LHC Black Holes

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Principles aren't deduced from observations. They are devised to conform to observations. Every now and then somebody devises a better set of principl es which explain a bigger set of observations, or fit better to the observa tions we've accumulated. It's a process of induction rather than deduction.

The math embodies the principles. If you don't understand the math - at som e level - you don't understand the principles. George Gamow's "Mr. Tompkins " books made some of the math accessible.

No. But it makes sense to devise laws that might be.

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Bill Sloman, Sydney

I don't see that as much evidence. If *all* the small black holes were swallowed up by larger black holes, why would any other matter have escaped the same fate? Why wouldn't the entire universe be inside one black hole?

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Rick C

Sorry, I used the wrong term. I meant to refer to inductive reasoning. As you say principles are designed to suit data that is collected. The math is fitted to the principle and used to make predictions. It is not an explanation of the principle.

That is nonsense.

Laws have to fit the data, fitting the previously designed laws is optional.

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Rick C

As I have said elsewhere, math does not "explain" things.

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Rick C

Yeah me too... mostly I delete that stuff. I figure if you want to understand something, go study it, and then ask questions. If you're only kinda interested...

Yeah I think that's my understanding, we only know about the surface, from far away.

Hmm well there are lotsa non-equilibrium processes where thermo (stat. mech.) doesn't apply. But in the end I think it always has to work.... fundamental conservation laws. Energy and Entropy. That's why I said the mechanism is not all that important* I figure an energy difference leads to a transfer of energy, eventually. (I guess there are "frozen" glass states.)

George H.

*mechanism: how do virtual particles couple to the BH. I have no f'ing idea, my guess is some gravitation thing. unless the BH is charged or spinning or has an excess of some other "thing".

In general, previously designed laws do fit existing data pretty well. The unexplained component of the orbital precession of Mercury wasn't large

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Bill Sloman, Sydney

Oh... Did you get the part about where all the entropy of the universe is? All the BH's are second, I think he said cosmic background radiation is third, and then comes the rest of us. :^)

George H.

Ask a theoretical physicist. Meanwhile, in the two black hole mergers that LIGO has detected, an appreciable proportion of the mass of two black holes was radiated away in the gravitational wave. Normal matter doesn't seem to be able to get rid of anything like as much energy as a gravitational wave .

Our galaxy currently seems to contain a few "hyper-velocity stars" which ar e moving fast enough to eventually escape our galaxy.

The current explanation is that they are halves of binary stars that got cl ose to our galaxy's central black hole - the process that broke up the bina ry gave half the binary enough energy to completely escape the central blac k hole.

This isn't compatible with all the universe ending up in a single black hol e (or at least not all that soon).

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Bill Sloman, Sydney

No. It's the way you express and manipulate useful explanations. If you can't follow the math, you don't understand the explanation.

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Bill Sloman, Sydney

It depends how you define "to explain". Taking it very narrowly physics does not explain anything, too. See the Feynman video about the "why" question.

Physics makes observations and builds a model (mostly a mathematical one) to _describe_ how somethings happens. At least if the model is good enough.

Our current model says energy is conserved, at least locally. So when energy is conserved and we get something out (one particle of the virtual pair) of lonely BH it must come from somewhere. If there is nothing else around it must come from the BH.

What happens with Hawking radiation is not very different from pair creation in strong EM fields. I remember being at GSI (Heavy Ion Research in Darmstadt, Germany). They sent heavy nuclei onto targets of other heavy nuclei (even Uranium onto Uranium). When these nuclei are very close you have a really strong electric field. In there real e+/e- pairs are created and detected. They take their energy from the electric field. This changes the interaction between the two nuclei, because of this lost energy.

Similarly the virtual pairs near a BH become real particles by taking the energy from the gravitational field. BTW the captured particle of the pair now also has potential energy in the gravitational field which gets release when it falls into the BH.

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Reinhardt

Yep, curved spacetime implies an energy density, just as within any region of spacetime you have an EM field there is an energy density that has its flux described by a Poynting vector.

