

List of Unsolved Problems in Physics - lukas
http://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics

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suprgeek
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...

~~~
whatshisface
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?

~~~
noobermin
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.

~~~
kamilner
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."

[http://www.scottaaronson.com/democritus/lec9.html](http://www.scottaaronson.com/democritus/lec9.html)

~~~
noobermin
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.

May be there is no "deeper" reason after all.

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new299
I always find reading over this article pretty entertaining, in particular
Sonoluminescence generally catches my attention:

[http://en.wikipedia.org/wiki/Sonoluminescence](http://en.wikipedia.org/wiki/Sonoluminescence)

As it seems like it should be practical to replicate it on a relatively small
budget.

~~~
stolio
Looks like they can reproduce it at will but they can't figure out why it
happens.

~~~
new299
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.

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bglazer
"Ecliptic alignment of CMB anisotropy

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

~~~
Ono-Sendai
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)

See
[http://en.wikipedia.org/wiki/Cosmic_microwave_background](http://en.wikipedia.org/wiki/Cosmic_microwave_background),
'CMBR dipole anisotropy'

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ggonweb
The list of unsolved problems by field
[http://en.wikipedia.org/wiki/Lists_of_unsolved_problems](http://en.wikipedia.org/wiki/Lists_of_unsolved_problems)

~~~
Beltiras
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."

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cmpb
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!

[1] [http://www.ligo.caltech.edu/](http://www.ligo.caltech.edu/) [2]
[http://www.ligo-la.caltech.edu/](http://www.ligo-la.caltech.edu/)

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mijoharas
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).

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lkbm
> The beauty and clearness of the dynamical theory, which asserts heat and
> light to be modes of motion, is at present obscured by two clouds.

Lord Kelvin's two clouds led us to relativity and quantum mechanics.

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spanko_at_large
"Whats a Z job?" "If you have to ask you can't afford it" -Beerfest

"What are some unsolved physics problems?" "If you have to ask you can't solve
them" -spanko

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calebm
I want a room-temperature superconductor!

