What's Going Wrong in Particle Physics? (This is why I lost faith in science.)

Transcript
0:00 · If you follow news about particle physics, then you know that it comes in three types.
0:05 · It’s either that they haven’t found that thing they were looking for. Or they’ve
0:09 · come up with something new to look for. Which they’ll later report not having
0:13 · found. Or it’s something so boring you don’t even finish reading the headline.
0:17 · How come that particle physicists constantly make wrong predictions. And what’ll happen next?
0:23 · That’s what we’ll talk about today.
0:29 · The list of things that particle physicists said should exist but
0:33 · that no one’s ever seen is very long. No supersymmetric particles, no proton decay,
0:39 · no dark matter particles, no WIMPs, no axions, no sterile neutrinos. There’s
0:44 · about as much evidence for any of those as for Bigfoot, though Bigfoot would probably
0:49 · have got me more views. Some particle physicists even predicted unparticles,
0:54 · and those weren’t found either. It’s been going like this for 50 years, ever since the 1970s.
1:00 · In the 1970s, particle physicists completed what’s now called the standard model. The
1:06 · standard model of particle physics collects all the fundamental particles that matter is
1:11 · made of and their interactions. When the model was completed, not all these particles had yet
1:17 · been measured. But one after other they were experimentally confirmed.
1:22 · The W and Z bosons were discovered in 1983 at CERN, the top quark was discovered 1995 at
1:28 · Fermilab. And the last one was the Higgs-boson which was found at CERN in 2012. It was the
1:35 · final nail in the coffin of the standard model. There are no more particles left to look for.
1:40 · But particle physicists believed there’d be more to find. Indeed, I’d guess,
1:45 · most of them still believe this today. Or at least they’d tell you they believe it.
1:50 · Already in the 1970s they said that the standard model wasn’t
1:54 · good enough because it collects three different fundamental forces:
1:58 · That’s the electromagnetic, the strong, and the weak nuclear force. Particle
2:03 · physicists wanted those to be unified to one force. Why? Because that’d be nicer.
2:10 · Theories which combine these three forces are called “grand unified theories”. You get them
2:16 · by postulating a bigger symmetry than that of the standard model. Grand Unified Theories,
2:22 · GUTs for short, reproduced the standard model in the range that
2:26 · it had been tested already but led to deviations in untested ranges.
2:31 · I’d say at the time grand unification was a reasonable thing to try. Because symmetry
2:37 · principles had worked well in physics in the past. The standard model itself was
2:42 · born out of symmetry principles. And even though Einstein himself – yes, that guy again – didn’t
2:49 · use symmetry arguments, we today understand his theories as realizations of certain symmetries.
2:56 · But this time, more symmetries didn’t work. Grand unified theories made a prediction,
3:02 · which is that one of the constituents of atomic nuclei, the proton, is unstable. Starting in the
3:09 · 1980s, experiments looked for proton decay. They didn’t see it. This ruled out several models for
3:16 · grand unification. But you can make those models more complicated so that they remain compatible
3:21 · with observations. That’s what physicists did. And that’s where the problems began.
3:28 · Next there was the axion. The standard model contains about two dozen numbers
3:33 · that must be determined by experiment. One of them is known as the theta parameter.
3:39 · Experimentally it’s been found to be zero or so small it’s indistinguishable from zero. If
3:45 · it was non-zero then the strong nuclear force would violate a symmetry known as CP-symmetry.
3:51 · That the theta parameters is zero or very small is known as the strong CP problem.
3:57 · It isn’t really a problem because the standard model works just fine with simply setting the
4:03 · theta parameter to zero. But particle physicists don’t like small numbers. It’s a feeling that I’m
4:10 · sure most of us have experienced when looking at our bank statements, but particle physicists
4:15 · are somewhat more accepting. They accept small numbers if there’s a mechanism keeping it small.
4:23 · The standard model has no such mechanism. This is why, to make the small theta-parameter acceptable,
4:30 · particle physicists added a mechanism to the standard model that’d force the parameter to be
4:36 · small. But a consequence of this modification was the existence of new particle, which Frank Wilczek
4:43 · called the “axion” in 1978. The name’s a pun on the symmetry-axis of the mechanism, and the name
4:51 · of an American laundry detergent, because, the axion-particle was a particularly clean solution.
4:56 · Unfortunately, the axion turned out to not exist. If the axion existed, neutron stars would cool
5:03 · very quickly which we don’t observe. With this argument, the axion was experimentally ruled out
5:08 · almost as quickly as it was introduced, in 1980. But physicists didn’t give up on the axion. Like
5:16 · with grand unification, they changed the theory so that it’d evade the experimental constraints.
5:21 · The new type of axion was introduced in 1981 and was originally called the “harmless axion”. It
5:28 · was then for some while called the “invisible axion,” but today it is often just called the
5:34 · “axion”. Lots of experiments have looked and continue to look for these invisible axions.
5:39 · None was ever detected, but physicists still look for their invisible friends.
5:44 · Wilczek by the way invented another particle in 1982 which he called
5:48 · the “familon”. No one’s found that either.
