context of special relativity, frame of references can not move as fast light cause Lorentz transformation doesn’t consider a f.o.r with boost greater than or equal to c as valid? – Muhammed Roshan Jun 21 '21 at 06:36
  • @MuhammedRoshan Yes. A Lorentz transformation where $\gamma$ is infinite or complex is not physically possible, although we can still consider its mathematical properties. If you have further questions you should put them in a new post. – gandalf61 Jun 21 '21 at 07:29

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is it possible for a frame of reference to move with the speed of light? Is there something preventing that?

This is not possible, and the first postulate of the special theory of relativity, is that the speed of light is a constant, which is $c \approx 3\times 10^8 ms^{-1}$, for all observers.

For example, if you are moving toward me at velocity $v$ and at some instant you switch on a light, both you and I will measure the speed of this light-pulse to be moving at $c$ and not the sum of speeds. That is, $$v' \ne c +v$$ but instead $$v'=c$$

Regardless of your motion as the observer, and regardless of the motion of the source, the speed of light will always be measured at this value.

This principle, the constancy of the speed of light, is one of the foundations of relativity.

joseph h
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  • This I know, and this is not what I asked – Muhammed Roshan Jun 21 '21 at 05:31
  • What exactly are you asking then? – joseph h Jun 21 '21 at 05:42
  • @josephh The question is about reference frames not rest frames. A reference frame is a mathematical construct and does not have to be the rest frame of a physical object. Don’t worry - the moderators who closed this as a duplicate question made the same mistake. – gandalf61 Jun 21 '21 at 05:57
  • Oh. He's not asking for a possible frame of reference that is at rest wrt to light. So should I delete this answer since it is not "on-topic"? thx. – joseph h Jun 21 '21 at 06:04