Why is faster-than-light travel 'impossible'?

[Linguofreak]

So, you mean something like that, rotating around the tether:

If so, how is this different from simply a circular tube?

A circular wheel with interior walls across the tube would work too.

What he's getting at is that, if you don't have any air or obstructions in the tube, and there is an object in the tube at rest relative to the center of the wheel, then there will be nothing to bring that object up to the speed of the rest of the wheel, so there will be no centrifugal force on it in a rotating frame.

 

[Fizyk]

I know! I don't know why people keep trying to explain this to me, I've never argued it :/

I'm sorry, I had the impression that you argued with it a bit at one point and wanted to straighten things out. Now I can't find that part of the discussion, so it looks like I was wrong.

Both of them are in non-inertial frames relative to the Earth, because they're being rotated around the Earth, which requires them to be accelerated by gravity (or the space elevator tether) in order to not fly off into space.

In general relativity, all reference frames in which Newton's first law is valid are considered inertial - that makes the orbiting twin's reference frame inertial (well, approximately, because the gravitational field isn't homogenous).

 

[ZombiezuRFER]

try spinning a circular tube with water in it. No matter how fast you spin it, you will not be able to get the water to stay against earths gravity. it does not generate centrifugal force.

 

[Topper]

... i think i just understood it, why it's not possible.
bevore, i don't understand the argument that the mass of an accelarting object would be infinite.

Thats no argument, because the mass of the fuel would also be infinite (verry heavy), and so also the thrust.

But now i understand:
The mass would be ony infinite from the relativ point of view.
So there are 2 point why you never can reach the speed of light, not relativ to another point of view and not from the point of view of the traveling object:

1. No one can reach the speed of light relativ to an observer, because for the observer, a clock on a traveling vessel would be go slower and slower if he come closer to the speed of light.
If the clock goes slower, also the acceleration of the object would be more less (it's the same as if the mass of the object goes higher), because a=dv/dt. But the dt goes higher and higher (because the clock goes slower) and it's infinity if the traveling object is reaching the speed of light.
[edit]
Or in other words:
The acceleration from the point of view of the traveling object is constant, but here the times go faster then at the point of the observer,
so the acceleration from the observer is more less.
[/edit]
2. No one can reach the speed of light from the point of view of the traveling vessel, because c is always constant, and it dosen't matter how fast you allready are.

 

[Hielor]

In general relativity, all reference frames in which Newton's first law is valid are considered inertial - that makes the orbiting twin's reference frame inertial (well, approximately, because the gravitational field isn't homogenous).

Newton's first law is always valid, but in the case of the orbiting twin, Newton's first law indicates that there is a force being applied (and thus, an acceleration, and therefore it's not an inertial frame).

If the orbiting twin were to continue in a straight line, as posited by Newton's first law, it would no longer be in orbit and would just zoom off into space. In order for him to stay in orbit, he must be accelerated toward the planet, and is thus not in an inertial reference frame.

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try spinning a circular tube with water in it. No matter how fast you spin it, you will not be able to get the water to stay against earths gravity. it does not generate centrifugal force.

Please re-read the following:

The assumption is made that all of the objects within the wheel are spun up at the same time as the wheel itself, not free-floating inside the wheel. Once you're spinning with the wheel, you will "feel" the "gravity" that it's generating.

Also note that with a spinning wheel in space, you're not fighting against Earth's gravity. Do the same experiment in zero-g and you'll see the water get pressed to the outsides.

 

[ZombiezuRFER]

Also note that with a spinning wheel in space, you're not fighting against Earth's gravity. Do the same experiment in zero-g and you'll see the water get pressed to the outsides.

Even still, nothing is pressing the water to the sides. The water would free float, and no artificial gravity would be generated.

 

[Dambuster]

Even still, nothing is pressing the water to the sides. The water would free float, and no artificial gravity would be generated.

Do you mean that the water will move frictionlessly against the sides of the container (or whatever), so that relative to something outside your rotating object, the water is stationary?

 

[Fizyk]

Newton's first law is always valid, but in the case of the orbiting twin, Newton's first law indicates that there is a force being applied (and thus, an acceleration, and therefore it's not an inertial frame).

In general relativity, neither gravitational force nor inertial forces are considered forces. They are just effects of using a curvilinear coordinate system.

Anyway, that's all just a matter of definition.

In order for him to stay in orbit, he must be accelerated toward the planet

From the point of view of general relativity, he is actually the one that is not accelerating. His world line is a geodesic (which an analogy to the straight line in curved space) and acceleration is defined as, roughly speaking, the curvature of the world line. The twin who is accelerating is the one in the elevator - the floor of the elevator accelerates him upwards - this actually makes him stay in place, but in 4-dimensional terms his world line is curved.

 

[Wood]

Even still, nothing is pressing the water to the sides. The water would free float, and no artificial gravity would be generated.

Take a look at these, they might help you to better understand fluids in microgravity.

 

[Izack]

Don't give me the 'I was tired excuse'...

Try saying that after spending five hours churning things through Hess's law after a 14-hour day of shoveling heavy snow up a ten-foot snowbank and pulling minivans out of icy ruts. :tiphat:

Thank you for the clarification, though. I probably shouldn't have posted without being able to fully explain the first time, sorry.

