Here's the way to go faster than light

1. Apr 16, 2006

Giulio B.

Sorry but was necessary to write an appariscent title to make you read.

---- We know it's impossible to reach light because the mass of the object moveing grows exponentially by the icreasing of speed ----

But, what happends in a gravitational field? the acceleration we wold get doesnt got influenced by the mass increasing, so we will never stop to increase our acceleration, also reaching and going over the speed of light.

So, if in the future i will meet a big star whit a grav field sufficient to do that, whit my spaceship i would be able to destroy the star (whit some ray...weapon etc) just after i reached c; then i will keep sailing at that speed forever, forward time and faster than god.
le's say that i placed some amazing nuclear bombs around the star and somebody else when i'll reach c will make it explode....neutralizing and fragmentising the gravitational field.
can somebody tell me if what i wrote is fantasy or something else? thanx.

2. Apr 16, 2006

Staff Emeritus
Nope. As I explained on your other thread, big finite forces won't do the job. It isn't just mass that goes to infinity as v approaches c (in fact some would argue that mass is invariant!) but energy and force required to acccelerate become inifinite too, while lengths and time inervals shrink to zero. Wake up and smell the coffee; massive objects travelling at c are physically impossible.

3. Apr 16, 2006

Giulio B.

wrong

m in a gravitational field doesn't influence the acceleration, because it is a constant, so m can grow to infinite and the acceleration would be always the same. so, if in that case the growing of mass is ininfluenced, for our purpose we can say that in a grav field, realtively to the acceleration it is a constant.

---> F=v x const.

As you said, reaching c needs infinite energy, but why? : because mass grows to infinite. (remember F=mxa?lol) so, if we don't care about mass, we neither care about energy.

now that i showed it matematically, i'm writing again the question:

Is there a rule that makes me unable to reach c in a gravitational field?Thanx for any answer.

Last edited by a moderator: Apr 16, 2006
4. Apr 16, 2006

pervect

Staff Emeritus
[reorganize]
I'm going to "fix up" my original reply some, to make it simpler and more clear by rearranging it a bit.

Objects with escape velocities greater than the speed of light do exist - they are called black holes.

There is a sense in which when one falls into a black hole that one moves "at the speed of light" at the event horizon. Certainly, one approaches the speed of light as measured by a stationary observer the closer one gets to the event horizon. At the horizon itself, one cannot emit signals back to the outside world, either.

This is not a particualrly productive or useful way to think about falling into black holes, though.

An important point to avoiding the "speed of light" issue is that stationary observers do not exist at or inside the event horizon of the black hole. The velocity of one infalling observer into a black hole will always be less than or equal to the speed of light relative to any other infalling observer. You will never see an example in which two observers at the same point in space have a relative velocity greater than 'c'.

The argument that one is "exceeding the speed of light" requires one to make the false assumption that stationary observers exist inside the event horizon of a black hole. They don't. The "greater than c" velocities occur relative to non-existent stationary observers, they do not exist between any physically existing observers.

I wonder if the OP has possibly been talking to Zanket, who has similar issues.

It's also definitely wrong to think that one's mass becomes infinite when one crosses the event horizon of a black hole, for instance. In one's own frame, one's mass is always constant. In the frame of an observer at infinty, the mass of a black hole will increase by the "energy at infinity" of an infalling object, which is a finite amount, not by an infinite amount.

Thus if a 1kg object falls into a black hole at rest from infinity, the masss of the black hole will increase by 1kg - slightly less, if the infalling object emits any gravitational radiation while it falls in. See for instance MTW's gravitation, pg 904, and /or the discussion of "the first law of black hole dynamics".

While this will undoubtedly be too advanced a book for the OP (original poster) to understand in detail, the fact that the OP is claiming a greater understanding of GR than a graduate level textbook MAY, if we are fortunate, incite a sense in the OP that perhaps there are aspects to GR he does not currently understand. I will not dwell on the less fortunate outomes.

I have to say that I feel I may have been a bit harsh here, but I get concerned when I see people like the OP here get off on the wrong track and not listen to explanations of why they are getting off on the wrong track (the OP was not listening much to SelfAdjoint, either). By expressing my objections a bit hashly I'm hoping to "wake him up" a bit to the magnitude of the problem he's attempting to solve, and the fact that yes, other people have thought about these issues, using much more advanced tools, and that it would be a good idea to study the literature more, and be a bit humble, and make a lot of effort to understand.

This only scratches the surface of the "black hole" issue, BTW.

A much better way to think about the space-time geometry of a black hole is that space becomes so twisted that the radial direction pointing nto the black hole becomes timelike.

This immediately shows why stationary observers don't exist inside the event horizon - we are asking observers to stop time when we demand that they keep their 'r' coordinate constant.

Using this geometric approach to the problem, there are no "forces" to worry about at all. Rather than experiencing a "force", objects simply follow a geodesic path in space-time. One still does run into some unpleasant coordinate singularities at the event horizon, but these can be eliminated with the correct choice of coordinates.

This is the approach that real GR uses (real GR as opposed to the popularized version) - but it takes quite a bit of mathematical sophistication to pull it off :-(.

Last edited: Apr 16, 2006
5. Apr 17, 2006

LnGrrrR

Once a black hole is created, is there any way to 'untwist' them, theoretically?

6. Apr 17, 2006

pervect

Staff Emeritus
Well, they will evaporate on their own, if you leave them alone long enough, according to current theory.

You can, in principle, split up a black hole into two black holes,subject to the law that the sum of the squares of the masses (the total area of the event horizon) always increases. This is unattractive, but not quite a dead end, because black holes do evaporate faster when they are smaller.

Thus you could theoretically break up a black hole of mass 1 into two black holes of mass > .707, which while they have a larger total mass, still both have less mass than the original black hole, and would hence evaporate more quickly.

Finally, if one had access to bulk exotic matter with a negative energy density, one could just dump it into the black hole, decreasing it's mass. Such exotic matter probably doesn't exist, but the various energy conditions that were thought to prohibit its existence have come under attack recently. (See some of the remarks I quoted from Visser in that other thread where were talking about some of the same issues, for instance).

7. Apr 17, 2006

Giulio B.

black holes evaporates?

so this means that is not true that when something reaches the horizont gets his story ended..

in what they should evaporate? particles, photons....
with an explosion or slowly?
why they do evaporate?

8. Apr 17, 2006