Monday, April 21, 2014

Away note

I will be traveling the rest of the week to give a lecture at the Sussex graduate school "From Classical to Quantum GR", so not much will happen on this blog. For the school, we were asked for discussion topics related to our lectures, below are my suggestions. Leave your thoughts in the comments, additional suggestions for topics are also welcome.


  • Is it socially responsible to spend money on quantum gravity research? Don't we have better things to do? How could mankind possibly benefit from quantum gravity?
  • Can we make any progress on the theory of quantum gravity without connection to experiment? Should we think at all about theories of quantum gravity that do not produce testable predictions? How much time do we grant researchers to come up with predictions?
  • What is your favorite approach towards quantum gravity? Why? Should you have a favorite approach at all?
  • Is our problem maybe not with the quantization of gravity but with the foundations of quantum mechanics and the process of quantization?
  • How plausible is it that gravity remains classical while all the other forces are quantized? Could gravity be neither classical nor quantized?
  • How convinced are you that the Planck length is at 10-33cm? Do you think it is plausible that it is lower? Should we continue looking for it?
  • What do you think is the most promising area to look for quantum gravitational effects and why?
  • Do you think that gravity can be successfully quantized without paying attention to unification?
Lara and Gloria say hello and wish you a happy Easter :o)

38 comments:

Juan F. said...

To your questions:

Sab1) Is it socially responsible to spend money on quantum gravity research?
My answer: Yes, we NEED quantum gravity, badly. If the human species has to master new technology, there is nothing more keen that "gravity-like technology".


Sab2) Don't we have better things to do?

My anwer: more useless and evil projects are funded out there. Quantum gravity is Science, not fiction or myth, so it should be granted and funded...

Sab3) How could mankind possibly benefit from quantum gravity?

My answer (I could elaborate it more, but this is not the right place): if humans are to go to space and interstellar space, there are few options to survive in space for a long time. Even if protected against cosmic radiation, we have to find a way to locally modify gravity in order to create "gravitational fields" by "induction". No current TESTED theory speaks or tells how we could generate induced gravitational fields with other forces. So, either we manage it, or to survive in low gravity we should use some another method (even genetic modification has been advocated by S.W.Hawking to survive in space).




Sab4) Can we make any progress on the theory of quantum gravity without connection to experiment?

My answer: likely yes, but unless we discover that the fundamental scale of gravity is lesser than Planck scale, we will be searching for indirect tests. However, stuff like the information paradox or what is the "picture" and realization of spacetime "quanta" should be elucidated as well theoretically.

Sab5) Should we think at all about theories of quantum gravity that do not produce testable predictions?

My answer: If you mean testable in reasonable time with suitable technology, likely not excepting as a mathematical training. The fact that the ether idea is "wrong" does not mean we should not know what was the classical ether and in what sense "something similar" (but not the same) is encoded in currently accepted theories. Theoretical physics must discuss any idea, experimentalists will tell what of our models/theories are right to "survive"...


Sab6) How much time do we grant researchers to come up with predictions?


My answer: researchers should be come up with predictions, but also with reasonable models. The art of prediction is not "easy", many physicists before us were aware of this thing...


Juan F. said...

Sab7) What is your favorite approach towards quantum gravity? Why? Should you have a favorite approach at all?

My answer: I have no favourite approach right know. Perhaps, I like those that predict some additional structures in spacetime or that goes beyond Lorentz invariance. Beyond the entropic mood, I like to study ANY possible and thought "extension" to SR both kinematically and dynamically (DSR, TSR, deformed -quantum/cosmological- spacetimes, de Sitter relativity, Finsler relativity, anisotropic relativities, multitemporal relativites, extended relativies, supersymmetric extensions of Poincare group, tensor superspace relativities,...)



Sab8) Is our problem maybe not with the quantization of gravity but with the foundations of quantum mechanics and the process of quantization?

My answer: I can not imagine an scenario in which only the quantum idea "win". The dualitiy revolution came to show us that classical/quantum dynamics are somehow mapped to each other, so, if we "improve" our knowledge of gravity, that will affect QM and QFT as we know it today. Even more, not only "emergent" spacetimes theories but also "emergent" QM groups have been founded. I dislike the word "emergent" because it hides that it is a cloak of "effective theories"...



