Wednesday, October 11, 2017

What could we learn from quantum gravity? [Video]


CapitalistImperialistPig said...

Did you do your own animations?

Bill said...

Lovely accent, Sabine, and a great video. A working QG theory would definitely change our lives, and I only hope to see it before I check out of here.

Sabine Hossenfelder said...


Well, yes, who else do you think did them? I'm still fighting with the software, but the number of glitches and problems has gotten smaller. I'm not happy, eg, about the resolution.

Charles Morrison said...

Enjoyed your video very much. I noticed that there were some links to other videos shown on the screen at the conclusion of yours. More specifically, do you have any thoughts you would care to share regarding the "Is Reality a Simulation?" video produced by Quantum Gravity Research in Los Angeles, California? I found it quite thought provoking although it did seem a little far afield from conventional research in this area but since you linked it I would be very interested in what you thought of it.

Tom Andersen said...

My favourite part is the end where gravity comes out unchanged and QM changes. There is very little work in that direction - that gravity may not quantize. Which would end QM in the same way that GR ended Newtonian gravity. Newton's gravity is still good enough for government work (eg NASA probes) but conceptually is all wrong.

Paul Hayes said...

I was hoping to see probability theory replace mysticism in physics before I check out of here. Oh well...

David Schroeder said...

Very nice overview of the current status of Quantum Gravity research. Having pedaled 5000 miles, with a quarter million feet of vertical climbing, in the past two warm seasons, I probably will still be hanging out here when significant advances are made in Quantum Gravity understanding.

Cliff Berg said...

Sabine - what are your thoughts of Erik Verinde's theory, and how it related to quantum theory versus GR?

akidbelle said...

Hi Sabine, many thanks; being deaf I apreciate the captions. Did you do it or Google?

Tom, I also liked this part where QM changes; but what tells you that a theory might be conceptually right?


Sabine Hossenfelder said...


I wrote about this here.

Sabine Hossenfelder said...


I uploaded the transcript, but the timing worked automatically! I probably would have broken down sentences in more meaningful parts had I done it by hand, but besides that it came out remarkably well.

peter cameron said...

awesome video, much admiration, tho would rather see You than your avatar.

Regarding being in two places at once, seems safer to me to say the wavefunction can be (almost) anywhere until it is collapsed. Where the energy of the wavefunction 'is' cannot be said unless it is collapsed - the wavefunction is not observable.

Regarding the empty space between your avatar's hands, geometric Clifford algebra (and particularly the Pauli and Dirac subalgebras) is well suited for describing the vacuum wavefunction in terms of the fundamental geometric objects of those two subalgebras.

Pauli wavefunction of space is comprised of
one scalar
three vectors (for the three directional DOFs of physical space)
three bivector pseudovectors (ditto)
one trivector pseudoscalar

Interactions of wavefunctions are described by the geometric product, generate the four dimensional Dirac algebra of Minkowski spacetime, the particle physicist's S-matrix

One can define such a wavefunction at any massive particles Compton wavelength, including the Planck length, examine the resulting scattering matrix,... this is perhaps a possible route into quantum gravity.

Louis Tagliaferro said...

It’s worth repeating that you have a rare ability to communicate complex physics to us layman in an understandable way that still preserves a good representation of the issues. I liked the video.

I personally think pursuing a solution that rewrites the dynamics of space-time to eliminate a physical curvature of space by gravity holds much promise in unifying QM with gravity. I believe there are empirical, fundamental assumptions in the observation of any displacement (change of position), that could from a solid basis for such a solution. Flat space-time is consistent with observations of the Universe as a whole and its malleability would naturally show a curvature when it followed the shape and strength of a mass’s gravitational field (i.e. the spherical shape of a star).

Tom Andersen said...


I think it's likely that history is a good guide and that no theory is conceptually perfect, except the 2nd Law of Thermodynamics.

"But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."
(Eddington, A.S., "The Nature of the Physical World," [1928], The Gifford Lectures 1927, Cambridge University Press: Cambridge UK, 1933, reprint, pp.74-75. Emphasis original).

peter cameron said...

Louis - If I understand correctly, agree with your remark regarding elimination of space curvature. If you take a look at Gauge Theory Gravity, you might find that approach interesting. Folks in that community claim to have proven the equivalence of their model in flat Minkowski spacetime with GR in curved spacetime back in the 1990s, and so far that claim seems to be holding up.

peter cameron said...

Louis - also, there an interesting paper
"Why Einstein did not believe that general relativity geometrizes gravity"
published by Elsevier, but think you can find it on the web if no paid access.

argument there is curved space was an interpretation introduced by the math folks (they were majority of those who understood the math early on) due to historical precedence of Riemann's speculations on gravity, that the math was just a tool Einstein used to connect gravity and inertia, felt that to call it 'curvature' was waffling.

In that view the apparent endorsement of the curved space view in his communications might often have been to some large extent simply speaking in the majority language of his audience.

Don Foster said...

Thank you for the broad overview of the state of play in quantum gravity research. It was useful to have clear presentation of the basic problem and a brief review of the wide range of theoretical approaches to its solution. Difficult to imagine the quantity of chalk dust that may have accumulated over the years in its pursuit.

The notion that spacetime is emergent is appealing because it suggests a more fundamental normative framework that might also give rise to the apparent universality of physical law – a curious property of the universe for which a rationale would be satisfying.

Another curious observable feature of the universe is the ubiquitousness of oscillatory or periodic dynamics occurring at temporal and spatial scales ranging over many orders of magnitude. While there is within present theory an explanation of each instance of these dynamics their prevalence at multiple scales seems to suggest a broader underlying principle. Would any of the candidate theoretical approaches to quantum gravity be likely to offer, as a byproduct, a rationale for the prevalence of periodic phenomenon?

And, my only quibble with the video is that your avatar would perhaps be more immediately sympathetic with brown lips rather than black. Strange what one notices.

Louis Tagliaferro said...

Peter - Thank you. I've unsuccessfully tried to find if anyone has pursued research in that direction. I'm excited to read what you referenced.