# Question about going faster than speed of light

1. Oct 21, 2007

### MagikRevolver

I was watching a show on TV that talked about why we couldn't go faster than speed of light because we would gain infinite mass (e=mc^2 I presume). The show said that if we wanted to go faster than the SoL that we would have to move space around us. Compress space in front and expand space behind. I don't know how far out we would be moving space or the universe around us, but if we moved a part of the universe to a spaceship (rather than moving the space ship) wouldn't that exert massive forces (g's) on everything except the ship? Also the show said that the forces would be exerted on the ship, but that doesn't make any sense since the ship isn't really moving, right? And also they said that if we wanted to counter g forces that we could use a diamagnetic field to push back in the opposite direction of the g forces (they said its a magnet strong enough to move living organisms, or the water in them). But that doesn't make any sense either since if you exert a force on something in one direction at the same time as another, wouldn't it just squash that object like a bug. Like the force of a foot at the same time as the force of the earth on say a cockroach?

2. Oct 21, 2007

### G01

I am no expert in space travel myself, but by going off of what you have said, I would be very skeptical about the information provided by this show. It seems, at best, misguided and convoluted, and, at worst, not really portraying actual science.

Last edited: Oct 21, 2007
3. Oct 21, 2007

### MagikRevolver

It was on the Discovery Channel. Which I would suppose is not the most accurate source of info. Seeing as how they do have a lot of misguided information on shows like Myth Busters and Dirty Jobs. However this particular show had some pretty well known astrophysicists and the like. Of course those physicists never exactly said what I stated, the narrator did. I don't know though, I would like a definite answer from some phys person here.

4. Oct 21, 2007

### MagikRevolver

It was on the Discovery Channel. Which I would suppose is not the most accurate source of info. Seeing as how they do have a lot of misguided information on shows like Myth Busters and Dirty Jobs. However this particular show had some pretty well known astrophysicists and the like. Of course those physicists never exactly said what I stated, the narrator did. I don't know though, I would like a definite answer from some phys person here.

5. Oct 21, 2007

### MrXow

There is no difference in motion and standing still, in fact it is impossible to tell whether or not you are moving at a constant velocity versus not moving. Also you cannot move faster than the speed of light as seen by another observer. Space time can move faster than the speed of light, but if spacetime is moving faster than the SoL around you, as long as it does not accelerate you would not feel a force

6. Oct 21, 2007

### MagikRevolver

Could you elaborate. I know space time is moving and faster than the speed of light, but how does that correlate to inertial force and gravity if one is then able to move the universe?

7. Oct 21, 2007

### MrXow

It does not correlate to a force. As far as I know, spacetime does not exert a force on the objects in it. There is a theorized Frame-dragging effect, but other than that spacetime does not exert a force on stuff in it.

8. Oct 21, 2007

### Loren Booda

If we use light as the standard of measurement, how can we both send and receive information to and from something traveling faster than light speed?

9. Oct 21, 2007

### MrXow

Nothing can be observed to move faster than the speed of light.

10. Oct 22, 2007

### JesseM

Assuming they're talking about relativistic mass rather than rest mass, this is correct, the relativistic mass goes to infinity as you approach the speed of light.

11. Oct 22, 2007

### MrXow

Really? So your saying light, a massless particle, has no energy.

12. Oct 22, 2007

### JesseM

No, light has zero rest mass m. However, the full equation for the energy of a moving particle is $$E^2 = m^2 c^4 + p^2 c^2$$, where p is the relativistic momentum $$p = \frac{mv}{\sqrt{1 - v^2/c^2}}$$, so if m=0 this simplifies to E=pc which is the photon's energy (and if the velocity of an object is zero in some frame, then in that frame p=0 so the energy simplifies to E=mc^2).

13. Oct 22, 2007

### curiouschemist

So if relativistic mass goes to infinity, that is the mass to an observer. What about the mass to the object itself? Say I'm cruising along at .9c, my relativistic mass is some 700 times normal, but I can still run, jump, and kick aboard my... uh... millenium falcon, which I wouldn't be able to do if I weighed ~70 tons. So what if we use a relative force? A moving force with the object, say, an impulse drive. Or a simple Hydrogen-Oxygen thruster on board, also moving at .9c? All attempts at approaching SoL or c that I know of are done in cyclotrons and thus use a stationary force/thrust propelling a particle. What if we strapped a rocket to that electron?