The reason spacetime is curving at all is because matter -> an enormous amount of energy. In some regions (i.e. close to the even horizon of a black hole) that energy density is enough, and the curvature great enough, such that it can cause one virtual particle of the pair to be sucked down the well, and the other blasted out in to free space so quickly that the uncertainty principle can't do a thing about it.

Hawking radiation has not been observed in a real black hole, and it is unlikely ever to be observed directly - the black holes we can detect are big, and the Hawking radiation will be totally swamped by radiation from the accretion disk.

But we can make "sonic black holes" as a simulation, using sound waves instead of light waves. And there has just been an experiment published that measured Hawking radiation from such sonic blackholes:

It is not proof of real black hole radiation, but it is suggestive evidence.

Unfortunately, general relativity is not amenable to nice mechanical explanations. And quantum effects are even less amenable to such explanations. And with black holes, you've got the most extreme situations for the relativity effects /and/ the most extreme quantum effects. You are never going to get a nice, clear, intuitive "bouncing ball bearings" picture here.

I too like a nice mechanical explanation - but here you are going to have to accept weird things if you want to understand. (Not that I am claiming to understand this stuff.)

You are, I think, imagining a black hole as a sphere bounded by its event horizon. It is better to think of it as a gravitational well without a specific size (but with decreasing gravitational force as you move further away), or alternatively as a dimensionless point or singularity. The event horizon is not a physical barrier or boundary - it is just the point at which the escape velocity is the speed of light.

So when one virtual particle escapes, it is escaping the black hole's gravitational well - it is not escaping the event horizon, because it was never inside it. And the other one - the one that falls into the event horizon - is not a new particle being added to the black hole, because it has always existed within the gravitational well.

There is no problem with light or anything else moving from the inside of the event horizon to the outside - but without additional forces acting on the object (or photon), the black hole's gravity will eventually pull it back. This is just the same as being able to throw a ball in the air here on earth - it is perfectly possible, and you can throw it as high as you like. But it will fall back down again unless it is captured by something else (such as hitting a satellite), or you throw it faster than the escape velocity for the surface of the earth.

A spaceship falling into a black hole will not notice anything special as it passes the event horizon, and it can still escape - provided it has powerful enough thrusters or makes it into the influence of another gravity field (such as of an orbiting star).

You can think of the particle anti-particle pair as being a single particle that moves forward for a bit in time, then moves backwards (this being equivalent to the anti-particle moving forwards in time). So it's just a little loop in space-time - nothing is created or destroyed.

Whether that image makes it easier or harder to understand is another question!

Hmm I thought virtual particles were supposed to be created and destroyed continously (rather than "always having been there").

That doesn't sound right either. I have always thought (read) that it is in fact impossible to escape no matter what. The light cones all point inwards or some such.

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John Devereux

I should perhaps have written that the particle that falls into the event horizon was created inside the black hole's gravitational well, from the gravitational well (from the energy it has from its mass) - thus it did not come from "outside" and move "inside" the black hole. It's energy/mass has always been part of the black hole and its gravitational well.

(It is really hard to make clear sense when discussing this sort of thing - I hope that helps a little. And I am merely an interested layman with this stuff - I apologise if I've got things wrong.)

It depends on the observer.

An outside observer viewing the spaceship will see time slowing down, and the ship appears to get smeared out on the event horizon rather than passing through it. But to an observer on the spaceship, there is nothing special about the event horizon. The passage of time is completely different to two observers. So the spaceship can turn around and fly out again, but the outside observer cannot see this happening as it has never seen the ship falling past the event horizon in the first place.

Quantum mechanics doesn't have a monopoly on weird and counter-intuitive!

a computer simulation of how things would "appear" to various observers, would go a long way help with understanding this stuff, more so than some equations.

M

That's true - but it can be pretty difficult to do. A typical "simulated video" only has one time stream - it is very hard to visualise time from different observers in a film.

But perhaps someone much smarter than me has done this already - I haven't searched for such videos.