## Neutrino Puzzles (Part 2)

Okay, I’ve drafted an update to my list of open questions in physics.

I eliminated a bunch of questions that seem to have been answered. It’s really remarkable how accelerator experiments in the last decade or so have settled questions in particle physics without discovering any new mysterious phenomena. The really big mysteries remain.

I have not gotten around to adding the new questions about black holes raised by LIGO. I have not gotten around to updating the sections on ultra-high energy cosmic rays or gamma ray bursters, both of which sorely need it. But I have updated the section on neutrinos!

Here’s the new version. I still need some more good new general reviews of neutrino experiments and theoretical questions. Do you know some?

What’s going on with neutrinos?  Why are all the 3 flavors of neutrino—called the electron neutrino, the muon neutrino and the tau neutrino—so much lighter than their partners, the electron, muon, and tau?  Why are the 3 flavors of neutrino so different from the 3 neutrino states that have a definite mass?  Could any of the observed neutrinos be their own antiparticles?  Do there exist right-handed neutrinos—that is, neutrinos that spin counterclockwise along their axis of motion even when moving very near the speed of light?  Do there exist other kinds of neutrinos, such as “sterile” neutrinos—that is, neutrinos that don’t interact directly with other particles via the weak (or electromagnetic or strong) force?

Starting in the 1990s, our understanding of neutrinos has dramatically improved, and the puzzle of why we see about 1/3 as many electron neutrinos coming from the sun as naively expected has pretty much been answered: the three different flavors of neutrino—electron, muon and tau—turn into each other, because these flavors are not the same as the three “mass eigenstates”, which have a definite mass.  But the wide variety of neutrino experiments over the last thirty years have opened up other puzzles.

For example, we don’t know the origin of neutrinos’ masses.  Do the observed left-handed neutrinos get their mass by coupling to the Higgs and a right-handed partner, the way the other quarks and leptons do?  This would require the existence of so-far-unseen right-handed neutrinos.  Do they get their mass by coupling to themselves?  This could happen if they are “Majorana fermions“: that is, their own antiparticles.  They could also get a mass in other, even more exciting ways, like the “seesaw mechanism“. This requires them to couple to a very massive right-handed particle, and could explain their very light masses.

Even what we’ve actually observed raises puzzles.  With many experiments going on, there are often “anomalies”, but many of these go away after more careful study.  Here’s a challenge that won’t just go away with better data: the 3×3 matrix relating the 3 flavors of neutrino to the 3 neutrino mass eigenstates, called the Pontecorvo–Maki–Nakagawa–Sakata matrix, is much further from the identity matrix than the analogous matrix for quarks, called the Cabibbo–Kobayashi–Maskawa matrix.  In simple terms, this means that each of the three flavors of neutrino is a big mix of different masses.  Nobody knows why these matrices take the values they do, or why they’re so different from each other.

For details, try:

• Paul Langacker, Implications of Neutrino Mass.

• A. Baha Balantekin and Boris Kayser, On the Properties of Neutrinos.

• Salvador Centelles Chuliá, Rahul Srivastava and José W. F. Valle, Seesaw Roadmap to Neutrino Mass and Dark Matter.

The first of these has lots of links to the web pages of research groups doing experiments on neutrinos.  It’s indeed a big industry!

### 37 Responses to Neutrino Puzzles (Part 2)

1. Toby Bartels says:

In naïve particle physics, every kind of particle just inherently has an associated mass. But with masses arising through interactions (such as the Higgs mechanism), they're not so inherent, and it makes sense that flavour eigenstates might not be mass eigenstates. In fact, maybe we're lucky that these both are apparently eigenstates of charge, lepton and baryon number, and things like that, so that we don’t have to say that the real mass eigenstate is actually a superposition of a positron, a mu neutrino, and a top quark!

Anyway, just as it seems to require explanation that neutrino masses are almost but not quite zero, whereas quark masses aren't so mysterious, maybe it requires explanation that the Cabibbo–Kobayashi–Maskawa matrix is almost but not quite the identity, and the Pontecorvo–Maki–Nakagawa–Sakata matrix isn't so mysterious in comparison.

