This weekend at Caltech we had a small but very fun conference: the “Physics of the Universe Summit,” or POTUS for short. (The acronym is just an accident, I’m assured.) The subject matter was pretty conventional — particle physics, the LHC, dark matter — but the organization was a little more free-flowing and responsive than the usual parade of dusty talks.
One of the motivating ideas that was mentioned more than once was the famous list of important problems proposed by David Hilbert in 1900. These were Hilbert’s personal idea of what math problems were important but solvable over the next 100 years, and his ideas turned out to be relatively influential within twentieth-century mathematics. Our conference, 110 years later and in physics rather than math, was encouraged to think along similarly grandiose lines.
And indeed people had done exactly that, especially ten years ago when the century turned: see representative lists here and here. I asked the organizers if anyone was taking a swing at it this time, and was answered in the negative. I was scheduled to give one of the closing summaries, and this sounded more interesting than what I actually had planned, so naturally I had to step up.
Here are the slides from my presentation, where you can find some elaboration on my choices.
And here’s the actual list:
- What breaks electroweak symmetry?
- What is the ultraviolet extrapolation of the Standard Model?
- Why is there a large hierarchy between the Planck scale, the weak scale, and the vaccum energy?
- How do strongly-interacting degrees of freedom resolve into weakly-interacting ones?
- Is there a pattern/explanation behind the family structure and parameters of the Standard Model?
- What is the phenomenology of the dark sector?
- What symmetries appear in useful descriptions of nature?
- Are there surprises at low masses/energies?
- How does the observable universe evolve?
- How does gravity work on macroscopic scales?
- What is the topology and geometry of spacetime and dynamical degrees of freedom on small scales?
- How does quantum gravity work in the real world?
- Why was the early universe hot, dense, and very smooth but not perfectly smooth?
- What is beyond the observable universe?
- Why is there a low-entropy boundary condition in the past but not the future?
- Why aren’t we fluctuations in de Sitter space?
- How do we compare probabilities for different classes of observers?
- What rules govern the evolution of complex structures?
- Is quantum mechanics correct?
- What happens when wave functions collapse?
- How do we go from the quantum Hamiltonian to a quasiclassical configuration space?
- Is physics deterministic?
- How many bits are required to describe the universe?
- Will “elementary physics” ultimately be finished?
Clearly I cheated somewhat by squeezing multiple questions into single problems. But the real challenge was thinking sufficiently big to come up with problems that people a century from now would agree are interesting. And I stuck to “elementary physics” — particle physics, gravitation, cosmology — just because I’m not competent to pick out the important problems in any other fields. Twenty-four, of course, because Hilbert had 23, and we had to go one better. There was certainly no shortage of candidates; I was coming up with more good problems and throwing out old ones right up until the last minute. Any obvious ones I missed?
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There is only one question of modern physics that anyone who isn’t specialist cares about and that is: Time? Maybe some of the questions you meantioned are part of that, but time is what people care about.
I’m even more interested in question #25. A generation ago when I was studying physics, some of Sean’s 24 questions hadn’t yet even been fathomed. What questions will we be asking some 30 years hence?
@Ben Lillie:
What is the ‘bastard S^2 symmetry’? I can’t find any reference to S^2 symmetry on google. S^2 the sphere isn’t a group.
is there something i’m missing? i can’t find anything on wikipedia about an S^2 symmetry
Some more;
Science assumes Law of Uniformity of Nature and Law of Causality.What if it is not so?
If our receiving light from distant stars prove the existence of objects in the past, what about our existence?Are we in the past, present or future?
Why can’t we conceive more geometric patterns other than what we have now,like Square,Rectangle,Modifications of these two,Circle?
What is Time and Space?
In the light of Quantum Theory,should we not change our Perception of Physics and our understanding of Laws as we know of them now.
Is spacetime a manifold – and if not why does it mostly look like one ?
Is there a fundamental mathematical idea that would revolutionize physics ?
Why is ordinary arithmetic so unreasonably effective ?
Is Math just a tool in the service of Physics ?
@eigenlambda
Zwicky coined the term “Spherical Bastard”, i.e. a bastard any way you look at them. Bit of a strained joke on my part, and you’re right, I should have called it the O(3) bastard symmetry.
On Space and Time Kindly read my blog under Time a Non- Linear theory at ramanan50.wordpress.com and offer your views.
This is a great list. Many of them stump me. Dumb me.
Comments:
#16. Is this the unfunny haha way of asking “I am a Boltzmann Brain?”
#21. This seems to be a very general question in statistical mechanics.
More questions (some of them wacko, but fun)
1. You should add a cool question about time (you are now a man about from eternity to here). How about “Does time have the same characteristics (effects) on all length scales?”
2. Maybe too fuzzy and weird is “Is there an elementary particle equivalent of a ratchet?” That is, are space and time different only because of the characteristics of what they contain? An answer might lead to a resolution between objectiivism and a weakened sort of subjectivism.
3. People (not just physicists) devote a lot of time thinking about time travel in the sense of going back to a previous point in time, but has anybody thought about how hard (quantitatively) it is for us humans stuck to earth to travel back to the same point in space?
Finally, I think that the acceleration of galaxies away from each other is the most important cosmological problem of the century. So is there an easy way to lay the following idea to rest?
4. The acceleration of the galaxies away from each other is caused by positive static electrical charges on matter in galaxies screened by an equal amount (but more spread out) of screening negative charges in intergalactic space, and this imbalance is a simple result of stellar formation and Boltzmann statistics (more fast electrons than protons leak out of galaxies into intergalactic space). Forgive me if I’m wrong but didn’t Poincare posit an electromagnetically dominated universe? Has Gravity’s century dispelled Poincare’s ghost?
