Thursday, May 10, 2012

Top Ten

This is a repost and update of a six year old post in which I listed what I think are the most interesting and pressing open problems in theoretical physics, or at least the area that I work in, quantum gravity. I thought it might be worthwhile to revisit. This list doesn't even pretend to be objective - it omits entire areas of theoretical physics - it is mainly a reflection of my personal interests; a summary of puzzles I find promising to spend brain time on.
  1. How can the apparent disagreement between general relativity and quantum gravity be resolved? Does it require to quantize gravity?
    (Still 1. Haven't changed my mind about that.) 
  2. Can we understand quantization?
    (Up from 9. The more I think about it, the more I believe our problem with quantizing gravity is in the quantum part, not in the gravity part.)
  3. Do black holes destroy information? What happens to the matter that collapses to a black hole?
    (I think we spent enough time on the black hole information loss problem. It would be fruitful to instead think about what happens to matter at Planckian densities. Down from 2.)
  4. Are the electroweak and strong interaction unified at high energies? If so, are they also unified with gravity?
    (That is, is there a theory of everything? Up from 8. I'm undecided whether or not unification is helpful to the problem of quantizing gravity.)
  5. Are the currently known particles of the standard model (SM) elementary? Are there more so far unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other? Why are the gauge groups of the SM what they are? Is it even possible to uniquely answer this question?
    (Formerly 8, minus the question for unification plus the question whether there's a unique answer.)
  6. Did the universe start with a big bang, a big bounce or something else entirely?
    (This is a reformulation of the earlier question number 4 which focused on singularities specifically. Down from 3.)
  7. Why do we experience 3+1 dimensions? Are there extra dimensions? Does the effective dimension of space-time decrease at short distances?
    (This is an extension of the earlier question 7, taking into account that dimensional reduction to 2 dimensions has received some attention recently.)
  8. Why is gravity so much weaker than the other interactions?
    (Up from 10.)
  9. Does dark energy exist? If so, what is it? Is the coincidence problem more than a coincidence?
    (Down from 4. I think that the dark energy puzzle is possibly a relevant hint for quantum gravity. But then, maybe not.)
  10. How do we correctly assign an entropy to gravitational degrees of freedom? Is this testable at all?
    (Newcomer.)
Dark matter has dropped off the list. I think we're well on the way to finding some direct evidence for dark matter. It will be difficult to pin down, but at this point it seems unlikely to me that it will be relevant for quantum gravity.

45 comments:

Glen Mark Martin said...

It would seem to me that the solution to #3 depends very strongly upon one's reference frame. From the standpoint of an observer exterior to the event horizon, anything going into the black hole never quite reaches it due to time dilation, but rather asymptotically approaches the event horizon. In essence, nothing happens to the matter because it never actually reaches the black hole. To an observer within the event horizon (poor soul), they would perceive a virtual event horizon that has a dip in it, where they are at the bottom of the dip, and they would perceive the same behavior as the first case, with the matter approaching but never reaching this virtual event horizon. As for an observer at the singularity, well, what does that even mean?

PTMR said...

What is the main evidence that gravity is weak? I know it looks like it is weak on macroscopic scales but the same holds for EM forces on large enough scales and we now it's simply due to cancellation.

So what prevents gravity from also being strong on short enough scales and mostly canceled on macroscopic scales?

For a hypothetical toy example let's imagine that each electric charge induces a strong local spacetime curvature whose sign depends on the sign of the charge. Now lets also imagine that this hypothetical effect is tiny bit stronger in case of quarks then it is in case of leptons. In this setup electrically neutral atoms would only induce very weak residual curvature.

Are there any obvious reasons why such ideas are ruled out by experiments? I know that this would make anti-matter exert anti-gravity, but AFAIK gravitational force exerted by anti-matter has not been measured.

Juan F. said...

Hello Beee!!!

On your 1st Question. Not Doctor Who?Just kididing...XP: How can the apparent disagreement between general relativity and quantum gravity be resolved? Does it require to quantize gravity?

