What Are Bee's Top Ten Unsolved Questions in Physics?

[marcus]

there has been a rumbling of lists in the Bloggery and Bee rose to the occasion at Backreaction http://backreaction.blogspot.com/
by presenting her LIST OF STUPID TITLES of professional articles on arxiv
http://backreaction.blogspot.com/2006/07/stupid-title-list.html
Bee also linked to a good discussion of major open problems in physics by John Baez:
http://math.ucr.edu/home/baez/physics/General/open_questions.html

Bee had some other lists as well, and some links to other people's lists of physics questions. It included thoughtful speculation by David Mermin
http://www.physicstoday.org/pt/vol-54/iss-2/p11.html
(begins with a not-very-informative list published in 2000 by the NY Times but then Mermin tells you his own ideas)
==================

this is just by way of introduction to Bee's TOP TEN PHYSICS QUESTIONS, which is a very handy list. It would be interesting to know how each of us would guess the answers. these are major unsolved problems, stated very concisely. One can't know the answers, but one can have suspicions and ideas about how it will go. I like this list so much I will copy it from here:
http://backreaction.blogspot.com/2006/07/top-ten.html#my

 

[marcus]

here is Bee's top ten
http://backreaction.blogspot.com/2006/07/top-ten.html#my

Unsolved Questions in Theoretical Physics
1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.

2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?

3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?

4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?

5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?

6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?

7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?

8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?

9) Can we understand quantization?

10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?

I want to add that imo solving 3 will solve 2, and 9 will help with 1.

====HERE ARE MY REACTIONS=====
Please, if you feel like it, put down your guesses too. Here are mine:

1. How can the disagreement be resolved? By implementing spacetime geometry and matter in the same 4D world model. Will this quantize spacetime geometry (i.e. gravity)? Yes---or else it will uncover something deeper from which quantum theory emerges.

2. No they don't destroy information. The matter comes out the bottom in a re-expansion. Evolution is unitary but time forks.

3. No there are no singularities. Singularities are just the failure of a theory and do not exist in nature. Gravity is repellent at high density and there is a bounce.

4.

5.

6.

7. We experience 3+1 dimensions because there are 3+1 dimensions. No, there are not "extra" dimensions.

8. There are no particles. The "particles" of the SM model are not elementary. Matter is an aspect of spacetime geometry (see recent Freidel, recent Baez, Smolin, and others). Electroweak and strong DO unify because all interaction of matter with matter and of matter with geometry is PART of the same comprehensive spacetime-matter model.

9. Yes. If you are good you will be allowed to learn the secret.

10. This was already asked as part of question 8. so I think Bee put it on just so she would have a full ten.

 

[Kea]

I'll have a go!

1)Machian principle from Gray categories. Quantum theory more fundamental - GR derived.

2)No - and stop thinking classically.

3)Clearly GR does not apply in such regimes.

4)DE - see Padmanabhan (think cohomology). Coincidence problem - new form of locality kills this paradox.

5)TeVeS MOND.

6)Flatness follows from the basic principle. Inflation (effective) - do old BC idea in new state sum context to get eqn of state.

7)Naive extra dimensions are a REALLY silly idea.

8)We're working on this one...but even Heisenberg knew that particles were not elementary.

9)Yes.

10)The broken parity cube helps 'generate geometry'. Of course masses are just quantum numbers.

 

[marcus]

Kea's answers seem interesting, so for easier reading I will put them with the questions
======================
Unsolved Questions in Theoretical Physics

1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.
Machian principle from Gray categories. Quantum theory more fundamental - GR derived.

2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?
No - and stop thinking classically.

3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?
Clearly GR does not apply in such regimes.

4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?
DE - see Padmanabhan (think cohomology). Coincidence problem - new form of locality kills this paradox.

5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?
TeVeS MOND.

6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?
Flatness follows from the basic principle. Inflation (effective) - do old BC idea in new state sum context to get eqn of state.

7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?
Naive extra dimensions are a REALLY silly idea.

8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?
We're working on this one...but even Heisenberg knew that particles were not elementary.

9) Can we understand quantization?
Yes.

10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?
The broken parity cube helps 'generate geometry'. Of course masses are just quantum numbers.

 

[Kea]

Ooooooh, Marcus, this is fun! Can we have more questions? GZK maybe?

 

[marcus]

... this is fun! Can we have more questions? GZK maybe?
...