Sab9) How plausible is it that gravity remains classical while all the other forces are quantized?

My answer: somehow, gravity IS purely quantum at Nature. It must. A different thing is that we can describe it as a classical field theory and a geometrical setting with riemannian or Riemann-Cartan disguise. In fact, the own non-renormalizability of old pure quantum gravity, I believe, says that canonical quantization fails, and it is related to both: the structure of the theory and the fact that G is dimensional as the Fermi Constant in the weak interaction. G in terms of Planck's constant and c is NOT fundamental, or it isn't, in a pure quantum gravity approach. We need to undersand how G arises from some other field, just as the Fermi constant (and the effective Fermi theory of beta decay) "emerged" from integrating out the right degrees of freedom the purely quantum electroweak theory. Therefore, we have to modify gravity at some scale in order to get the right theory. String/M theory does it, LQG does it, some other theories also do it


Sab10) Could gravity be neither classical nor quantized?

My answer: NO. I don't like this question. Gravity IS classical in the sense of the newtonian theory or in the general relativistic version (relativistic gravity would be a better name for general relativity). And since gravity IS a fundamental interaction, it should have a fundamental quantum and be quantum in the deepest and tiniest "space of space". Classical/Quantum theories are right, aren't they?, in their domain of energies. So, if quantum gravity is "something very different", it makes hard to me how to say that classical or quantum theories are "wrong". Gravity IS a field. As classical field, it has two "effective" theories. Newtonian gravity is the lowest effective theory, and GR is the highest effective theory of (classical) gravity. Any fundamental theory of quantum gravity MUST explain the link classical/quantum better, not to deny it both. Information? Maybe...


Sab11) How convinced are you that the Planck length is at 10-33cm?

My answer: I think you mean "the fundamental" Planck scale instead of the effective "Planck scale". LHC explores energies up to 0.1 zm, so quantum gravity could hide any place between the normal Planck length or larger (smaller sounds crazy). Without experimental hints on the contrary, combination of "fundamental constants" give us that length, and it seems inevitable unless extradimensions or other weird effect applies...

Juan F. said...

Sab12) Do you think it is plausible that it is lower?

Planck length lower?Bigger? Again, I think it is in the end a phenomenological question. Unless some new physics appear in the LHC or new experiments, "constants" are cruel...

Sab13) Should we continue looking for it?

My answer: Yes. Test any theory that is possible. Make any experiment you can imagine.


Sab14) What do you think is the most promising area to look for quantum gravitational effects and why?

My answer: CMB polarization, cool neutrons interferometry, neutrino oscillations precision measurements (neutrino astronomy), gravitational waves searches. CMB polarization hints that quantum gravity IS and WAS very relavant; cold neutrons interferometry can aid us to test gravity WITH QM faces; neutrino oscillations can reveal patterns from quantum gravity (even the relic neutrinos from the Big Bang can tell us a lot of our past); gravitational waves and the measurements of their properties are fundamental to find deviations from GR induced by the true QG theory...


Sab15) Do you think that gravity can be successfully quantized without paying attention to unification?

My answer: Any success in the quantization of gravity with shed light into the way QM is "fundamental" and how the running of the "fundamental constants" run with energy, so at least in the context of a renormalization group programme, quantum gravity will affect how the running of G (or the dimensionless analogue) will merge into a single point with the remaining interactions.

Uncle Al said...

Is it socially responsible to spend money on quantum gravity research? Yes United States' $12 trillion in Afghanistan purchased nothing. California High-Speed Rail cannot cost less than $92 billion: 800 miles, 24 stations, average estimated speed now ~50 mph from boasted 220 mph. Science costs chump change.

Don't we have better things to do? Perform a geometric Eötvös experiment. Get off the planet or near-term smother as a species. We desperately need a gravitation loophole.

How could mankind possibly benefit from quantum gravity? How could mankind possibly benefit from charitably feeding billions of the congenitally inconsequential?

