Thursday, November 15, 2018

Modified gravity, demystified [video]

Here is the promised follow-up on my earlier video about dark matter. This time I explain how Modified Newtonian Dynamics gives rise to flat rotation curves and what’s the deal with the Tully-Fisher relation. Fixed my make-up issues but now I put the microphone in the wrong place, hence the noise from my shirt. Sorry about that. Click on “CC” in the bottom bar to get English captions.

The problem with the language of the automatic transcription disappeared as spontaneously as it had appeared. As you can see, if it works, it works remarkably well, except that it’s missing all punctuation.

Update: Now available with German and Italian captions. Click on gear icon/subtitles in bottom bar to change language.


  1. Thank you very much, Sabine for your informative videos.

  2. Great video as always!

    (BTW, I'll ask again, what are you using, technically, to make these? I mean equipment and software. Part of that is curiosity, part is that I'm setting up a talk series and am interested in recording them well.)

  3. DvDs have radial constant angular velocity (disk galaxies) or tangent velocity. Disk galaxies exhibit angular momentum non-conserving flat rotation curves.

    Spatial isotropy plus Noether's theorem obtain angular momentum conservation. Baryogenesis' Sakharov criteria include ~0.1 ppb spatial chiral anisotropy to diverge Big Bang matter/antimatter formation. Expect ~0.1 ppb Noetherian leakage of angular momentum conservation (Milgrom acceleration).

    A vacuum trace left foot embeds pair-of-shoes molecules with different energies. Extreme geometric chiral divergence maximizes the divergence. Measure as simultaneous divergent frequency microwave rotational spectra. Maximum line amplitude requires ground state single line excitation via ~2 kelvin rotational temperature and prolate rotationally symmetric top rotors.

    One day, DOI:10.1002/anie.201704221 running crafted molecules. Look, and heal physics.

  4. Great video and quite well presented! Funfact: pronouncing Einstein's name with German [scht] insteadt of Englisch [s-t] unveals in a most sympathic way your German nativity. After all, Einstein was born and raised in Southwest Germany and lived in Switzerland for a long time, so he sure pronounced his own name like you did...

  5. Something to watch tonight. Thanks.

    sean s.

  6. One modest hint: try to avoid too strong gestures, especially making a fist - is's very difficult, I know and various people in (German) TV - providing i.e. weather forecasts or financial news from stock market - have been fighting those unwillingly hand movements for years withtout ongoing successes: one main reason they're holding those cards ('moderation sheets') in their hands all the time: watch news anchorman Ingo Zamperoni firmly warping them or Anja Kohl, working without them, grimly trying to prevent her hands from wagging or - even worse! - raising the index finger... :-)

  7. This is a great video and I'm going to recommend it to several friends. Hope that some teachers pick up on it.

    And I, a USian, don't find the gestures and occasional slight Germanism at all off-putting. Quite the opposite.

  8. I found that video very clearly presented and fascinating! No subtitles were necessary.

    I have always doubted the theory of dark matter just because it feels arbitrary, and because you need precisely the right distribution of DM to make it work reliably. You seem to be condemning the Dark Matter theory too.

    A proper alternative theory of gravity would feel more attractive (but General Relativity exceeds my mathematical abilities, so unfortunately I guess this theory will too).

    Out of interest, what does happen with clusters of galaxies - do they move in accordance with Newton's law, or in yet another way?

  9. Dr Hossenfelder,

    this was an interesting video. I think you carefully make your statement, compare the different approaches and their predictions, and expect your statements to be judged in the end, when you finished your statements. And you would also be open to discussion with people who think you are wrong, but who also make their statements carefully and take you seriously.

    But would you also regard an approach as scientific if someone quickly pounces on what they see as a weak point, but does so with sloppy reasoning that only works if one uses the loose, shifting meanings of everyday language instead of a more exact, nuanced approach? Someone who doesn't even look at the comparison of the predictions, since they would be wrong anyway? Someone who doesn't even see the need for testing which approach would be correct since he thinks everything would already be solved by his (sloppy) reasoning?

    I think that while people shouldn't blindly follow a new approach, they should at least show some curiousity in possible tests of the approach, and what the consequences would be if the approach was the currently best one. And not just for things you personally like such as Modified Gravity, but also for things you dislike such as the symmetric approach to General Relativity (instead of insisting without even testing that it must be asymmetric).