Your answers were fun---more than that: provocative, consciousness-raising ----not sure what the word is.

but no one else answered yet. maybe top ten questions don't work here at PF right at the moment.

or no! maybe what we should do is NOT WAIT for other people to put their answers. we should just discuss the replies (yours and mine) that we already have

the trouble is that several of your answers, when I scan back at what is written in blue, look so SENSIBLE!
I can't imagine how anyone could have any doubt! Isn't that strange. these look like the obvious answers, so who could disagree and how is controversy possible?

And then what you said for #10, I don't understand at all, or #4, so I can't disagree with those either. Either it is self-evident or incomprehensible----like so many things.

 

[Kea]

And then what you said for #10, I don't understand at all, or #4 ...

Hi Marcus

I think 4 needs the most work. 10 is clear, but involves lots of diagrams.

 

[Kea]

...so who could disagree and how is controversy possible?

Well, we could be completely wrong, I suppose. Maybe we're all deluding ourselves into thinking it actually makes sense after too many years of confusing ourselves silly.

It does seem to me, though, that any other candidate picture has to at least have a decent go at answering all the questions. Maybe we should make up a String theory answer list - but I can't get one to make sense!

 

[Kea]

The broken parity cube helps 'generate geometry'.

To write down a (2-topological) spin foam model including masses we just need to remember that instead of functors made out of tetrahedra we now have functors using Street's orientals in the next dimension up. The problem with horizontal composition of 2-cells forces one to put in parity cubes (bits of weak cohomology).

The 4-arrow inside the cube obeys a nice Stasheff polytope cocycle condition...related to a modified tetrahedron equation, solutions to which have naturally been studied by some of the 3D lattice model gurus:

The modified tetrahedron equation and its solutions
G. von Gehlen, S. Pakuliak, S.Sergeev
http://arxiv.org/abs/nlin/0303043

Zamolodchikov's Tetrahedron Equation and Hidden Structure of Quantum Groups
Vladimir V. Bazhanov, Sergey M. Sergeev
http://arxiv.org/abs/hep-th/0509181

Thus we get our 14j (or whatever) symbols. Piece of cake. Well, I'm off to a conference tomorrow, so have fun.

 

[marcus]

Arrrgh, Kea! Arrgh arrgh arrgh!

 

[Kea]

Here is a Crans paper that talks about the ordinary tetrahedron equation:

Lie 2-Algebras
Alissa S. Crans
http://arxiv.org/abs/math/0409602

Maybe John can tell us more about this.

 

[Kea]

It has not escaped our attention that the weak cohomology thus indicates the existence of a mass gap, a cosmological constant and three generations in the standard model.

 

[f-h]

I like you Kea. You do Algebra (even when you do geometry) and it's pretty, but I still don't understand a fraction of it.

Still interferes with my idea of doing physics by digging deep in the known and working theories rather then building high out of pure abstraction, but damn it looks pretty. Building into the sky is always so much clearer then digging deep. ;)

I'll have a go with some wild (minimalist, conservative) speculation:

"1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3."

No quantization of GR required, though it supplies good hints. some modifications to quantum mechanics in the "Problem of time" sense, but Hilbertspace+Operator algebra is sufficient. Both ordinary QM of particles and geometry of spacetime are limits of a unified form of degrees of freedom.

"2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?"

No, though not all information may be accessible in all timeslices.

"3) Are there really singularities in GR (inside black holes/big bang)? "

I think you meant in reality, in GR, yes of course. See the singularity theorems.

"If so, how can we understand what happens there?"

Theres no problem with that as long as censorship holds. It's just a bit of infinity brought closer to home. I really would like to emphasize this point, people always say that singularities indicate the breakdown of GR, as far as I have seen there is nothing in the singularities ordinarily encountered in GR that renders the theory inconsistent. These singularities could be a perfectly consistent way for reality to operate.

"If not, how are they avoided?"

Our notions of space and time break down. See unified degrees of freedom above.

"4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?"

Cosmological constant. A term in the effective action of GR.

"5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?"

TeVeS MOND.

6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?

Flatness arises from renormalization.

7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?

Because you can't tie your shoes in more (or less) then 3 spatial dimensions.
As the concept of geometry breaks down for the fundamental DoF, so does the notion of dimension. (Stringtheorists 10=11)

"8) Are the electroweak and strong interaction unified at high energies?"