• John Baez says:

Since charge is invariant under the Poincaré group, if you had an eigenstate of mass that was a superposition of particles of two different charges, you could project it down to the two charge eigenspaces and get two different mass eigenstates that both had well-defined charge. So, without loss of generality we can always assume mass eigenstates have well-defined charge!

The same is true for lepton and baryon number, at least to the extent that these are actually conserved. (A proton acts like a mass eigenstate, but if protons decay it’s not quite.)

Physicists talk about related issues using the buzzword “superselection sectors”.

Anyway, just as it seems to require explanation that neutrino masses are almost but not quite zero, whereas quark masses aren’t so mysterious, maybe it requires explanation that the Cabibbo–Kobayashi–Maskawa matrix is almost but not quite the identity, and the Pontecorvo–Maki–Nakagawa–Sakata matrix isn’t so mysterious in comparison.

Good point! Here are the absolute values of the entries in the Cabibbo–Kobayashi–Maskawa matrix:

$\begin{bmatrix} |V_{ud}| & |V_{us}| & |V_{ub}| \\ |V_{cd}| & |V_{cs}| & |V_{cb}| \\ |V_{td}| & |V_{ts}| & |V_{tv}| \end{bmatrix} = \left[\begin{array}{rrr} 0.9740 & 0.2265 & 0.0036 \\ 0.2264 & 0.9732 & 0.0405 \\ 0.0085 & 0.0398 & 0.9992 \end{array}\right]$

You can see it’s close to the identity, but somewhat less so for the lighter quarks.

Here are the absolute values of the entries in the Pontecorvo–Maki–Nakagawa–Sakata matrix, but with error bars for 3 standard deviations of error, because it’s much less well known:

$\begin{bmatrix} |U_{e 1}| & |U_{e 2}| & |U_{e 3}| \\ |U_{\mu 1}| & |U_{\mu 2}| & |U_{\mu 3}| \\ |U_{\tau 1}| & |U_{\tau 2}| & |U_{\tau 3}| \end{bmatrix} = \left[\begin{array}{rrr} 0.801 - 0.845 & 0.513 - 0.579 & 0.143 - 0.156 \\ 0.233 - 0.507 & 0.461 - 0.694 & 0.631 - 0.778 \\ 0.261 - 0.526 & 0.471 - 0.701 & 0.611 - 0.761 \end{array}\right]$

It’s much farther from the identity. So, some people are trying to cook up theories where lighter fermions have some reason to get more mixed. But you’re right: maybe we should think of mixed rather than non-mixed as the default!

• Toby Bartels says:

Since charge is invariant under the Poincaré group […]

So why doesn't this work with flavor? I can't even think how the Poincaré group is supposed to act on flavor at all.

So, some people are trying to cook up theories where lighter fermions have some reason to get more mixed.

That seems like a good thing to think about, since that phenomenon is happening across the board. So I guess that I'm only saying that perhaps the question isn't so much why light fermions mix so easily as why heavy fermions find it so hard to mix. It's as if inertia applies to flavor.

• John Baez says:

Toby wrote:

So why doesn’t this work with flavor? I can’t even think how the Poincaré group is supposed to act on flavor at all.

Instead of thinking about how the Poincaré group acts on flavor, it’ll probably get the right neurons in your brain firing if you say the sentence “flavor is not conserved”. The Hamiltonian of the Standard Model doesn’t commute with flavor, since it contains lots of flavor-changing interactions involving the W and Z bosons. Things like this:

The strong and electromagnetic forces conserve flavor, just not the weak force. This is why physicists noticed early on that certain ‘strange’ hadrons take a surprisingly long time to decay into hadrons. Before quarks were known, the anomalously long lifetimes of these hadrons led Murray Gell-Mann to posit an approximately conserved quantity, ‘strangeness’. He later went ahead and hypothesized the existence of quarks: the up, the down, and the strange quark. Strange particles contain strange quarks, and they can only decay into particles made of the lighter up and down quarks using the weak force! He won the Nobel for his prediction of the $\Omega^-,$ a particle containing 3 strange quarks, and thus—by some unfortunate choice of conventions—strangeness -3:

So the approximate conservation of flavor played—and still plays—a big role in particle physics. It’s just that damned weak force that violates it. But it’s also the reason the Sun is shining, so don’t turn it off!

