# Near Light Speed/Time Travel

#### guss

I'm not sure if this is the right forum for this, sorry if it isn't.

I was presented with the idea that to travel in time, we could make a train track around Earth. On the track, there would be a train that traveled at 99.999% of the speed of light and would travel in time.

A girl gets up on the train, and starts running in the same direction of the train. However, she cannot break light speed because time on the train slows down. But this made me confused (obviously the train and the girl running forward are completely hypothetical and the girl is just used as an example).

How would the girl break light speed, or add to the speed of the train, because wouldn't she be travelling relative to the train, and not relative to Earth?

If this is not the case, then how is the speed of light measured, because the planet is orbiting around the sun, and the sun obviously has some movement, etc.

Can anyone explain this to me?

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#### bcrowell

Staff Emeritus
Gold Member
In non-relativistic physics, velocities add, so the girl's velocity relative to the dirt equals her velocity relative to the train plus the train's velocity relative to the dirt.

It doesn't work this way in relativity. In relativity, the equation for combining velocities is more complicated http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity , and it always gives a result less than the speed of light.

#### guss

In non-relativistic physics, velocities add, so the girl's velocity relative to the dirt equals her velocity relative to the train plus the train's velocity relative to the dirt.

It doesn't work this way in relativity. In relativity, the equation for combining velocities is more complicated http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity , and it always gives a result less than the speed of light.
Yeah, I knew that. Sorry, my question was a bit confusing.

If the train can travel 99.99% of the speed of light relative to the dirt, then how can the girl running effect time in the train, or add on to the total velocity of the train if she is running relative to the floor of the train?

Also, how do scientists measure the speed of light, if the planet is always rotating and moving, and therefore the maximum speed of light relative to the "testing facility" would be inconsistent depending on how the planet is moving?

#### Dr Lots-o'watts

No matter where you are, how fast you're going, or when you do it, the speed of light is always measured to be the same value. Whether you're on a train, standing on a sidewalk, free falling in an elevator, blasted off into space, sitting in a lab on a rotating and revolving planet, everyone measures the same value for any beam of light, no matter how fast the light source is going. There is no such thing as adding any velocity to the speed of light: c + anyspeed is still c. That's an experimental fact and it's the basis of special relativity.

Now by fixing that postulate, time and distances automatically become functions of speed (Lorentz transformations).

#### guss

No matter where you are, how fast you're going, or when you do it, the speed of light is always measured to be the same value. Whether you're on a train, standing on a sidewalk, free falling in an elevator, blasted off into space, sitting in a lab on a rotating and revolving planet, everyone measures the same value for any beam of light, no matter how fast the light source is going. There is no such thing as adding any velocity to the speed of light: c + anyspeed is still c. That's an experimental fact and it's the basis of special relativity.

Now by fixing that postulate, time and distances automatically become functions of speed (Lorentz transformations).
Interesting, thanks.

I got excited for a second, thinking that we could build some kind of device that would constantly measure the speed of light in x, y, and z direction, and therefore we could determine our movement relative to nothing in the universe. Too bad that will never happen :p

#### MillerGenuine

Just in case you didnt know, the speed of light can not be exceeded by anything with MASS. If it doesnt have mass it can theoretically go faster than C. Im not sure if you have experience with the equations, but the reason any particle with mass can not go faster than light is because as you approach C, the amount of force required to increase its velocity even the slightest amount becomes extremely large. Eventually as you arrive at C, the amount of force needed to increase the velocty becomes infinite.
And just as "Dr Lots-o'watts" post said, the speed of light is the same in any frame of reference, which is why it is a constant, and we can just call it C.
To make you feel better i've read bio's on einstein that said as a kid he always wondered what would happen if you were traveling near the speed of light, pointed a flashlight straight ahead and turned it on. Seems like the same question you have, so hey, great minds think alike.

#### Dr Lots-o'watts

...some kind of device that would constantly measure the speed of light in x, y, and z direction, and therefore we could determine our movement relative to nothing in the universe...
This is roughly what the Michelson-Morley experiment tried to do (at least x and y). It showed that the speed of light was the same in any direction.

#### guss

This is roughly what the Michelson-Morley experiment tried to do (at least x and y). It showed that the speed of light was the same in any direction.
But it sounds like that's still possible if we shoot objects that have mass at the speed of light. So, wouldn't this already be happening at the LHC since they shoot protons?

And thanks Miller.

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