Personally, I believe quantizing gravity is unavoidable! However, gravity is different from the rest of the forces. It couples to everything, even to itself! I agree this is likely one of the biggest challenges of we, theorists. And something must have scaped our attention that makes the problem harder that what it should! I also believe that QM and GR, both, will be modified somehow when the correct QG theory arises...Are QM and GR emergent as the new "emergence" paradigm defends? REally, it is not new at all. It is a new twist to the effective theory paradigm, not a new one (only some subtle details that maybe make it different subtly for outsiders).

On the 2nd question: Can we understand quantization?

Zeilinger, an expert in QM, thinks, I belive, that there is quantization since information itself is quantized. However, I agree with you in that there is no new thing about what is the answer to What is a light quantum? However, beyond that, do we really need to knot what are quanta? Quanta are "real", I mean they are a quite predictive model for the photoelectric effect, interactions,...Of course, if it were probed that quant have substructure (at what today we think are fundamental/elementary particles) or they could be derived from some bigger framework, it should be an advance. Today, the most conservative physicists will defend the quantum states reality and that they are a complete description of the physical reality.
You puzzle me when you say that the point is to understand quantization itself instead of quantization of gravity. In my opinion, they are the two sides of a same coin. I read a new paper though about the main issue of QM foundations and quantum theories. It related all the stuff to the issue of the numbers used in the mathematical framework. But on general grounds, yes...Quantization itself is yet a complete mystery. But numbers are into the game!




Your 3rd problem: Do black holes destroy information? What happens to the matter that collapses to a black hole?

IN the last 15, perhaps 20 years (from the time around 1992 when holography was born) I think that most of people believe that information is conserved. Of course, the details are important, but seeing information like energy, likely, we will find that any conceivable solution involves some general picture about how information is stored on horizons and what happens at Planck scale to matter fields. Perhaps even to some energy scale between TeV and the usual M_P~10¹⁹GeV.

4th issue. Are the electroweak and strong interaction unified at high energies? If so, are they also unified with gravity?

Most of people assume that the chain of unification is electromagnetic--->electroweak--->GUT--->Unified theory. IN the last months I have become more and more convinced that approaches like SUSY or similar lacks some other ingredient or are wrong in something essential since we have no hints on new particles beyond, maybe, the neutrinos. I am now more convinced that forces are unified two-by-two, and not three-to-one as conventional GUTs assume. Of course I can be wrong, but if some stuff related to AdS/CFT is to be right, gravitation shares with the strong force much more than the other forces (in spite the striking similarity, yet misunderstood, of Coulomb and Newton forces). My scheme would be strong-gravity unification at scale X, and then unification to scale Z of electroweak and gravistrong force. I have never read something like this, even in the RG approach, excepting the asymptotically safe approach, strongly similar to asymptotically free QCD. Of course, the nature of the fixed point in UV gravity has to be yet confirmed and checked but it is more than nothing.

Juan F. said...

5th. Are the currently known particles of the standard model (SM) elementary? Are there more so far unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other? Why are the gauge groups of the SM what they are? Is it even possible to uniquely answer this question?

-Preonic or subconstituent theory has been always come back in some form. New particles? I don't know, LHC will tell if there is something else beyond the Higgs about 126 GeV( I like 126 more than 125 GeV). The question, I think, is even more general, why 3 generations?Where do gauge symmetries come from? Indeed, it is likely to affect the 2nd question and the 1st.




6th. Did the universe start with a big bang, a big bounce or something else entirely?

If Dark energy is real and not an artifact of current models/theories, we could think there is some vacuum pressure -like in the (quantum) space-time fabric that manifests at higher and lower scales. What puzzles me the most is that if we identify DE with vacuum energy we get the CC problem. And even SUSY or any proposed symmetry until know has FAILED to explain that. The only way to get some realistic CC is plugging by hand an exponential-like term exp(-1/g²) of non-perturbative Nature that could "solve" the mismatch. However, it only shifts the fine-tuning problem to handle a non-perturbative stuff we are not really very familiar with. But it is one of the few alternative I see at this moment (beyond a true radical new idea that can solve the problem).