Can we make any progress on the theory of quantum gravity without connection to experiment? Absent experiment, theory does not know how to succeed. Euclid cannot navigate the high seas for having a weak founding postulate. Parameterized Euclid is no better.

Should we think at all about theories of quantum gravity that do not produce testable predictions? How much time do we grant researchers to come up with predictions? 40 years of quantum gravitation, SUSY, and dark matter are sterile. Phys. Rev. Lett. 112, 161301 (2014) is a huge fuming pile with no pony. Something is deeply, fundamentally wrong - but not its derivation. Vacuum symmetry toward boson photons is not exact toward fermionic matter (quarks, hadrons). Bosons and fermions are not interchangeable.

What is your favorite approach towards quantum gravity? Why? Should you have a favorite approach at all? I'm a bench chemist. My favorite is anything that gives high yield. Zero-% is intolerable.

Is our problem maybe not with the quantization of gravity but with the foundations of quantum mechanics and the process of quantization? As assumed vacuum symmetry does not exactly obtain toward mass. This screams as parity violations, symmetry breakings, chiral anomalies, Chern-Simons repair of Einstein-Hilbert action. Repair the weak assumption.

How plausible is it that gravity remains classical while all the other forces are quantized? Could gravity be neither classical nor quantized GR is a subset of Einstein-Cartan-Kibble-Sciama gravitation containing chiral spacetime torsion plus achiral (or racemic - Ashtekar and Immirzi) spacetime curvature. Test for a trace chiral vacuum background (chiral spacetime torsion) only active toward fermion quarks with a geometric Eötvös experiment. The worst it can do is succeed, explaining everything.

How convinced are you that the Planck length is at 10^(-33) cm? Do you think it is plausible that it is lower? Should we continue looking for it? No comment.

What do you think is the most promising area to look for quantum gravitational effects and why? Define spacetime geometry with test mass geometry, then write theory.

Do you think that gravity can be successfully quantized without paying attention to unification? Given empirically valid founding postulates, everything will click into place. Perform a geometric Eötvös experiment, left-handed versus right-handed single crystal alpha-quartz test masses. Physics has nothing else remaining that is not proven failure.

Juan F. said...

In my opinion, for "business" and "laymen", the key question is Why quantum gravity matters for Technology and society, and the applications it could surely provide...
To a scientist, the fundamental issues are How to test the "true" quantum gravity theory and its implications of the unification programme. In this respect, theoretically, the main question is if quantum gravity stands apart form the remaining interactions OR if quantum gravity is the milestone of the whole unification quest...In summary, for QG theorists and business men matter:
1) What IS quantum gravity in the sense what are they d.o.f. and what are the spacetime "atoms" and how to test it?
2) What applications quantum gravity has and how could we build them?

Giotis said...

Off the top my head here is a couple of topics for discussion:

What is the minimum set of fundamental rules that a theory of QG has to respect?

What is the ultimate set of criteria that will allow you to decide whether a theory of QG is successful as a theoretical construction?

hush said...

To Lara and Gloria:

Happy Easter and Hi!

From my twin and I.

Post Script:
We wrote letters because there were no computers when we were the age you both are now.

Theophanes Raptis said...

Just to complete this questionnaire. What are we but somewhat more intelligent apes trying to figure it all out?

http://www.youtube.com/watch?v=GQcN7lHSD5Y

http://www.youtube.com/watch?v=Y15NnGZIBuM

Zephir said...

/*Is it socially responsible to spend money on quantum gravity research?*/

What is socially irresponsible is to ignore the cold fusion and overunity energy research on behalf of basic science research. The quantum gravity research is just deeply confused and imbecille (as I explained here many times, it's subject are all phenomena between quantum mechanics and relativity scales, i.e. the chemistry, biology and classical mechanics). The research of Tesla scalar waves technologies belong into quantum gravity too. With ignorance of this physics the physicists bring the world on the verge of global nuclear war.

Zephir said...

