Hacker News Reader

Comment by FartyMcFarter | Hacker News Reader

> This seems like a fully general (and somewhat unconvincing) argument against doing theoretical physics.

Not exactly, since this quote definitely doesn't apply to all theoretical physics:

> experiments are not possible, not now and probably not in the next 10000 years.

The error is that, simply because, even if true, that an experiment can't be done for the next 10000 years, this doesn't mean that someone else could think of a different experiment that could fulfill the initial prerequisites and provide a workable outcome/answer

I think the writer is being too fatalistic on that simply because he/she can't come up with a workable experiment, then no one else can. I think that simply because of all the nonlinearities of "life", we are bound to have someone else come up with maybe a different experiment which answers the question, just picture that there are several, several ways to prove General Relativity with experiments not just "the one"

At least for cosmic black holes, there is no way to measure the correlation of hawking radiation with the objects composing the black hole unless you can detect and store information about all objects going into the black hole and all radiation coming out, and then look for correlations between these. Even assuming you could store and process this literally stellar amount of information, you would have to be extraordinarily lucky for the correlation to arise immediately. More likely, you would need to do this for the entire lifetime of the black hole, and only after it's (mostly) evaporated could you run the analysis. Even granted the galaxy-sized computer that could do so, you would need to wait a few thousands of billions of years or more to reach that state.

Einstein had no positive evidence to suggest direct tests of gravitational waves would ever be possible. His arguments were entirely about consistency and theoretical parsimony, as are the arguments about the black hole information paradox. If we take the view that we shouldn't work on physics we can't test, or can't test for a very long time, then we are led to the conclusion that Einstein shouldn't have worked on gravitation waves. I find that conclusion untenable.

I think that author is simply saying why they don't want to work on this problem any more, not necessarily that nobody else shouldn't be working on it anymore. Einstein did what he wanted for his own good reasons, he was right and extremely insightful on most occasions and wasn't driven to 'put his name in the history books' but because he was working on interesting problems. Once the problems no longer seem that interesting it's time to move on.

I disagree. The author is very pointedly saying that research on this problem is worthless and a waste of time. See the last paragraph, for example.

Which contains this bit: "I am not talking about this because I want to change the mind of my colleagues in physics."

That seems to be pretty clear to me.

The full paragraph is: "Why am I telling you this? I am not talking about this because I want to change the mind of my colleagues in physics. They have grown up thinking this is an important research question and I don’t think they’ll change their mind. But I want you to know that you can safely ignore headlines about black hole information loss. You’re not missing anything if you don’t those articles. Because no one can tell which solution is correct in the sense that it actually describes nature, and physicists will not agree on one anyway. Because if they did, they’d have to stop writing papers about it. "

It's hard for me to read this is as anything other than "I am not talking about this because I want to change the mind of my colleagues in physics [because they're too far gone]." That is, she believes the research is worthless and the problem shouldn't be investigated. She just doesn't think she can convince people of this.

But what she believes may simply be wrong. It's just one persons opinion and regardless of motivations ascribed to others people need to justify their choices to themselves and themselves alone but some people apparently need to do so publicly because otherwise it somehow doesn't count.

This is just one interesting sub-problem in physics and if you choose for a career in theoretical physics you know that not everything that you set out to do you will achieve and I suspect that in this particular case it is a let down of fairly large proportions.

So you get a lot of internal struggle to justify the choice, the sunk cost issue is massive and your colleagues are going to go and continue without you. All that needs to be justified to the 'self', both on the off chance that they will come up with a solution when you gave up as well as about all of the time that you now feel that you have wasted.

And in the realm of self justification 'I will stop working on this problem because I no longer feel like working on it' is a lot harder to sell to the ego than 'I will stop working on this problem because I feel that it can't be solved in a meaningful way'.

I'm fine with it, either way, I've seen enough people struggle with career choices made in their 20's when they were in their 40's not to recognize the symptoms: you have reached the halfway mark in your life, what do you have to show for it? And if the answer is 'not much' then that can be a problem. But it doesn't have any value for others, that's all just window dressing and ego-placating.

Gravitational waves are just a consequence of the Einstein field equations for which he had experimental evidence for (the light bending from the sun during the eclipse)

GR made predictions that were testable at the time and Einstein was keenly interested in testing them. See https://einsteinpapers.press.princeton.edu/vol5-doc/609

Yes, but gravitational waves themselves were not testable. To get from the confirmation of the other predictions to belief in gravitational waves, for which you had no direct evidence (until recently), you need to apply exactly the non-empirical criteria (parsimony, consistency, etc.) that Hossenfelder (taken literally) seems to think is not sufficient for doing theoretical physics in the absence of direct evidence.

Then you are misunderstanding her position. She is not claiming that a convincing solution to the BHILP is impossible. If you came up with a theory of quantum gravity that can be measured in other regimes and that also solves the BHILP, perfect: we now have good reasons to believe that your theory is indeed THE solution to that problem.

However, if you come up with a theory that makes the exact same predictions as QM and GR except for the BHILP, then your theory is not very interesting, since we will never be able to test this theory, and there are other inconsistencies between QM and GR that you haven't solved, so we can't just rest on our laurels and say physics is over.

I don't think I misunderstand her position. She seems to take the position you give:

"However, if you come up with a theory that makes the exact same predictions as QM and GR except for the BHILP, then your theory is not very interesting, since we will never be able to test this theory, and there are other inconsistencies between QM and GR that you haven't solved, so we can't just rest on our laurels and say physics is over."

I think this is wrong. There are non-empirical reason reasons we might prefer the new theory to the old one. For example, we know QM are GR are inconsistent, so if I give you a parsimonious, consistent theory that captures both QM and GR in the appropriate limits, then that's a great reason to prefer it. In principle, we could prove theorems (and some theorems of this form have been proved) that pin down the possible resolutions of the GR/QM inconsistency. If we tighten the net so that only one theory remains, then obviously we should choose to believe that theory.

Now, will that actually happen? I don't know. But merely waving your hands and going "not testable" is not enough, because we all accept that mathematical consistency ought to have a huge influence on theory choice. You need to make some argument about BHILP specifically saying that theoretical arguments will never produce productive physics.

> For example, we know QM are GR are inconsistent, so if I give you a parsimonious, consistent theory that captures both QM and GR in the appropriate limits, then that's a great reason to prefer it.

Absolutely agree. But BHILP is not the only inconsistency, so if you only solve that one and leave the others, but complicate the math or add other elements that can't be tested, it's not a particularly compelling theory.

So, why not work directly on the other inconsistencies, which might be directly testable, and see if this one disappears that way?

To be clear, when I say other inconsistencies, I am referring to things like making QFTs work in a non-flat spacetime, and/or reconciling the linearity of QM (without Born's rule) and the non-linearity of GR.

> So, why not work directly on the other inconsistencies, which might be directly testable, and see if this one disappears that way?

These are not mutually exclusive options. The community works on both.

Again, I agree that so far, practically speaking, work on BHILP has not been super compelling. I take issue with the stronger stance that Hossenfelder seems to take, that we know a priori that work on BHILP will be worthless because experimental data is not forthcoming. ("And that’s why I stopped working on the black hole information loss paradox. Not because it’s unsolvable. But because you can’t solve this problem with mathematics alone, and experiments are not possible, not now and probably not in the next 10000 years.")

Reply on news.ycombinator.com