The one (listed under other problems) which always struck me since the first time I heard it:
"How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse or quantum decoherence, give rise to the reality we perceive? "
I still vividly remember the first time I heard the "collapse of the wavefunction" and openly challenged our poor Solid-state electronics professor - it led to a looong discussion at the end of which he got flustered enough that after shifting thru entanglement, decoherence, Measurement problems, many worlds interpretation, Bohm, time travelling interactions, philosophy, etc he pretty much kicked me out of his office.
I think I might have partly contributed to him seeking a transfer to another dept. the next year...
There is a bit of a parable I hear told sometimes when this topic comes up:
A man is being taught how computers work, and is presented with the design of an adder in the form of a few Boolean logic expressions. Thinking back to the true "reality" of addition, he rejects the design of the adder on the grounds that nand gates have no apparent connection to the sensible reality of having 5 apples and 3 oranges and 8 fruits.
This would be considered a ridiculous objection in every field but physics, where misunderstandings following from it have become the basis of many bestsellers. For some reason, we consider the math involved in physics to be exempt from the otherwise universal sense of abstraction that all the other fields enjoy. We don't go looking for tiny calculators in bacteria capable of plotting out exponential population growth, so why should the weirdness of quantum math cause so much trouble?
I'm not so sure this is a fair comparison. Anything that grows proportionally to its population will grow exponentially, math just says it will, and you can't avoid that. You can look at the bacteria itself and see that bacteria divide, and so, you expect this sort of growth, and so, it mathematically follows.
It just isn't that way for QM, nothing in nature says that systems have to evolve by a hermitian operator on a fock space, we just know that it gives us the right results.
I think part of the issue is that Physicists really care about why, not whether something fits the model.
I would disagree with you there, actually. As Scott Aaronson says:
"Quantum mechanics is what you would inevitably come up with if you started from probability theory, and then said, let's try to generalize it [...] As such, the theory could have been invented by mathematicians in the 19th century without any input from experiment. It wasn't, but it could have been.
[...]
In this lecture, I'm going to try to convince you -- without any recourse to experiment -- that quantum mechanics would also have been on God's whiteboard. I'm going to show you why, if you want a universe with certain very generic properties, you seem forced to one of three choices: (1) determinism, (2) classical probabilities, or (3) quantum mechanics. Even if the "mystery" of quantum mechanics can never be banished entirely, you might be surprised by just how far people could've gotten without leaving their armchairs! That they didn't get far until atomic spectra and so on forced the theory down their throats is one of the strongest arguments I know for experiments being necessary."
I don't really like arguing back and forth, but the way I read that is he is describing calculations in QM, not the "why" behind QM. QM is not a mystery in calculationally, it is a mystery in interpretation.
The generalization of probability theory to include "negative" probabilities, really, amplitudes is just the method in quantum mechanics. It's like knowing what a derivative is to obtain exponential population growth. That's just mathematics. No one says people who have solved dN/dt=aN predict that bacteria will grow exponentially, it's just how a system acts like that would behave, and we found an example.
But, I've thought a little more about it, I think my original comment was touching a personal bias more than making an argument. I asked, "why does the time evolution operator have to be on a fock space and why do P and X commute like ~i?" I'm not sure physics can answer beyond that at this point, since it matches experiment, we should or could just accept it the way it is.
> why should the weirdness of quantum math cause so much trouble?
The deep issue is not the weird math - that's just math and it's not even particularly weird - mostly just differential equations and complex numbers. It's the weird behaviour of physical reality that's puzzling. Take for example entangled photons going through detectors a long way apart. The state of the detector at one end has something like a faster than light effect on the outcome at the other end. It's easy to write equations for it but how on earth does it work?
yep, I find it interesting that on a relatively small budget (hobbyist level) you can reproduce an experiment which exhibits unexplained physical phenomena.
It gives me some home that there's still interesting experimental physics to be done on a low budget. Potentially by hobbyists outside of a traditional industrial or academic framework.
Huh? The luminisence is for very short periods of time, but I don't believe it's particularly challenging to detect. I would imagine it's more challenging to setup the chamber. The first paragraph of the wikipedia article should tell you it's not that hard to detect:
"H. Frenzel and H. Schultes put an ultrasound transducer in a tank of photographic developer fluid. They hoped to speed up the development process. Instead, they noticed tiny dots on the film after developing and realized that the bubbles in the fluid were emitting light with the ultrasound turned on."
Other articles suggest photomultipler tubes have been used, with which you can detect single photons. PMT are relatively cheap (I've seen them for ~30USD on ebay), the motivation seems to be to get good time domain resolution. Even so other pages suggest that around 500,000 photons are produced. This should be relatively tractable.
Obviously it's not as easy as making an LED flash, but I can't see that it would be intractable for a hobbyist.
Some large features of the microwave sky at distances of over 13 billion light years appear to be aligned with both the motion and orientation of the solar system. Is this due to systematic errors in processing, contamination of results by local effects, or an unexplained violation of the Copernican principle?"
Wait what? An unexplained violation of the Copernican principle? That's pretty crazy. What is being referred to? The article on the CMB says that it's isotropic to one part in a thousand
The anisotropy of the CMBR is easily explained in terms of red/blue shift due to the velocity of the earth, solar system, galaxy etc.., as long as you don't mind violating special relativity. (in some sense)
I find the "Unsolved in AI" funny. One sentence is enough for the entire article: "We have no idea what the problem is but when we do we should come back and make this list."
Reading through these makes me really miss my undergraduate days as a physics major.
Only marginally related:
If you ever have a chance to go to one of the LIGO [1] sites (Laser Interferometer Gravitational-wave Observatory), do go take a tour! I've been to the one in Livingston, Louisiana [2], and it is simply amazing. The technology used and the science involved is very interesting. It's also kid-friendly!
It's been a while since my physics degree but aren't the "cosmological constant problem" and the "vacuum catastrophe" two different phrasings for the same problem? reading through to the linked pages doesn't help me in understanding what the difference between the two is (or if there is any).
I still vividly remember the first time I heard the "collapse of the wavefunction" and openly challenged our poor Solid-state electronics professor - it led to a looong discussion at the end of which he got flustered enough that after shifting thru entanglement, decoherence, Measurement problems, many worlds interpretation, Bohm, time travelling interactions, philosophy, etc he pretty much kicked me out of his office.
I think I might have partly contributed to him seeking a transfer to another dept. the next year...