Question Answer(s)
If Schwarzchild didn't realize his formula determined the radius of the event horizon, what did he think it did measure
live answered
I didn't hear the zound of black holes merging. Please can he play it again?
This was caused by a bug in the interface between the presentation software and Zoom. We will try to fix this in the recorded version which will be available later
As you fall into a black hole looking at the outside universe, shouldn't you see time "accelerating" because of time dilatation and see the whole infinite time as you pass the horizon?
Hope my verbal answer to your v good question was clear?! If not please ask me for more details...!
Does the QM process you describe by which black holes loose mass happen in reverse? i.e. they release negative energy and gain positive energy and thus gain mass? If not, why not?
live answered
If Hawking radiation can be generated from either particles or anti-particles, how come the mass of the black hole is always decreasing? Surely if a particle is captured by the BH, and an anti-particle released the mass should increase?
Both particles and anti-particle have positive energy! Anti-particles only have opposite values of charges, such as electric charge.
Is there an equation or equations that smoothly merge(s)
quantum gravity with classical gravity? live answered When two black holes collide, is there a Lagrange point
between them where the forces from each cancel each other out? What happens to the geometry of event horizons during collision?
During a black hole collision the event horizons are no longer spherical but distort and merge -- this is seen in the remarkable modern numerical calculations of BH mergers done by Franz Pretorious at Princeton
Why is it that only positive energy particles escape, with negative energy particles falling into the black hole leading to mass reduction of the black hole. Is it possible for negative energy particles to escape, with positive energy particles falling into the black hole?
I hope my verbal answer to your v good question was clear?! If not please ask me for more details in chat...!
On the event horizon telescope image - do they really "see" the event horizon, or is it the innermost stable circular orbit?
live answered I'd be interested to hear more about why the tunelling
effects are asymmetric in the sense that only positive energy particles can escape. Could John recommend a paper if there's not time to go into it now?
Yes see https://arxiv.org/abs/hep-th/9907001 by Wilcezk etal
Can we say anything about what are the microstates of a black hole? My (undergrad) understanding is that a black hole is defined purely by mass, charge and spin, so what structure can it have to give rise to microstates?
The "macro-states" of a BH are defined by mass, charge and spin -- this is the content of the classical "no-hair"
theorems (which fail in the QM case!). The fact that these macrostates are so few is one of the reasons the entropy of BHs is so large. We don't in general have a good
understanding of the details of the microstates -- only in very special cases. This is a topic of a vast amount of current research (we are making progress...!).
Are black holes actually spherically symmetric? Schwarzschild BHs are spherically symmetric, but not rotating (Kerr) BHs which are the case seen astrophysically Do you think Loop Quantum Gravity sheds any light on
quantum black holes?
Personally I think that the insights from LQG are currently much weaker than other approaches -- not just string theory, but even the old Gibbons-Hawking approach of Euclidean Quantum Gravity
how do you view the work done in labs to attemp to
recreate BH. Do you think condensed matter techniques can be useful to study BH in general?
live answered
You described the entropy of the black hole by having a qubit per 4 plank areas on the surface. Why qubits rather than a classical bit?
As I mentioned, there has been a lot of recent progress in understanding the true quantum nature of BHs, especially how pure quantum states evolve to pure quantum states. A crucial aspect of this is that for a BH created from an pure QM state, the "horizon degrees of freedom" are in fact qubits. (There are arguments that this is generally true, though the qubit density matrix might be that of a mixed state.)
Question Answer(s)
Could you explain again how a black hole can allow violation of baryon conservation?
Imagine forming a BH from a very large number, N>>1, of neutrons. This BH slowly Hawking evaporates -- first by emitting phtons and gravitons, and then more massive particles as the BH gradually gets hotter. Once the temperature gets higher than the mass of the baryons (neutrons, protons, ... and their antiparticles) they get emitted too. Crucually the Hawking emission rate of neutrons and anti-neutrons is exactly the same (this fact is connected to the BH "no-hair" theorems). Since the emission is a statistically random process it might be that by accident one gets slightly more neutrons than anti-neutrons, but this gives a final neutron number that is (at most) ~ \sqrt(N). The same is true of the emssion of protons and anti-protons. Thus the final neutron +_ proton (ie, baryon) number is O(\sqrt{N}) << N if N was initially large, so baryon number is not conserved! But as I mentioned there are many open questions about details of how this BH violation of baryon number feeds into new interaction terms!
Could Grav Wave radiation emitted by colliding BHs be a significant proportion of dark matter?
Hi James! Since gravitons are massless they can't be the dark matter as dark matter but be made of particles with strictly positive mass. But one point of the second of my papers I put in my talk was that the gravitational waves from an early epoch of colliding BHs could make a significant contribution to the so-called "dark radiation" content of the universe. This is potentially observable in near-future experiments/observations!
Is AdS/CFT correspodence a mathematical ‘accident’ or does it have physical/observable meaning and
consequences?
