zachf

9 ms·

31.
by zachf 4y ago
I have a PhD in physics. I gave my assessment of the article in the first reply above. You're welcome to disagree, but since it seems you're mostly interested in rejecting the article on the basis of who published it instead of th
32.
by zachf 4y ago
Is this what the kids call "cancel culture"? I'm judging the quality of the OP article, not the sum total of everything that they've ever published. The politics of the editors of Scientific American may not align perfec
33.
by zachf 4y ago
I have a PhD in a closely related field and I'm not sure what you mean--this sentence is well-formed and basically correct, if slightly simplified to appeal to laymen.
34.
by zachf 4y ago
Peter Woit has strong opinions but they're often not representative of the physics community as a whole. In my opinion, which as a physicist myself I think matters as much as Woit's (i.e. not much), Scientific American is variable
35.
by zachf 4y ago
Periodicity in time is unusual to think about in physics, partly because you start to get wacky results. If you could establish a CTC in nature, it would allow for computers that can efficiently (i.e. in polynomial time) solve not just NP p
36.
by zachf 4y ago
Gravity (more precisely, a dynamical metric) is absolutely a field in the QFT sense. The problem is that its unrenormalizable. That’s a technical term that roughly means the theory doesn’t make predictions to arbitrarily high energies. As a
37.
by zachf 5y ago
Strictly speaking it’s none of them, because those are idealized perfectly symmetrical spaces with no matter in them, only dark energy, and our universe (happily) has matter in it :). But it’s very, very close to flat, except not quite perf
38.
by zachf 5y ago
Think about 2D surfaces. Which ones are the most symmetric? A flat plane is a very nice space: every point is as good as any other (there’s nothing intrinsic to any point to distinguish any point from any other, except arbitrarily), and no
39.
by zachf 5y ago
I can see why it might look like that from the outside, but the set of mathematical results that led up to it actually come from the angle of, “let’s try to prove that traversable wormholes and the negative energy densities required to crea
40.
by zachf 5y ago
There's a good reason why most people who specialize in quantum gravity work in string theory: violating any axioms of string theory leads to theories which are self-evidently inconsistent with themselves and/or experiment. It&#x2
41.
by zachf 5y ago
It’s a very strongly supported conjecture, but it has not been proven rigorously. As I said it would be extremely surprising if It were not true because of a frankly absurd amount of evidence. The second claim you make is false though— 3D h
42.
by zachf 5y ago
Well my main point is that AdS/CFT duality is asserting an exact equivalence between quantum field theories and string theories. So for example N=4 SYM, a quantum field theory viewed from one perspective, is exactly equal to a string t
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by zachf 5y ago
As a nitpick, negative energy densities are indeed uncommon but they aren’t unphysical. In fact, every quantum field theory admits at least one state with negative energy density—its a theorem. But even in quantum field theory there isn’t e
44.
by zachf 5y ago
Sure, experiments will always be the ultimate arbiter of truth. But if you don’t “visit the territory”, you’ll never know that your map is wrong. The map has been challenged in many many ways that seemed more impossible than large scale QC,
45.
by zachf 5y ago
I agree with everything you said in the first two paragraphs. But string theory is not “motivated by nothing but cool looking math”. String theory is the generalization of quantum mechanics to higher dimensional fundamental objects, i.e. be
46.
by zachf 5y ago
The question about firewalls is such a good one. In short, physicists identified this problem involving three parts of an evaporating black hole, and all three seemed to be entangled according to the usual understanding, but that was bad be
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by zachf 5y ago
A titanic intellect, he will be missed. Overall a fine remembrance piece, but weird that Woit insists on rehashing his tired arguments against naturalness at the end. There’s such a trivial counterargument to Woit’s objection that any numbe
48.
by zachf 5y ago
I'm going to have to butcher it a bit to make this fit in a comment :) Basically the thing you need to think about is the energy associated with the field itself. For the electromagnetic fields, the energy in the field is like (E^2 + B
49.
by zachf 5y ago
So you're right that (low-energy) gravitational physics is understood because you know how to write down the action. The question is whether the sum over paths in the path integral should also include a sum over metrics. That sum over
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by zachf 6y ago
Well the argument I presented shows that at least one state has a negative energy density at any given point (I suppressed the dependence on position, T = T (x) )— but the argument does not prove that this state has lower total energy than
51.
by zachf 6y ago
Good question! You can definitely add a constant to the Hamiltonian without changing the local physics, but I was referring to the stress-energy tensor (i.e. energy density), not the Hamiltonian (total energy). The stress tensor can be loca
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by zachf 6y ago
Penrose’s model of the brain needs to be more powerful than quantum computation, so that it can actually solve NP problems in P time and similarly amazing feats. If you’re interested in CS and quantum physics and whether there could be any
53.
by zachf 6y ago
Its a short and elegant proof set in pure theoretical general relativity (GR). The idea is that if there’s a sphere of space where if you try to emit light rays and the light rays don’t initially start separating, then they can’t start sepa
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by zachf 6y ago
The “physicist” explanation that I heard (meaning non-rigorous but good for building intuition) is that at every point where the derivative vanishes, for suitably random functions, every direction you move in will either be a direction wher