In some sense, yes. They are both effective forces arising in the low energy limit after all.

"If so, are the currently known particles of the standard model (SM) elementary?"

No. The very concept of particle is not fundamental.

"Are there more yet unobserved particles?"

Yes.

"Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?)."

Accidental.

"Why are the gauge groups of the SM what they are?"

A handfull of generic features of the underlying theory, plus the fact that at low energies all theories look like renormalizable theories via Wilsons argument.

9) Can we understand quantization?

Do we have to? I never liked it to begin with.

Can we understand the opposite process though? How a classical low energy theory arises.

10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?

Mass is generated effectively via the Higgs mechanism. What it effectively arises from? Same as above.

 

[selfAdjoint]

Still interferes with my idea of doing physics by digging deep in the known and working theories rather then building high out of pure abstraction, but damn it looks pretty. Building into the sky is always so much clearer then digging deep. ;)

If you get too woozy from the stratospheric math, take a hard analysis break with "[/URL] by Schnabl. All the old time goodies; Bernouilli numbers, Stirling numbers of the first kind, the whole down-and-dirty works.

(Thanks to Cosmic Variance for the link)

 

[marcus]

Both Kea's answers and FH's seem interesting, so for easier reading I will put them with the questions
======================
Unsolved Questions in Theoretical Physics

1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.
Machian principle from Gray categories. Quantum theory more fundamental - GR derived.
No quantization of GR required, though it supplies good hints. some modifications to quantum mechanics in the "Problem of time" sense, but Hilbertspace+Operator algebra is sufficient. Both ordinary QM of particles and geometry of spacetime are limits of a unified form of degrees of freedom.

2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?
No - and stop thinking classically.
No, though not all information may be accessible in all timeslices.

3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?
Clearly GR does not apply in such regimes.
I think you meant in reality, in GR, yes of course. See the singularity theorems.
"If so, how can we understand what happens there?"
Theres no problem with that as long as censorship holds. It's just a bit of infinity brought closer to home. I really would like to emphasize this point, people always say that singularities indicate the breakdown of GR, as far as I have seen there is nothing in the singularities ordinarily encountered in GR that renders the theory inconsistent. These singularities could be a perfectly consistent way for reality to operate.
"If not, how are they avoided?"
Our notions of space and time break down. See unified degrees of freedom above.

4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?
DE - see Padmanabhan (think cohomology). Coincidence problem - new form of locality kills this paradox.
Cosmological constant. A term in the effective action of GR.

5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?
TeVeS MOND.
TeVeS MOND.

6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?
Flatness follows from the basic principle. Inflation (effective) - do old BC idea in new state sum context to get eqn of state.
Flatness arises from renormalization.

7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?
Naive extra dimensions are a REALLY silly idea.
Because you can't tie your shoes in more (or less) then 3 spatial dimensions.
As the concept of geometry breaks down for the fundamental DoF, so does the notion of dimension. (Stringtheorists 10=11)


8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?
We're working on this one...but even Heisenberg knew that particles were not elementary.
In some sense, yes. They are both effective forces arising in the low energy limit after all.
"If so, are the currently known particles of the standard model (SM) elementary?"
No. The very concept of particle is not fundamental.
"Are there more yet unobserved particles?"
Yes.
"Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?)."
Accidental.
"Why are the gauge groups of the SM what they are?"
A handfull of generic features of the underlying theory, plus the fact that at low energies all theories look like renormalizable theories via Wilsons argument.

9) Can we understand quantization?
Yes.
Do we have to? I never liked it to begin with.
Can we understand the opposite process though? How a classical low energy theory arises.


10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?
The broken parity cube helps 'generate geometry'. Of course masses are just quantum numbers.

Mass is generated effectively via the Higgs mechanism. What it effectively arises from? Same as above.

 

[Careful]

Well, we could be completely wrong, I suppose. Maybe we're all deluding ourselves into thinking it actually makes sense after too many years of confusing ourselves silly.

It does seem to me, though, that any other candidate picture has to at least have a decent go at answering all the questions. Maybe we should make up a String theory answer list - but I can't get one to make sense!

