Geoff Penington
@quantum_geoff
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Assistant Professor at UC Berkeley. Quantum Information and Quantum Gravity.
Joined June 2017
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In 1993 the Superconducting Supercollider was cancelled. Estimated cost: $8 billion. An exodus of physicists left to Wall Street, bringing fancy maths and dubious risk management. 15 years later the global financial crisis cost ~$20 trillion. This is why you don't defund physics!
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This thread is mostly technical correct but conceptually not the best way of thinking about things imo. Let’s start with classical field theory and classical gravity. In both cases, there are nontrivial local observables. 1/
A crucial difference between quantum field theories and gravity is that in QFT you can define local observables, but in gravity you can't.
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Mathematical physics is what happens when mathematicians try to do physics. Physical mathematics is what happens when physicists try to do mathematics.
Mathematical physics is a subfield of physics, Physical mathematics is a subfield of mathematics and physical mathematics is a subfield of mathematical physics.
Just in case there was any confusion.
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Any methodology that concludes physics is a humble field has gone really wrong at some point. Maybe they should have instead asked people whether they could do every other field better than the experts in it…
Philosophy as a field shows other signs of delusion. In a survey asking PhD holders to rate fields by brilliance and importance, philosophy was the most narcissistic field.
Most scientists see philosophy as unimportant. That guy arguing otherwise? Probably a philosophy grad.
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Sat down to do some calculations about this rather than just tweeting from the hip in the replies. The theory is controlled by a single new parameter D_0 which has units of length^4. The decoherence of matter fields is controlled by D_0... 1/
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/
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As a working physicist, anything that explains the nature of reality, or from which everyday physical properties emerge, is by definition physical. If everything emerges from consciousness, consciousness is physical. So physicalism is tautologically true
No. Physicalism is a position in philosophy of mind that 52% of philosophers accept and 32% reject. Have some respect for other academic disciplines please.
@skdh
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@theGEDInstitute
If you're proposing spending an extra $8 billion on high school physics education then I'm 100% in favour
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It would be if you just gave us our 22 billion dollars dude. Large extra dimensions are sitting at 20TeV bro. We’ll be directly probing quantum gravity before you know it…just give us this one more collider. I promise bro. Just one more…
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Really good explanation by Suvrat of why string theory remains important to quantum gravity and theoretical physics more broadly, even as (probably rightly) fewer people work on explicitly stringy calculations today than in the 80s/90s
Despite so many naysayers, why does string theory continue to attract young physicists?
This is an excellent explanation why.
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@martinmbauer
Feels more basic than that? It’s easy to extend the operation of “adding something to itself x times” to noninteger x: we call it multiplication by x. The real mistake is that the RHS depends on x in two ways (size of each term + number of terms). And both affect the derivative
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There's been some controversy over this generally excellent article by
@philipcball
, so I thought I would write some thoughts. Full disclosure: I'm friends with many of the authors of this paper and was acknowledged in it for a couple of tangentially related discussions 1/n
In case you missed it the first time: here's how to make two black holes in a lab and send a qubit down a wormhole connecting them. It uses trapped ions - but all the same, if the AdS/CFT conjecture is true, that's literally what you'd be doing.
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Everybody, including the White House, now seems to be relying on the IHME model to predict what's going to happen with coronavirus. Which is pretty scary. Because it's bad. Like really bad. (Note: I'm not an epidemiologist and I would love to have one tell me why I'm wrong) 1/n
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Apparently the Nature traversable wormhole paper encoded one Majorana fermion from each SYK model in each physical qubit. So they sent information “through a wormhole” from a set of qubits to different operators on exactly the same set of qubits! (Still an interesting demo)
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A cool blog of stories of people who have left physics and what they went on to do. Probably most useful for early career people (undergrad-postdoc) trying to figure out what options exist outside academia
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If quantum states were simply probability distributions (at a mathematical level) there would be no need to worry about whether they are physical or not. QM is a generalisation of probabilities in the same way that complex numbers generalise real numbers
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This is incredibly inappropriate and frankly disturbing. 70% of kindergarteners lack the vocabulary to come up with more than one new set of pronouns a week. A common core guidelines suggest not introducing the pride flag pledge until second grade.
i’m a kindergarten teacher and every day I make my students choose new pronouns and when they recite the pledge of allegiance they have to say “pride flag” instead of “flag of the United States of America” and “gay” instead of “God”
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A beautiful fact about functions of Grassmann numbers is that they are always exactly equal to their Taylor series expanded to first order. This is even better than functions of ordinary numbers, which are only always exactly equal to their Taylor series expanded to second order!