7th. Why do we experience 3+1 dimensions? Are there extra dimensions? Does the effective dimension of space-time decrease at short distances?

Ah, extra dimensions(EDs), that idea that boost Sci-fi and fiction-Science as well. I am open minded with them, because I believe that we have restricted to very limited options till now. I read sometime ago a paper on "variable" spacetime dimension and one even wondering about if D=\infty. Extra time-like dimensions have been anatem from many researches, but I think one should be open for thinking more about them. However, at last experiment will say if there is extra dimensions or even what are their topology (or the topology of our own dimensions, something that is usually disregarded). We have not exhausted all the options. Compatifications in circles is a new form of ptolemaic epycicles. If some ED theory works, I presume, if right, it could explain the origin of current families. Knowing that electron is about 0.5 MeV, the muon 105.7 MeV, and the tau 1777 MeV, it is obviously seen that the spectrum is NOT equally spaced, as usual KK towers arise. So, the "Simpson-doughnout" Universe for EDs can not work for leptonic or quark families/generations.

8th. Why is gravity so much weaker than the other interactions?

I believe that it is the same question that the one in 4th. I mean, 8th is 4th. Any unification scenario has to handle with a logical answer to this question.

Juan F. said...

9th. Does dark energy exist? If so, what is it? Is the coincidence problem more than a coincidence?

Well, if you think dark energy is the reason we observe a positive acceleration as different cosmological experiments, and that there is a CC-like stuff. Dark energy must exist. There is no scape. Of course, the question I consider more precise is the second you make What is Dark Energy? A rolling (slowly) quintessence scalar-like field permeating the whole space? Some other thing we have not invented. However, the striking quite precise modeling by a Hooke-like (a.k.a. an inverted Harmonic Oscillator) and an "almost" constant CC is at least quite surprising. I am sure Einstein himself would have been surprised with the 1998's discovery!


10th. How do we correctly assign an entropy to gravitational degrees of freedom? Is this testable at all?

That is, I presume you are thinking about the microscopical origin of the BH entropy. I agree that is a big question, but it is also tied to the Nature of quantizing gravity, but of course, it has its own impartance to be an independent question. I would go further. Is the entropy of the gravitational field a Shannon-Boltzmann entropy or something more general? I am quite sure that answering this question implies to understand entropy better than nowadays, and that non-extensive and/or more general entropies are involved in the final answer. By one reason, Bekenstein-Hawking entropy shows that entropy for the gravitational field has a different "scaling" with respect to usual fields. Unless you think Bekenstein -Hawking result is wrong (I don't believe so), we need some new statistical physics for the gravitational field microstates. And then, the next question would be, why is gravity holographic ( in the sense S~A) while for other forces or general thermodynamics we get S~V?

Let me know what you think about my comments.

Best wishes, and great selection of Quantum Gravity issues in your post! Congratulations once again!

Uncle Al said...

1) "apparent disagreement with [empirical observation]"

http://www.mazepath.com/uncleal/erotor1.jpg
Two geometric parity Eötvös experiments, 90 days, 5×10^(-14) differential sensitivity.
http://www.npl.washington.edu/eotwash/publications/pdf/lowfrontier2.pdf
Apparatus and its orthodox (null output) applications.
http://www.mazepath.com/uncleal/shoes2.png
Geometric parity calorimetry experiment, 1 day, 1.2×10^(-15) differential sensitivity.
http://www.mazepath.com/uncleal/orbit.png
24-hour sinusoidal response cycle (plus constant differential background fit divergence).

3) "matter that collapses to a black hole" Matter collapses without limit as its local gravitational field inflates space into maximum elliptic geometry. A net finite number results.
4) "Are the electroweak and strong interactio... unified with gravity?" Gravitation is geometry. It does not unify; its quantizations fail, (3).
5) "standard model (SM)" SM assumptions of vacuum symmetry toward fermionic mass are empirically untrue, (1). The SM is fundamentally defective (e.g., parity violations plus symmetry breakings; SUSY, MSSM, NMSSM, and see-saw sillinesses).
6) "Did the universe start with a big bang..." Yes.
7) "Why do we experience 3+1 dimensions?" The holographic universe is a Schlegel diagram of higher dimensions. Local 3+1 supports chirality, sourcing the strong arrow of time.

