can you really fall into a black hole? or, fun with time dilation.

September 27, 2012


As we discussed many times on Weird Things, black holes are the most amazing and terrifying things in the universe we know, and they’re not shy about gathering every law of physics we’re sure we understand, then laughing at them and doing something completely different. Well, not completely different per se, but the incredible heat and gravity of these objects makes time and space flow in ways they can’t anywhere else. One of these extreme phenomena is time dilation induced by gravity. We talked about the extreme effects of time dilation at relativistic speed over the last few years and mentioned that you could technically cross the entire universe in a human lifetime if you were traveling at 99% the speed of light. And the same effect applies when you’re exposed to an extremely gravitationally powerful object. Time would continue normally for you if you’re falling into a black hole, but to an outside observer, you’d be frozen in time and he won’t see you spaghettified and turned into quark-gluon soup past the event horizon.

Or at least that’s the theory which one physicist says might be wrong. According to his view, any particle falling into the black hole would never actually cross the horizon because the dilation is so extreme as to keep it falling until the black hole evaporates. Unfortunately for him, this really doesn’t sound even remotely right since that would prevent black holes from accreting mass. We know they do exactly that. If particles could never fall into a black hole, there would be absolutely no accretion and black holes would have the same mass with which they were created. It could be possible but it would make explaining hypermassive 10 billion solar mass beasts very, very difficult. You’d need a significant portion of a galaxy to implode in on itself just right, circling into itself over eons without enough gravitational nudges and tugs from the various stars and solar systems inside to maintain some semblance of equilibrium. And that just doesn’t sound right. It’s a lot more straightforward to assume that supermassive black holes are born maybe a thousand or so orders of magnitude smaller and work their way up through galactic collisions, gaining most of their mass during massive cataclysms rather than steady feedings.

The root of the problem with this paper lies with its author seemingly forgetting that dilation has an observable effect from the outside while time for the object in question continues as if nothing happened. Were the test particle in the paper see another particle going at the speed of light right next to it, it wouldn’t keep pace with it; the other particle would seem as if it was flying away from its point of reference at the speed of light. He achieves his result by removing a metric he doesn’t seem to have any grounds to remove, and while describing how a black hole accretes a good amount of matter, then evaporates over time due to Hawking radiation, he says that a test particle will just fall until the black hole unraveled into nothingness. These flows of events seem to contradict each other, unless I’m missing something crucial, and since the paper describes opposite outcomes to the same process, methinks it’s staying put on arXiv. The whole point of a black hole’s event horizon is that it must eventually be crossed and nothing can escape it, and once something crosses the event horizon, it’s effectively inside the black hole. If your paper doesn’t get the definition of this critical juncture right, it’s pretty much bound to be flawed.

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  • Robert

    Some things may be right and some wrong. I have already pointed out to the author of the paper that Hawking radiation would also be impossible as one of the matter/antimatter pairs could not fall into the black hole. A black hole could still have all the mass just about to but not quite falling into the event horizon from our reference frame and accumulate or accrete mass. The mass would not need to be past the event horizon and arguably couldn’t be. But travel with the mass and it is probably a different story, the singularity is inevitable. The paper could be rewritten with different conclusions but is nevertheless a promising contribution.