It’s this sort of unjustified slander of physicists by mathematicians that really gets to me! We KNOW that sometimes the function is at a critical point and then you quadratically approximate it instead…
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I don't want to pick on this
@philipcball
tweet, but I've always felt like the distinctions between different subfields of physics tend to be overstated and are to some degree relics of a previous era 1/n
@cas_group
@skdh
Modern physics is in rude health and full of great experimental work. Modern *high-energy* physics, on the other hand, is... less so.
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Here’s a relativity thought experiment I’ve always enjoyed: suppose you have two rigid rods. One is moving at speed v close to c and is aligned perpendicular to its motion. The other is stationary and aligned at 45 degrees to the first rods motion. 1/
I am trying to give my students one relativity puzzle per lecture. The first one I gave was: devise a thought experiment that shows lengths cannot contract perpendicular to their motion. It is open-ended, but there is a beautiful answer, I think from Taylor and Wheeler. (1/5)
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@Kaju_Nut
It is generally agreed that, if you can eg create entanglement by placing a probe particle in such a superposition of potentials, it would prove gravity is a quantum mechanical force. It is one of the winning conditions of my/Carlo Rovelli’s bet with
@postquantum
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This is actually a completely legitimate question that captures an important apparent problem in Snell’s law (and e.g. the principle of least action and even path integrals) as usually stated. The fastest path from a to b depends nonlocally on stuff far from a. 1/
The first question when a kid hears about Snell's law is "How does the light *know* how to take the fastest path in different media". It's an intuitive question, yet we do not ask how an apple "knows" to fall down? Perhaps it has to do w the fact light-refraction isn't intuitive?
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Honestly proud of most mathematicians here for rejecting pedantic and overly rigid notational conventions in favour of what is clearly actually being meant in context. We will make physicists of you yet :)
This one is *only* for mathematicians and physicists, and only those who picked A or B. (Everyone else please just wait for results)
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This is clearly correct, but also the central limit theorem is really just a baby version (technically embryonic might be more accurate) of the renormalisation group, which is far far cooler still
Sorry to get political but the central limit theorem is much much cooler than the fundamental theorem of calculus.
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Pet peeve: people saying exponential when all they mean is superlinear. The number of relationships only grows quadratically with number of people. Good thing too or it would be impossible to have a group of more than ~10 people where everyone knows each other
Our eldest is 7. Our 4th is 2 weeks old. The house is alive and noisy. Dinners are full of questions and arguing and stories
It’s hard to convey, but there’s something greater than the sum of the parts
Partly it’s obvious? There are an exponential # of relationships forming
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If you allow superdeterminism then it’s not possible to learn anything from any experiment ever about what the world is like when you’re not doing experiments
It's not possible to close the superdeterminism loophole with a Bell-type test
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Ok poll answers time: a standard fact about general relativity is that the path of longest proper time between two spacetime points is always a timelike geodesic - ie the worldline of a free falling observer and not a locally accelerating observer like Bob 1/
Inspired by the recent special relativity poll by
@WKCosmo
: Q1. Alice jumps down a vacuum tube through the center of the earth and reappears 90 minutes later having followed a timelike geodesic to the opposite side of the earth and back. Bob stays at the top. Who ages more?
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@felix_led
It’s proportional to the number of lumens produced by the light bulb that appears over someone’s head when they have a really good idea
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Since I’m apparently spending my day defending string theory I’ll keep going. (Disclaimer: I’m not a string theorist although much of my work is closely connected to it.) People often seem confused about the fact that string theory doesn’t uniquely determine low energy physics 1/
@quantum_geoff
Yes, I see that multiplicity as a breathtakingly heterodox idea (and also obviously a bad one).