Let me confess a ``secret''. Some six years ago - when I had to give a presentation for a course - I gave a lecture about the de Rahm theorem in which Cech cohomology and more abstract ideas concerning long exact cohomology sequences associated to chain complexes played an important role. I was enchanted by the elegance and simplicity of the ideas involved, studied some part of Saunders Mc Lane's book and looked into some papers about quantum gravity based upon quantum logic. I thought about using these abstract cohomology ideas in connection to the inverse problem in causal sets but in a way that is not based upon sprinklings of course (that was ``cheating'' in some sense). But, there was a big but. Causal sets are generically not like (topological) manifolds at all - manifoldness being a very delicate property - so whatever definition one would come up with, it would generically give ``bad'' results. Likewise, one immediately realized that finding a dynamics which would result in a manifold -like universe on sufficiently large scales is far from trivial - actually almost a miracle (although the Sorkin-Rideout dynamics has partial succes here; note that these observations are not bounded to causal sets).

Therefore, it occurred to me that these ideas - despite how beautiful they are - are not going to give us much chance for deeper insight in nature - at least not in the forseeable future. They are simply too ``untestable'' and general (and unfortunately didn't shed much new light on QM). For example, the preon idea is cute but it occurs to me that this view is again an effective picture ((10^{15})^4 orders of magnitude larger) of a fundamental Planck scale dynamics (a common idea to most QG approaches). Is it realistic to expect a natural reason to exist why this should be so ? Also, I was remembered at the blissfull properties of the continuum which makes it so much easier to write down dynamical laws; and why should the continuum decription not be adequate ? Why should a renormalization transformation (rescaling) reveal (flat ?) manifold like behavior when the foam is crazy as hell ?

Is the world truly that crazy ? Isn't it better to understand less wild alternatives first, at a deeper level and test them by a realistic experiment (not about how many angels ... ) ? Again, I and many others (I believe) would truly appreciate it if you could give an example which would make this somehow more plausible.

Careful

 

[Kea]

I like you too, fh.

You do Algebra (even when you do geometry) and it's pretty...

Well, that's category theory - geometry and algebra - and logic.

Still interferes with my idea of doing physics by digging deep in the known and working theories rather then building high out of pure abstraction...

There is no substitute for digging and digging. Remember that I have the advantage of being a little older than you, and I did my time.

 

[Kea]

I was enchanted by the elegance and simplicity of the ideas involved, studied some part of Saunders Mc Lane's book and looked into some papers about quantum gravity based upon quantum logic...

Ideas have their time. For a long while now, people have thought about these things. But only in 1995 did Gordon, Power and Street figure out how to define tricategories, and they are still not very well understood.

But, there was a big but...manifoldness being a very delicate property...

You are right, Careful. A great deal of work remains to be done before we can say we understand 2-topologies. We are still awaiting a new PhD thesis by a mathematician who has been working on these structures.

Is the world truly that crazy?

As always, yes.

 

[Hurkyl]

So just what is a 2-topology? Is it like a topological category, or a Grothendieck topology, or something else?

 

[Kea]

So just what is a 2-topology?

A Grothendieck topology is a proper 1-topology (meaning it knows about topos theory) because it is a topology for 1-categories. Like I say, no one really understands the mathematics of 2-topologies. However, for practical physicists it shouldn't be that complicated. There are hints to the right way to do it in Baratin-Freidel.

Actually, even 2-topology is the wrong word, because we are into 3-categories here. That is, the topos point of view is telling us to think more carefully about categorification (going up one dimension).

 

[Careful]

***
You are right, Careful. A great deal of work remains to be done before we can say we understand 2-topologies. We are still awaiting a new PhD thesis by a mathematician who has been working on these structures.
***

For sure, a desirable thing would be to find very robust criteria for manifoldness (measure theoretical sense). Even more difficult: one has to find criteria which can be implemented by physicists (*practical* ideas) - that is where it might become impossible !

***
As always, yes.
***

Well, I am not a specialist in Bell tests - although I know quite a bit more about them than most do. It is my impression that it is going to be terribly difficult to perform a test violating locality without appealing to some fair sampling and/or perfect measurement device hypothesis (and basically I don't believe that it will ever be done). Moroever, it is known from (realist) theories how both assumptions can be violated without ``conspiracy'' (there are still more subtle possibilities but I am not very well ``into'' these). On the other hand, I never believed that looking for higher symmetry groups was the right way to go (the high energy physicist's strategy), so my picture of the world is still crazy enough, but not *that* wild as you imagine it to be. Or let me put it more accurately, I do not think that it *should* make much of a difference if you make it that wild or not. Unfortunately, releasing any background structure is setting yourself back for many decades on issues which you could assume to be true to start with (and in contrast to some, I do not think that this is a necessary thing to do - so you would still not be adressing directly the key issues).