14. Oct 22, 2007

### pervect

Staff Emeritus
The mass that is the property of an object (the invariant mass, and/or the relativistic mass in the proper frame of the object, aka the rest mass) does not depend on velocity at all.

So you can't tell by an experiment in a closed room what your velocity is. You certainly won't feel heavier, nor will you notice time passing in any different manner than normal.

As far as force goes, the concept of force requires modification to work with relativity. I favor the 4-force approach, which treats forces as a 4-vector.

See for example this wikipedia stub for a general idea.

15. Oct 22, 2007

### curiouschemist

Okay... I get it barring the gammas and lambdas that weren't really explained, but how does this prevent a rocketship with infinite fuel from reaching the speed of light just by running its thrusters constantly in a vacuum for the appropriate amount of time?

16. Oct 22, 2007

### MrXow

JesseM i was saying that to point out the flaws in iwillsurvive, obviously light has energy, or else the photoelectric effect (along with other things) would not work

17. Oct 22, 2007

### JesseM

This is related to the fact that velocity addition doesn't work the same way in relativity as it does in Newtonian physics--see here for details. Suppose I'm on a ship which is moving at 0.8c relative to you, and then I shoot out a rocket from the ship, which in the ship's own rest frame is moving at 0.6c in the same direction that the ship is moving relative to you. Using Newtonian assumptions you'd expect that the rocket would be moving at 0.8c + 0.6c = 1.4c relative to you, but in relativity it doesn't work that way, instead the rocket will be measured to be moving at (0.8c + 0.6c)/(1 + 0.8*0.6) = 1.4c/1.48 = 0.956c. This has to do with the fact that each frame uses rulers and clocks at rest in that frame to measure distance/time, and in relativity my ruler is shrunken in your frame while my clocks are slowed-down (and also out-of-sync in your frame if they're synchronized in mine).

So, by the same token, if I'm moving at 0.8c relative to you, and then I hit the ship's afterburners and accelerate until I'm now moving at 0.6c relative to someone who was previously at rest alongside the ship, then in your frame I've only accelerated by 0.956c - 0.8c = 0.156c. Basically what this means is that even if I can accelerate for a constant rate as experienced on board the ship, in terms of the G-forces I feel on board or in terms of how fast my speed will be seen to be increasing by someone instantaneously at rest relative to me at that moment, then as seen by a non-accelerating observer in a constant inertial frame my speed is not increasing at a constant rate, instead the speed is increasing more and more slowly as I approach the speed of light in their frame. Also, if they used a laser at rest in their frame or some other similar means of pushing my ship along, the energy expended as measured in the laser's frame would get larger and larger for each increase in speed as my ship got closer to the speed of light.

18. Oct 22, 2007

### JesseM

Oh, OK...but I didn't see iwillsurvive saying light doesn't have energy, just that "if our mass is 0 we also not require energy to attain speed of light according to e = mc^2" which is close to correct, although in reality a massless object can never have any speed except light speed, so it doesn't need to "attain" it.

19. Oct 23, 2007

### curiouschemist

I think I see now... to the outside observer, even if you did constantly accelerate, you would be constantly accelerating more and more slowly as time slowed down to the outside observer, so as you approach the speed of light, time slows down for you, but speeds up for everyone else, so you could theoretically accelerate until the universe ended because you'd be preserved by your own time distortion field from going so frickin fast. By the time you hit the brakes (which would be an incredibly long process that would eventually have time accelerating back up to normal all around you), the universe would have progressed through time all around you, so you effectively would have time traveled, but unless you were going in a big circle you'd be light years from where you started and, looking back, time wouldn't have progressed that much because the light wouldn't have gotten there yet... unless you had some electrons with you that are paired with others from where you started and you were making observations by their spin states.

But let's say you time travel until the end of the universe... by then the laws of physics may have changed, eh? Thanks JessieM, I think you've brought me peace with this issue.