Natural Order and Science

[Alex K]

Why can't black holes be worm holes?

That's a very subtle question. In theory, if you assume a black hole which has existed forever and take a close look at the maths, it will contain a wormhole. Especially when it is electrically charged or rotating, the singularity in the middle will in theory be spread out and turn into a horizon leading to a parallel universe.

Black holes which form e.g. from star collapse will however not form the region inside it which would lead through a wormhole.

One can however simulate what falling into a rotating black hole would look like:

http://jila.colorado.edu/~ajsh/insidebh/rn.html

In this vid you fall into a charged black hole and then through various horizons marked by different colors in the small map, and the flashes are when you see your old or the new universe at superhigh blueshift (which would in reality probably vaporize you), and in the end I think we come out at the new universe, but looking back at the hole. The gridlines are just to illustrate the distorted geometry of the event horizon.

https://vimeo.com/8724840

That this looks so crazy scifi is even cooler considering that this is a mathematically accurate simulation.

This is what the mathematical structure of such an eternal electrically charged black hole that we just fell into looks like: The wavy line is i think the path you would follow if you jumped in, e.g. stating in the patch called "Universe".

[ignoramus]

What a crazy wonderfully complex universe we live in!

I was really asking because I left my wallet in the delta quadrant and was wondering if there's a quick way to retrieve it? Had at least 35 bucks in it!

[I_am_not_mafia]

Alex, I have a few questions about blackholes and physics that I have been pondering and would be grateful if someone who actually knows about this stuff could help explain to a lay person like me.

If a blackhole does not contain an entry to another universe and isn't a wormhole, but is a closed system, then what happens to all the light that falls inside? Is there a limit to how many photons can fit into a finite part of spacetime?

If you combine two neutron stars then you can form a blackhole because the gravity increases to the point whereby light cannot escape. How do we know that the singularity isn't actually just a really big neutron star? Or even formed from something even denser than a neutron star, say like a quark star?

Continuing on from this, I understand with normal stars that there is gravitational pressure pulling stuff together and outward pressure maintaining a certain size. Nothing can go faster than the speed of light, but can there be enough outward pressure equal to the gravitational pull strong enough to stop light escaping?

Why is it that information can not be destroyed in quantum mechanics? Is this because information = energy?

How much wood would a woodchuck chuck if the woodchuck chucked wood at near light speed?

Thanks.

[Alex K]

Alex, I have a few questions about blackholes and physics that I have been pondering and would be grateful if someone who actually knows about this stuff could help explain to a lay person like me.

If a blackhole does not contain an entry to another universe and isn't a wormhole, but is a closed system, then what happens to all the light that falls inside? Is there a limit to how many photons can fit into a finite part of spacetime?

You have - perhaps unwittingly - raised one of the central problems of current quantum gravity research, the "black hole information problem".

Black holes grow in size when you throw something in. It has been shown by Bekenstein and others that, as seen from the outside, the amount of stuff (actually: information) that can be contained in a volume of space is proportional to the surface of the volume before a black hole is formed. So from this perspective, there is nothing too strange going on, more stuff needs more volume. Even disproportionately more volume.

However, if you would jump right in into the black hole, you would disappear in something like a singularity in finite time, so an arbitrary amount of stuff appears to get concentrated there. What happens at that would-be-singularity is an unsolved question as long as we don't have a firm grasp on what the correct quantum theory of gravity looks like.

Normally, one would say "forget about the singularity, it's separate from the universe and has no effect", but Nature doesn't allow us to entirely forget about it: what is usually called the information paradox arises because black holes don't seem to live forever, but evaporate by radiating random noise called Hawking radiation. Once the black hole is entirely evaporated, there is no "inside" region of space anymore that is cut off causally, the singularity has also dissolved, and any information about what was thrown in seems to get entirely eradicated ---- UNLESS the Hawking radiation is not fully random but subtly carries information -- people are currently still wondering how this information transfer could possibly work, and Hawking and collaborators have just recently stirred the pot a bit with their "Firewall" hypothesis which I don't understand.

If you combine two neutron stars then you can form a blackhole because the gravity increases to the point whereby light cannot escape. How do we know that the singularity isn't actually just a really big neutron star? Or even formed from something even denser than a neutron star, say like a quark star?

I'm not sure I get your question. My answer from the hip would be "because it's not big enough to house a neutron star". There are limits on how small e.g. a neutron star can become before gravity overwhelms the degeneracy pressure exerted by the neutrons.

Continuing on from this, I understand with normal stars that there is gravitational pressure pulling stuff together and outward pressure maintaining a certain size. Nothing can go faster than the speed of light, but can there be enough outward pressure equal to the gravitational pull strong enough to stop light escaping?

In a normal star, the gravitational pressure is not yet as large as it will be later in the collapse. Since gravitational pull increases at shorter distances, the more the star collapses, the stronger the gravitational pull becomes like in a vicious circle.

It goes as follows - the radiation keeps the star stable, when that goes out, it collapses until the electrons say "hey, we're fermions, we like to have our separate spots. If you want us to move closer, give us energy or piss off". This at first works and the electron degeneracy pressure brakes the core collapse and you get a white dwarf. If the star is heavier the collapse continues until the neutrons say "hey we're fermions, same as above!", which stabilizes the core and it becomes a neutron star. If the star is even heavier, however, the gravity will at this point be so strong that it releases enough energy to squish the neutrons together - at this point the collapse goes on unhindered until there is a black hole. Whether you have a quark star phase in between doesn't change the principle.

