No, negative masses have not revolutionized cosmology • Hacker News Board

Re: No, negative masses have not revolutionized cosmology

#51

Post by ajkjk » Fri, Dec 07, 2018, 9:48 PM UTC

eutropia wrote: post #31

ajkjk wrote: post #25

Earlier quoted context omitted.

The argument presupposes that a person either believes (if they're not a physicist) or knows (if they are) that a spin-2 field must obey those rules, and that gravity is a spin-2 field, which makes the conclusion follow naturally. The math for this would not make any sense to a layperson, but is widely accepted. Proposing a new theory of negative mass means proposing much more significant alterations to the underlyin…

True. I'm not a physicist, so help me understand -- we've only ever observed fermions with spin 1/2 and bosons with spin 1 (and recently, Higgs at 0). We've postulated that gravitons would have to be spin-2 because of how the math works out (I don't understand the math but wikipedia suggests that its because gravity is defined by the use of 2nd order tensors) but we've not confirmed the existence of gravitons. Hopefu…

I am not an expert on this either, though I guess I hope to be someday. But when Hossenfelder says that Spin-2 necessitates like charges attracting, I believe her. I've also heard that result elsewhere.

My understanding is that the spin of a field or particle is more of a result of the equation (specifically, the Lagrangian) which governs its dynamics. This is irrespective of whether you consider it as a field or as a quantized particle of that field; either way the Lagrangian has certain symmetries. The Fermion Lagrangian has symmetry on 4pi rotation, but not 2pi, which (confusingly) we call spin-1/2. I suppose that the GR Lagrangian has symmetry under pi rotations, which corresponds to spin-2.

(that would make sense if the stress-energy tensor is contracted with two vectors; it would essentially boil down to the fact that (-x^T) M (-x) = x^T M x if you wrote everything as matrices. But while I have studied GR I haven't studied it as a field theory so I'm not sure it's this simple.)

So the Spin-2 thing is not too questionable. I don't anything about how to turn that into a statement about gravitational charges, though.

Re: No, negative masses have not revolutionized cosmology

#52

Post by mbell » Fri, Dec 07, 2018, 9:52 PM UTC

lisper wrote: post #48

mbell wrote: post #23

Earlier quoted context omitted.

Maybe? We don't know, also Einstein wasn't a fan: > Einstein originally introduced the concept in 1917 [2] to counterbalance the effects of gravity and achieve a static universe, a notion which was the accepted view at the time. Einstein abandoned the concept in 1931 after Hubble's discovery of the expanding universe. > Einstein reportedly referred to his failure to accept the validation of his equations—when they ha…

It was actually a double-blunder. Einstein missed the opportunity to predict the expanding universe (and, as a corollary, the big bang). And then, when it turned out the universe was expanding, he retracted the CC despite the fact that it actually turned out that there is a (non-zero) CC, the expanding universe notwithstanding. He got the right answer, but he abandoned it because he got it for the wrong reason.

> he retracted the CC despite the fact that it actually turned out that there is a (non-zero) CC

Dark Energy being the CC is just _one possible explanation_, it hasn't been tested.

Re: No, negative masses have not revolutionized cosmology

#53

Post by sunstone » Fri, Dec 07, 2018, 10:01 PM UTC

throwawaymath wrote: post #35

jetrink wrote: post #34

Earlier quoted context omitted.

As I read that paragraph, I thought to myself that the author should read Sabine Hossenfelder's recent book Lost in Math: How Beauty Leads Physics Astray . In that book, Sabine argues quite convincingly that our aesthetic judgements of new theories can mislead us. She says of simplicity, > This dream still drives research today. But we do not know whether more fundamental theories necessarily have to be simpler. The…

> As I read that paragraph, I thought to myself that the author should read Sabine Hossenfelder's recent book Lost in Math: How Beauty Leads Physics Astray. The author is Sabine Hossenfelder :)

In which case she probably has read the book.

Re: No, negative masses have not revolutionized cosmology

#54

Post by rytill » Fri, Dec 07, 2018, 10:03 PM UTC

khawkins wrote: post #12

Occam's razor is a heuristic used in the investigative process, not a principle. While it's fair to point out that a theory makes everything more complicated and thus might make it less likely to be sound, it's not fair to push back on the first bit of research into a theory because they haven't had the time to fully develop it. Indeed, a scientific revolution generally starts by treating as false an assumption previ…

It's not totally a heuristic, though it's intractable to use it the real way. Which theory has a shorter program that generates it?

Re: No, negative masses have not revolutionized cosmology

#55

Post by poopchute » Fri, Dec 07, 2018, 10:03 PM UTC

rwilson4 wrote: post #45

jetrink wrote: post #39

Earlier quoted context omitted.

I realized when I reached the bottom of the post. It is odd to me to see her saying that the new theory is disqualified for not being as simple as alternatives after reading her book.