BTW You've so beautiful children..! Did you ever think about how the Ukrainian crisis threatens the Western Europe and how this crisis is powered with fossil fuel crisis which is fueled with ignorance of cold fusion and scalar wave technologies with mainstream physics? Everything is connected with everything more closely, than you may even realize. What I'm sadly lacking is the basic self-preservation instinct, if not social responsibility here.

Joe Marchione said...

In short, I believe solving the quantum gravity question is crucial to our fundamental understanding of nature. Its hard not to think of gravity in the classical sense as pure a potential and in the quantum sense as a form of weak coupling monopole that is directly related to physics at the Planck scale. Its all important. All of it.

A. Mikovic said...

The problem of QG consists of 3 problems. The simplest problem is to find a path integral or canonical quantization formulation which is based on a smooth manifold geometry (for example, asymptotic safety models or loop QG) or some generalization of the former (i.e. loop manifold in the case of string theory or a PL complex in the case of state-sum models),
such that in the appropriate limit the theory gives GR plus small quantum corrections and that the path integral can be defined non-perturbatively.

This can be achieved, as demonstrated in string theory as well as in the case of state-sum models. The second problem is to address the problem of the elementary particle spectrum within a QG theory, and so far it was only the string theory that gave some answers. One can also add the problem of the fate of an evaporating black hole, but there are strong indications that the simplest solution is a unitary evolution without a singularity formation.

The third problem is more of a conceptual nature, and it is related with the difficulties of applying the quantum mechanics formalism to quantum cosmology. The questions of the meaning of probability measurements without observers, creation of an universe and meaning of time are of this type. This requires a generalization of QM and this is the most difficult one.

nemo said...

Is it socially responsible to spend money on quantum gravity research? Don't we have better things to do? How could mankind possibly benefit from quantum gravity?

- Yes it is! No, we don't. Quantum gravity is the most powerful insight that human being may have about the Nature.

Can we make any progress on the theory of quantum gravity without connection to experiment?

- Yes we can. Once that quantum gravity has been understood most of the present mystery of Science will be resolved.

Should we think at all about theories of quantum gravity that do not produce testable predictions?

- No. As I said, once we understand qg will have wonderful key to investigate e resolve a huge amount of mysteries of Science.


How much time do we grant researchers to come up with predictions?

- as first it must solve the minsters of dark energy and dark matter. If it works with that you are in good position and you may dedicate all you life to fully unveil the mystery.

What is your favorite approach towards quantum gravity? Why? Should you have a favorite approach at all?

- General relativity. Space-time is not real. It is just the best way we have to describe and inquiry Nature. Nothing else. Space-time quantization is a chimera. The best approach in QM is the field theory and knot theory. If you have doubt about space-time, remember the entanglement. Locality a non locality is a wrong to look at Nature. What's is really important is the relation between things.


Is our problem maybe not with the quantization of gravity but with the foundations of quantum mechanics and the process of quantization?

- if we remove space-time what we have? All fields we may measure, all coupled together. That's what we call space-time.

How plausible is it that gravity remains classical while all the other forces are quantized? Could gravity be neither classical nor quantized?

- we have to forget these points. Space-time is needed just to discover the law of Nature and make the proper relations between things.

How convinced are you that the Planck length is at 10-33cm? Do you think it is plausible that it is lower? Should we continue looking for it?

- I suppose I've already replayed.

What do you think is the most promising area to look for quantum gravitational effects and why? Superconductivity. It show quantum effect that belongs from the force that are inside the matter, where all field works together.

Do you think that gravity can be successfully quantized without paying attention to unification? No, it would be silly.

Phillip Helbig said...

This is the most important: "How plausible is it that gravity remains classical while all the other forces are quantized? Could gravity be neither classical nor quantized?"

Be radical; think about the possibility that all other forces might be unified but that gravity is just separate. Is it somehow necessary that gravity be unified with the other forces.

Say hello to Peter Coles if you see him; you are united in the fact that I comment on both of your blogs.

Phillip Helbig said...

"Did you ever think about how the Ukrainian crisis threatens the Western Europe and how this crisis is powered with fossil fuel crisis which is fueled with ignorance of cold fusion and scalar wave technologies with mainstream physics?"

