Black holes
Published by: Wrathnar the Unreasonable on 23rd Jan 2012 |
View all blogs by Wrathnar the Unreasonable
Read a annoying article today about black holes. They came out with
the usual twaddle about a star collapsing to a 'infinite density at
an infinitely small point'. Allow me to deconstruct that with my
bare teeth.
A 'point' is just a location. It can't be a object, cos it's dimensionless - it has no length, width or height, so there's no space there for anything to exist. It's no more than a set of coordinates.
As for being 'infinitely small', nothing can be infinitely small. Spacetime is grainy - that is to say, there is a minimum size that anything can be. Just as energy can only be emitted in discrete quanta (ie, you can't have half a photon), spacetime comes in building blocks of a definite minimum size - the Planck length. A single grain of spacetime is 1.616 x 10 to the power of minus 35 metres across (quite small, but not infinitely so).
This means that a body undergoing gravitational collapse to form a black hole can only shrink to a sphere one Planck length in diameter. It can't shrink to a smaller size, cos there isn't one. It could only occupy a single grain of spacetime, which can't be any smaller. If it vanished to a dimensionless point, it would cease to exist, and couldn't therefore have a gravitational field.
The article also said that spacetime becomes 'infinitely curved' around the singularity. What is that even supposed to mean? Spacetime could only wrap itself around the collapsar's ultimate grain, which has a small but not infinitesimal size.
They also said that 'nothing can escape from a black hole'. Well, I know of at least two things that can: quantum entanglement (a consequence of Wolfgang Pauli's exclusion principle) and gravity.
There's a hypothetical scenario where a astronaut has fallen into a black hole. As long as it's a large one (eg, the black hole at the centre of the Milky Way galaxy), he should be able to survive for a good long while before being torn apart by tidal forces (poor sod). But they say "Unfortunately, he can't describe what he sees inside the black hole to anyone outside, cos it's impossible to transmit any information across the event horizon." Well OK, you can't use quantum entanglement to transmit info (the fabled SciFi QED - Quantum Entanglement Device, which would supposedly make instantaneous communication possible over distances which would otherwise involve a time-lag due to the limitation of the speed of light for conventional communication methods such as radio) cos quantum entanglement is 'non-local', ie independent of spacetime. Any QED info would arrive at all points in space and time - imagine a page of Morse code, where every single dot and dash was printed all over the entire page: it would be a mess, and there would be no way to extract any information from it.
There is another possibility, though, using gravity waves.
Classical physics defines gravity as a apparent force caused by the curvature of space. Quantum mechanics disagrees, and says gravity is a actual force mediated by virtual particles called gravitons. But both sides of physics agree that there's something called 'gravity waves'. Quantum mechanics says all particles can be treated as waves, and classical physics says moving distortions of space (waves) can be generated by agitation of massive objects. Gravity waves could escape from a black hole, cos the hole's gravitational field won't act on them. So the hypothetical astronaut could use a gravity wave generator to create a coded signal which could be picked up from outside the event horizon - so much for the Cosmic Censorship hypothesis (up yours Roger Penrose!).
Well, got that out of my system, feel much better now!
A 'point' is just a location. It can't be a object, cos it's dimensionless - it has no length, width or height, so there's no space there for anything to exist. It's no more than a set of coordinates.
As for being 'infinitely small', nothing can be infinitely small. Spacetime is grainy - that is to say, there is a minimum size that anything can be. Just as energy can only be emitted in discrete quanta (ie, you can't have half a photon), spacetime comes in building blocks of a definite minimum size - the Planck length. A single grain of spacetime is 1.616 x 10 to the power of minus 35 metres across (quite small, but not infinitely so).
This means that a body undergoing gravitational collapse to form a black hole can only shrink to a sphere one Planck length in diameter. It can't shrink to a smaller size, cos there isn't one. It could only occupy a single grain of spacetime, which can't be any smaller. If it vanished to a dimensionless point, it would cease to exist, and couldn't therefore have a gravitational field.
The article also said that spacetime becomes 'infinitely curved' around the singularity. What is that even supposed to mean? Spacetime could only wrap itself around the collapsar's ultimate grain, which has a small but not infinitesimal size.
They also said that 'nothing can escape from a black hole'. Well, I know of at least two things that can: quantum entanglement (a consequence of Wolfgang Pauli's exclusion principle) and gravity.
There's a hypothetical scenario where a astronaut has fallen into a black hole. As long as it's a large one (eg, the black hole at the centre of the Milky Way galaxy), he should be able to survive for a good long while before being torn apart by tidal forces (poor sod). But they say "Unfortunately, he can't describe what he sees inside the black hole to anyone outside, cos it's impossible to transmit any information across the event horizon." Well OK, you can't use quantum entanglement to transmit info (the fabled SciFi QED - Quantum Entanglement Device, which would supposedly make instantaneous communication possible over distances which would otherwise involve a time-lag due to the limitation of the speed of light for conventional communication methods such as radio) cos quantum entanglement is 'non-local', ie independent of spacetime. Any QED info would arrive at all points in space and time - imagine a page of Morse code, where every single dot and dash was printed all over the entire page: it would be a mess, and there would be no way to extract any information from it.