• Toby Bartels says:

OK, I see your point, but I don't like your example! That weak interaction doesn't preserve flavour per se, but it preserves both fermion generation and electric charge, and the flavour of a quark or lepton (although not a baryon) is determined by its generation and charge. Since the CKM matrix is so close to the identity and neutrinos are so hard to detect, generation is conserved to a good approximation, even in weak interactions. So the real reason that flavour eigenstates (which, since charge is invariant, are the same as charge+generation eigenstates) need not match mass eigenstates are the exceptions: on the quark side, interactions like the one shown but with the down quark replaced with a strange (or bottom) quark; on the leptonic side, neutrino oscillation.

Incidentally, as I've been reading more about generation-mixing, I see that flavour eigenstates are the same as mass eigenstates for quarks, but this is something of a convention, convenient for the strong force but messy for the weak force. There is a slightly different basis for quark generations whose eigenstates don't quite match the mass eigenstates, and if we wrote quarks this way, then the weak interactions would conserve these alternative generations, but quarks would undergo spontaneous mixing like neutrinos. Conversely, if we describe neutrinos using the mass eigenstates ν₁, ν₂, and ν₃, then there is no oscillation between these, but instead a W boson will sometimes (in fact quite often) mediate between an electron and a ν₂ or even a ν₃ neutrino instead of a ν₁ neutrino, just like it will sometimes mediate between an up quark and a strange quark or even a bottom quark instead of a down quark. This seems like a better overall approach, in that there are fewer fundamental interactions (at the cost of making the weak interaction more complicated to describe).

There's also the mystery of why flavour and mass eigenstates match exactly for the charged leptons if they don't need to. Except that it looks like this is also a matter of convention; there is yet another way of looking at things in which flavour and mass eigenstates match exactly for neutrinos, weak interactions still preserve generation, and charged leptons undergo oscillation now. But of course nobody would want to use this basis, since it makes it complicated even to talk about electrons, the most commonly encountered fundamental fermion of them all!

• John Baez says:

Very interesting stuff, Toby.

Incidentally, as I’ve been reading more about generation-mixing, I see that flavour eigenstates are the same as mass eigenstates for quarks, but this is something of a convention, convenient for the strong force but messy for the weak force. There is a slightly different basis for quark generations whose eigenstates don’t quite match the mass eigenstates, and if we wrote quarks this way, then the weak interactions would conserve these alternative generations, but quarks would undergo spontaneous mixing like neutrinos.

Where did you read this? I’m having trouble finding sources that go into detail on this business of “what’s reality and what’s just a conventional choice of basis” when it comes to Yukawa interactions that give rise to fermion masses.

• Toby Bartels says:

Physics StackExchange, mostly. The answer at https://physics.stackexchange.com/a/76849/ explains that it's just a convention to say that the weak and mass states of up-type quarks and charged leptons are the same, how we could have made this convention for down-type quarks and neutrinos instead, why we have the freedom to make this convention, and why we could make both conventions simultaneously (at least for leptons) if neutrinos were massless. Similarly, it's a convention to say that the flavour states are the mass states for the down-type quarks but the weak states for the neutrinos. Much of this is also at https://physics.stackexchange.com/a/135441/ where it's also explained why the conventions for quarks and neutrinos, while confusingly different, are each reasonable in their own way.

At this point, my thoughts are as follows: In theory, the most natural states are the mass states, which (as Vanadium noted on the Physics Forums, in an excerpt that you quoted in a comment on the other neutrino puzzle post) are the only ones that are truly particle states in the sense that they have plane-wave solutions as free fields. In the interaction picture, mass states evolve to mass states in strong and electromagnetic interactions (and presumably also in gravitational ones), and even in weak interactions mediated by Z bosons, but not in weak interactions mediated by W bosons. The two mixing matrices describe this interaction in the basis of mass states.

