sabine1

Interview with a Theoretical Physicist: Sabine Hossenfelder

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We are pleased to introduce Sabine Hossenfelder. Sabine is a theoretical physicist from the Frankfurt Institute for Advanced Studies. Sabine also authors a very popular physics blog called backreaction.

Reading your blog over the past 6 months or so, one gets a feeling that your views about (particle) dark matter versus modified gravity have changed during that time, or are at least in a state of flux. Where do you stand on this right now, and why?

Both particle dark matter and modified gravity have pros and cons and I don’t think the case is remotely close to being settled.

Given the fact, that physical models are commonly regarded as beautiful and in a sense minimal, how can we explain or classify ever expanding parameters: Super Lie algebras increasing gauge groups, increasing dimensions? Does it possibly mean we haven’t found the right language yet, or is real world description actually more complicated than the models we’d prefer?

I have never heard of the fact that “physical models are commonly regarded as beautiful and in a sense minimal” and even if that was so I don’t know why it would matter. Yes, quite possibly it’s a pretty bad idea to rely on the aesthetic sense of humans to find a good theory to describe nature.

What is your view on the absence to date of encouraging LHC results for supersymmetry? Should we reassess any fundamental assumptions and how might this impact future experiments?

We should have reassessed the fundamental assumptions 30 years ago. I’m not a prophet and can’t tell you how such a reassessment would impact future experiments.

What direction should experimental particle physics go?

The major problem in foundational physics research isn’t the experimentalists, it’s the theorists. We need theorists to come up with good ideas for where to look for new physical phenomena, but theorists merely amend models they should have buried long ago. It’s clearly not working, but evidently we fail to learn the lesson. I’m afraid it’s a systemic problem that will be hard to fix.

What are your views on Loop quantum gravity, and should top Universities like Princeton, Harvard, MIT, Stanford sponsor a loop quantum gravity research group and faculty?

Loop quantum gravity is even more detached from physics than is string theory, and the math is less interesting on its own right. No, I wouldn’t recommend to any university to set up a research group on loop quantum gravity. If some department is interested in quantum gravity, they should set up a research group on quantum gravity phenomenology, and hire people to take on the question how to find experimental evidence that gravity is even quantized. (And, at some point, find out which theory of quantum gravity is the correct one.)

Can we expect anything exciting for theory in the experimental developments that will follow LIGO’s observation of a black hole merger? I mean developments in the most general sense – including the Japanese, Indian and space versions of LIGO.

LIGO will likely deliver lots of new and exciting data for the theory of solar system formation, eg, or the population density of black holes and or neutron stars, not to mention precision tests of general relativity.

Share with us some of the experiences you’ve had running the “Ask a Physicist” program.

Everyone who calls us is very passionate about physics. I think I wasn’t previously aware how much fascination our research brings with it. Or maybe I had forgotten about it. Either way, it’s good to be reminded of it.

Where do you get your inspiration from and how do you stay motivated when much of your work and questions may not be answered till after your passing?

I am happy to contribute small pieces that I hope will one day help to solve a larger puzzle. Inspiration I find plenty in the literature – it’s full with open problems that are waiting for someone to tackle them. The more difficult part is making a wise decision for what is the best project to spend time on. This decision also has to take into account what pays the rent.

I do go through phases where I lack motivation to continue with my research. In these cases I usually spend some time working on something entirely different. My music videos are examples for this. Also, writing helps a lot.

It seems that there is more anti-science and misinformation in the media, which probably trickles down to our students. How can we better train our students to be more resistive to this?

I think that all students should get a rudimentary education in logical fallacies, and cognitive and social biases, as well as means to prevent them.

In your March 22, 2017 blog entry, the impression was given that you may leave academia in a few years. If this is true what are all the reasons you may want to leave, whether there is any minimal change that can keep you in academia and what that minimal change would be, etc.

The blog entry was about perverse incentives in academia. I have considered leaving academia on and off for many years because of this. If you want to succeed in academia you have to play by the rules and my best chances at success would require me to work on topics I do not think have much promise. I don’t see the point in doing that.

Would you recommend a young theoretical physicist to direct his career into a research of supersymmetry, and why?

I’d recommend to all young physicists that they shouldn’t put all their eggs in one basket. Concretely, they should build up their research so that they can, if necessary, specialize in various different directions. Regarding supersymmetry in particular, I have no idea why anyone at this point would want to go into this research area. It’s overpopulated already, and there’s zero evidence it has anything to do with the real world.

What does the future hold for you?