There is another possibility, though, using gravity waves.
Classical physics defines gravity as a apparent force caused by the curvature of space. Quantum mechanics disagrees, and says gravity is a actual force mediated by virtual particles called gravitons. But both sides of physics agree that there's something called 'gravity waves'. Quantum mechanics says all particles can be treated as waves, and classical physics says moving distortions of space (waves) can be generated by agitation of massive objects. Gravity waves could escape from a black hole, cos the hole's gravitational field won't act on them. So the hypothetical astronaut could use a gravity wave generator to create a coded signal which could be picked up from outside the event horizon - so much for the Cosmic Censorship hypothesis (up yours Roger Penrose!).
Well, got that out of my system, feel much better now!
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26 Comments
As an aside I often get worked up when people say they put a slide rule over something to measure it. A slide rule is a calculator, not a measuring device. Arrrggghh.
Minxie, infinity is dimensionless. So minus infinity is still infinity, half infinity is still infinity and for that matter twice infinity is .... you get the picture.
As to the gravity wave communication method. Again, gravity waves can escape black holes. Kind of by definition that one, the hole can't pull in more mass if the gravity waves aren't escaping, so it must be true even if no-one has been up close and put a slide rule on one. I am a measurements bloke myself but this can be taken as read. However, in order to use them to carry data they would have to be modulated. In order to do that there would have to be a gravity wave generator (we might call that a planet or something) and amplifier (otherwise a bit of technology yet to be postulated).I assume he wobbles the planet sized thingy in time and space to add his data to the gravity carrier wave.
On the other hand the increased mass of his gravity wave generator will cause increased acceleration towards the black hole reducing his available time for living. Maybe.
I have missed these blogs of yours, don't stop.
I would have thought that if something had collapsed in on itself and was pulling all sorts of other stuff into the void where it once was, then the astronaut would be squished immediately and not torn apart after a 'good long while'.
I have 2 questions; how long is a good long while and how much are they paying the poor astronaut?
Actually, talking about attractive bodies, perhaps.... oook, slap, OK. I'll behave.
Minxie: you may be thinking of a neutron star.
Alan: If I'm right about G-waves, I should get a nobel prize or something. I posted about it on a physics questions site, but they couldn't answer. they did congratualte me for a good question, tho, and left a open invite for someone to answer. That was over a year ago, no answer yet!
Geri: you're right, the accretion disc could scald the astronaut quite badly on the way in - gets hot enuf to emit Xrays!
But the technicalities of it aren't really the important point - after all, we don't currently have the technology to travel to a suitable black hole anyway. The point of this 'thought experiment' is to decide whether it is theoretically possible for information to escape a black hole. The current thinking is that it isn't (see Stephen Hawking's 'A brief history of time'), but I feel i may have come up with a theoretically possible method. Even the simplest little bit of information being transmitted back across the event horizon would be enough to cause a major rethink in cosmology!
Wrathnar, are you confident in your assertion that "Nothing can be infinitely small"?
Nothing - as in "no thing" - is as non-existent as it's possible to be in any dimension and is therefore an infinite in its own right. 'Nothing' is even less than a 'Point', which, by your own definition is a co-ordinate and therefore 'Something'.
Everything under the sun is classifiable; even the unclassifiable may be classed as such.
But as you say ... this is a thought experiment; based entirely on Stephen Hawking's Black Hole Theory which - along with the Big Bang - remains just that ... a theory. You may go round in circles to your heart's content but sooner or later - if you want to know the truth - you will have to consider the possibility of many dimensions science has not yet begun to consider. Strings don't come into it.
If a gravity creating mass is whacked in an interstellar fashion then it will cause a pertubation in its gravity field, which is otherwise a constant force. It's nothing like this but think of it as a bit Dopplerish. Tony's suggestion of the astronaut hopping around is interesting. I don't think it will create a wave as such, rather it will simply move the centre of mass around. However, shifting the centre of mass could be sensed by a directional receiver and could therefore carry a binary data stream.
So there remain two questions, both of which have possible solutions. The first is developing a receiver. Sensing gravitational shifts is probably possible to detect via crystal structures. A crystal lattice structure would be perturbed by any gravity shift and by 3D monitoring it should be possible to achieve directional sensing. This leaves getting meaningful data out in the time remaining to the astronaut before he is crushed. Recent advances in data compression mean that we are able to convey ever increasing amounts of information on less and less data. Those of you with iPods are used to choosing the compression factor on your tunes, for example. If this trend continues in a relatively linear fashion then by the time we are advanced enough to reach a black hole I expect that complex messages along the lines of "It's dark in here" or even "Oh shit, I'm in trouble" could be done in a couple of bytes or less.
I like your musing, Alan. It seems doable.
Any of you physicists heard any more on the Hadron Collider anomaly - the particle that appeared to travel faster than the speed of light?
Ama: Since the particle was only travelling a tiny fraction faster than light, there's still a fairly strong possibility of experimental error. They are presumably trying to recreate the results, and since we haven't heard anyhting, it seems likely they haven't succeeded.
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