 

[ZombiezuRFER]

Do you mean that the water will move frictionlessly against the sides of the container (or whatever), so that relative to something outside your rotating object, the water is stationary?

Friction is not artificial gravity.

 

[Linguofreak]

Newton's first law is always valid, but in the case of the orbiting twin, Newton's first law indicates that there is a force being applied (and thus, an acceleration, and therefore it's not an inertial frame).

If the orbiting twin were to continue in a straight line, as posited by Newton's first law, it would no longer be in orbit and would just zoom off into space. In order for him to stay in orbit, he must be accelerated toward the planet, and is thus not in an inertial reference frame.

Under general relativity, with time being a dimension and space-time being curved, the definition of a straight line changes.

http://en.wikipedia.org/wiki/Geodesic_(general_relativity)

 

[ZombiezuRFER]

Take a look at these, they might help you to better understand fluids in microgravity.

http://www.youtube.com/watch?v=BxyfiBGCwhQhttp://www.youtube.com/watch?v=kpMwBGbBo6Y&feature=channel

Wasn't the point, it was about if it stick to the sides of a spinning tube.

 

[Quick_Nick]

Friction is not artificial gravity.

The idea is not to create gravitons. It's to simulate gravity, which this does.
And water will indeed stick to the sides if you do it right.

 

[Wood]

You can see, there is no sides to sphere there, only the surface tension, and the artificial gravity effect is still produced. The bubbles migrate towards the centre and the tea particles migrate to the edge. As long as an object is rotating, it will feel the angular accelation. The water inside your tube is rotating, hence feels the artificial gravity.

 

[Keatah]

Excuse my simple view on this. If light travels at light-speed, then it should have infinite mass. It clearly does not. Imagine pointing a flashlight at a window. The light punches through it, and shatters it! Then sucks all the glass in like a black hole. Clearly this does not happen.

So, at zero relative velocity, like has zero mass? Ok. And at light-speed it has "some" mass, enough to push a solar sail around? Right?

Maybe we could like build a powerful flashlight and use that for thrusters!

You know, I bet someday there'll be a loophole to this whole theory and FTL will become commonplace.

 

[Wood]

As to why FTL travel is impossible in special relativity and how it relates to causality, I found this article to be very helpful.

 

[Quick_Nick]

Excuse my simple view on this. If light travels at light-speed, then it should have infinite mass. It clearly does not. Imagine pointing a flashlight at a window. The light punches through it, and shatters it! Then sucks all the glass in like a black hole. Clearly this does not happen.

So, at zero relative velocity, like has zero mass? Ok. And at light-speed it has "some" mass, enough to push a solar sail around? Right?

Maybe we could like build a powerful flashlight and use that for thrusters!

You know, I bet someday there'll be a loophole to this whole theory and FTL will become commonplace.

AFAIK it never has mass, just energy and momentum. (Energy happens to be equivelent to mass though) And using a flashlight would be ineffecient. Best to just use the sun and solar sails. However, it has been proposed before to shoot beams of EM at spacecraft for energy or whatever (basically the same idea as yours) but it's still far from practical.

 

[Linguofreak]

Excuse my simple view on this. If light travels at light-speed, then it should have infinite mass. It clearly does not. Imagine pointing a flashlight at a window. The light punches through it, and shatters it! Then sucks all the glass in like a black hole. Clearly this does not happen.

So, at zero relative velocity, like has zero mass? Ok. And at light-speed it has "some" mass, enough to push a solar sail around? Right?

Pretty much, although a lot of scientists don't like to call "relativistic mass" mass. They call "relativistic mass" energy (since it's the total of kinetic energy and rest mass), with mass meaning rest mass.

Maybe we could like build a powerful flashlight and use that for thrusters!

We could, but it would take about 1,700,000 horsepower to get enough thrust to lift a one pound object against Earth's gravity.

 

[Keatah]

So you're saying that gravity magically appears out of nowhere and causes spacecraft to move?

It does not matter whether the acceleration comes from gravity or some other source. The fact remains that one of the two has accelerated and the other has not.

Consider a simplified case: Two hollow spheres of gravitationally insignificant mass in an empty vacuum, each of which has a smaller solid sphere inside at the exact center. One of the hollow spheres begins accelerating away from the other at a rate of 2g.

What happens to the solid spheres inside the hollow ones? One of them will bump against the side of its container, one will not. One of the two is undergoing acceleration and the other is not. You're complicating things by introducing gravity where it is irrelevant. In this case, it is quite obvious which one has accelerated, and there is no way to say that it is the other way around.

..And you could detect very easily how the change in velocity was effected. Was it from a gravity source? Was it from a rocket thruster? A gravity source would accelerate both the inner solid sphere and the outer hollow sphere just about equally. The spacing would be the same.

A rocket thruster would only move the outer sphere, leaving the inner one to crash against it when under power..Then both would be moving under power. So there, you have a detector.

Interesting how gravity reached through and grabbed both spheres. I wonder how this would work at lightspeed? And how gravity would propagate? I keep hearing it is instantly, and also takes time.