Why is it that information can not be destroyed in quantum mechanics? Is this because information = energy?

Information is measured by entropy, which is a quantity which is related to energy, but not quite the same. Entropy is related
to the amount of energy transferred and the temperature.

Quantum mechanics posits that that the state of a system evolves as dictated by the Schrödinger Equation, and the SE has certain mathematical properties which contradict that. This is a bit technical - so if you start with a state in which you know the wave function (maybe because you prepared it), the SE equation allows you to tell the evolution of the wave function. Hawking radiation is a random thermal thing which cannot be described in this way, so quantum mechanics seems to break.

How much wood would a woodchuck chuck if the woodchuck chucked wood at near light speed?

[I_am_not_mafia]

Thanks Alex, that clarified quite a bit for me. Especially the inverse square law of gravity increasing in strength, I should have remembered that. One question though, if nothing can go faster than the speed of light and not even light can escape from a black hole, then how can black holes evaporate by radiating random noise called Hawking radiation? Why do physicists think that this happens?

[Alex K]

Thanks Alex, that clarified quite a bit for me. Especially the inverse square law of gravity increasing in strength, I should have remembered that. One question though, if nothing can go faster than the speed of light and not even light can escape from a black hole, then how can they evaporate by radiating random noise called Hawking radiation? Why do physicists think that this happens?

p.s. with Einstein, it is only approximately an inverse square, because gravity itself begets gravity in Einstein theory, which is not a thing in Newton theory. This means that once gravity gets strong enough, it boosts itself until it becomes more severe more quickly than a simple inverse square would. Hence you get extreme phenomena such as black holes.

The derivation that is most illustrative is as follows: near the event horizon, quantum field theory makes it so that pairs of virtual particles appear. Usually, if nothing is nearby, they would simply disappear again and nothing happens. If they appear near the horizon, it can happen that one of them falls behind the horizon, whereas the other one can leave. What we see as hawking radiation is then the one that got lucky

One might now wonder why something falling in can let a black hole decrease. Now, virtual particles are not bound by E = m c^2, and if a pair appears ex nihilo, one of them will in fact carry negative energy to balance out the positive energy of the other. Only events where the negative-energy one falls in will actually happen. We did the calculation in my relativity lecture, but that was 11 years ago and I forgot how the details went

[Alex K]

What a crazy wonderfully complex universe we live in!

I was really asking because I left my wallet in the delta quadrant and was wondering if there's a quick way to retrieve it? Had at least 35 bucks in it!

Better forget your wallet, your money was probably already assimilated to buy some Borg booze.

[I_am_not_mafia]

p.s. with Einstein, it is only approximately an inverse square, because gravity itself begets gravity in Einstein theory, which is not a thing in Newton theory. This means that once gravity gets strong enough, it boosts itself until it becomes more severe more quickly than a simple inverse  square would. Hence you get extreme phenomena such as black holes.

Bloody hell I didn't know that at all. Hence the reason I suppose that calculations of black holes require the use of infinity. Does anyone have any ideas as to why this is a feature of gravity?

The derivation that is most illustrative is as follows: near the event horizon, quantum field theory makes it so that pairs of virtual particles appear. Usually, if nothing is nearby, they would simply disappear again and nothing happens. If they appear near the horizon, it can happen that one of them falls behind the horizon, whereas the other one can leave. What we see as hawking radiation is then the one that got lucky

One might now wonder why something falling in can let a black hole decrease. Now, virtual particles are not bound by E = m c^2, and if a pair appears ex nihilo, one of them will in fact carry negative energy to balance out the positive energy of the other. Only events where the negative-energy one falls in will actually happen. We did the calculation in my relativity lecture, but that was 11 years ago and I forgot how the details went

Can I just say that Physics, or rather than universe, is fucked up.

[robvalue]

That all sounds fine, but I'll wait until Harris gets back to confirm it.

[Alex K]

Bloody hell I didn't know that at all. Hence the reason I suppose that calculations of black holes require the use of infinity. Does anyone have any ideas as to why this is a feature of gravity?

"Why" questions are always a difficult concept in physics. The mathematical reason is that (for example as opposed to Maxwell's equations) Einstein's field equations are not linear, i.e. twice the energy does not simply give you twice the effect. If we wanted to know why they have this structure, we'd need a deeper principle from which they are derived that we can point to, otherwise, physics and nature just are what they are.

In the case of gravity, one can start from a graviton field with spin-2 and try to make it linear such that like newton's gravity, twice the mass simply makes twice the gravity. People, notably Deser, have shown that in order to have the resulting theory fulfil certain conditions such as the local conservation of Energy, one is forces to include all the nonlinear parts as they are present in Einstein's full field equations.

Of course, one can math all one wants, in the end nature decides what's correct and what is not, and it looks like the weird nonlinear Einstein equations are correct. The gravitational wave signal from LIGO tells us once more that this complex structure with gravity making gravity is really there in nature, otherwise such a collapse would not look like this.