# Stopping a Body (travelling at the speed of light) [closed]

While, I read that if a body accelerates to the speed of light would have an infinite mass and therefore, infinite energy would be required to maintain the speed.

A question popped up in my mind:

Assuming we've a body traveling at the speed of light (and having infinite mass), how difficult would it be to stop that body? What if it collides with a planet, would it stop or will it continue moving?

EDIT: Only infinite source (or tremendous source) of energy would be required to reach the speed of light. Once reached at that speed, I guess, nothing would try to stop it in a vacuum (except colliding or gravitational bending).

## closed as off-topic by Qmechanic♦May 31 '15 at 5:50

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• Mainstream physics in this context means that a massive body has finite (rest) mass and speed less than the speed of light in vacuum. – Qmechanic May 31 '15 at 8:23

It would not have infinite mass. Relativistic mass is device used to introduce beginners into relativity. Mass is invariant (doesn't change due to reference frame) in more advanced relativistic studies. Aside from that, one does not merely need infinite energy to reach c. One also needs infinite time. Therefore, your question is if we wait until infinity is over and then we do the impossible (moving at c) (it is not possible for something with mass, but it is REQUIRED for something without mass), what happens? Physics can not answer such a question because you are asking what happens to a system if we ignore is rules.

Also, $E = mc^2$. Wherever you are getting this energy, the mass there must be infinite. An infinite mass in a finite space would form a black hole.

• Oh, I see, thanks. I had forgot that infinite mass would form a blackhole. – Areeb May 31 '15 at 4:28
• What do you mean by infinite time though? – Areeb May 31 '15 at 4:29
• @userLTK I didn't say that a super accelerated object would form a black hole. I started my answer by saying that mass is invariant. – Jimmy360 May 31 '15 at 6:33
• @userLTK I said that the source of his energy would form a black hole (i.e. a battery with infinite charge or something of the sort) – Jimmy360 May 31 '15 at 6:35
• @Jimmy360 what do you mean, no warp speed? I have seen the movies - they do it all the time! – Floris May 31 '15 at 13:16

Infinite energy would be required to reach that speed - not to maintain it. But you can never reach the speed of light... only ever get close. Like they do in the new LHC - individual protons getting energies in the TeV range. At these energies, they have a gamma of about 7000 - meaning they have 7000 times their "rest mass". That's heavy, but nowhere near "infinite".

By definition, it is impossible for massive particles to reach the speed of light. Particles without mass (photons) do it routinely - and when they hit the Earth, we call it "sunlight".

• Thanks a lot. Will wait for more answers to clear the confusion. And, then accordingly choose the wisest answer. – Areeb May 31 '15 at 4:07