Sorry, I noticed the link to the slides themselves after I posted. You already have a cool question about time, and my comment about #16 also applies to #17. If #16 and #17 are Boltzmann Brain questions, my take is that simulations be damned, there’s nothing fruitful to be learned by the Russel paradox and the simple answer is that it is easier (by some large cardinal number) to make a big universe with many observers (but much smaller than that cardinal number) than a very small self aware universe.
Liked that post! I’ve also written about these questionable assumptions just here
http://bruceleeeowe.wordpress.com/2009/08/01/our-questionable-assumptions/
I think these are nice…
more fiendish questions
Is it even possible to explain why space is 3d without also explaining Hydrogen ?
Why do particles have a complex phase ?
Is the good behavior of the universe contingent on Alternativitity ?
A lot of people have jumped on the question “10: How does gravity work on macroscopic scales?” and glibly said “Don’t you believe in GR” or related answers.
This totally glosses over the fact that GR only describes how gravity *behaves*, but doesn’t do squat to explain the mechanism.
How does the Andromeda Galaxy create a gravitational field that affects our galaxy? What’s the mechanism? Do gravitons exist? If so, where do they fit in the Standard Model? If it’s just curved spacetime with no mediator particle, how do other locations “know” that a mass is in motion? Consider tidal effects on earth due to the sun and moon – as things move in their orbits, what mechanism makes the curves in spacetime move to match? So many questions, so little time….
I see some questions that hit all around it, but unless I’m missing something fundamental and a question that you included that I don’t understand covers this, there is an obvious question that is missing from your list:
What CAUSED the big bang?
I get the impression that physicists don’t want to think about this question either because they think it was god, and they don’t want to be ridiculed, or they think any attempt to investigate the question is in the realm of theology instead of physics and they don’t want to be ridiculed. Well, I don’t buy the “god” theory, but neither do I believe the question is inherently unanswerable.
We understand what causes TNT to explode, what causes a volcano to erupt, what causes a big cloud of hydrogen in space to ignite in a fusion reaction, and what causes that cloud of hydrogen to eventually explode and spew heavy metals all over the local galaxy. What we don’t know, as far as I’ve heard, is what caused the big bang. (Please correct my ignorance if the question has already been answered.) It seems like a very interesting question to me. After all, if “that” caused it, what’s stopping “that” from causing another one halfway between the Earth and the Moon the day after tomorrow? Or is nothing stopping it?
Everybody seems to be grappling with #10. I would be willing to bet ( unless I am missing something here) that he is referring to how General Relativity either needs to explain or must still be valid despite the discovery of “dark energy”. The universe is expanding at an increasing rate….. Why? Gravity is supposed to be an attractive force so why isn’t the expansion slowing down? Does gravity just ‘magically’ generate negative pressure at large distances (i.e. macroscopic perspectives)? If so, why? If not, what is this other energy and how is it related to gravity? General relativity doesn’t address this but it should.
@ DP in CA (Post 39)
I would imagine most physicists avoid the question “What caused the big bang” because it is an inherently nonsensical question. If we can conclude that time is inseparable from space (i.e. space-time) and we also conclude that space-time didn’t “exist” until after the expansion, then words like ‘before’ are inconsistent and meaningless.
Hell, I would argue that the question, “How does one correctly think about and/or phrase questions relating to the big bang?” could be added to the list, provided of course that I thought it was possible to answer, which I don’t.
Was Lee Smolin there? I just happen to be reading his 2006 ‘The Trouble with Physics’, decrying the general lack of progress in the field over the past 3-4 decades since the Standard Model, a troubling groupthink regarding string theory, and his 5 great problems in theoretical physics (1. unification of GR and QM into quantum gravity; 2. foundational problems of QM (relationship between the real and the formal worlds); 3. unification of the particles and forces; 4. the bases for the setting of the free constants in the Standard Model in nature; and 5. explanation of dark matter and dark energy). It’s interesting to consider the philosophical debate between someone like Lisa Randall and Smolin – both believe theory has to be tied to experimental data, not just beauty of the theory – but she seems to promote building up understanding from solving small problems, while he seems to think that approaching from small problems or big problems depend on the times, and these times call for thinking about the big problems.
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The current Big Bang Model is a QFT in a curved spacetime. Unfortunately, no such theory is mathematically well-defined; in spite of this, theoreticians claim to extract information from this hypothetical theory. On the other hand, the super-classical limit of the not mathematically well-defined QED in a curved spacetime is the mathematically well-defined Einstein-Maxwell-Dirac system. (One could get a similar system for the standard model.) As a super theory, EMD violates the positivity condition in the Penrose-Hawking Singularity Theorem. Thus, it is possible that there would be complete solutions without any singularities-Yau has in fact constructed some. Furthermore, it is known that the Einstein-Maxwell-Dirac system admits of solitonic solutions, i.e., classical electrons and photons. This is the kind of theory Einstein was hoping for. EMD is also a totally geometricized theory as a non-commutative geometry; here, the charge e and the mass m of the electron are geometric invariants of the non-commutative geometry analogous to pi!
I think the ultimate questions do hover on the edge of nonsense. Decrying questions like “what caused the big bang” seems to violate the spirit of a conference that encourages experts to ask big questions. If they are to go out on a limb in asking these questions, we must keep our part of the bargain by not mercilessly attacking them for doing so. The fundamental questions, i.e. “why is there something instead of nothing” are the most interesting, the reason that many people care about theoretical physics to devote their lives to it, and the most difficult to state as a question using concrete language that may lead to a falsifiable theory.
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