Careful

 

[Chronos]

I infer these are the nominees for the 10 most interesting unanswered questions in physics. I can't help but wonder what the 10 most uninteresting unanswered questions might be – e.g., How do clothes dryers invert shirts?

1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.

Gravity cannot yet be quantized because, unlike GR, quantum theory is incomplete. GR should, IMO, naturally emerge from a correct and complete quantum theory

2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?

This is an entropy question in essence. Destruction of information could be construed as a form of negative entropy [sort of like negative mass]. I think you could just as easily ask – Do black holes destroy time? The answer is no -, information, like time, becomes frozen at the event horizon. It leaks back slowly via Hawking radiation until the balance is released when the black hole finally evaporates.

3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?

Yes, there are singularities in GR, but I think they are mathematical artifacts that will disappear when we learn how to correctly quantize gravity. I vote for the Planck density as the cutoff.

4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?

Dark energy works for me in the form of Einsteins cosmological constant. I'm not so sure about the coincidence problem. I think the 'time' when acceleration appears to kick in is observer dependent. I do not see how you can avoid this conclusion without conferring Earth a priveleged location. Explaining this is more difficult. Could it be a relic of inflation?

5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?

Dark matter works fine. It emerged much sooner after the big bang than did baryonic matter, hence the energies required to produce it in the laboratory may be inaccessible. I am confident, however, we will find it. Keep in mind it took about 2000 years to 'find' the atom.

6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?

Inflation rules. It was a phase transition. Electroweak symmetry breaking is probably a different aspect of the same mechanism responsible for the matter – anti-matter imbalance.

7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?

Dimensions are a human concept and 3+1 dimensions are the minimum necessary to describe the observable universe. Extra dimensions are unphysical, but useful mathematical tools.

8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?

Even gravity should unify at sufficiently high energies. There are no truly elementary particles. Particles are low energy phase transitions. I would guess the Higgs boson followed by the DM particle represent the highest energy levels at which any 'particles' can exist. That does not necessarily mean they are the most massive, they could just as easily be extremely light particles. Some parameters of the standard model must necessarily be what they are without a casual explanation. I suspect you don't need as many as currently envisioned, but we surely need at least one – and probably no less than 4. Why 4? Perhaps I have a crackpot reason I choose not to share [one for space, one for time, one for gravity, and one for the electroweak/strong force: is that lame or what?].Gauge groups arise from the irreducible physical parameters. I do however think some of the gauge groups are unphysical and will eventually be discarded.

9) Can we understand quantization?

Not at present. While current renormalization procedures do a nice, and sometimes superb job of approximating results, current renormalization procedures are mathematically unsound. This is another indicator quantum theory is incomplete.

10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?

I like the Higgs boson. Its a good thing the Planck mass is much larger than the elementary particle masses. The universe would be a very dull place [i.e., no humans to observe it] if elementary particles were very much closer to the Schwarzschild limit.

 

[CarlB]

I see that we are being invited to speculate. (And to buy charge preamplifiers from cremat.com)

here is Bee's top ten
http://backreaction.blogspot.com/2006/07/top-ten.html#my

Unsolved Questions in Theoretical Physics
1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.

The disagreement is not just "apparent" it is deep and significant. It can only be resolved by unifying the two theories. Both are correct and beautiful in their own domains, but both are getting a bit ragged trying to keep up with the latest corrections. It's time to look for a simple theory that will allow both to be correct, but only in their own domains.

Our primary problem is that in our physics, the concept of time is too simple. Instead of being able to divide time into past, present and future, as is intuitively obvious, our physics is written as if all times are equivalent. Then our savants write books on where the arrow of time comes from. Some of them try to convince us that the past and future are just an illusion. Some of us are convinced.

To try to quantize gravity is a mistake. We do not yet truly know what "quantize" means, so extending the technique to a domain many orders of magnitude away from the domain quantization was developed in will be a waste of effort. What we need to do is to understand quantum systems.

2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?