I’ll have three music videos about physics topics coming up in the next months which I am super-excited about right now! I’ve also written a
popular science book which I hope will come out in Spring 2018, but the date isn’t yet fixed. If I find the time, I’d like to think more about the covariant version of Verlinde’s emergent gravity that I recently wrote a paper about. It worked much better than I anticipated and I find this intriguing. In the next months, I’ll be working on a project on the phenomenology of quantum gravity (following up on a paper I wrote some years ago).

Thanks for your time Sabine! We look forward to your future research and those physics music videos!

Sabine’s Recent Blog Posts

  • How do you prove that Earth is older than 10,000 y... by Sabine Hossenfelder on Nov 22, 2017
    Mesosaurus fossil. Probably dating back to the early Permian Period, roughly 300 million years ago. [Image source] Planet Earth formed around 4.5 bi...
  • Away Note by Sabine Hossenfelder on Nov 12, 2017
    I am overseas the coming week, giving a seminar at Perimeter Institute on Tuesday, a colloq in Toronto on Wednesday, and on Thursday I am scheduled to...
  • Naturalness is dead. Long live naturalness. by Sabine Hossenfelder on Nov 10, 2017
    I was elated when I saw that Gian Francesco Giudice announced the “Dawn of the Post-Naturalness Era,” as the title of his recent paper promises. T...

 

I have a BS in Information Sciences from UW-Milwaukee. I've helped manage Physics Forums for over 14 years. I enjoy learning and reading about new science developments. I have a lovely wife and a cat named Mason.
14 replies
  1. kodama
    kodama says:

    What are your views on Loop quantum gravity, and should top Universities like Princeton, Harvard, MIT, Stanford sponsor a loop quantum gravity research group and faculty?

    Loop quantum gravity is even more detached from physics than is string theory, and the math is less interesting on its own right. No, I wouldn’t recommend to any university to set up a research group on loop quantum gravity. If some department is interested in quantum gravity, they should set up a research group on quantum gravity phenomenology, and hire people to take on the question how to find experimental evidence that gravity is even quantized. (And, at some point, find out which theory of quantum gravity is the correct one.)

    Reference https://www.physicsforums.com/insights/interview-theoretical-physicist-sabine-hossenfelder/

    i'm glad you asked the question i've asked.

    i'm sure abhay ashketar or smolin or rovelli would disagree.

    given sabine's answer

    what about loop quantum cosmology's contact with observation in CMB and other observable?

    what about loop quantum gravity reproducing hawking entropy and radiation?

    what about asymptotically safe gravity, noncommutative geometry, causal dynamic triangulation etc?

    should universities that do have LQG research groups and LQG researchers like penn state shut down their LQG research group and fire abhay ashketar et al?

    since the LHC found no evidence of SUSY, and proton decay has found no evidence of proton decay as predicted by GUT's

    Should universities that do not currently have a string theory research group, set one up in light of current LHC SUSY results?

  2. UsableThought
    UsableThought says:
    Greg Bernhardt

    Martin, where have you seen such comments on the models?

    Even as a layperson reading pop-sci accounts, I retain a strong impression that over the years, some inside physics have complained of the un-tidyness of various modern models; that smacks of an aesthetic argument.

    Beyond that, mathematics famously has aesthetic appeal; and since physics relies so heavily on math, it wouldn't be surprising that some researchers and authors inside physics would say the same, at least in pop-sci presentations. I did a quick Google and "symmetry" seems particularly mentioned. Here are just a few hits out of many:

    – A PBS-related article: Symmetry: How Beautiful Math Makes Elegant Physics

    – A particular physicist's pop-sci book: Deep Down Things: The Breathtaking Beauty of Particle Physics (re: the standard model)

    – Another book, with a Penrose forward: Fearful Symmetry: The Search for Beauty in Modern Physics

    Feynman too seems, in his recorded remarks and interviews, to hint here & there at aesthetics; the difference is, I doubt he would have insisted on it in relation to usefulness. Indeed, in his lecture on harmonics, he says this:

    In this one discovery of the Greeks, there are the three aspects: experiment, mathematical relationships, and aesthetics. Physics has made great progress on only the first two parts.

  3. martinbn
    martinbn says:
    Greg Bernhardt

    Martin, where have you seen such comments on the models?

    I didn't mean the latest speculative models, I mean mathematical models in general and throughout history. For example Dirac (and many others) has always maintained such a view.

  4. UsableThought
    UsableThought says:
    martinbn

    For example Dirac (and many others) has always maintained such a view.