I think it is much more socially irresponsible to put faith in things like cold fusion. This delusion detracts one from solving the real problems. I remember an interview with someone who, during the Vietnam War, got a shaman to levitate the Pentagon. Or so he believed. This guy felt that he had done something against the Vietnam War (though what, exactly, considering that the Pentagon didn't notice anything, I don't know). He could have made a real difference by doing something actually useful.

Sabine Hossenfelder said...

Phillip,

I, and many others, have thought about this question and the problem is that it's hard to make it mathematically consistent, as I am sure you know. I am not willing to give up mathematical consistency, that would seem an extremely implausible step to me. Best,

B.

johnduffieldblog said...

Is our problem maybe not with the quantization of gravity but with the foundations of quantum mechanics and the process of quantization?

IMHO your problem is that you do not appreciate the curved-space geometry of classical electromagnetism. Or that curved spacetime equates to inhomogeneous space. Or that the photon is effectively a graviton.

How plausible is it that gravity remains classical while all the other forces are quantized?

Not at all. In the current-and-wire the electromagnetic fields almost cancel, but not quite. When you stop the current, the electromagnetic fields almost cancel, but not quite.

Vincent van der Goes said...

Out of these, in my humble opinion the most interesting question is the second last one:

What do you think is the most promising area to look for quantum gravitational effects and why?

Most of these questions are related to the need for empirical evidence; therefore, I feel that the essential question is this: where to look for it.

Jerry Lisantti said...

The program of events for this graduate school session is impressive. Any chance of these lectures/discussions being put online?

hush said...

What do you think is the most promising area to look for quantum gravitational effects and why?


Is anyone reminded of the past?

What do you think is the most promising area to look for nuclear effects and why?

Transmutation. (Guess)

Why stop there?

What do you think is the most harmful area to look for nuclear effects and why?

Weaponizing. Abuse.




What do you think is the most harmful area to look for quantum gravitational effects and why?

Weaponizing. Abuse.

What do you think is the most promising area to look for quantum gravitational effects and why?

Constraints. Self preservation.

Tom Andersen said...

You can build everything you need out of Classical GR. No other tools required.

The problem with physics today is that everyone thinks it has to be new equations, new fields and new particles.

The amount known about the solution space of GR is of zero measure. Add Godel to that and you have an infinite playground. Particles, fields, everything can be constructed from gravity.

The first gravitational particle discovered was the Schwarzschild solution. It took decades before anyone believed they could be found in nature.

David Brown said...

"... favorite approach towards quantum gravity?" The favorite approach should be whatever best explains Milgrom's acceleration law. On the basis of overwhelming empirical evidence, Milgrom is the Kepler of contemporary cosmology. Note that Kroupa is scheduled to give a talk at the IAS on Thurs., 24 April 2014.
Astrophysics Events, School of Natural Sciences, IAS

L. Edgar Otto said...

David,
The Milgrom ideas while conceptually clever are unlikely to explain the higher reach of gravitational theory so that approach as with imagined particles some tweak that the universe's acceleration data matches their theory will leave us cold.
But it is among the first of falsifiable new physics as was Kepler's model until a Newton or Einstein came along to tie it all together. :-)

Zephir said...

/*..This delusion detracts one from solving the real problems..*/
Yep, I'm aware it's the main problem with acceptation of cold fusion. It just steals the social credit, grants and money the physicists engaged in alternative research. Which is why I consider the mainstream physicists as the culprit most responsible for delay of cold fusion research - not the alleged fossil fuel lobby or someone else.

L. Edgar Otto said...

Zephir,
Everyone of the questions raised by Sabine in this post is important.
Cold fusion is as only as useful as its possibility and mathematical consistency as quantum theory that grounds it as a physical possibility. Can that show a gain in energy to that put in?

Zephir said...

/* Cold fusion is as only as useful as its possibility and mathematical consistency as quantum theory that grounds it as a physical possibility.. */

Are you serious? For example, we already have dozens of practical applications of HT superconductors, despite we still have no working theory for it. What's worse, even if we would have a working theory of HT superconductors, we still may not be able to prepare them. The practical applications of cold fusion can behave similarly, in addition they have anything to do with quantum gravity in its present state.