This allows us to define weak states as the states that arise from a mass state through an interaction mediated by a W. Now, because this changes the electric charge, people want to change the name of the particle, which works out because they come in generations. So we define the down weak state as what you get when the up mass state emits a positive W (or absorbs a negative one), the electron neutrino weak state as what you get when the electron mass state emits a negative W (or absorbs a positive one), etc. This means that all six quarks and all six leptons have both a mass state and a weak state, which are all different.

However, everybody ignores the weak states of the up-like quarks and of the charged leptons, because it's never helpful to use the weak state on both ends of an interaction. People also take the weak states of the neutrinos as the main ones and describe the mass states as superpositions of the weak states, even though they still take the mass states of the charged leptons and of the quarks (both up-like and down-like) as the main ones and describe the weak states (when they mention them at all) as superpositions of the mass states.

The reason that neutrinos are treated differently is that we are pretty much only interested in them as something that results from an interaction with a W boson. Charged leptons and especially quarks get involved in a lot more stuff, but neutrinos don't do much else. It's more convenient to be able to ignore mixing when working with leptons and only have to deal with it when working with quarks. Except that you then get this fictitious force of neutrino oscillation; that's where the bubble under the wallpaper went.

• John Baez says:

Thanks for the nice overview, Toby! I believe you’ve sketched out a good answer to the question by “Clueless”. It’s funny how hard it seems to be to find this material clearly explained in many introductions to the Standard Model or neutrinos. I think I just half-understand it myself.

2. You mention that a lot of progress has been made since your 1997 list, and that you have eliminated the “answered” questions. May I request a blog post (or series) that reprints the answered questions from 1997 together with a brief description of their solutions?

I also notice that perhaps half a dozen times you distinguish between “mathematical physics” and “physics per se”. Of course, much ink has been spilled on this distinction. (Within mathematics, there was famously the intentionally provocative article by Jaffe and Quinn and the many responses it provoked.) But I would love to hear what you see as the difference in the context of major open questions?

• John Baez says:

Theo wrote:

May I request a blog post (or series) that reprints the answered questions from 1997 together with a brief description of their solutions?

I don’t think I’ll have the energy to write a blog article about that. These questions are so famous, and their answers so celebrated, that it’s almost boring to say more about them now. But I’ll do a quick and dirty job here. Here are the two main questions I deleted:

Q1: Is there really a Higgs boson, as predicted by the Standard Model of particle physics? If so, what is its mass? If not, what breaks the symmetry between the electromagnetic and weak forces, and gives all the elementary particles their masses?

A1: Yes, there is really a Higgs, just as predicted by the Standard Model, and its mass is 125 GeV. It was discovered in 2012. Peter Higgs and François Englert won the Nobel in 2013 for their prediction of this.

Q2: Do gravitational waves really exist?  If so, can we detect them?  If so, what will they teach us about the universe?  Will they mainly come from expected sources, or will they surprise us?

A2: Yes, gravitational waves really exist. As the FAQ noted, they had already been indirectly shown to exist by the Hulse–Taylor binary pulsar observations, so the challenge was to detect them directly. LIGO detected them in 2016. Weiss, Barish, and Thorne won the Nobel in 2017 for their work on this experiment. By now LIGO and the European detector Virgo have detected 50 gravitational wave events, which mainly seem to come from collisions of black holes, though there’s also been one from a neutron star collision.

The big surprise is the great number of collisions of black holes with masses roughly 20–80 times that of the Sun! People didn’t think there were many black holes in this mass range. Some even doubt LIGOs results, on this basis.

The blue ones here are the ones detected by gravitational waves.

Note that both Q1 and Q2 were mainly questions of the form “is what everyone thinks actually true?” As such, their answers were a bit anticlimactic—but more so with the Higgs than the gravitational waves. I need to add some new open questions about the large black holes we’ve been finding!

• John Baez says:

Theo wrote:

I also notice that perhaps half a dozen times you distinguish between “mathematical physics” and “physics per se”. Of course, much ink has been spilled on this distinction. (Within mathematics, there was famously the intentionally provocative article by Jaffe and Quinn and the many responses it provoked.) But I would love to hear what you see as the difference in the context of major open questions?