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Would gravity being fundamentally classical surprise me more than Leicester City’s 2016 title? Yes. Yes it absolutely would. (Note: as general life advice, you should never take a bet at 5000:1 on, because the chance you are an idiot is >>1/5000. But where’s the fun in that…)
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/
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@skdh
What is supposed to be new or surprising here? It’s standard knowledge that “singularity” in the theorems means “obstruction to extending lightrays beyond finite affine length” and that it’s unclear to what extent this has to be related to a traditional curvature singularity 1/
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This guy might be the biggest clown on this site. Apparently you now have to either a) explicitly quote someone or b) “sufficiently reword” a single sentence describing their results and citing them. Otherwise you’re committing plagiarism
EXCLUSIVE:
@RealChrisBrunet
and I have obtained documentation demonstrating that Harvard President Claudine Gay plagiarized multiple sections of her Ph.D. thesis, violating Harvard's policies on academic integrity.
This is a bombshell. 🧵
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Most of modern theoretical physics has been heavily influenced by string theory. One representative example is quantum phase transitions (a major subfield of CMT). The mathematical tools for studying this (conformal field theories) were largely developed for string theory 1/
@quantum_geoff
Wouldn't string theory be more "heterodox", given that A) no one uses it for anything, and B) some string theorists have called for overturning mainstream ideas of empirical validation? AFAIK, the LCG people haven't done the latter; they seem more mainstream in their ideas.
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It’s this sort of unjustified slander of physicists by mathematicians that really gets to me! We KNOW that sometimes the function is at a critical point and then you quadratically approximate it instead…
My favorite part of Oppenheimer was the mathematician telling them not everything can be linearly approximated and then they fired him
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Not remotely close
This is always a question I have asked myself: Given the knowledge that Einstein had by 1906, could a currently existing AI have invented GR and realized that it needed Riemannian geometry and tensors to figure it out? If not, what would it take?
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The invention of GR is arguably the single greatest feat of intellectual endeavour in human history. If we had AIs capable of the same feat, the process of theoretical and mathematical research right now would look very very different (and arguably most humans would be redundant)
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Thread to hopefully clarify some confusion. There are two different issues involved here: a choice of classical background and a choice of boundary conditions. A theory of quantum gravity should not depend on the former. It should depend on the latter (like any quantum theory) 1/
@quantum_geoff
@gomes_ha
@karch_andreas
@CburgesCliff
@ThomasVanRiet2
I agree that the two cases (old school string theory and AdS/CFT) use the classical "background/boundary" in different ways. But crucially, both are using the background/boundary, to classically define the time and how we measure evolution. 1/
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Judge me by the enemies I have made
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One example of interesting progress using such observables is a paper I put out last year w Chandrasekaran, Longo and Witten on semiclassical de Sitter space (). See also Witten’s recent review paper 12/
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@skdh
I don’t understand this. If I take the only copy of a bit string and throw it into a black hole, the black hole information problem is precisely whether the information in the bit string can eventually be decoded from the radiation or not
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Was a fun chat with Suvrat and Vasu. If there was one thing I wanted to get across (not sure how well I did) it’s that while measuring the Page curve requires exponential many experiments and the ability to carry out precisely the same experiment multiple times…1/3
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@Quasilocal
Listening to mathematicians talk about abuses of notation is like listening to Mormons tell each other it was OK that they accidentally saw their wife naked because it only lasted a second before the lights went off. Notations can be abused to extremes you cannot even dream of!
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I can't speak for anyone else, but to me a job in academia is only worthwhile if I get to work on whatever I happen to currently be interested in. If people stop wanting to pay me to do that, that's fine. There's plenty of other interesting things to do with your life...
It's about time scientists become a little more self-aware and have a hard look at how social reinforcement and funding opportunities influence their supposedly oh-so-pure interests.
