# Falling object at c

Gedenke experiment: Falling object upto c?

Bonjour,

I would like to know the relativistic position expression for a falling object that could reach the speed of light.

Thanks.

P.S.: Suppose the only things, that exist in the universe, are a BIG attractive mass and the attracted "object".

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Nothing with mass could be accelerated to c.

Even if it were to go into a black hole, starting an infinite distance away, once it crosses the event horizon, it is in a different spacetime.

Gedenke experiment: Falling object upto c?

I shall agree with your mass issue.

Suppose, isolated in the universe , a small energetic object, with zero initial relative speed, somewhere far-far-far away from a non-negligeable gravitationnally attractive accumulation of energy (both with mass equivalence (m & M) for gravitational purpose ).

"m" shall be accelerated by "M", following gravitational effect, right?

While m's speed is within non-relativistic speed, I got no problem to express acceleration, speed and position equations.

What would be these equations when relativistic speed's effect is non-negligeable? (Should I ask this in Theoretical forum?)

P.S.: ............

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For relativistic versions of those, some tinkering with the equations here: http://www.wikipedia.org/wiki/Relativistic_equation

Also, http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

note that it has the equations you are looking for.

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Staff Emeritus
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Welcome to Physics Forum, Imagine!

For relativistic versions of those, some tinkering with the equations here: http://www.wikipedia.org/wiki/Relativistic_equation

Also, http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

note that it has the equations you are looking for.

Phobos, merci pour votre mot de bienvenue.

Brad, I red the rocket page. I also understand the origin of the following restriction: "The acceleration of the rocket must be measured at any given instant in a non-accelerating frame of reference traveling at the same instantaneous speed as the rocket".

Thanks, that was exactly what I searched.

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