In the context of the FAQ I was trying to make a distinction between questions about the physical world and questions about mathematics. Of course one can argue a lot about the borderline: for example any experiment involves a lot of interpretation, and thus a lot of mathematics. But I wanted naive readers to be clear that I was talking about some very different kinds of questions here.

The questions I called “physics per se” are questions about the physical world around us. The questions I called “mathematical physics” cannot be definitively answered by studying the world around us. E.g., the question about whether the Navier–Stokes equation has global-in-time solutions for all smooth initial data cannot be answered by observing fluids, though one might get useful hints this way. (For one thing, actual fluids aren’t precisely described by these equations!)

In short, the mathematical physics questions are really math questions that happen to be interesting to physicists.

I think of all this as somewhat orthogonal to the Jaffe–Quinn article and the ensuing fight over mathematician’s ideals of rigor versus physicists’ standards of rigor. A “physicist’s proof” that the Navier–Stokes equation has global-in-time solutions for all smooth initial data would still be a very different thing than watching water swirl around in a bowl and noticing that it never explodes.

3. Scott Centoni says:

Is this the link you want for “Boris Kayser, Neutrino Mass: Where Do We Stand, and Where Are We Going?”
https://arxiv.org/abs/hep-ph/0306072

• John Baez says:

Yeah, that’s what I want! Thanks! It’s strange how bits rot.

I’d prefer something similar that’s not 17 years old, but I haven’t been able to find it yet.

• John Baez says:

Oh, here we go!

• A. Baha Balantekin and Boris Kayser, On the Properties of Neutrinos, 2018.

4. https://www.physics.upenn.edu/~pgl/neutrino/jhu/jhu.html exists while the URL offered for “• Paul Langacker, Implications of Neutrino Mass” does not. It’s not in great shap showing its age of 25 years.

• John Baez says:

I want something equally good but not 25 years old!

5. And the Kayser piece is at: https://arxiv.org/abs/hep-ph/9810513 rather than the arxiv.gov URL.

6. Maybe: https://arxiv.org/pdf/hep-ph/9811460.pdf for the Langacker contribution?

7. Or 13 years later he wrote: https://arxiv.org/abs/1112.5992

• John Baez says:

Thanks! I should read this. But for the FAQ I’d rather have surveys of the problems and data that aren’t so tightly wedded to a particular speculative theory. In this article “mechanisms for generating small neutrino masses are surveyed from a top-down (superstring) perspective.”

8. ecoquant says:

A tangential comment (which seems to be the rule for me here for some reason): I’m heavily engaged supporting the Einstein@Home Project which, among other assessments, uses the prodigious computing capability unleashed by the BOINC platform to detect and study LIGO-derived gravity wave measurements for observational astronomy.

BOINC has made a significant impact on climate prediction, created by Prof Myles Allen. (I also run climateprediction.net.)

If you have spare cycles on a big workstation, especially if you have spare CUDAs, check it out.

I like contributing to observational astronomy because, well, while these days biopharmaceuticals are all the rage, observational astronomy needs some love.

9. Phillip Helbig says:

“Okay, I’ve drafted an update to my list of open questions in physics.”

Cosmology and astrophysics has the most. Some brief comments.

I doubt that anyone who works in the field believes that there really was an initial singularity.
This could be split into the question of small extra dimensions and large extra dimensions and/or into something like Kaluza-Klein extra dimensions and extra dimensions containing Tegmark’s Level II multiverse.
The much simpler question, whether, assuming a trivial topology, what the sign of the curvature is, is still not answered. Some say that if it is too close to call, then it doesn’t matter. However, positive curvature implies a finite universe, which is a big—shall we say, infinite—difference. Keep in mind that, in FRW models, an infinite universe was also infinite at the big bang. If anyone wants to bet, I’ll bet. :-)

5., 6. Again, if anyone wants to bet, I will. What is interesting is that all the new cosmological data can be explained without the need to introduce anything really new. In particular, there is no evidence that dark energy is more than the cosmological constant.

