# Experimental Black Hole Physics

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1. Feb 25, 2015

### wabbit

(I don't know if this is the best place to ask this, if not please feel free to move it elsewhere or delete it.)

A number of Quantum Gravity papers explore the evolution of Black Holes and their potential transition to White Holes, and some discuss the possibility of astronomical observation of the phenomenon. I have also seen descriptions of experiments on BH analogues with optics or acoustics. However I haven't seen descriptions of proposed experiments on actual Black Holes.

My question is, how close is experimental physics to being able to study actual Black Holes in the lab - i.e. engineer micro-BHs, presumably in large accelerators, and observe them to test such theories? Are there proposals in this area? Or is there a reason this might not be feasible in principle, or in practice for the foreseeable future?

Thanks!

Last edited: Feb 25, 2015
2. Feb 25, 2015

### Staff: Mentor

Some theories suggest gravity might become strong on small scales, lowering the Planck mass as smallest possible black hole - maybe enough to produce black holes at the LHC. Those models are quite exotic, but interesting enough to be considered in analyses (example).

There is also the more violent approach (page 9), but purely hypothetical for now.

3. Feb 25, 2015

### wabbit

Very interesting, thanks. The first paper sounds more hypothetical to me than the second, but it gives an idea of the process in the context of an accelerator.

Staying wiith standard BH, you point out something that presumably rules out accelerators as a BH factory any time soon, i.e. that the smallest possible BH would be planck mass - not sure about the numbers but this sounds like it may be just too many (15?) orders of magnitude above LHC for consideration in the next...how many years?

The proposed "violent approach" paper's method of actually using gravitational collapse of a spherical laser beam (!...) might not be that far off perhaps, considering that the goal here would not be to produce long lived BH, instead short life/low mass would be better. Not sure how the numbers work out for attempting to produce BHs just a little above planck scale and enjoy the instant fireworks...

Last edited: Feb 25, 2015
4. Feb 26, 2015

### ChrisVer

That's only true if the Planck scale (currently known to be around $10^{19}GeV$) is not that large, but -due to extra dimensions- gravity gets diluted appearing weaker than it is. If that's the case, you can drop the Planck scale by several orders of magnitude, dropping it even to some TeV. That is why there was such a fuss about LHC creating a black hole...but that is not true so far.

For the last approach I am skeptic. How would we assemble that large structure into space? leave aside the large interferometers that it suggests.

5. Feb 26, 2015

### wabbit

Right, and the ATLAS paper gives an idea about how this might be used in the LHC if it turns out to be true. Nothing wrong with that of course, just not the aspect I am most interested in: I was more aiming for "what can we expect assuming standard known physics and minimal extrapolations thereof." i.e what are the technological perspectives.

A couple more thoughts on that after going through the two papers mfb linked:

A few numbers here (corrections more than welcome) :
BH size goal: say 5-50 m_p ~ 0.1-1 mg ~ 5e16-5e17 TeV ~ 2-20 MWh ~ 10-100 GJ

Assuming these are in the correct ballpark,
- The amount of energy required is nothing special to produce or store.
- Peak power could be a hurdle : given the extremely short lifetime of such BHs, presumably the production process must happen in a very short time too.
- Getting it into the incredibly tiny volume required might be the truly tough one.

Possible technologies to extrapolate from:
- Particle accelerators : LHC is 7 TeV, we need 1e16 times more, not realistic.
- Lasers : NIF is 2 MJ, we need 5,000-50,000 times that - getting closer. Of course the confinement volume required is another story... The laser beam collapse in the "violent method" paper appears to fall in this "extrapolate NIF" category, though they work with BHs about 1e12 times as massive as ours so the volume constraint isn't necessarily the biggest one they face.
- ?

This is starting to look not completely out of reach... Another 50 years? Much more? Much much much more?

Last edited: Feb 26, 2015
6. Feb 26, 2015

### wabbit

I am now having doubts abut the laser collapse method - is it really feasible in principle ? It would seem to require focusing a laser beam to an infinetisimal fraction of a wavelength, is there a reason this should be possible ?

7. Feb 26, 2015

### wabbit

OK I suppose a spherical laser does in fact converge to a region of a size comparable to the wavelength, so the spherical collapse is a pipe dream.
To get a conclusion of sorts - back to accelerators, extrapolating some kind of Moore's law from last 80 years I get something like 1000 years, give or take an order of magnitude, before we can use them to study BHs.
Looks like there's a reason we don't see many detailed proposals : )

8. Feb 26, 2015

### Staff: Mentor

With current technology, such a collider would need a length of thousands of light years, and even with very optimistic assumptions for proposed improvements you don't get to the size of the solar system. There is no Moore's law for accelerators. You would have to disassemble one of the larger moons just to get the raw materials for the structure, not even accounting for all the rare elements the components would need (stone is not a good material for superconducting magnets).

You are limited by the wavelength of the lasers, black holes close to the Planck mass wouldn't work well. That is the main reason for the larger mass suggested in the paper.
That is probably one of the easier problems. Just send it to space in pieces and put them together similar to the ISS.