  10. I love these informative videos. Please continue them!

    Question: the observed baryonic mass of galaxies and galactic clusters seems to be heavily dependent on an empirical mass-luminosity relationship. How accurate is that relationship, and could at least a portion of the hypothesized dark matter content be explained by the slop in the relationship?

  11. Sabine, I enjoy your mini-lectures, but this sequel didn't seem to add much to your original. I'm not complaining, and I might find out I'm wrong if I re-watch the original.

    Your German English is charming and easy to understand. When you say that you fixed your make-up issues, are you referring to your original video? I didn't notice you were wearing make-up.

    For whatever it's worth, I think your hand and arm gestures are charming, not in a distracting way. Uli seems to prefer a talking head. If you learn ventriloquism, you wouldn't even need to move your lips. Then you would appear to be a super-evolved human who can project your thoughts. Or just record a separate audio track and stare into the camera.

    By the way, I found evidence of your American-style sense of humor in your book, and I'll also point out that I find it in your videos. For example, this is how you begin your video on string theory: "Today I want to talk about string theory, not because I think that's so interesting, but because I think it's not interesting and I wish we could stop talking about it." You emphasize the last part by putting on a pout.

    Your music videos are entertaining, charming, and humorous too.

    At this point, I must conclude that your denials about enjoying American-style humor are intended to be humorous. But maybe I'm stating the obvious, and shame on me for taking you seriously.

  12. Hi Sabine, the video in your blog is the original one, not the MOND video.

  13. MOND is really not based on sound physics and mathematics although it seems to touch in a phenomenological sense, on some profound theory of Gravity. The derivation of the Tully-Fisher relation in MOND is forced. The square of the velocity should not be equal to a square root following strict mathematical principles implying that this derivation is forced. There must be a derivation that naturally comes from the correct theory of gravity.

  14. Excellent again Sabine. Thank you.

  15. I have a dumb question.
    Some ten years ago during a nonlinear dynamics class we discussed gasses with gravitational interaction, basically reaching Jeans' instability ( for those who don't know the term).

    Afterwards, the professor said let's now assume that we're looking at a galaxy, and let's look at the gravitational force on a star at a given distance from the center of the galaxy. Depending on the mass distribution in the galaxy, the effective mass in the gravitational force changes, and you can easily recover the flat profile just with regular matter.
    In your simplified discussion in this video you don't consider the mass distribution at all, you seem to imply that once you're far enough away the effective mass doesn't change (and this sounds wrong).
    Is this just in order to simplify the discussion, or is it actually close enough to the truth?

    Side-comment: I assume people have done simulations of disk galaxies with Newtonian gravity and no dark matter, and that it didn't fit the results (otherwise it would be a result worth mentioning). In other words: using the luminosity of galaxies as a function of distance from the center as a proxy for mass density, did anyone work out the effective mass that appears in the gravitational force?

  16. Thank You for the video!

    I, also (like some people before me) completely disagree with the notion of You having exaggerated gestures. I find them completely fine. I also guess that You don't care what people say either way and will do Your own thing regardless :D

    This last thought makes it difficult to bring my second point - but I think there is a possibility for objective improvement here (as opposed to completely subjective and culture-dependent gestures). The thing is - maybe You could consider training Your vocalization a little bit? The way teachers, actors and singers are taught? It's useful not only for videos, but also for conferences :D It is infinitely easier to listen to someone not using a microphone, especially when acoustics of the room are BAD, and even if one still have to use a microphone, a sharp, clear and reasonably accented pronunciation makes a lot of difference. Just a thought to consider.

    Third - about Mr Stuart - I can't help but think there should be a bot automatically responding to such comments with 'Have You read the book?" :D

  17. Unknown,

    First, it seems I constantly say this, but could you please use a pseudonym. There are dozen's of "unknowns" in my comment sections. At least you could add a number. Like Unknown553 or something.

    Second, I think you must have misunderstood this. You can recover the profile by adding matter. But we know that it can't be regular matter because we'd see this. That's why the stuff is called "dark matter".