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@Irish_Atheist
@lastpositivist
I think that's unfair. It's not the 90s anymore. The film won't claim the white man was the first to kneel: he will just be the main character and him kneeling will be the defining moment that fixed racism.
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@Noahpinion
Lot easier to come up with the idea of a vaccine after you've been taught how germ theory and acquired immunity work
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I assumed these questions were obvious but quite funny parodies of silly unclear IQ test questions but everyone (both the people complaining about them and the people trying desperately to answer them) seem to be taking them seriously
A question indicative of (very approximately) 110 IQ
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@martinmbauer
Sickening. I can excuse antisemitism but I draw the line at wick rotations of - - - -
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@JosephPConlon
Is “holography” (in the original broad sense - not just AdS/CFT) too vague to qualify? Otherwise I would argue that it’s more likely to be true and possibly deeper than string theory. Strings are definitely the most consequential.
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Genuinely godlike political talent. Anyone else would have overplayed it. The look of apparently shock at where his hands were and then quickly moving on was a work of art
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Embarrassment to the field of philosophy that ontological argument isn’t universally regarded as solely a case study of “stupid shit people used to take seriously” tbh
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Never understood what philosophers could possibly mean by the term. But I’m skeptical anyone who actually understands physics would feel that eg quantum fields are “physical” whereas a mind is some other more ghostly thing
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If someone used quotation marks in a physics paper, I’d think they were taking the piss out of the person they were quoting. The product of science isn’t particular combinations of words: it’s ideas. If you cite the latter, no one gives a shit about the former
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The Force Awakens was a boring vision executed well. The Last Jedi was an interesting vision executed terribly. Rise of Skywalker was a boring vision executed terribly
THE LAST JEDI AND RISE OF SKYWALKER WERE EQUALLY BAD
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To be even more precise: it’s like the way the quaternions generalise the complex numbers. In both cases the new feature is noncommutativity: suddenly you’re allowed to have AB =/= BA
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@skdh
is pretty close to one! “Derivation” is maybe too strong, but it’s certainly a fairly compelling argument combining general relativity and quantum mechanics consistently necessarily leads to holography
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We were able to define an algebra of local observables associated to an observer’s worldline and show that it has an entropy that is given by the generalised entropy (=entropy + horizon area) of the observer’s cosmological horizon. 13/13
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@Quasilocal
The real trick is figuring out how to end up with formulas relating finite quantities to finite quantities such that all the ambiguities involved in separating a formula into divergent and finite pieces don’t matter
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Otherwise, let's just celebrate the cool progress being made in physics, because fundamentally we are all on the same team of wanting to understand the universe in all its crazy glory. 11/11
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This may be field dependent but in my experience the physicists with the most (and deepest) knowledge also do the best research. Closest to this is someone like Steve Shenker (doesn’t publish much, knows things) but the one paper he writes a year is always brilliant so not really
There should be room in academia for professors who don't publish, but who know things. Not dead wood, who stopped learning in 1995. I mean: leaves for the summer, reads all summer rather than write, smokes a pipe on the quad and will talk to you about stuff.
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Probably because I’m sufficiently far from the field that I didn’t have actual pride/trust-in-my-own-expertise at stake, I experienced the opposite of
@gravity_levity
. My snap judgement was “lol no”. Nothing I read changed that. But part of me still really wanted to believe…
Now that LK99 seems to be over, I feel like I learned something a bit sad about myself.
Throughout the ordeal, I mostly founding myself rooting inwardly to be proven right, rather than rooting for the outcome that was most interesting scientifically.
:-/
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Another example is of course AdS/CFT which came out of string theory but has taught us a huge amount about strongly coupled QFTs, black holes etc. that have nothing to do with string theory. And then of course there’s it’s influence on mathematics 2/
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So there doesn't seem to be any parameter space that isn't excluded. Of course, there was a lot of handwaving here (even assuming I didn't screw up anything major) so there may well be a small open window. But I don't think it's big!
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Actually if it’s value is a bajillion dollars then by definition its quantum value is e^bajillion
trillions? meh. quantum computing will be the first bajillion dollar industry.