Nature 463 210 (2010)
Phys. Rev. D 71 057501 (2005)
Phys Rev Lett. 91(24) 247404 (2003)
Chem. Phys. Lett. 173(5-6) 485 (1990)

8) "Why is gravity so much weaker" Gravity is geometry not virtual gauge vector boson exchange.
10) "How do we correctly assign an entropy to gravitational degrees of freedom? Is this testable at all?" NICE! (1).

Chemistry can't help you with (2) and (9). Everything else, including the end of dark matter, is rectified in 24 hours on a bench top.

Arun said...

Dear Bee,

Whenever you have the spare cycles, a little more detail into what shaped the change of your thinking?

Phil Warnell said...

Hi Bee,
It’s nice see these shifts in your priorities and also some of your thinking, which doesn’t surprise me much as I’ve never thought of you as one who would be caught standing still. However I must admit the switch that intrigues me most is that you now suspect that QM should be more carefully scrutinized in terms of the difficulties to have QM and GR to mesh. This has me mindful of Lucien Hardy's new paper, which is just the latest one of a string of new results brought forth as of late which seems to point to it not being long perhaps as to when it might become decidable if the wave function is a true element of reality, as opposed to being considered simply as a symptom of it. The decision of this I’ve always thought to be a real potential game changer, that is as to how this whole quantum gravity quest thereafter proceeds. However I don’t think it will have things to become less weird, yet actually much weirder; so much so that I might come to be reminded one should be careful what it is that we wish for.

“Information! Whose information! Information about what!”

-J.S. Bell, “Against Measurement”, Physics World, Vol. 3, No. 8. , p 34 (1990)

Best,

Phil

Robert L. Oldershaw said...

I would agree with your choice of #1 as a key question in theoretical physics today.

If nature has a fundamental symmetry known as discrete scale invariance, with scale including mass as well as space-time, then one does not need to quantize GR in order to have a unified model of the microcosm and the macrocosm.

Gravitation would behave exactly the same in both domains, which are merely separated by a very large and discrete change of MLT scale.

But that's too simple an answer, right?

RLO
Discrete Scale Relativity

Roy Johnstone said...

RE: Question #10, gravitational degrees of freedom can/should be considered when quantifying entropy, particularly in the cosmological sense, along with other latent or inactive d.o.f.

This can potentially help resolve the so called conundrum of the improbably low entropy of the initial cosmic state, if considered as part of a *cyclic* model. Penrose's ccc model is a recent novel example of this where there are inactive gravitational d.o.f. following the conformal transition to a new epoch.

Viewed in the past light cone, there would be "big bang like" surface appearing to be low entropy but with at least the same total information content as the previous epoch. This could explain things such as the evolution of natural laws(why these laws?) and why we find ourselves here now.

Think of it like the biological life cycle, the embryonic state taken in isolation would seem highly improbable & low entropy compared to the most common state a being would be randomly found in (fully developed). But that embryo contains all the information needed for the fully developed adult. So that once the cycle and timescales are known and entropy is measured as total information content, including inactive d.o.f's(eg DNA), probablities become completely different.

Eric said...

Regarding item 10, a rhetorical question:

What is the entropy between all the air molecules within a tornado or hurricane?

What is the entropy between all the air molecules at the intersection between a low and a high pressure atmospheric system?

Finally, if we are in the middle of an intense low or high pressure atmospheric condition, but not at an intersection between the two, why don't we feel any kind of force on us? How would that translate into high entropy while still encapsulating a high energy system?

Bee said...

Hi Eric,

Well, since it's a rhetorical question, you don't want an answer. I'd like to remark though that I can't make sense of the term "the entropy between." Entropy is an extrinsic quantity, there's "entropy of" not "entropy between". Sorry for the nitpicking ;p Best,

B.