9. Feb 27, 2015

### wabbit

And this appears to kill his proposal too. Attometer-wavelength lasers? Good luck with that:)

Last edited: Feb 27, 2015
10. Feb 27, 2015

### ChrisVer

The ISS is around 450 tonnes, say 10^3 tonnes because I want to see the optimistic scenario, and it took some decades to complete... not 10^10 tonnes Also the ISS has already created a lot of trash on the outer atmosphere, this would be a mess to watch after a while if you want to keep sending matterial out there. I think that's the reason he writes that it should be built around the sun.
The order of magnitude is way beyond imagination ... sounds fun but it is still scifi.

And I don't understand what lasing mass is...

Last edited: Feb 27, 2015
11. Feb 27, 2015

### Staff: Mentor

That is a "launch stuff to space"-problem, not an assembly problem. Sure, currently this is not feasible, and if we don't find something beyond rockets you would probably get the raw materials from somewhere else (asteroids, moon, ...).

12. Feb 27, 2015

### ChrisVer

It is also an assembly problem, exactly because you can't move out all this matterial from earth. You can't assemble pieces that you can't have. (well this is a "logic" problem)

Absolutely agree. If I would have to compare it with ISS, I have a deep feeling that we will be able to reach the Planck scale in accelerators before completing such a project.
If for moving and assembling 1000 tonnes took us ~10 years, then for 10^10 tonnes it would take us around ~1 billion years.
On the other hand, the accelerators seem to improve by 1 order of magnitude per "new one", if its construction takes 50years to complete, we will be able to reach the Planck scale in something like 1000 years. I could even allow this number to reach 1,000,000 years just in case things go wrong or become even harder.

13. Feb 27, 2015

### wabbit

Regarding accelerators, beyond the practical issues, is there an issue in principle? This is unclear to me, but let's say we smash protons. The size of the proton is 1fm, so can we really hope to get colliding protons within the radius of a BH i.e. well below attometer? This is the same problem as with lasers really. Or does this size play no role here?
It does work in stars but that's with a spherical shell again, not an accelerator geometry.
Sorry I'm just confused here,..

Last edited: Feb 27, 2015
14. Feb 27, 2015

### ChrisVer

If you are able to collide protons with energy = Planck scale 10^19 GeV, then you will create a black hole. It doesn't have to do with the size (in fact any elementary particle with that energy will become a singularity) of the proton , since the things that interact at high energies are the protons' partons (quarks+gluons) which are "point-like" particles. The problem with the accelerators/colliders is that you need extremely large power to accelerate the protons to this energy.

15. Feb 27, 2015

### wabbit

Thanks -
To clarify : the quarks/gluons are only confined within the radius of the proton, so their localization is extremely spread out (compared to Planck scale), but this doesn't matter ? Or, even as point particles they have a wavefunction with some spatial extent : again, this plays no role ?

Edit : presumably their high momentum may eliminate the issue in the direction of motion, but it doesn't help in the transverse direction ? My mental picture is of two wavepackets shaped like very thin, very large pancakes colliding.

Last edited: Feb 27, 2015
16. Feb 27, 2015

### Staff: Mentor

@ChrisVer: Material launched to space increased from absolutely nothing to hundreds of tons per year in 70 years. Those extrapolations just don't work.
10 orders of magnitude? The LHC design energy is a factor of ~8 above the Tevatron energy, and is expected to reach that 30 years later (1986 -> 2016).

What you actually collide at high energy are quarks and gluons inside the protons, and they are point-like. Unlike the laser approach, you just have to collide two particles, which means every collision has a chance to produce a black hole if the energy is sufficient. This chance is very small, and reflects the relative sizes of protons and the size of a possible black hole.

No, it will not! A proton will never become a black hole just from being viewed in a different coordinate system (which is all you do if you accelerate a single proton).

17. Feb 27, 2015

### ChrisVer

I fixed the 10 to 1... I had even written 10GeV orders of magnitude at first ...

Well I never mentioned that the proton is an elementary particle, and a collider wouldn't have "one" particle, but at least two (the accelerated one and the target, or both accelerated). However what I wanted to mention was this:

Last edited: Feb 27, 2015
18. Feb 27, 2015

### Staff: Mentor

The point is not whether something is elementary or not, the point is the wrong quantity: energy in the rest frame is relevant, not the energy in an arbitrary system.

19. Feb 27, 2015

### wabbit

I must admit I am still missing something :
When our two point particles collide, they have enough energy to make a BH, so that's a start.
But then we must still show that they are localized within its Schwarzschild radius, a few Planck lengths in our case.
In a classical case, this localization is unambiguous ; but what does it mean for quantum particles ? My understanding is that it doesn't have anything to do with being pointlike, but to the spatial extent of the wavefunction or something like that.
What I fail to see is what tells us that this localization condition is in fact realized.
You state that it is enough that the two particles collide for the BH to form and I have seen this mentionned many times so I am not saying it doesn't - But I still fail to see why.

Hopefully I have made myself a little clearer so you might see where my error lies... Thanks a lot for your patience.

20. Feb 27, 2015

### Staff: Mentor

As we do not have a quantum theory of gravity, it is not possible to make very clear predictions. What has been done for the LHC is called "black disk approximation": you take a disk with the radius of the black hole as cross-section. This would roughly correspond to the classical case, and it should give the right order of magnitude for the cross-section no matter how exactly quantum gravity works.

So basically, the point where their wavefunctions overlap, you "measure" the distance (gravity performs that measurement), and if it is smaller than the Schwarzschild radius (or diameter? Not sure, but that is part of the unknown numerical prefactor) in your model you assume it gives a black hole.