In the sense of GR, yes, black holes destroy information. But GR is incomplete. In particular, GR assumes that there is no universal time. If one can convince oneself of this, then anything is possible, going back in time, etc. The fact that none of this is seen in our world should put pause to the people who believe this, but they seem to have no problem swallowing the claim. You take two theories, GR and QM, and extend them BOTH to way way outside the realms in which they are verified, and then, lo and behold, the result is that you get weird consequences. And this decades after everyone knew that the two theories were fundamentally incompatible. This is not physics, it is barely mathematics.

My guess is that the matter that collapses into a black hole is emitted from the black hole as radiation that travels somewhat faster than light. I don't think that this process conserves energy, in the usual way we measure it (which is based on a physics determined by relationships observed only at very very low energies). I'd tell you how to derive these superluminal particles (see comment on inflation below), but you're too busy and probably know too much.

3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?

No, there are no singularities. The density of matter reaches a maximum. To understand what goes on there you need to first understand that we are very small and weak and the universe is very big and made of very sturdy material. Even our whole galaxy is very tiny, and the processes contained in it very weak. The universe is a very tenuous thing.

The reason that physics is simple is because it is all done as linear approximations. The reason linear approximations work so well is that we are very small, our energies are low, while spacetime is very big and its inherent strength is very large. When we use GR to model spacetime, it is GR that gets all non linear and curvaceous, not spacetime.

It used to be known that in QM it was impossible to create a hidden variables theory. The big boys provied it impossible. Then Bohm showed how to do it. Whoooops. The same thing is going on in GR. It used to be known that spacetime was inherently non Euclidean. Now several people have shown that it is possible to get GR on a flat spacetime.

So what should you believe? That moving around matter can alter the spacetime that has existed for billions of years? Or that spacetime is unalterable by the puny efforts of our tiny little galaxy? Now that we have theories that have it both ways you can decide for yourself. No mathematics will make the choice for you. But Occam's razor will tell you that you should assume that spacetime cannot be destroyed until it is proved otherwise to you. Note that it is only if you assume that spacetime is weak enough to be bent by matter that you have to explain how it is that we're not already stuck in a big black hole.

4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?

When we look at the list of known particles, it is quite evident that there must be some further order that is organizing them. When that order is better known, these issues will likely be resolved.

5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?

I'll guess that dark matter does not exist, and that instead, Milgrom is right with MOND. Everyone would love the theory except it doesn't have a theoretical underpinning. As long as QM and GR run around loose in their own restricted domains it will not be possible to derive MOND.

The MOND theory is about how very low accelerations work. An acceleration is a change in velocity. Let's do our QM in a box. Velocities are quantized, therefore changes in velocity are also quantized so acceleration is quantized. Under this assumption, a MOND effect when dealing with very low accelerations is not that surprising.

In QM, when we model a particle in a box, we let the dimensions of the box approach infininity so that the number of velocities become infinite. Or do they. The number of velocities is always countable, but we're hoping that the limit will be an uncountable number of velocities? Nope, mathematics doesn't work like that. And besides, the universe is finite, it is unphysical to take a limit as the dimension of the box goes to infinity.

6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?

Inflation is the solution for the homogeneity problem. Inflation is the natural consequence of assuming that the usual elementary particles are composite and the preons travel faster than light. I'd show you the mathematical details but you either don't have enough mathematical sophistication or you are too busy.

When physicists talk of electroweak symmetry breaking, the underlying assumption is that of perturbation theory. The world is inherently non perturbative, and looking at it through perturbation colored glasses causes some serious confusion. The assumption of symmetry breaking in the elementary particles is that these particles should all be equivalent at high enough energies, and it is only in the cold conditions now seen that the symmetries are lost. The assumption is that the "vacuum" can choose anyone of a number of orientations, and that the choice of orientation determines which particles are which.

You are too busy for me to explain to you why it is that the vacuum is an unneeded artifact of QM. Ask Julian Schwinger's ghost if you don't have the time for me to explain it to you. The short description: the vacuum state only exists to the extent that you convince me that you can split a density matrix state description into a state vector state description. I can demonstrate at great length that the density matrix formulation is superior, for example see: https://www.physicsforums.com/showthread.php?t=124904 but this will have little effect because you are too busy and know better.

The primary disadvantage of the state vector formalism is that it gives you great freedom to define the internal symmetries of your particles. This means that if you want to suppose that the structure of the elementary particles arises from simpler subparticles, you have to look through huge numbers of possible types of subparticles. Geometric density matrix theory is very very restrictive and leads you to a unique solution. But again, you have very little time.