L. Edgar Otto said...

Where is the ground? Zephir
How is this different from heterodyneing over complex number amplification? Not a bad enigineering application and testable even without deeper theory. But would it work in a strong gravitational field? Would the flashlight change color? Would the light come out any faster than c? Is cold fusion possible beyond Boron?
Where do the neutrons go? Define random. And why do we need even a single input when supernova and black holes in isolation can spew out heavier atoms as materialized "aether waves"?

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MarkusM said...

Dyson lately gave a quite detailed talk about the problems with detecting a graviton:
https://www.youtube.com/watch?v=l-FSFtoeagc

Nevertheless, Kraus and Wilczek think that one could see QG signatures in cosmology:
http://frankwilczek.com/2013/gravitons3.pdf

Best.




L. Edgar Otto said...

Markus,

We can observe them on the surface of stars & the Earth too. So why can they not see that?

MarkusM said...

L. Edgar Otto,
Gravity, yes, but not the quantum of gravity, the graviton.
Best.

L. Edgar Otto said...

Zephir,

The leading social responsibility question in Sabines list is the most important of our day.
I think it amounts to not do much an area of inquiry is not pursued or dismissed but how we can add to the wisdom rather than define ourselves as victims in emotional bias that merely gives us a place in line rather than anyone else to be part of inquirey of worse it simply amounts to consuming the economics based on unclear ideas of ethics as thieft.
We need not stand up for ourselves although emotional stability can have diminishing returns, esteem issues rescently indicating lower IQ - it is my flaw ameliorated only by I am prone to stand up for others.
That said, your ideas from an engineering standpoint with my objectivity opens up wider possibilities.
Theory is the design to find leader that can speed inquirey along.
I can quite imagine now a whole new class of electronics from your concerns and examples - to be tested of course.
New variations on transformers and antennas the QM gravity questions as physics may apply on a higher level than what we now explore. And this amounts to that we may actually be able to modulate the probability amplitudes.
It seems this poet has grained much from all inputs of dialog as hopefully us all. I would not discourage anyone from inquiry who respects the art of it and the value that may be in our shared experiences of a personal nature. Best

Marie and Mitch said...

I like your questions for discussion, and I would like to share my thoughts.

In order to find a successful theory of quantum gravity, I believe both general relativity and quantum mechanics may need modification. In my opinion, this modification may involve our most fundamental understanding of physics: Newton's first law of motion.

Is Newton's law of inertia complete? Or is there more to the story? The concept of inertia was originally developed by Galileo, but that concept was different than Newton's. Based on Galileo's original concept of inertia, I have developed ideas that would modify Einstein's relativity in a way that suggests a simple theory of quantum gravity. I have several papers published on these ideas, and several others accepted for publication. Anyone interested can view my video on YouTube, but please bear with me; it's not all that professional. It is titled "Galileo's Return and the Fall of Einstein's Relativity Theory".

https://www.youtube.com/watch?v=8ZzMz5XMIFw

My video is less than 12 minutes long and is only a condensed version of the whole story. Hence, it may raise more questions than it answers.

Phillip Helbig said...

"I have several papers published on these ideas, and several others accepted for publication."

Can you give links to the papers? Where were they published?

Marie and Mitch said...

I believe that many (if not all)strange interpretations of today's physical theories, particularly with quantum mechanics, come from the practice of developing purely mathematical theories without good philosophy.

I've sent you 5 attachments and 2 links via email. I hope you find them interesting.

Marie and Mitch said...
This comment has been removed by the author.
Svik Vik said...

Basic physics will always be of interest. Of course you have to do some practical work first to pay for it all and help people who need it.

But we need some measurement first. Like what is the gravity between two neutrons as they pass each other closely. Or neutrons going through as carbon tube grating.

Or the frequency of a quantum tuning fork (100 atoms or less) as a large mass passes by.

There must be some way of measuring short range gravity as is should be large 1/R.

hush said...

"Or the frequency of a quantum tuning fork (100 atoms or less) as a large mass passes by." - Svik Vik

What are you predicting here?