It is a duality of descriptions -- so anything one can do one way, you can do the other way *in principle*. But very often one description makes some properties much more
manifest, and leads to predictions for the behavour of the system (say a strongly coupled quantum condensed matter system) that are not accessible from the alternate
Dear Sir / Madam, would it be possible to discuss a research idea on Black Hole inforation paradox that I recently
published on Researcgate?
https://www.researchgate.net/publication/346630333_The _black_hole_information_paradox_in_the_Lagrangian_form alism
Yes just send me an email and I'll think about it once I have some time (not until late next week at earliest I'm afraid!).
Could a rocket that converts all its mass into directed energy escape a black hole?
Not if it is inside the event horizon! Even in the case you mention the maximum speed is limited by the speed of light, and even light (namely "pure radiation") cannot escape the interior of a BH.
Doesn't it take infinitely long for a black hole to form from the viewpoint of an external observer?
Yes this is correct for a classical BH! - and this is a source of a huge amount of confusion in the literature and in books. Corresponding to this infinite length of time for a *perfect* BH to form the entropy of classical would-be-BH is infinite. (I emphasise that to a very very good *approximation* a classical BH forms in finite time.) These two infinities are connected to each other! In the true quantum case the formation time is finite and so is the entropy. Hope this helps?!
Dear John could you point to the paper you mentioned on CM systems that could help to study some properties of Black Holes, thank you
see for instance
https://arxiv.org/abs/1911.06314
-- but there are other, possibly more "realistically realisable", suggestions too. I'm not an expert on this particular issue so I don't know how to reliably evaluate the proposals.
You've said that there are a vast number of quantum micro-states, but do we know what those states are, i.e. what it is that distinguishes them?
We have partial understanding in limited special cases (namely supersymmetric or nearly supersymmetric BHs in string theory in higher dimensions), but not in general. This is a topic of a massive amount of current research.
Why is the mass of a photon zero when it has finite energy and energy is mass?
E=mc^2 is only true for strictly non-zero mass particles at rest. The general fomula is E^2 = (c^2 p^2 + c^4 m^2). For a photon m=0 and thus one finds E=c p where p is the magnitude of the momentum of the photon.
Question Answer(s)
What processes might generate black holes of sufficiently small mass that they can evaporate in ~Gyr? Could we observe a BH evaporation in practice, e.g. via gravitational waves?
There are many early-universe (*not* current universe) processes that could produce such small BHs. For instance enhanced quantum fluctuations from inflation, or colliding bubble walls in a strongly first-order phase transition, or lots of other things! It is a very interesting question if the gravitational waves from such BHs could be observed. This was one of the subjects of the 2nd of my papers that I mentioned -- see https://arxiv.org/abs/2004.00618 This deserves much more study though!
I’m intrigued by this notion of fixing a topology and triangulating the manifold. Should we think of this as a computational technique or a more fundamental Ansatz as to how spacetime is constructed?
live answered
If the quantum universe is "very four dimensional", what is
the consequence for string theories? live answered What would be the "Hamiltonian" equivalent of the the
"Path integral" approach to the summation of manifolds? live answered If GR is treated as a QFT it is non-renormalisable. But I
thought being renormalisable was a requirement for for QFTs to be acceptable?
live answered
Many thanks John! Another Q for J M-R or Luis ... is there a fundamental reason the charges of Quarks are exactly in 1/3 multiples of charge fo electron. Consequence of GUT symmetry groups?
That is a very good and important question. Yes one reason could be that there is a GUT symmetry group. But
currently we have strong reasons to think that there are much deeper reasons for this "charge quantization" -- in fact closely related to the violation of all non-space-time global symmetries by quantum gravity effects!! I can explain latter if you'd like
why do gravitons have to be point like? live answered Why not use 3 dimensional strings / objects rather than 1
dimensional objects? What has led theorists to focus on a somewhat artificial object like a point particle or string?
live answered
What is the equivalent of an instanton (a field configuration over 4D space-time of the fields for quantum field theory) in String theory which is a quantum theory with the fields
Is the 1-2+3-4+... result independent of how you regularise
it? If not, how do we know that's the correct regularisation! live answered ANO Q if I may .... Is there any evidence from String Theory
or elsewhere that the number of particle generations should be 3? Many thanks!
live answered How do fields emerge from strings? As in the reduction of
string theory to QFT. live answered
Could the vast numbers of vacuo solutions represent the
multiverse? live answered
Surely loop quantum gravity is another possible way to
unify General Relativity and QM? live answered Will it be newly thought out physics experiments that will
help to find the single unique string theory of the future? live answered Can you say in simple terms what the approach of loop
quantum gravity is? (which I understand to be an alternative)
live answered Why do you say particles are points? In particular Quarks
have a measured size. live answered
The answer why the quarks have charge plus or minus one-third isn't really answered by the fact that they must be to give one fo proton and zero for neutron. It doesn't explain why an electron has exactly three times. Is there a more fundamental argument why the charges come out in these ratios?
live answered
Would you mind explaining a bit more what those blue rectangles are which you mentioned when you introduced string theory? Sorry I forgot what they were called!
live answered Is the string theory approach to gravity fundamentally
perturbative on a fixed background Minkowski space? Is there a “sum over topologies” approach analogous to Prof Wheater’s talk?
live answered If the lack of observed super symmetries or the 6 extra
dimensions doesn't falsify string theory, is there anything that could?
