I wish I deserved the heretic moniker, but isn’t asking whether spacetime is quantum or classical just common sense? After all, general relativity (GR) - our theory of gravity and spacetime - is special. It isn’t a gauge theory, and gravity isn’t a force. 1/

https://twitter.com/TOEwithCurt/status/1833612758824124655

(i) Gravity is not a force, because it can be described by a universal geometry in which all other fields live. In this sense, gravity is unique. This is the equivalence principle. Test particles just free fall, and so no force is acting upon them. 2/

Folks, something seems to be happening... We show that our theory of gravity is valid down to the shortest distances . and that it can explain the expansion of the universe and galactic rotation without dark matter or dark energy 1/arxiv.org/abs/2402.17844
arxiv.org/abs/2402.19459 I'll post details when I'm back from lectures, but just wanted to say that I'll give a public lecture March 15th about the theory, and we're hoping to fundraise to bring a remarkable student to join our team. Tickets and ways to support our research at 2/ucl.ac.uk/oppenheim/gi.s…

The cover of this week's @newscientist features an article by @lyndie_chiou on six experiments to test the quantum or classical nature of space-time. It's interesting to think about what assumptions go into those experimental proposals. 1/

https://twitter.com/newscientist/status/1717172523043872896

The proposal to search for gravitationally mediated entanglement would certify the quantum nature of the graviton because a classical mediator cannot create entanglement. It has minimal assumptions, only requiring the interactions to be local. 2/

I have a question, about the use of a term to describe theories which couple quantum and classical degrees of freedom. The term has also been used as a slur for mixed-race people, so I'd like to check-in -- is it appropriate to use? 1/ I'm especially interested in hearing from mixed-race scientists, about how they would personally view the phrase. I've also posted this question on bsky, and I'd encourage people to also answer there if possible. 2/

🧵: Every student of mathematics knows the famous story of Ramanujan and Hardy – but it seems that @RishiSunak, @SuellaBraverman, and the @ukhomeoffice have missed its crucial message. 1/10en.wikipedia.org/wiki/Srinivasa… In 1913, a self-taught Indian clerk named Ramanujan living on 20£/year wrote to the renowned mathematician G.H. Hardy at the University of Cambridge. Recognizing Ramanujan's incredible genius, Hardy made arrangements for his journey to the UK. 2/10privatdozent.co/p/ramanujans-f…

I'm always amazed at the faith that string theorists have in the AdS/CFT holographic conjecture! Even calling it a conjecture rather than a correspondence risks you being treated like a heretic. But the evidence that it holds around black holes seems very thin to me. 🧵 1/

https://twitter.com/karch_andreas/status/1616822687766138881

First (i), the main ingredients of the conjecture have changed significantly in a few years. The entropy of a boundary region of the CFT was dual to the area of a bulk extremal surface, then to the extreme of the area + bulk entropy, and is now given by "the island formula". 2/

If you're into quantum mechanics, you probably love Feynman path integrals. But path integrals exist for classical mechanics as well 🤯, and now we have hybrid classical-quantum path integrals! Z. Weller-Davies and I derive them in scirate.com/arxiv/2301.046…. A path integral 🧵 1/ The standard Feynman path integral gives the amplitude of going from an initial state to a final one, by summing over all paths connecting the two states, weighted by the exponential of the action times i. 2/

My bet with @carlorovelli and @quantum_geoff has finally been signed! The question is whether space-time is best described by quantum theory. I think it's possible that our next theory of gravity will be radically different from anything we've seen before. 1/ Gravity is the bending of space-time. Maybe space-time is best described by quantum theory, but maybe a better approximation is that we should treat it classically (see arxiv.org/abs/1811.03116), or maybe it can only be described by something else entirely. 2/

I don't know whether space-time geometry is fundamentally quantum or classical, but @carlorovelli has given me 5000:1 odds, so I'm taking him up on it. When he looses the bet, he has promised to give me 5000 of some item, but what item should I choose? 1/ Carlo offered to give me 5000 coloured balls so I can play bazinga: and if he wins, I give him one red ball. But I don't play bazinga, and he seems unexcited by my proposals: 50 ml of olive oil or wine? .00002 bitcoin? a packet of crisps? My home? 2/

Congratulations to Roger Penrose, Reinhard Genzel and Andrea Ghez, winners of the 2020 Nobel Prize for their contributions to our understanding of black holes! Here is Roger's hands during his talk @ucl, discussing his singularity theorems (w/ Stephen Hawking). 1/

https://twitter.com/postquantum/status/1133801871804645376

Penrose and Hawking showed that under reasonable assumptions about the matter in our universe, there will always be a place where space-time comes to a violent end, and our known laws of physics breaks down, creating a black hole. 2/

@AlmheiriAE @quantum_geoff It seems crucial in arxiv.org/abs/1911.11977 to assume that the bulk theory is dual to some ensemble of unitary theories. Because the bulk calculation itself computes S(\sum p_i \rho_i) and you want to argue that an experimenter could infer \sum p_i S(\rho_i) by learning 1/n @AlmheiriAE @quantum_geoff which boundary theory (i) they live in. So the bulk calculation alone suggests that the entropy of the Hawking radiation keeps increasing. This is what I mean by needing to assume holography (or more accurately, that the underlying theory is unitary). Do you agree? 2/n

Looks like the worst kept secret in quantum technology has finally leaked into the mainstream press ahead of publication. While this is a milestone, it is *very* far from being a quantum computer that can compute anything useful. ft.com/content/b9bb4e… The Google device solves a "problem" called random sampling. Essentially, you run a very short and random quantum circuit and then measure the output. We believe (although we can't prove) that a classical computer would not be able to produce the same distribution of random 2/

Isn't this like saying quantum mechanics isn't broken because all the interpretations are broken? 💔 My main criticism of the paper is that what they call "Consistency" is the ability to reason about measurements which might in the future become "undone" by someone making a 1/

https://twitter.com/skdh/status/1042716838042447872

measurement on our brain. And we already knew from Bell's theorem that we can only reason about measurements which have been performed. So maybe this paper merely extends our notion of what constitutes an element of reality in QM to not only be the result of a measurement but 2/