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

[Artlav]

isn't acceleration all relative? ie, one object is accelerating when compared to another, but the same can be applied to the other?

True, you're accelerating relative to the astronauts when the shuttle lifts off.
But who feels the G forces?

 

[ZombiezuRFER]

well, in a ship creating 1g of acceleration, either the ship is moving and the people inside feel the g generated by the acceleration, or the people are accelerating at 1 g into the ship.

 

[Fizyk]

I just came up with a more sophisticated problem regarding the twin paradox, involving gravity.

Let's say we have two twins again, but now one of them sits in a space elevator on a fixed altitude above the surface of the Earth, while the other one orbits the Earth at the same altitude. Which one ages slower?

The problem is, two intuitive approaches give different results this time:

1) The orbiting twin goes at about 8 km/s (well, that depends on the altitude, but let's say it's quite low), so he should age slower than the other one.
2) The twin sitting still is the one in a non-inertial frame (he feels the gravity, while the orbiting twin is in free fall = doesn't feel gravity), so he should be the one who ages slower.

I'm pretty sure the solution number 2 will be correct, but I've got to calculate this

EDIT:

well, in a ship creating 1g of acceleration, either the ship is moving and the people inside feel the g generated by the acceleration, or the people are accelerating at 1 g into the ship.

That's kinda the gravity-acceleration equivalence Ship is moving = normal acceleration, people are accelerating = gravity.

EDIT2:
Calculated it, turns out that after one orbit the time that passed is:
[math]t_1 = \frac{2\pi r}{c} \sqrt{\frac{rc^2}{GM}-3}[/math] for the orbiting twin
[math]t_2 = \frac{2\pi r}{c} \sqrt{\frac{rc^2}{GM}-2}[/math] for the sitting-in-the-elevator twin.
So, the one that ages slower is actually the orbiting twin.

 

[ZombiezuRFER]

at least inside a closed system. for things like the shuttle launch, which is accelerating? Both the shuttle and the earth are "accelerating" away from each other. To an observer on the ground, the shuttle is accelerating, to the astronauts, the earth is accelerating simultaneously as they are. I hope that made sense.

 

[Rtyh-12]

Yes, but on Earth, you don't feel the 3g of acceleration... in the Shuttle you do.

 

[ZombiezuRFER]

Yes, but on Earth, you don't feel the 3g of acceleration... in the Shuttle you do.

Yeah... its just that its all about the point of view. Its kind of the reason I find Artificial gravity via rotation implausible, at least when its the great big wheel centrifuge.

 

[Rtyh-12]

The 'great big wheel centrifuge' is perfectly correct...

 

[ZombiezuRFER]

I say no. Its just spinning. If its just spinning, why would it generate artificial gravity? (this is fairly hard to explain, gimme a little leeway.) While the centrifuge spins, what makes things stick to the "floor?" The only way the great big wheel would work is if you made it so it had floors at angles, which angle I'm not sure, but as a guess, 45 degrees up from the centrifuge walls. Each floor would need to be its own closed system to function as well. I'll draw up an MS paint graph later tonight.

 

[Fizyk]

While the centrifuge spins, what makes things stick to the "floor?"

The centrifugal force.

Have you ever been on a carousel? If yes, have you felt the force trying to push you off the carousel? That's what would create the artificial gravity.

 

[ZombiezuRFER]

thats in an atmosphere. centrifugal force does move things outward, but it first needs another force to make things stay on whatever is generating centrifugal force.

 

[Fizyk]

What does atmosphere have to do with it? The reason for the centrifugal force is that the frame of reference is spinning, that's all.

Imagine you spin a ball on a string. The string has to pull the ball to the center to make it go in circles - that's centripetal force.

Now imagine there is a fly sitting on the side of the ball that is directed to the center. The fly isn't connected to your hand with a string, so it doesn't pull the fly, and yet the fly goes in circles too. That's because the ball pushes the fly, creating the centripetal force this way.

But the fly can look at this from another point of view. It can say that it's not the ball that pushes it, it's some mysterious force that pushes it to the ball. This is the centrifugal force.

It doesn't matter if you spin the ball in the atmosphere or in a vacuum. If something goes in circles, there has to be a centripetal force that pulls it to the center, and it can always be interpreted as a centrifugal force from another point of view.

 

[ZombiezuRFER]

on a cable is different. A wheel just makes centrifugal force useless.

 

[Fizyk]

The only difference is that instead of a piece of string, the source of the centripetal force will be whatever connects the rotating wheel to the rest of the ship. Substitute the wheel for the ball and the crew for the fly, and you get exactly how this works.

 

[ZombiezuRFER]

I'll make a graph to demonstrate my point better. What I'm saying is unless your wheel is closed off in sections, you wont get artificial gravity. If you spin a wheel in space, you wont get centrifugal force pushing everything off to the floor of the wheel.

 

[Artlav]

unless your wheel is closed off in sections.

Please define "closed off in sections".
What does that looks like?

 

[ZombiezuRFER]

As in the wheel isnt one big curving tube, but sections. You have individual rooms that take up an entire section of the wheel, aka segmented into rooms.

 

[Artlav]

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

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

 

[jimblah]

Faster than light, I can do that.

Being an absurd optimist, I think the light barrier will be cracked, I'd even put money on it but the bookies would write in so many caveats. In another thread i did mention that gravity acts instantaneously, and waited fro the howls of protest. it was this problem that kept Newton awake at night, and note that Einstein takes the same view. There's an e.m speed limit of c but gravity happily ignores that.

We need all the optimism we can get. I would love it if mankind found a work around to the FTL barrier.

 

[Hielor]

Hielor: imagine you were in a coma, you wake up and you find yourself in a closed room without windows. You feel the normal 1g gravity. The point of the gravity-acceleration equivalence is that you might as well be on a rocket which accelerates at 1g, and you have no way to tell (without looking outside) which possibility is true.

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

1) The orbiting twin goes at about 8 km/s (well, that depends on the altitude, but let's say it's quite low), so he should age slower than the other one.
2) The twin sitting still is the one in a non-inertial frame (he feels the gravity, while the orbiting twin is in free fall = doesn't feel gravity), so he should be the one who ages slower.

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.

Yeah... its just that its all about the point of view.

Acceleration is not "all about the point of view." It's not relative, because the points of view are different--the astronauts in the shuttle experience the 3g acceleration, but the Earth doesn't, so we know which one is accelerating and which one is not.

I'll make a graph to demonstrate my point better. What I'm saying is unless your wheel is closed off in sections, you wont get artificial gravity. If you spin a wheel in space, you wont get centrifugal force pushing everything off to the floor of the wheel.

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.

 

[T.Neo]

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

As a centrifuge, nothing, except that if you walk from one end of a segment to another, you will feel as if you are walking up and then downhill.