Eric said...

Hi Bee, perhaps I should have rephrased it, but I think you probably get the gist.

Best,
Eric

Bee said...

Hi Phil,

I have to admit that the whole discussion about the "reality" of the wavefunction passes me by, as I have no clue what reality is supposed to be anyway. To me our models are exactly that, models, meant to be useful in the first place. Either way, I welcome all scrutinizing of the foundations of QM just because I believe there's something to be found there. Best,

B.

Bee said...

Hi PTMR,

That gravity might be stronger on short distances is an idea which has been around for a while, it's the idea that fundamentally the Planck energy is lower than we might think from extrapolating long distance measurements to microscopic scales. Either way, you still have to explain where the large gap in scales comes from, even if it's not fundamentally there. There are no negative gravitational charges. Anti-matter can't antigravitate, we already know that. Both for symmetry reasons and because virtual matter-antimatter pairs do actually have observable consequences. This has been known for a long time, which is why nobody ever was really excited about measuring the gravitational force on anti-matter. Either way, I believe people are working on some experiment. Best

Bee said...

Hi Juan,

4 and 8 are not the same question. Even if all interactions are unified fundamentally, you still have explaining to do why we arguably observe so large differences between the relevant energy scales. You can hope though that if you know what the unification looks like, that will solve the hierarchy problem too.

Reg the entropy, of course I don't think the Bekenstein-Hawking result is wrong, but I think its interpretation is less clear than people want it to be. Best,

B.

Bee said...

Dear Arun,

I am terribly bad at explaining what shapes my thinking, which is one of the reasons I have an eye on what research says about cognitive biases and how the media influences us. I have little idea what my unconsciousness is doing when I'm not looking.

That having been said, for some points I can come up with reasons.

The black hole information loss: My paper with Lee convinced me that the reason we're still thinking it's a problem is that we're misunderstanding the final phase of evaporation. I think the actual problem is the singularity and once we've managed to work this out, unitarity will be restored.

The theory of everything makes a rise because of the question of uniqueness that I think is relevant to how we think of ourselves as being part of nature.

Dark matter dropped off the list because evidence has been accumulating that makes it increasingly unlikely it's a modification of GR. More likely, it's some sort of matter we just haven't seen. Which, I should add, I think is plausible, but somehow unexciting, at least to me.

Best,

B.

Zephir said...

/*..How can the apparent disagreement between general relativity and quantum gravity be resolved? Does it require to quantize gravity?..*/

The general relativity and quantum mechanics perspectives are different four-dimensional slices of hyperdimensional world. If you extent these theories into higher dimensions, their solutions will become more and more similar. AdS/CFT correspondence partially accounts into it already, but it's five-dimensional theorem only. The quantized gravity will be natural result of this reconciliation.

Dense aether model it explains the water surface analogy: you can observer the water like the dense environment full of chaotically moving particles (quantum mechanics view) - or like the flat space-time membrane, the motion at which is not affected with underwater nearly at all. Both views describe the same surface though.

Zephir said...

/* What happens to the matter that collapses to a black hole? */

The existing models tend to ignore the evaporation of matter into neutrinos during accretion, which I do believe can pass through even horizon at certain level. Roughly one half of matter is evaporated in this way. The rest of matter is dispersed into quantum fluctuations (gravitons), which do form the interior of black hole in similar way, like the vacuum fluctuations are forming the space-time in our part of Universe.

Bee said...

Hi Glen Mark,

Well, yes, you have to define suitable physical quantities of course, you always have to. It is generally expected that quantum gravity avoids the formation of a true singularity, thus my question. Best,

B.

Phil Warnell said...
This comment has been removed by the author.
Phil Warnell said...

Hi Bee,

I can understand your perspective on this, which is by in large the normal pragmatic scientist’s way of looking at things; which is sensible, that is in so far as science being a philosophically centered discipline where all models and with them their elements of reality are to be taken as provisional. Never the less I’ve always thought that such models must be taken at least seriously enough so that they give one sufficient contact with reality as to allow things to be further scrutinized, rather than having the philosophical foundation of the theory have all such inquiry forbidden. The way Albert Einstein used to put it was (I paraphrase), reality should be dictating to theory, rather than theory dictating to reality.

