A week ago, six panelists plus host Jim Holt (of the New Yorker) debated cosmology during the World Science Festival in New York:
The panelists included the fathers of inflation Alan Guth and Andrei Linde, philosophers Barry Loewer (Rutgers) and his occasional collaborator David Albert (Columbia), cosmo-philosopher George Ellis (South Africa), and – as Melvin overlooked – UC Davis string cosmologist Veronika Hubeny. She's Czech American.
She was telling me these things in Colorado in 1999 so I might have forgotten but I think that she was still a kid when her dad Dr Hubený, an astronomer, emigrated to the U.S. with his family, but it's fun to hear that behind her refined English, you may still safely discover the pure Czech accent which is somewhat close to the Russian or German or Hungarian one but still different. I guess that they had to speak Czech at home. ("Hubený" means "skinny" and Czech daughters and wives modify the surname to the feminime adjective, "Hubená", which Veronika didn't do in order to agree with the American myth that the surnames of all members of a family should be exactly the same.)
Well, Veronika made a lot of sense. I haven't quite listened to the whole video but I have heard a significant portion and I would probably subscribe to every word of hers. Well, the moderator Jim Holt was an interventionist of a sort – he isn't quite capable of keeping his mouth shut – and Veronika seemed like the primary victim of it.
So Hello Giggles, The Mary Sue, ABC10, Teen Vogue, and Mic.com agreed: Veronika has been misplained things!
The top moment appears around 1:05:30 in the video when a woman from the second row screams very comprehensibly: "Let. Her. Speak. Please". Everyone is entertained.
Well, the weird verb "misplained" really means "[a female was] explained [something] by a male" which the feminists consider politically incorrect, probably because only females should have the right to explain something, and women should never be forced to listen to a man's explanation. OK, I don't agree with any idea that a feminist is proud about. But if we adopt the definition of "masplaining" as "explaining by a man", Dr Hubeny was surely being mansplained something.
The real problem which I see is that a more ignorant and stupider person (Mr Holt) was trying to explain something to a more informed and more intelligent one (Dr Hubeny). You know, she was ultimately allowed a lot of space for a rather detailed monologue about the nature of holography in quantum gravity. Holt protested: Holography must be wrong because shadows of the objects A,B,C only contain some "truncated" or "simplified" information about the actual objects A,B,C. So it cannot be a one-to-one map.
Except that in quantum gravity, the map actually is one-to-one. The lower-dimensional theory contains exactly all the physics that is distinguishable in the higher-dimensional one. Veronika was clarifying these things nicely. She said that the shadow in quantum gravity is "clever". Well, one comment that she could make and that should be comprehensible to everybody is that the "clever shadow" is actually closer to a hologram than a shadow (and that's why the principle in quantum gravity is called "holographic": the mechanism is analogous to the real-world holograms on films although these two holographies aren't "exactly" the same). It contains some quasiperiodic patterns – like parallel lines – and the distance between the lines or the periodicity "knows" about the depth of the object in the new, holographic dimension.
At any rate, she did very well. The shadow is "clever" and both theories are remarkably exactly equivalent. Jim Holt asked "which side is the real one". Well, both sides of a duality – and the holographic AdS/CFT correspondence is a duality – are exactly equally real and equally true, she stressed. David Albert was forced to join the discussion and he indicated that he considers some description more real than others. However, he admitted that he was familiar with the Fourier transform – the representations of a wave function in the position space or the momentum space may be "equally real". But he can only live with it if there is a clear simple mathematical equivalence between the two descriptions.
However, this added condition is utterly irrational. The map doesn't have to be simple or straightforward for the two equivalent descriptions to be truly equally good. And in fact, theoretical physicists only use the word "duality" if the map or "change of variables" needed to get from one description to another "at least somewhat difficult". In the case of holography, it seems that it is very difficult – much more complex than the Fourier transform. We can't really write a straightforward universal map that would provide us with a dictionary between the two descriptions. But both of them are still exactly equivalent.
Veronika also said that a particular observer may find some observables more natural than others. So Mr Albert is a localized object in the bulk who finds the locality in that space important which is why he wants to describe the quantum information in terms of quantum fields of an effective field theory that is localized in the higher-dimensional coordinates of the bulk space. But this bulk description isn't preferred in general. It is only preferred within the choice of an observer or some phenomena he may be interested in.
Also, Veronika pointed out another issue that helps to "resolve" Holt's would-be paradox about the "complicated object in the bulk and the simple shadow". While the shadow is "clever" and contains some extra features that remember things about the objects in the bulk – so the boundary theory tries to go in the direction of the bulk objects and become able to hold more information – the other side actually does some job for the two to unite, too. In fact, the locality in the bulk – and by that word, I mean the ability to independently change objects at places A,B,C... in this situation – isn't perfect. It is violated by gravity.
If you try to place too many memory chips to too small a volume, their mass must be large enough and they ultimately attract each other, gravitationally collapse, and create a black hole that prevents you from placing additional chips next to the previous ones. Or the volume occupied by the black hole may exceed the original volume that you have reserved for the chips. So gravity says that if lots of information live in the region A, the information you may add to region B isn't arbitrary. You should better not place too much matter into too small a volume etc.
Because the "freedom" to pick the state of individual regions in the bulk independently of others isn't unlimited, there exist some restrictions in the bulk. The maximum number of nats (or bits) that you may place into a volume doesn't really scale with the volume. It only scales with the area of the surface – these are the Bekenstein or holographic bounds. And this fact makes it easier for the "clever shadows" or "holograms" on the boundary to remember everything that is physically meaningful about the objects in the bulk.
You know, I would say that it's often being assumed that women tend to talk gibberish in debates about physics. And it's assumed because it's true in many if not most cases: women are often invited in order to have any women and therefore increase the diversity which implies that the quality of their thought is lower in average. But there are exceptions and Veronika Hubeny is one of them. Mr Holt (and the 2+1 philosophers), you should have been better at using the opportunity to listen and learn from her.
I won't comment on the rest of the debate. What Linde said was overwhelmingly clever, what Guth said was mostly clever, and what the philosophers said was mostly weird and reflected e.g. the "infinite-phobia". In the early parts of the debate, Veronika also calmly explained that there was nothing wrong about a theory that predicted that infinitely many things of some kind exist. A stupid philosopher like the the philosophers who participated at a debate may take the baby's attitude and insist that every object in a theory must be built from sand in a sandbox and personally touched by the baby's or, equivalently, philosopher's hand and it's too expensive to buy the infinite amount of sand etc.
Except that this requirement is, well, childish. Theories may predict lots of things that babies can't make out of sand in the real-world sandboxes. Real-world sandboxes don't contain an infinite amount of sand, the Universe may. Sandboxes may contain crystals of sand that look discrete, the Universe may be smooth at any level. Sandboxes don't contain 10500 valleys, the stringy landscape may. An objective reality classical description may be enough to capture most "normal" features of a sandbox but an observer-dependent quantum description is needed for the Universe around us. And so on. It is absolutely irrational to prefer theories that are sandbox-friendly in this sense, let alone ban the theories that are not sandbox-friendly. Needless to say, this is a simple point that is too hard for most philosophers, "philosophers", and other laymen to grasp. If their totally stupid person's method to imagine or "touch" something in a theory fail or don't work, they immediately assume that something must be wrong about a theory – they are too arrogant to consider the alternative, correct explanation that something is wrong with themselves and their limitations.