Theoretically

Okay lets say you could open a wormhole in front of your space ship, but you couldn't fly through it, in fact no matter and not much energy could come through. About all that could filter across is the force of gravity.

So let's say you open the other side of the wormhole above a star. Now you have the force of gravity pulling you forward, and you might close and reopen it every nanosecond so you don't fly through it.

How fast could you theoretically end up going?

I think there was an article in JBIS in the 90's along the same lines - create a gravity gradient from front to back. In effect you alter the properties of Spacetime and although in your altered spacetime you didn't exceed the speed of light, to an outside observer you would be going faster than light. Or something like that. I didn't understand the maths.

Well I fiddled about with the gravity equations and I can't figure out if you keep accelerating because there is no drag in space or whether there is a maximum speed based on the size of whatever you opened up your wormhole over. Could you get to relativistic speeds by opening it near a black hole?

Another option for something like this would be dark matter, that interacts with normal matter mostly through gravity, a useful property if you want to create a gravity gradient. Dark matter highways?

Perhaps this image might not be a bad way to describe it:



How fast would that falling guy eventually go if he was in a vacuum?

Well, as the speed increases, length contracts. So, wouldn't the space ship contract more and more in the direction of travel as the speed increases, thus experience less and less of a gravity gradient the closer it gets to the speed of light?

There is a book - "Black Holes and Timewarps" (I think) by Kip Thorne that may be useful in determining what might go on.

Why would that cause it to experience less gravity? What I'm wondering is does the energy imparted by gravity grow less for some reason as you go faster, without friction getting in the way.

@James ta I'll check that out.

Basically until we have a good understanding of quantuum gravity, there's no concept (no matter how plausible) that is going to actually let you go faster than light. With quantuum gravity - I think its probably possible.

I think of it like this - its takes an infinite amount of energy to reach the speed of light. Even though the gravity well is still imparting energy, speeding you up, you still never have an infinite supply, so you never reach the speed of light.

Excellent, thanks guys. The idea here isn't to go ftl, it's to give a stronger theoretical backing to something like Alastair Reynolds' lighthugger starships, which do .999999c or something silly like that. Alastair got around it by postulating that superbrains tapped into the energy of the big bang to produce these tremendously long acceleration bursts, your basic handwavium.

The problem of course is that to achieve anywhere near a few percentage points of lightspeed, you need unfeasably huge energies, to the extent that you could easily produce any items you needed from raw matter, so interstellar trade in the star wars sense is kind of pointless.

So I thought, what if you don't actually produce the energy and blast it out the back, but rather hitch hike on something else's energy. Nothing really goes that fast, and if it does, you seriously don't want to be in front of it, but is there a loophole whereby you can ride down an eternal gravity gradient?

We already know that there are particles which interact mostly through gravity (dark matter), its a short step from there to fiddling wormholes to leak gravity only, then you should conceivably have your very near light drive, with the only energy needed being that to produce these limited wormholes, which while still significant would be orders of magnitude less than needed to propel a vessel to those speeds, sort of like a sailboat that can move without any propellor or engine.

So you could use this to approach arbitrarily close to lightspeed?

Wouldn't quantum gravity have to be consistent with relativity, in which case it wouldn't be possible?

How do you open a wormhole? Last I checked, the "opening" at each end is a sigularity, which tends to have a Black hole wrapped around it. So, if you can compress enough to squeeze through that, twice, it can be used as a gravitational shortcut that can circumvent the universal speed limit. For obvious reasons, gravity doesn't propogate through it, nor light, nor anything else. When you come out, you have to speed to escape the event horizon of the singularity on the other side. Even Gravity doesn't move that fast.

Now, I've heard, and read a lot of thought experiments with Einstein-Rosen bridges, where you create two quantum entangled singularities, then move them at near light speeds to a far enough distance to time travel, but that's all they are, thought experiments. Until we have the tools to make the tools to make the tools to make a black hole, let alone move them, it has about as much practical value as locking a cat in a box with a cyanide pellet, and a coin.

What is there to stop an accelerating object into a black hole from gaining a insane relitivistic mass from gravity? At what point does the drawing force of gravity not touch inertia? If I weigh infinity and am in motion how do you stop me?

If I accelerate negitive mass does it gain negitive mass?

If my craft is half negitive mass do I have zero inertia?

If I am destroying mass to accelerate into a black hole will I gain thrust potential as I gain mass? Ie fission?

Are you perhaps referring to this fellow: Miguel Alcubierre? He hypothesized the Alcubierre metric which allows an object to 'move' faster than c without essentially moving at all, but by creating a space-time bubble around the spacecraft which 'moves'.
http://arxiv.org/pdf/gr-qc/0009013.pdf

I have a friend, who has just finished third year physics (that's the last year in undergrad in Aus), and used this guy's paper in a general relativity assignment.

That is what happens as matter passes thru the event horizon. And its how cosmic rays are created from the small amount of matter and energy that manage to escape with tremendous energies.