”I am, in fact, firmly convinced that the essentially statistical character of contemporary quantum theory is solely to be ascribed to the fact that this [theory] operates with an incomplete description of physical systems.”

-Albert Einstein, “Albert Einstein: Philosopher-Scientist , from The Library of Living Philosophers Series”, Cambridge University Press, 1949

Best,

Phil

Bee said...

Hi Phil,

Yes, I can fully agree with that. People may differ on what they think is promising to spend time on, but one should be very careful to discourage other modes of inquiry based on nothing but ones own convictions. As the saying goes, errare humanum est, sed... Best,

B.

Zephir said...

/* .. Did the universe start with a big bang, a big bounce or something else entirely? */

The Big Bang model is the result of the light scattering at the density fluctuations of vacuum. This scattering is itself wavelength dependent and it leads to the illusion of inflation, particle horizon and initial singularity of the Universe. What we can see at distance is analogous to the look at the water surface with its own ripples - at the certain distance everything will be blurred into singularity. But does it mean, no water surface exists there? Of course not, this perspective is observer location dependent. The observer sitting inside of this distant part of Universe would see us in exactly the same way from his own perspective.

Zephir said...

/*..Are there extra dimensions?..*/

It depends on the definition of the extradimensions. If we accept, that the strictly 3D space allows constant speed of light and inverse square law dependence for all forces, then every gravitational lensing would violate this dimensionality, not to say about many forces, which don't follow the inverse square law (like the Casimir force, nuclear forces and various dipole forces). In this sense the extradimensions are all around us.

/* Why do we experience 3+1 dimensions?..*/
Inside of dense field of hyperdimensional particles the spreading of energy is most intensive by principle of least action, when these particles are represented with just 3D hyperspheres (the volume/surface ratio of spheres of variable dimensionality goes through maximum for just 3D spheres). It means, just this number of dimensions enables us to see as much as possible from the observable Universe.

Juan F. said...

Hi Bee!

On your reply:

"4 and 8 are not the same question. Even if all interactions are unified fundamentally, you still have explaining to do why we arguably observe so large differences between the relevant energy scales."

OK, but at least they are same of the same "big picture/problem".

Yes, I was thinking indeed in the lines you remarked here:

"(...)You can hope though that if you know what the unification looks like, that will solve the hierarchy problem too.(...)"

That is precisely the point. Of course, you can split the issue, in the way you did it. But probabley, their solution is linked. Agreed, I suppose.


I also agree with your thought on entropy: "(...)Reg the entropy, of course I don't think the Bekenstein-Hawking result is wrong, but I think its interpretation is less clear than people want it to be. (...)"

Bekenstein-Hawking result is remarkable, but it is not completely understood! I am just worried that some "stringers" hard defenders do affirm that their theory is the only one that can explain the logarithmic corrections to BH. That is not true. Or at least, not so tight like they use to argue! Indeed, as far as I know, the Bekenstein-Hawking law corrections have been calculated from strings only for certain classes of BH, not for all. Though, it is striking they guess the "expected" result. Let's see if some interesting counterexample can break that line. I didn't like at all that from the last paper by A.Sen some people attacked LQG so hard. It is not deserved.



I have always stayed on touch with strings since my time as teenager, but time has made me think that string as we know can not be the whole story, or, at least, its current approach can only be an effective theory of quantum gravity with "less substraction problems" than usual QFT.

LQG is trying to build a QFT free of metric using spinors and twistors since some months ago. It is an interesting counter strike to the most critical sector (from strings).

Robert L. Oldershaw said...

Hi Phil,

RE: "The way Albert Einstein used to put it was (I paraphrase), reality should be dictating to theory, rather than theory dictating to reality.