It is important to distinguish between what Peebles, in his latest book (read it!), calls the astronomers’ dark matter, the cosmologists’ dark matter, and the particles physicists’ dark matter. How these are related to the dark matter required in the ΛCDM models of structure formation is not entirely clear. Another aspect of this is why MOND works. In other words, why do many galaxy-scale phenomena which can be explained by dark matter, often requiring additional assumptions, fine-tuning, and so on, happen to be such that a simple one-parameter fit, with many independent and agreeing determinations of that free parameter, works amazingly well?
There are serious claims that the horizon problem isn’t what many think that it is. One of my next projects is to look into this. Stay tuned.
I think that the answer is clear, assuming that the ΛCDM model of structure formation is correct.
I thought that this had been solved, but perhaps I was wrong.

In the particle-physics section, this one

Are there important aspects of the Universe that can only be understood using the Anthropic Principle?

could just as well go into the astrophysics-and-cosmology section. One thing to keep in mind is the question itself, as some will claim that an aspect of the Universe which can be understood only by using the Anthropic Principle is not an important aspect. :-)

• Phillip Helbig says:

“Okay, I’ve drafted an update to my list of open questions in physics.”

I saved the text posted above, so I know that the blog software caused most of the numbers to disappear, so I’m now re-editing it. Understanding how blog software works via reverse engineering is almost as difficult as answering the questions on the list. :-(

Cosmology and astrophysics has the most. Some brief comments:

Question One. I doubt that anyone who works in the field believes that there really was an initial singularity.

Quesion Two. This could be split into the question of small extra dimensions and large extra dimensions and/or into something like Kaluza-Klein extra dimensions and extra dimensions containing Tegmark’s Level II multiverse.

Question Three. The much simpler question, whether, assuming a trivial topology, what the sign of the curvature is, is still not answered. Some say that if it is too close to call, then it doesn’t matter. However, positive curvature implies a finite universe, which is a big—shall we say, infinite—difference. Keep in mind that, in FRW models, an infinite universe was also infinite at the big bang. If anyone wants to bet, I’ll bet. :-)

Questions Five and Six. Again, if anyone wants to bet, I will. What is interesting is that all the new cosmological data can be explained without the need to introduce anything really new. In particular, there is no evidence that dark energy is more than the cosmological constant.

Question Seven. It is important to distinguish between what Peebles, in his latest book (read it!), calls the astronomers’ dark matter, the cosmologists’ dark matter, and the particles physicists’ dark matter. How these are related to the dark matter required in the ΛCDM models of structure formation is not entirely clear. Another aspect of this is why MOND works. In other words, why do many galaxy-scale phenomena which can be explained by dark matter, often requiring additional assumptions, fine-tuning, and so on, happen to be such that a simple one-parameter fit, with many independent and agreeing determinations of that free parameter, works amazingly well?

Question Eight. There are serious claims that the horizon problem isn’t what many think that it is. One of my next projects is to look into this. Stay tuned.

Question Nine. I think that the answer is clear, assuming that the ΛCDM model of structure formation is correct.

Question Thirteen. I thought that this had been solved, but perhaps I was wrong.

In the particle-physics section, this one

Question Twelve. Are there important aspects of the Universe that can only be understood using the Anthropic Principle?

could just as well go into the astrophysics-and-cosmology section. One thing to keep in mind is the question itself, as some will claim that an aspect of the Universe which can be understood only by using the Anthropic Principle is not an important aspect. :-)

• John Baez says:

By the way, I think if you write 1) rather than 1. the number won’t disappear even if it’s at the very start of a line. Let me check:

Here’s a 1.:

1.

Here’s a 1):

1)

• John Baez says:

Hmm, neither one disappeared for me.

Will this disappear?

2) Will this disappear?

This time the 1. disappeared: it’s being interpreted as markup, and it’s getting translated into this:

<ol> <li>Will this disappear?</li> </ol>

but then for some idiotic reason the ordered list does not have visible numbers in it!

So it’s safer to use numbers like 1), 2), etc. for a numbered list, due to this bug.