    Following up on this – Wikiquote has some particularly toothsome remarks by Dirac along these lines; they are taken from a 1963 article by him in Scientific American. The article was reprinted in 2010 and can be found here; the title is "The Evolution of the Physicist's Picture of Nature". These quotes are pulled directly from the article; bold is mine:

    . . . There is one other line along which one can still proceed by theoretical means. It seems to be one of the fundamental features of nature that fundamental physical laws are described in terms of a mathematical theory of great beauty and power, needing quite a high standard of mathematics for one to understand it. You may wonder: Why is nature constructed along these lines? One can only answer that our present knowledge seems to show that nature is so constructed. We simply have to accept it.​

    . . . It seems that if one is working from the point of view of getting beauty in one's equations, and if one has really a sound insight, one is on a sure line of progress. If there is not complete agreement between the results of one's work and experiment, one should not allow oneself to be too discouraged, because the discrepancy may well be due to minor features that are not properly taken into account and that will get cleared up with further developments of the theory.​

    Clearly Dirac was expressing a preference only; his argument goes no deeper than that. So even in his time others might have dismissed such a view as irrelevant, just as they might today. This is besides the fact that beauty is subjective.

  5. UsableThought
    UsableThought says:

    A related question might be whether Occam's razor (which I would argue is rooted in aesthetics) has any real utility; this blog piece presents a good argument against its over-use in physics and other branches of science: Why The Simplest Theory Is Never The Right One: Occam’s Razor Has A Double Edge. Here's a quote; the bold is mine and highlights the relevance to this thread:

    Theories with the fewest assumptions are often preferred to those positing more, a heuristic often called “Occam’s razor.” . . . But there are numerous reasons to suspect that this simple “theory of theories” is itself fundamentally misguided. Nowhere is this more apparent than in physics, the science attempting to uncover the fundamental laws giving rise to reality. The history of physics is like a trip down the rabbit hole: the elegance and simplicity of Newtonian physics has been incrementally replaced by more and more complex theories. At the time of writing, this has culminated in M-Theory, positing no less than 10 dimensions of space and the existence of unobservably small “strings” as the fundamental building block of reality. It seems safe to assume that the fundamental laws of reality will be even more complex, if we can even discover them.​
  6. leo vuyk
    leo vuyk says:

    There are no LHC results for supersymmetry, if we live inside a real material bubble of a raspberry shaped CP symmetric multiverse. Then super symmetry has even a multiple long distance entangled form down to each quantum.

  7. Lapidus
    Lapidus says:

    Nice overall interview. But this must probably the dumbest thing she ever said:

    I have never heard of the fact that “physical models are commonly regarded as beautiful and in a sense minimal” and even if that was so I don’t know why it would matter. Yes, quite possibly it’s a pretty bad idea to rely on the aesthetic sense of humans to find a good theory to describe nature.


  8. Denis
    Denis says:
    kodama

    i'm sure abhay ashketar or smolin or rovelli would disagree.

    Why should one care?

    kodama

    what about loop quantum gravity reproducing hawking entropy and radiation?

    That's more of a counterargument, given the trans-Planckian problem with Hawking radiation. Ok, to have some Hawking-like radiation you don't need much, all you need is that you have to prevent stability (stable stars do not Hawking-radiate), while everything else somehow radiates. So, it is not really a counterargument. But it is nothing which should somehow count as positive.

  9. Denis
    Denis says:
    UsableThought

    A related question might be whether Occam's razor (which I would argue is rooted in aesthetics) has any real utility;

    It has. There is Popper's criterion of empirical content: We should prefer theories which make more falsifiable predictions.

    A theory with more free parameters to fit with observation has typically less predictive power.

  10. kodama
    kodama says:
    Denis

    Why should one care?

    That's more of a counterargument, given the trans-Planckian problem with Hawking radiation. Ok, to have some Hawking-like radiation you don't need much, all you need is that you have to prevent stability (stable stars do not Hawking-radiate), while everything else somehow radiates. So, it is not really a counterargument. But it is nothing which should somehow count as positive.

    i was replying to this claim

    Loop quantum gravity is even more detached from physics than is string theory, and the math is less interesting on its own right. No, I wouldn’t recommend to any university to set up a research group on loop quantum gravity. If some department is interested in quantum gravity, they should set up a research group on quantum gravity phenomenology, and hire people to take on the question how to find experimental evidence that gravity is even quantized. (And, at some point, find out which theory of quantum gravity is the correct one.).

  11. UsableThought
    UsableThought says:
    Denis

    It has. There is Popper's criterion of empirical content: We should prefer theories which make more falsifiable predictions. A theory with more free parameters to fit with observation has typically less predictive power.

    Can you give a couple of examples? That would be helpful.