”I am, in fact, firmly convinced that the essentially statistical character of contemporary quantum theory is solely to be ascribed to the fact that this [theory] operates with an incomplete description of physical systems.”

There is a quiet revolution going on in QM research. For, example:

1. Copenhagen Interpretation mostly discredited. (Nature)

2. QM shows that Rydberg atoms conform to the Rutherford atom model based on an analogy to the Solar System and celestial mechanics. (Physical Review A)

3. Increasing evidence that the Schroedinger wavefunctions are real physical entities. (Nature)

4. Shocking new results in the double-slit experiment, reported in Science last year.

Einstein was right, but nobody listened.

Nature will vindicate his vision in the final analysis, regardless of what Swift's "confederacy of [Platonists]" believe.

Robert L. Oldershaw
Discrete Scale Relativity

William said...

Hi Bee, I like the list and am glad to see the update. I disagree on dark matter, however. In fact, I would put dark matter at number 1 or 2. If dark matter is indeed a different form of matter (or "a gravitational substance different from ordinary matter"), which seems to me to be possible, then it can open the way for radically new concepts concerning the nature of matter and gravitation ... which are issues which quantum gravity is groping around in the dark trying to answer.

William said...

"6. Did the universe start with a big bang, a big bounce or something else entirely? "

Or does it make any sense to posit that the universe ever "started"?

William said...

What about "where's the antimatter?"

Isn't that be within the realm of theoretical physics? I'd rank it as a top question.

An equal amount of antimatter must exist. As a matter of logic, physics and symmetry, it must exist. So where is it? Great mystery, at the heart of the universe.

I imagine a symmetrical matter-antimatter event in time and space with the creation of two opposing bubbles, one matter the other antimatter ... similar to galactic sized gamma-x-ray bubbles recently detected but on a scale of such magnitude that most locations in either bubble would experience their universe as flat. ... Just a thought to try to picture where antimatter might be.

"Everything gotta be some place," as the saying goes.

Bee said...

Hi William,

Reg 6, yes, that's what I mean. You can put a t_0 somewhere and call that start, the question is was there anything before that, and if so, what. Best,

B.

Bee said...

Hi William,

Reg dark matter, the thing is that from what we know already it's not radically different from normal matter. It's just some type of matter that couples more weakly. Finding and understanding it might help with unification and by that also help with quantum gravity, but I think the possibility is remote. That is in contrast to dark energy, which we know can't be some type of matter, which is why it's still on my list. Best,

B.

Giotis said...

" it is mainly a reflection of my personal interests; a summary of puzzles I find promising to spend brain time on."


But you need a theoretical framework at Planck scale to even start thinking about these questions.

But as far as I understand you don't have a favourite pet theory at Planck scale.

So how do you work on these questions? Using only QFT and semiclassical approximation? You can't go very far with these tools.

Rhys said...

William said:
"An equal amount of antimatter must exist. As a matter of logic, physics and symmetry, it must exist."

This is just nonsense. Stating a proposition confidently does not make it true. There are plenty of models which generate an asymmetry from symmetric initial conditions, and the theoretical understanding of how this comes about goes back decades. We just don't know exactly how (or, technically, whether) it happened in the actual universe.

David Brown said...

In connection with question #1, is J. Christian of Oxford the greatest theoretical physicist since *******? Is SU(8) the fundamental gauge group of our universe?

Phil Warnell said...
This comment has been removed by the author.
Phil Warnell said...

Hi Robert,

Even though one of my favourite scientists of all time being Einstein doesn’t have me to think he was always right ,as for example finding QM formalism would necessarily mandate spooky action at a distance had it to be incomplete. That is arguably the CI interpretation of this formalism is incomplete, yet entanglement is now recognized as a demonstrated character of nature. The fact is Einstein didn’t so much care about being found to be wrong, yet rather thought it worse to think one is right for the wrong reasons.

“Most people say that is it is the intellect which makes a great scientist. They are wrong; it is character.”

-Albert Enstein

Best,

Phil

Bee said...