• John Baez says:

Philip wrote:

Question One. I doubt that anyone who works in the field believes that there really was an initial singularity.

Nor do I, but this is based mainly on preferences rather than any sort of evidence. In an earlier version of the open questions I asked if gravitational waves exist. Of course every expert would say “of course! – because general relativity is correct and the Hulse–Taylor binary pulsar is doing what general relativity predicts”. And yet it was very satisfying to actually detect gravitational waves, to feel sure that they exist. Similarly, some sort of solid evidence that there’s no initial singularity would count as big news, even if it’s only confirming our guesses.

Remember also that this FAQ is for nonexperts. Many nonexperts believe that an initial singularity exists because they’ve heard people say that’s what general relativity predicts. They will be interested to learn that this is not really a fact.

Question Two. This could be split into the question of small extra dimensions and large extra dimensions and/or into something like Kaluza-Klein extra dimensions and extra dimensions containing Tegmark’s Level II multiverse.

I’m reluctant to talk about the details of currently fashionable theories for which there’s no evidence. When people reread my FAQ on the Wayback Machine in a few centuries, I would not like it to be cluttered with old theories that seem silly in retrospect. For example, if I were writing this in 1860, I could have asked whether it’s a change in the modulus of elasticity or the density of aether that causes refraction of light. I could have asked if Cauchy’s labile aether theory is correct—it was much better supported by evidence than string theory is now. But if I were smart I would have asked “does the aether exist, and if so what are its properties?”

• Phillip Helbig says:

“I’m reluctant to talk about the details of currently fashionable theories for which there’s no evidence”

Good point. But you do mention string theory. :-D

I’d be crazy not to mention it. I just don’t dwell on the details, like asking “Is the universe described by heterotic string theory compactified on a Calabi–Yau manifold? Is the universe described by an intersecting D-brane model?” etc.

Similarly, around 1850 I’d be crazy not to ask if there exists an aether, and if so what it’s like. But I might not have asked about the relative merits of Cauchy’s three different aether theories (all of which were supported by a lot more evidence than string theory is now).

By the way, how does one get the quote bars?

You can use HTML:

<blockquote> This is a quote done using HTML. </blockquote>

gives

This is a quote done using HTML.

You can also use Markdown:

> This is a quote done using Markdown.

gives

This is a quote done using Markdown.

There are unfortunately various glitches.

• Wyrd Smythe says:

(Especially if using the WordPress Reader rather than the actual website.)

10. Aaron T says:

Yes, the GRB section is in need of an update, if that’s your goal, as it seems you wrote it before the launch of Swift (+ then Fermi), which revolutionized the field (~1500 afterglows from Swift to date), and ended up settling many questions. In addition, GRBs have since been detected at increasingly higher energies (at GeVs with Fermi/LAT) and then at TeV energies starting ~2 years ago with the ground-based Imaging Atmospheric Cherenkov Telescopes (https://en.wikipedia.org/wiki/IACT) like MAGIC and HESS following Swift triggers.

The origin of the two classes is pretty well established at this point (long:collapsar/hypernova, short: compact object merger), and Swift has discovered a 3rd (ultra-long) class, which seems to be made up of some particularly super giant star classes and even the occassional Tidal Disruption Event. Most of these inferences came from detailed afterglow studies, population modelling and contextual evidence based on host galaxy properties, although obviously the GW detection of GW170817 coincident with GRB 170817 solidly confirmed the compact object merger progenitor hypothesis for at least that short GRB.

The hope of using GRBs themselves as standard candles (like SN Ia) with which to do higher redshift cosmology/Hubble constant mesurements hasn’t really panned out due to the extremely large event-to-event variability and still uncertain microphysics. Neither has the hope to see Lorentz violation in GRB spectra (obviously).

