Comment by tsimionescu | Hacker News Reader

Comment by tsimionescu | Hacker News Reader

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> Sure, but I don't need to appeal to this fact to know there's a problem that must be solved. The infinities are enough.

By that logic, you shouldn't believe in black holes / GR at all, right? In practice, we can always replace infinities with some arbitrarily large numbers that don't grow all the way to infinity because of yet-unknown physics (probably quantum gravity, in the case of black holes).

> But again, even if MM couldn't have been performed, Einstein still would have had good theoretical reasons (consistency with Maxwell's equations) to posit SR. And indeed, this was the path the discovery seems to have actually taken.

I don't think this would have been promising if the MM experiment seemed a hundred years away, and in general I don't think SR would have been as compelling in that case. I suspect we would have still had arguments about an aether if experiments on the speed of light in different inertial frames had remained beyond reach.

> Sure, but this is a difference in degree, not in kind. You accept that theoretical arguments for new phenomena that are not directly testable (at present) are acceptable if they're convincing enough. The question about BHILP under this view (which I agree with) is then about how convincing the theoretical arguments are.

Sure, it's a difference in degree in the end. The plausibility of having an experiment "soon" is the degree here. I don't think that Hossenfelder would argue that if an experiment is only possible 2 years from now, or maybe even 20 years from now, you shouldn't work on some theoretical subject. But, when that time horizon stretches well beyond your lifetime and the lifetime of your students, it's perhaps time to reconsider.

I also know for sure she doesn't have a problem with doing theoretical research where you don't yet know how something would be testable, as long as you plan to define an experiment as well. She explains this in some detail when discussing her own work on superdeterminism - where she plans to first define a concrete model, and then come up with experiments which could invalidate that particular model - instead of giving up a priori because "there's no known way to test such a theory".

> By that logic, you shouldn't believe in black holes / GR at all, right? In practice, we can always replace infinities with some arbitrarily large numbers that don't grow all the way to infinity because of yet-unknown physics (probably quantum gravity, in the case of black holes).

I don't see how this follows. The ultraviolet catastrophe (or other divergences) says we have to fix something, it doesn't say that we should choose a weird ad hoc fix.

> I don't think this would have been promising if the MM experiment seemed a hundred years away, and in general I don't think SR would have been as compelling in that case.

It probably wouldn't have been as compelling, I agree. Don't get me wrong; the gold standard is empirical evidence. But the invariance arguments and procession of mercury would be grounds for taking it seriously. Einstein's argument for the theory depends only weakly on MM.

> Sure, it's a difference in degree in the end. The plausibility of having an experiment "soon" is the degree here. I don't think that Hossenfelder would argue that if an experiment is only possible 2 years from now, or maybe even 20 years from now, you shouldn't work on some theoretical subject. But, when that time horizon stretches well beyond your lifetime and the lifetime of your students, it's perhaps time to reconsider.

This seems like an unnecessarily narrow view of what constitutes worthwhile physics. First, because the experimentalists are very clever, and there's no knowing what indirect tests they might propose. But mainly because, if we can make theoretical progress on an important question, why not do that (even if empirical data is not forthcoming)? This view suggests that a physicist in 1915 (or 1925, etc.) should not try to work out properties of gravitational waves, for example, which seems obviously ridiculous to me. (The direct confirmation came about 100 years later.) If the theoretical motivation for doing so is solid, why not?

I agree that BHILP paradox is probably a different case, one where we might really have no shot of saying something useful theoretically. But this requires getting into the specific details of BHILP. These general statements about what is and isn't good physics because of near-future testability all seem clearly suspect.

> I don't see how this follows. The ultraviolet catastrophe (or other divergences) says we have to fix something, it doesn't say that we should choose a weird ad hoc fix.

My point was that we have accepted GR even though we know it implies a divergence at the center of a black hole, which we know exists. We assume the math is probably mostly right, we do have a huge curvature at that point, but it can't be literally infinite. We assume that if we had a slightly better theory (probably quantum gravity) that would fix the divergence without changing too much else.