Hi Giotis,

That I don't have a pet theory, doesn't mean I refuse to consider answers offered by any approach to qg. Quite the contrary, I like to see what each of them has to offer. I think in the present circumstances the best way forward is to look for phenomenology, which is why I work on that. Best,

B.

Nemo said...
This comment has been removed by the author.
Neil Bates said...

Should have noted earlier, I wonder why not including "the quantum measurement paradox" (or problem, I like the QMP phrasing since I am high for that in Google search!.) Despite misleading talk by some, we don't really understand how that happens. Is it implied by another/s point?

Bee said...

Because most physicists, including me, don't think there's anything paradoxical about decoherence.

Neil Bates said...

Bee, the argument presented on behalf of the decoherence interpretation (the idea that decoherence actually explains "measurement" the exclusionary outcome, if that's what you meant) is circular. First it makes the mistake of taking the existence of exclusionary statistics already for granted ("put in by hand" into in the density matrix.) Sure, they are "found", but if you are trying to explain B from A, then you have to simply present "A" and then *derive* B from it.

Then the argument compares the different types of statistics, once they're already there. But, it doesn't derive a ny kind of "statistics" from the wavefunction in the first place. That sort of circular argument should be considered scandalous. And what happens to the other alternatives? MWI is full of trouble (just see them thrashing around trying to justify the Born probabilities. The internal bickering is awful, but at least many of the critics are calling *those* arguments "circular".)

Disorderly wave functions should just be the same intrinsic "sort of thing", still spread out. All decoherence changes is the type of statistical pattern *if* something is able to localize them, it doesn't explain *why* they end up trapped in a particular small spot like one detector among many. Penrose more gently articulated the same criticisms, so it's not just me.

No, the measurement problem is not explained or truly understood, at all. That's what motivates some minority holdouts like those who believe in de Broglie-Bohm, objective collapse like Ghirardi–Rimini–Weber theory, gravitational collapse etc, all from people who appreciate that you can't just rearrange waves by themselves to get localizations. So why do they? Who knows, it just doesn't make sense.

More importantly, I devised an experimental test of the claim that "decoherence turns superpositions into mixtures." The supposed mixture output can be recombined to show there were still superpositions. QIS specialist Ian Durham took an interest and is reviewing it these days. I'll be happy to send you a copy.

In any case, how about a big measurement problem thread here? Let everyone hash it out. Cheers.

Bee said...

Hi Neil,

Of course it doesn't explain the selection. But why should I care what happens to "the rest." I believe I've written about that dozens of times, I don't think mathematical models are real. They describe reality, and they might do so better or worse. It's a good model, and it works just fine. Maybe it's not fundamentally correct, but I don't see what there is to learn from poking around on this. As I wrote in my post, I believe that there is more to learn from asking if quantization itself is fundamental. You don't have to share my opinion. In fact, I'd find it rather funny if you did. Best,

B.

Neil Bates said...

Bee, thanks for your charming and diplomatic reply to my possibly overbearing comment. Yeah, in practice most physicists have given up on worrying about the big bad MP and concentrate on practical applications such as QIS. I like you saying, "I don't think mathematical models are real." Indeed!

In any case it seems to me that continued argument, grounds to disagree and lack of clear insight; do imply something still being "a problem" - but it's your list. I hope my experiment can stir things up, but meanwhile the status quo muddles along.

bill said...

The first question I would like answered is whether motion is continuous or discontinuous?

Next, is there anything smaller in space-time than a system? Is a binary system the simplest system?

If motion is discontinuous and the simplest system is a binary system, does that point to a parallel universe to our space-time universe. Do they interact as a binary system? Does Relativity only apply to space-time while will quantum physics applies to the parallel dimension.

Could that explain Bell's Theorem? Information travels faster than light in the parallel universe, clicks back to our space-time universe, and the spin has instantly changed.

Could the Uncertainty Principle govern this binary universe system? Quantum probability gates could exist at the intersection of the two universes at the Planck scale. Sub particles in space-time could click off entering the parallel dimension, then click on returning to space-time being informed by quantum probability gates governed by the Uncertainty Principle.