It's not disqualified for not being simple, it's disqualified for not making testable predictions. Given two theories, neither of which make testable predictions, the simpler one (and the one that has been studied more thoroughly) is the more reliable. Otherwise, why not just say that the missing mass is unicorns? And the energy driving the accelerated expansion of the universe is the energy of their love? There's no…

The author of the negative mass theory said that it will be testable via the cubic kilometer ice array in Antarctica when it comes online (though I'm not sure how). He mentions it in his chain of 17 tweets that summarize the theory .

Re: No, negative masses have not revolutionized cosmology

#56

Post by coldtea » Fri, Dec 07, 2018, 10:25 PM UTC

rwilson4 wrote: post #45

jetrink wrote: post #39

Earlier quoted context omitted.

I realized when I reached the bottom of the post. It is odd to me to see her saying that the new theory is disqualified for not being as simple as alternatives after reading her book.

It's not disqualified for not being simple, it's disqualified for not making testable predictions. Given two theories, neither of which make testable predictions, the simpler one (and the one that has been studied more thoroughly) is the more reliable. Otherwise, why not just say that the missing mass is unicorns? And the energy driving the accelerated expansion of the universe is the energy of their love? There's no…

>It's not disqualified for not being simple, it's disqualified for not making testable predictions

That's not uncommon. In their early development, most theories don't make testable predictions, especially when they attempt to expand the status quo, and not just add some incremental detail...

Re: No, negative masses have not revolutionized cosmology

#57

Post by rwilson4 » Fri, Dec 07, 2018, 10:26 PM UTC

poopchute wrote: post #55

rwilson4 wrote: post #45

Earlier quoted context omitted.

It's not disqualified for not being simple, it's disqualified for not making testable predictions. Given two theories, neither of which make testable predictions, the simpler one (and the one that has been studied more thoroughly) is the more reliable. Otherwise, why not just say that the missing mass is unicorns? And the energy driving the accelerated expansion of the universe is the energy of their love? There's no…

The author of the negative mass theory said that it will be testable via the cubic kilometer ice array in Antarctica when it comes online (though I'm not sure how). He mentions it in his chain of 17 tweets that summarize the theory .

Then we'll find out when it comes online! If the new theory is compatible with past observations, and correctly predicts something that GR gets wrong, that will be a huge finding! But the press has gone wild prematurely, as they often do. As others have mentioned, that seems to be the real issue to which Dr. Hossenfelder objects.

Re: No, negative masses have not revolutionized cosmology

#58

Post by sgt101 » Fri, Dec 07, 2018, 10:32 PM UTC

khawkins wrote: post #12

Occam's razor is a heuristic used in the investigative process, not a principle. While it's fair to point out that a theory makes everything more complicated and thus might make it less likely to be sound, it's not fair to push back on the first bit of research into a theory because they haven't had the time to fully develop it. Indeed, a scientific revolution generally starts by treating as false an assumption previ…

History : Dun Scotus (really) probably invented Ockham's razor - William was his student and used it to avoid the thing that all medieval philosophers needed to avoid, namely being tied up and put on a bonfire.

Why did Dun invent it (probably)… well here's the thing, Christianity has a God with three faces, son, father and holy ghost thing. Why? The answer is - don't multiply entities beyond necessity, so God has the number of faces necessary to do the job, no more, no less.

I'm wittering on because this is where that heuristic came from, literally it's angels on pins stuff. So don't invest in it, I'd bet a bit that if we got Dun and Bill together with a few pints of mead they'd laugh themselves silly to here that 21st Century physics pins any weight onto their measure.

The Greeks wouldn't have, the Chinese didn't, why do we?

Re: No, negative masses have not revolutionized cosmology

#59

Post by vymague » Fri, Dec 07, 2018, 11:10 PM UTC

btilly wrote: post #28

Obligatory XKCD: https://xkcd.com/955/ When you read breathless science journalism about the latest revolutionary paper, remember how it usually works out. Enjoy it as entertainment if you wish, but don't get your hopes up.

Reminds me of the SUSY bets.

Re: No, negative masses have not revolutionized cosmology

#60

Post by guitarbill » Fri, Dec 07, 2018, 11:23 PM UTC

rwilson4 wrote: post #41

mbell wrote: post #14

I'm not a physicist, just an interested bystander but some of these arguments don't seem to hold water to me: > There’s a more general point to be made here. The primary reason that we use dark matter and dark energy to explain cosmological observations is that they are simple. > A creation term is basically a magic fix by which you can explain everything and anything. Dark matter is an unknown 'substance' that inter…

I believe the author's argument is: however simple or complicated dark matter and energy are, negative (inertial) mass just makes things even more complicated, without offering any new (testable) insights. The author herself points out that she (and the GR community) may be wrong about a lot of this stuff (e.g. that gravity is mediated by a spin-2 field), but absent a testable hypothesis, negative masses don't actual…

> "it’s highly problematic to introduce negative inertial masses because this means the vacuum becomes unstable. If you do this, you can produce particle pairs from a net energy of zero in infinitely large amounts. This fits badly with our observations."

Basically, if you mess the the equation, you have to be very sure you aren't simulating something silly. Which is easy to do, unfortunately I've done it often.

I still need to read the original paper in detail to confirm, but if the post is correct, the N-body simulation might have some issues.