So a result of analyzing the elementary particles from the point of view of state vector formalism is that there will always be huge amounts of unphysical (gauge) freedom running around. One of the features of that sort of freedom is that you can convince yourself that relationships that, in fact, are very discrete and exact, are instead, continuous. The principle of symmetry breaking is one of these.

As an example of the very (unpredicted by standard model) discrete relationships between the elementary particles, take a good look at the current experimental limits on the MNS matrix. Is that the result of symmetry breaking? As long as you assume it is, without specifying what it is that the symmetry breaking comes from, you will always have billions of ways of describing the particles.

The baryon / antibaryon asymmetry comes from the fact that spacetime is an elastic solid. In an elastic solid, vibrations consist of regions of higher and lower densities. So long as the vibrations are very small (i.e. our current low energy regime) the two types of vibrations are symmetric. But at high enough amplitudes, the symmetry is broken by nonlinearities in the elasticity.

In an elastic medium, there are always two classes of moving waves, longitudinal and transverse. The longitudinal waves always travel faster, typically by a factor of about sqrt(3). In spacetime, the transverse waves travel at the speed of light. The longitudinal waves are too high energy to be commonly observed, but were responsible for the inflationary era.

7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?

There is one hidden dimension. Tightly curled up, it corresponds to proper time. Have we noticed it? Of course. Einstein noticed it in 1905. Why don't more physicists believe in it is a better question.

As soon as you work through the consequences of a single hidden dimension, with the assumption that matter and energy consists of waves in the elastic medium defined by that manifold, you can derive the special theory of relativity and a good bit of quantum mechanics. But rather than do this, isn't it better to forget about proper time, and instead develop two different theories, QM and, from SR, GR, that are incompatible? We wouldn't want to put any theorists out of business here.

8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?

No, the hope that the strong and electroweak interactions are unified at high energies is a result of the hopes and dreams of perturbation theory. The currently known particles are not elementary. They're not even one stage away. The parameters of the standard model are exactly determined by the physics of the interactions that bind the particles of which they are composed.

The gauge groups arise from the splitting of density matrices into spinors. Density matrices already eliminate the U(1) gauge symmetry. Suitably generalized, one can similarly eliminate the other gauge symmetries. To get them back, simply split the generalized density matrix back up into spinor form. Hey, you're too busy to understand this.

9) Can we understand quantization?

Yes. Most literally, quantization is the process that takes a wave and allows it to be interpreted as a particle. This is the measurement process. But to understand it, we have to step outside of time as it is used in physics, and instead contemplate time as it is felt by observers. That is, we have to think of time as separated into past, present and future. The wave function is the event as it exists in our future. The particle is the event as it exists in our past. The measurement is the process of evolution of the universe that allows an event that was in the future, to become a part of the past. Note that this evolution is not the unitary evolution of QM, which has only to do with how one evolves a wave state at one time to a wave at another time.

10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?

The fundamental particles should have Plank mass energies. Binding two particles into one produces a new particle with a mass less than the sum of the masses by the binding energy. If the binding energy is on the order of twice the Plank mass, then binding two Plank mass particles together will give a deeply bound composite particle with very low mass.

A group of Plank mass particles are bound together. The result has a lower mass than the Plank mass, and this is caused by its having a smaller strain on spacetime. A number of such groups can then combine again, possibly creating a new bound state that has even lower energy. The process continues, with each new hierarchical level having lower characteristic energies.

The hierarchy between the masses of the particles mostly arises from density matrix theory. In density matrix theory, only the spin-1/2 particles have clean and simple geometrizations. That suggests that using a scalar Higgs particle to give mass to the leptons is unrealistic.

To replace the Higgs with a spin-1/2 type particle, you must make it as an effective scalar, and that means replacing the usual bilinear mass term with a doubly bilinear mass term:

$$m h \psi^\dag \psi \to m^\dag m \psi^\dag \psi$$

In the above, note that in going to a purely spin-1/2 formalism we hope to lose the arbitrary constant mass term. The result is that instead of masses of elementary particles being simple, the square roots of their masses are simple. This gives the Koide formula, which can also be expanded to cover the neutrinos (i.e. predict the neutrino masses):
http://www.brannenworks.com/MASSES2.pdf for example.

Carl