By contrast, we couldn't say "the theory that predicts the ultraviolet catastrophe is mostly right, we're just missing a tiny piece to stop the divergence", because we had good experimental evidence that atoms don't collapse almost very quickly at all. So, while the infinity alone was a motivation to look for a better theory, it was the extreme contradiction with experiment that motivated a fundamentally different theory instead of an adjustment.

> But the invariance arguments and procession of mercury would be grounds for taking it seriously. Einstein's argument for the theory depends only weakly on MM.

Looking this up some more, I had a mistaken impression. The independence of the speed of light from the velocity of the source was already relatively well established by 1905 - in fact, this was part of the motivation for Maxwell's equations from 1865. Maxwell's equations had been thoroughly tested and were well accepted, especially because of experiments by Hertz in 1900. However, the dominant concept was still that light is a wave in the luminiferous ether - but several attempts to measure the interaction with this ether had produced null results. Even what we now call the Lorentz factor had been experimentally measured through experiments on the speed of light in a moving medium (then thought to be a drag coefficient between the medium and the ether).

Einstein then came up with the idea that constancy of the speed of light should apply in any reference frame, not just some special frame defined by the ether. That means that not only is the speed of light independent of the speed of the source (which isn't that strange for a wave), but it's also independent of the speed of the observer. This was only experimentally proven much later, in 1932, by the Kennedy-Thorndike experiment.

Still, SR followed from theories that were well proven empirically, and had obvious experiments that could falsify it.

> This view suggests that a physicist in 1915 (or 1925, etc.) should not try to work out properties of gravitational waves, for example, which seems obviously ridiculous to me.

Well, I think here there is a difference between trying to work out consequences of an otherwise well-established theory (such as working out exactly how gravitational waves would look like in GR, or arguing based on Maxwell's equations) and trying to come up with a new theory that modifies existing ones (such as grand unification or some solutions to BHILP).

> Still, SR followed from theories that were well proven empirically, and had obvious experiments that could falsify it.

Sure, my point is just that reasoning based on thought experiments, parsimony, etc., played a key role in its discovery.

> Well, I think here there is a difference between trying to work out consequences of an otherwise well-established theory (such as working out exactly how gravitational waves would look like in GR, or arguing based on Maxwell's equations) and trying to come up with a new theory that modifies existing ones (such as grand unification or some solutions to BHILP).

I agree, but this claim gets a bit far from what I take Hossenfelder's point to be (as does the UV catastrophe stuff). You and I both accept that certain theoretical activities are productive, even in the absence of empirical evidence (e.g. working out the consequences of a well-established theory). Hossenfelder's argument, as best I can tell from that blog post, implies that she does not accept these activities as worthwhile. I think there is an interesting conversation to be had about exactly where we should draw the line between worthwhile and not worthwhile theorizing when we don't have data forthcoming, and I think there's a good case to be made that the study of BHILP falls on the "not worthwhile" side of that line. But Hossenfelder doesn't make that case, or seem to admit any nuance: If there's no data, you're just "lost in math" and not doing productive work.

> If there's no data, you're just "lost in math" and not doing productive work.

I don't think that's her position, from what I've seen in many other videos. She is a theoretical physicist, after all, not an experimentalist.

I think her position can better be summed up as "if there is no data, and no realistic possibility of getting data, then you're just 'lost in the math' and not doing productive work".

Of course, "realistic" here is both subjective and a matter of degrees, but at least it means it should be a consideration you decide for yourself before investigating a field.

Additionally, I think, though I haven't necessarily seen her be explicit about this, that she would only apply this test to working on new theories, not to exploring the consequences of already-tested theories. Of course, even if I am right that she would make this distinction, the line between "new theories" and "exploring the boundaries of existing theories" can be pretty thin and subjective in places as well.

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