    Meanwhile I recently started reading Philosophy of Science: A Very Short Introduction, a primer by Samir Okasha; and he happens to raise a similar point to the one I was groping for; he doesn't use the phrase "Occam's razor" but to say "parsimony" is to say essentially the same thing:

    The idea that simplicity or parsimony is the mark of a good explanation is quite appealing, and helps flesh out the abstract idea of IBE [inference to the best explanation]. But if scientists use simplicity as a guide to inference, this raises a deep question. Do we have reason to think that the universe is simple rather than complex? Preferring a theory which explains the data in terms of the fewest number of causes seems sensible. But are there any objective grounds for thinking that such a theory is more likely to be true than a less simple rival? Or is simplicity something that scientists value because it makes their theories easier to formulate and to understand? Philosophers of science do not agree on the answer to this difficult question.​

    All of which echoes something that Hossenfelder says in the interview that this thread is about – here is that quote:

    I have never heard of the fact that “physical models are commonly regarded as beautiful and in a sense minimal” and even if that was so I don’t know why it would matter. Yes, quite possibly it’s a pretty bad idea to rely on the aesthetic sense of humans to find a good theory to describe nature.​

    We might like the "feel" of parsimony (I certainly do); we might like Popper's argument that theories ought to be falsifiable (despite criticisms by some of that argument); but these sorts of things are preferences on our part. Hossenfelder seems to be implying what might be an interesting caution though she doesn't go into detail.

  12. Denis
    Denis says:
    UsableThought

    Can you give a couple of examples? That would be helpful.

    Simply add a parameter to a theory. Say, instead of ##G=T## use ##G=T+c T_1## with some constant c. The original theory is simpler, by Occam's razor "Don't add entities without necessity", which includes such additional terms. Now let's look at the empirical content. An observation which falsified the new equation falsifies the old equation too, given that it is a particular case, with ##c=0##. On the other hand, it is possible that observation fits with the second equation for some ##c >0##, but not for ##c=0##. In this case, the first equation would be falsified, the second not. So, the simpler equation is more falsifiable, thus, has more empirical content, and should be preferred by Popper's criterion.

    Of course, one can invent examples in the other direction too, the ultimate most simple theory would be "we know nothing", with zero empirical content. But the case where the more complex theory has more parameters to fit, and therefore less empirical content is, I think, quite typical. It goes back to Popper himself, and is, therefore, part of established standard scientific methodology.

    How to give a meaningful explanation of aesthetic feelings like beauty of a theory is much less clear. And beauty can be very misleading. A lot of mysticism has some beauty, so that beauty can lead us into mysticism.

  13. leader
    leader says:

    Simple and beautiful models. These are what Einstein was also fond of.
    Apart from the ever.changing number of parallel universes, universe is quite simple to understand as from creation. The creation of matter and anti-matter through the radiation magnetic moments with the macro magnetic fields is the only model to explain the boson-higgs bosons, fermion transformations even a phenomenon taught in Radiography Testing courses today to explain the high energy radiation-matter interactions at MeV's to be resulted in electron positron pairs. At the cosmogenic origin the same interactions gave the quark, anti-quark pairs to form the matter and anti-matter galaxies.
    Besides that, I suggest you to reckon seriously on the role of the Unified Fields as regards the dark energy to answer the missing gravitational energy holding the galaxies together. This dark energy is nothing but the gravitational force induced by the ever expanding cosmogenic magnetic fields hemselves inducing the electric fields that give rise to the unified gravitational fields as explained by the mathematical sophistications on the second Maxwell's equations.

  14. dadobaggins
    dadobaggins says:

    Physics lately, 30 years or so, I won't argue about the dates, often seems to intrude on philosophy. I say this cause I actually feel uncomfortable just thinking about the physics concepts flying around in the 2017's and the implications. It hurts emotionally if I think or feel too hard about it. It reminds of something between the matrix and a hollow deck and being trapped in amber. How real does real have to be to be satisfying. If I don't feel like I'm in a pod and connected to wires, but feel like I am in a real world that I have some small influence in and on I feel ok. But, I'd have to largely be unaware of a reality like a cheap 1 room apartment pod of wires and peeling wall paper. It's gotta be better than that. I don't want to think about it much. Makes me shiver. I much prefer the illusion, if there is a sad but true reality behind our existence. I believe in God because I want to believe, that someone understands what's going on. Even if my certainty, it might be argued, rests on an illusion of perception of a thought and dream, I am satisfied if I feel not alone. In other words my reality can't just depend on me. There really are others out there? I'm not alone? I think God knows loneliness like that and turned himself into 3 beings, and liked that fellowship so much he kept on making more and more and never intends to stop. That's what I would do.

    love dadobaggins

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