Most attention nowadays concentrates in two areas:
1) detecting very high redshift bursts (highest so far is a Swift burst at z=9) in order to probe the Epoch of Reionization and the formation/death of the first population of massive stars (as yet undetected Pop III stars: https://en.wikipedia.org/wiki/Stellar_population#Population_III_stars) and
2) finding very nearby (z<<0.1) short bursts that can have detectable gravitational wave signals associated with their compact object merger. This was previously thought unlikely, as the (beamed) event rates in such a small local volume are low and the beam/jet opening angle small, but GW/GRB 170817 showed that for sufficiently nearby events the GRB can actually be detected appreciably off the jet axis (~30 deg), thus driving up the joint event rate, albeit with 3-4 order of magnitude less luminous in gamma-rays.
So significant effort (from myself and many others) is going in to deep searches for faint short bursts, which then seed targeted searches for GWs on the LIGO/Virgo side, and vice versa.

• John Baez says:

Thanks—that’s very helpful! I know that a huge amount of progress has happened with gamma ray bursters in the last decade, but I don’t really understand it: I just read news stories here and there, so I don’t have a good picture of the current picture. So this is one of the sections I’m the most scared of updating… but it needs to be done, and I’m sure it’ll be fun when I get into it.

Can you list a few of what experts consider to be the biggest open questions? That’s what I need to write about.

• Aaron T says:

Here’s a few open questions, biased obviously towards what I think about, but poorly attempting to mind-model my colleagues as well:

1) What is the actual radiation emission process for the prompt (read: gamma) emission. Is it synchrotron cooling? synchrotron self-Compton? compton upscattering of thermal photons? other?
2) What is the short GRB “extended emission” and does it come from a post-merger non-BH remnant like a super/hypermassive magnetar?
3) What is the precise jet launching mechanism? Jet angular structure? What fraction of progenitor scenarios successfully launch a jet? Does the particle acceleration mechanism also produce high energy neutrinos? What processes drove the 1.6 second delay between merger and GRB in GW 170817? Is this delay ubiquitous?
4) What fraction of the apparent short GRBs are actually relatively local, but extragalactic (read: < ~30 Mpc), Giant Magnetar Flares, and not cosmological compact object mergers? (wait for a press release next week)
5) Are the Soft Gamma Repeater sources also prolific producers of Fast Radio Bursts (see Swift discovered https://en.wikipedia.org/wiki/SGR_1935%2B2154)
6) Can collapsars also efficiently generate the r-process elements? (like the NS mergers can)
7) Are there other progenitors/other source classes of GRB-like phenomena? So far we’ve found TDEs, exploding stars, colliding NSs, NS quakes, red dwarf flares…. Can NS-BH mergers generate GRBs like their NS-NS brethren? can the earliest massive (pop III) stars generate GRBs?
8) Is the TeV emission from GRBs ubiquitous and only preferentially detected from nearby sources?

I tried to stick to questions on GRB phenomenology itself, and there are certainly things I am leaving out. Of course there are even bigger open questions in e.g. neutron star and black hole physics, connections w/ GWs, testing GR, cosmology etc. that GRBs can be used as a tool to help study. These are some of the most exciting areas in high energy astro right now, in my opinion.

Fast Radio Bursts are in a similar position now to what GRBs were when you wrote your original summary.

Might be helpful to peruse the schedule of one of the last major pre-covid GRB conferences:

http://yokohamagrb2019.wikidot.com/program

Let me know if anything isn’t clear, or if you are interested in more direct physics questions.

• John Baez says:

Thanks, this is great! Now I’m becoming a bit less terrified and a bit more fascinated by this subject.

At some point I’ll post a blog article about the gamma ray burster section of the “open questions in physics” FAQ. I’ll also post one about fast radio bursts. I will probably many more questions then!

Right now, alas, it’s the first week of class and I have to knuckle down a bit. But I know what I want to read about.

Of course there are even bigger open questions in e.g. neutron star and black hole physics, connections w/ GWs, testing GR, cosmology etc. that GRBs can be used as a tool to help study.

Can you list a few of the ‘biggest’ of these bigger open questions? I’m hoping ‘big’ means not just ‘complicated’ but also ‘fundamental’ and ‘exciting’. The open questions FAQ is supposed to focus on really deep, exciting questions; anyone who gets serious about studying them will wind up needing to learn about a lot of smaller, more technical but also more manageable questions.

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