    I am using a disk galaxy as example because in this case it's particularly obvious that something doesn't add up. Yes, you are right, disk galaxies even with the disk added, no dark matter, do not fit the data. You need dark matter to make it fit. The disk helps somewhat but not much. As I say in the video, neglecting it is a good approximation, at least to get across the problem.

    The literature is full with inferred dark-matter distributions that stem from the mismatch between the normal matter profile and the observed velocities. You don't have to dig deep to find references.

  18. The MOND potential sqrt{GMa_0}log(R/GM) is of the same form as what one find for two dimensions, or a charge on a line. Look up Petite’s demonstration of the orbits of drops around charged knitting needles performed on the ISS. These are on YouTube. This suggests some physics of one dimension lower than three.

    The low acceleration a_0 = sqrt{Lambda/3} is also curious. This might have some sort of holographic implication with the cosmological horizon. This then has some similar sense with Verlinde’s dark matter proposal as a space time effect.

  19. It would be really great to see you one day together with the team of "Urknall, Weltall und das Leben" maybe in a discussion with Josef Ga├čner and Harald Lesch on various topics (dark matter, beauty in physics, string theory etc.). I think you would be such a great enrichment for this youtube series though it would be in German only.

  20. Put me in the camp of enjoying your delivery the way it is, both voice and gestures. Good use of graphics too.

  21. It appears like you prefer MOND to dark matter because it is more beautiful :D
    Oh, horror!

  22. panais,

    I don't prefer MOND to dark matter and I don't think it's beautiful. I do not know where you got this impression from. Personally, I side with most other theorists in thinking that it's dang ugly. What I am telling you is that for galaxies MOND is the simpler explanation. As I go to great pains to stress in my book relative simplicity is basically Occam's razor and a good scientific criterion. If you think otherwise, you have misunderstood something.

  23. Hi Sabine,
    Since you find MOND ugly, but you appreciate relative simplicity, wouldn’t it be simpler with schemes that modify inertia rather than modify gravitational force or artificially assume an amount of unseen dark matter?

  24. Thanks for the video. It is accurate and concise.

    My (likely fruitless) hope is that somehow theoretical physics will get us a resolution of this Dark Matter/MOND/Dark Energy puzzle before new observations show us what is really happening. The upcoming James Webb IR telescope and others may well bring an answer. Theoretical physics thus has less than a decade to show if it's still working, by showing us what 95% of the Universe is made of.

    Physics lost soundly to astronomy in the first round, absolutely no predictions for the dark sector were made, like high-temperature superconductivity, physics has been relegated to catch-up.

  25. Again, we have to abandon an idealized four-dimensional geometry of spacetime that extends everywhere. Since spacetime is a so virtual frame (and we don't know why it behaves as a 3+1-dimensional geometry in the first place), it is possible that at larger scales it's more like 2+1-dimensional or similar. What keeps "coherency" of these dimensions to be treated separately at normal scales might be "broken" at larger ones.
    It's, it's... so virtual, just an averaged "projection" of a discrete, causally evolving network. At least in my humble opinion.

  26. Sabine,

    I have to disagree with your initial assumption that the rotation curve for a galaxy can be calculated without reference to the dispersed matter in the disk. That is simply the Keplerian method which works quite well for the solar system (to a first approximation) but cannot possibly suffice for a galaxy that is vastly different in both scale and structure.

    In your answer to @Unknown above you say,"...disk galaxies even with the disk added, no dark matter, do not fit the data." But that is because the disk is being added to the Keplerian calculations, yielding the Newtonian expectation curves, which are still inadequate, since only the centripetal forces are being evaluated. What is missing is the self-gravitational force of the disk system itself.

    There is nothing comparable to a galactic disk system in the solar system. It should not be surprising, therefore, that solar system specific modeling techniques are not adequate for galactic structures.

    While MOND may be a more satisfactory kludge than Dark Matter, it is still a kludge. MOND's virtue is that it points vaguely to the problem that McGaugh's RAR data makes explicit - we aren't calculating the expectation curves properly. The observed distribution of baryonic matter in the galactic disks account for the observed rotation curves. We just need a better analytical framework for calculating them properly. The fault is not in the galaxies, it is in our math.

  27. bud rap,

    Then I suggest you go and integrate the system with the added disk, solve the galactic rotation curve problem, and become famous.


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