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Incredibly stupid tweet. Everyone realises that if you pack the court it will be repacked the opposite way every time the other party holds the senate/presidency. Some of us just have a low enough time preference/sufficient reasoning skills that…1/
if you think the supreme court should be packed you either have extremely high time preference or extremely poor ability to reason through multiple stages of an iterative game
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Finally I want to list some things that such experiments would not do: prove the AdS/CFT correspondence, prove string theory is the underlying theory of our universe, creat a black hole by any usual meanings of the words, pave the way to Star Trek 13/13
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The man invented Seiberg-Witten theory folks. If he has a perspective on something I’m sure it’s worth hearing…
Doctor: may I give a different perspective?
Many Americans learned about how far off their concept of “science” was from COVID. They thought that science was something they could trust. They trusted their vaccines. Their FDA. The CDC.
And then they saw the COVID show….
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Public service announcement: if you ever find yourself negatively polarised (from either left or right) into thinking malaria nets are bad, you should probably take a long hard look at exactly how you are coming up with all of your opinions. Also you should definitely log off
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@SapientToasters
@JosephPolitano
Median household income is about $70k not $121k (that was net worth). And financial assets (which are pretty liquid) are $26k which is more like 10 paychecks
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@WKCosmo
I think they’re both just examples of students being thrown off by the terminology. If people understood what the statements actually mean they would realise they are totally unsurprising
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I generally thought Three Body Problem was quite good and certainly much better than the books, but a woman walking out of the control room for the “Oxford particle accelerator” to go kill herself by jumping into super-K was hilarious
@Samuel_Gregson
I had to shut it off after that woman walked from the control room right into super K.
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OTOH loop quantum gravity has had essentially no impact on anything else in physics. And after several decades there is no significant evidence that it is capable of describing any sort of semiclassical universe 3/
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@skdh
I’m impressed Roy Kerr is still writing coherent papers at 89, but I don’t know what’s new there except some examples of lightrays that are extendible (they just hit a Cauchy horizon at finite time)
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Of course in reality, it’s just a mathematical rewriting of a causal theory with a state space consisting of the light’s initial position and direction. But that part is generally not explained to kids. And if they don’t know it they are right to feel unhappy and confused! 7/7
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@postquantum
@Perimeter
@preskill
Daniel Harlow stealing a quick look at Ted Jacobson's answer for the BH entropy as usual
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This needs to happen. Nothing wrong with Harvard selling access to courses etc for funding. But, in a world where much of the value of a Harvard degree is the signalling effect of admittance, it’s basically corruption for donors’ kids to be indistinguishable from other applicants
A modest proposal: if you're not supposed to claim "Harvard" because you went to the extension school and it dilutes selectiveness of the brand, then you should also be required to report what list you were admitted from into the "selective" program
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Dude’s idea of how plagiarism works is based on undergraduate class projects, which to be fair is probably the last real work he did
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To be clear: there are many many reasons (most empirical, some more philosophical) to think that minds emerge from quantum fields and not the other way around. But either way both are obviously physical!
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@yudapearl
@evgeniyzhe
@kareem_carr
Of course, it may very well be convenient to introduce e.g. the notation h := p to succinctly describe the evolution process. But that is an emergent phenomenon. At the fundamental microscopic level there is no notion of causality - only dynamics.
