# Black holes & time travel

1. Jul 13, 2010

### @$@D hey people..!! I need to ask that whether time travel is possible ?.If speed of light is achieved..however besides this i also think that if speed of light is achieved nothing could stay at that speed virtually....what do you people think..?? Secondly, i was thinking that how do we know that BLACK HOLES are in a Circular Form.. i mean that it can be a shape less or even posses some other shape.... Also why cant we see a black hole as because the might emit some kind of radiations from them which can be used to plot an image or computer image of it.. 2. Jul 13, 2010 ### zhermes Backwards time-travel is not possible. Traveling the speed of light is not possible. The event horizon for a (non-spinning) black-hole is spherical because it is spherically symmetric, what determines the shape is only the distance away from it--which is constant. Black holes are defined as the things that (themselves) can't emit radiation. They do emit radiation from their horizons, its too weak to detect. 3. Jul 13, 2010 ### Chronos Black holes emit hawking radiation, but, it is much cooler than the cmb. Sort of like trying to detect a star adjacent to the sun. Time travel is possible, but, only in one direction - the future. A black hole is defined by its event horizon. Only a schwardschild [non-spinning] black hole has a perfectly spherical event horizon. Kerr [spinning] black holes are believed to have a bulge in their event horizon [the poles are flattened]. It is highly probable the vast majority of, if not all, blacks holes spin. 4. Jul 14, 2010 ### Nabeshin It's worth noting that you can have black holes which do not correspond to a simple solution of a kerr-schild spinning hole. For example, immediately after two black holes merge the common hole is quite elongated and different. In such situations (really, in situations where there is any deviation from a stable state solution such as schwarzschild or more generally kerr-schild), the black hole will "oscillate", dominantly in a quadrupolar moment, and radiate away gravitational waves until it relaxes to a stationary solution. Basically, anything that's not spherically symmetric will soon become spherically symmetric. 5. Jul 14, 2010 ### zhermes What of this did I not already say? And the poles are not flatted, the schwarZSchild radius is the MINIMUM distance. For a spinning black hole, the event horizon is EXPANDED in the plane of rotation. Once on the subject (though fairly irrelevant to the initial question), an interesting note is that the side of the black-hole rotating towards an observer will appear larger than the other side. Check and mate. 6. Jul 14, 2010 ### @$@D

Thats what i am asking that how do you know that it is spherical and symmetrical.when they donot emit any radiations and radiations from its horizon is very hard to detect ..then what has lead us to conclusion that black holes are spherical ...also If black holes are attracting things towards it. why isn't the milky way center black hole is attracting glaxcy planets...
secondly what about the Particle Acclerator .wont it give any help in time travelling.....

7. Jul 14, 2010

### zhermes

I suppose we have no observations to confirm that BH's are spherical, but a strong body of theory. Okay, here's a way to think about it:

A small asteroid is non-spherical because there isn't enough gravity to pull-it / mash-it into a sphere. Larger and larger asteroids, comets, moons become more and more spherical because their gravity gets stronger and stronger, and there are no forces which would lead to creating (significant) asymmetries.
Once you get to a star its VERY spherical, because thats the lowest energy configuration in a gravitational field. Once you're too massive to support yourself as a star, there is nothing preventing your collapse---and a spherically symmetric (for the most part) collapse into a black-hole. We know of no force to prevent the matter in a BH from shrinking to an infinitesimal point--so there's definitely (to our knowledge) nothing that would make it a-spherical.

The closer you get to a point, the more spherically symmetric you'll be. Black-holes are as close as you can get.

8. Jul 14, 2010

### zhermes

The BH at the center of the galaxy IS attracting everything towards it, thats part of why the galaxy remains bound together, and rotating nicely in a spiral--just like the planets around the sun.

Particle accelerators have nothing to do with time travel (before someone else chimes in: sure, the particles being accelerated move more slowly in time from an outside perspective, but still). You can't travel backwards in time.

9. Jul 14, 2010

### Chronos

Bear in mind we are talking about the event horizon, not the infinitesimal, singularity thing inside the event horizon. The event horizon is the only aspect of a black hole that has a meaningful geometry.

10. Jul 14, 2010

### arrow564

Hi all,
This is just some food for thought that I have been contemplating for a while. We live in a universe with three dimensions of space and one dimension of time. Matter can only exist in three dimensions to our knowledge (and without getting into the micro) length, height and depth. When matter enters a black hole this rule, and physics, brakes. The other wise three dimensional matter becomes one dimensional, by this who is to say that the rolls are not simply switched? If matter is made into one dimension, then time might be made into three dimensions. Forwards, Backwards and alternate time lines.

When you take a multidimensional universe and force it to become one dimensional, or a singularity, just about every rule is broken. Why cant the same apply to time?

11. Jul 15, 2010

### qraal

Breaks. And, no, the 'rule' doesn't break. The singularity is where 'the rules' supposedly break down, but for that reason many physicists think singularities are excluded by physical processes we have yet to discover - that's the motivator for research into things like Quantum Gravity.

'Spaghettification' doesn't mean the dimensions disappear, but that matter gets squeezed by intense tidal forces caused by space-time curvature very near the (supposed) singularity.

No to the first bit, but there are theories which involve extra time dimensions, so who knows? Produce a physically observable consequence of that idea and you might unify GR & QM, as well as win a Nobel Prize.

Again, doesn't happen as far as we currently know. And if it does, it happens behind an Event Horizon so that we can't see what happens...