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We can then define local observables based on their relation to the observer. This is called relational QM. It is hard and not well understood (just like the rest of QG!) in general. However in simplified settings (such as the semiclassical limit) we can make progress 11/
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Anyway, I'm not really sure what my point is here. But basically: if you want to critique something, critique a research program, not an arbitrarily defined subfield. Is the motivation not interesting? Is the strategy for achieving the goal wrong? Is it just too hard? 10/n
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3. No one in any sort of leadership position in this country has any training in, understanding of, or interest in maths, statistics or any other technical field. They have no ability to know when to trust a model and when not too. 16/16
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@postquantum
@karch_andreas
@CburgesCliff
@ThomasVanRiet2
Every classical theory ever quantised - from a free particle to quantum field theory - involved a choice of boundary conditions. This is completely different from defining a theory by a perturbative expansion about a particular classical background (a la old school string theory)
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@WKCosmo
@JosephPConlon
And indeed the Noether charge associated to translations of the time coordinate is conserved. It’s just that in GR time translation is a gauge-symmetry and so that Noether charge is always zero (up to a boundary term). In that sense in GR (bulk) energy is conserved but zero
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Ok, since I said yesterday that I didn’t see anything objectionable at all about Gay’s so-called plagiarism, I should maybe update based on new information. This new stuff is bad and you’re not meant to do it. 1/
In her 1997 thesis, for example, she borrowed a full paragraph from a paper by the scholars Bradley Palmquist, then a political science professor at Harvard, and Stephen Voss, one of Gay’s classmates in her Ph.D. program at Harvard, while making only a couple alterations.
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The difference between field theory and gravity is that coordinates are physical in field theory but not in gravity. So in field theory we can specify the location of our local observable by its coordinates. In gravity we need some other method. 3/
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@michael_nielsen
My initial reaction was that it was all a totally fake smear campaign (stuff undergrads aren’t meant to do because it defeats of the learning exercise but no one cares about in the real world), but some of the later stuff crossed the line into “unacceptable but not fireable” imo
Ok, since I said yesterday that I didn’t see anything objectionable at all about Gay’s so-called plagiarism, I should maybe update based on new information. This new stuff is bad and you’re not meant to do it. 1/
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It’s also good to know that, if every particle accelerator in the world starts seeing massive violations of the standard model, we have one month to figure shit out before all funding gets cut because “science is broken”
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CERN announced today that it is continuing with preliminary plans for a new collider (the FCC) that would be much larger than the LHC. Unsurprisingly, people are now arguing about this online. This is a famously productive activity, so I thought I would join in! 1/n
Here is something that should worry you. Each time I give a public lecture people come up to me and say they agree with me that building a bigger collider is currently a nonsense idea. It's a huge investment with little scientific benefit and basically no societal relevance 1/
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So what’s the solution? We live in a universe with quantum gravity and do local measurements all the time? The answer (like in classical gravity) is that we need to include the observer in the system. This time we don’t just care about their location, but also their energy 10/
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@Noahpinion
asks why we only hear about heterodox economics and not heterodox physics. Certainly no one uses the term but I think it’s fair to say that loop quantum gravity (and various other smaller programs) fill the corresponding niche 1/
Heterodox econ supporters claim that mainstream macro has failed. But mainstream macro just got us (mostly) out of an inflation, without causing a recession. What more could you want???
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@eshaghoulian
@Zugzwang_Sor
This. But also it’s not even clear that playing it safe maximises p(job). If there’s one ball left and you need a six to win there’s no point trying to hit it along the ground, even if that would maximise the average number of runs you score
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Physically however the more relevant method of fixing a particular point in spacetime is simply as the location of the observer themself. That is after all the only location they can actually do measurements. And that method is genuinely completely local 5/
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This seems very confused to me
The only two "arrows of time" we have don't match
The second law of thermodynamics gives us an arrow of time for entropy.
But contrary to what some assert, no, it can't explain why we experience, or perceive, our arrow of time.
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I tried being degenerate once and three months later I was found lying in a ditch begging for one more hit of a Weierstrass function. Just say no.
@weltbuch
We don't tolerate any degeneracy in physics I'm afraid. Round these parts people believe in good, clean, honest, healthy living. None of your weird mathematician perversions
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One such method involves describing the location by its distance from some (infinite) boundary (often called “dressing” to the boundary). This dressing makes the observable “nonlocal”, but only because we used nonlocal geometry to specify a particular location 4/
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@CburgesCliff
@Kaju_Nut
That very much used to be my attitude. I’ve since been convinced by working on stuff involving vN algebras etc that there’s interesting physics in this stuff. In the case of Haag’s theorem it’s that “nice” continuum states in the free theory are singular in the interacting theory
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