5:51 · Yet another flawed idea that particle physicists came up with in the 1970s is
5:56 · supersymmetry. Supersymmetry postulates that all particles in the standard model have a partner
6:02 · particle. This idea was dead on arrival, because those partner particles have the same masses as
6:08 · the standard model particles that they belong to. If they existed, they’d have shown up in
6:14 · the first particle colliders, which they did not. Supersymmetry was therefore amended immediately,
6:20 · so that the supersymmetric partner particles would have much higher masses.
6:26 · It takes high energies to produce heavy particles, so it’d take big particle colliders to see those
6:32 · heavy supersymmetric particles. The first supersymmetric models made predictions that
6:38 · were tested in the 1990s at the Large Electron Positron Collider at CERN. Those predictions
6:45 · were falsified. Supersymmetry was then amended again to prevent the falsified processes from
6:51 · happening. The next bigger collider, the TeVatron was supposed to find them. That didn’t happen.
6:57 · Then they were supposed to show up at the Large Hadron Collider. And that didn’t happen either.
7:02 · Particle physicists continue to change and amend
7:06 · those supersymmetric models so that they don’t run into conflict with new data.
7:11 · The reason particle physicists liked supersymmetry, besides that it neatly
7:16 · abbreviates to SUSY, was that they claimed l’d solve what’s known as the “hierarchy
7:22 · problem.” That’s the question of why the mass of the Higgs boson is so much smaller
7:27 · than the Planck mass. You may say, well, why not? And indeed, there’s no reason why not.
7:33 · The mass of the Higgs boson is a constant of nature. It’s one of those free parameters in the
7:40 · standard model. This means you can’t predict it, you just go and measure it. Supersymmetry doesn’t
7:47 · change anything about this. The Higgs boson mass is still a free parameter in a supersymmetric
7:53 · extension of the standard model, and you still cannot predict it. Supersymmetry
7:58 · therefore does not “explain” the mass of the Higgs boson. You measure it and that’s that.
8:04 · Then there are all kinds of dark matter particles. A type that is particularly popular is called
8:09 · “Weakly Interacting Massive Particles”, WIMPs for short. Experiments have looked for WIMPs
8:15 · since the 1980s. They haven’t found them. Each time an experiment came back empty-handed,
8:21 · particle physicists claimed the particles were a little bit more weakly interacting,
8:26 · and said they need a better detector. There are more experiments that have
8:31 · looked for all kinds of other particles which continue to not find them. There are headlines
8:36 · about this literally every couple of weeks. The PandaX-4T experiment looked for light
8:42 · fermionic dark matter. They didn’t find it. The STEREO experiment looked for sterile neutrinos.
8:49 · They didn’t find them. CDEx didn’t find light wimps, HESS didn’t find any evidence
8:56 · for WIMP annihilation, the MICROSCOPE experiment didn’t find a fifth force,
9:00 · An experiment called SENSEI didn’t find sub GeV dark matter. And so on.
9:06 · The pattern is this: Particle physicists invent particles,
9:11 · make predictions for those invented particles, and when these predictions are falsified,
9:16 · they change the model and make new predictions. They say it’s good science
9:21 · because these hypotheses are falsifiable. I’m afraid most of them believe this.
9:27 · But just because a hypothesis is falsifiable doesn’t mean it’s good science. And no,
9:33 · Popper didn’t say that a hypothesis which is falsifiable is also scientific. He said
9:40 · that a hypothesis which is scientific is also falsifiable. In case you’re a particle physicist,
9:46 · here’s a diagram that should help. Example: Tomorrow you will receive
9:51 · 1000 dollars from my friend the prince of Nigeria. Falsifiable but not scientific.
9:56 · The best way to see that what particle physicists are doing isn’t good science
10:01 · is by noting that it’s not working. Good scientists should learn from their failures,
10:07 · but particle physicists have been making the same mistakes for 50 years.
10:12 · But why is it not working?
10:15 · I’ll try to illustrate this with a simple sketch. If you understand the following two minutes,
10:20 · you can outsmart most particle physicists, and you don’t want to miss that opportunity, do you.
10:25 · Suppose you have a bunch of data and you fit a model to it. The model is this curve. You
10:32 · can think of the model as an algorithm with input parameters if you like, or just set
10:37 · of equations that you work out by hand. Either way, it’s a bunch of mathematical assumptions.
10:42 · If you make a model more complicated by adding more assumptions, you can fit the data better,
10:48 · but the more complicated the model becomes, the less useful it will be.
10:52 · Eventually the model is more complicated than the data. At this point you can fit anything,
10:58 · and the model is entirely useless. This is called “overfitting”. The best model is one that reaches
11:05 · a balance between simplicity of the model and accuracy of the fit. Let’s suppose it’s this one.
11:12 · If you get new data and the data do not agree with what was previously your best model,
11:17 · then you improve the model. This is normal scientific practice, and this is probably
11:23 · what particle physicists think they are doing. But it’s not what they are doing. The currently
11:29 · best model is the standard model and all the data agree with it, so there’s no reason to amend it.
11:36 · Here is what they are doing instead. Let’s imagine that this curve is the standard
11:41 · model and this is all the existing data. And imagine we have a particle physicist,
11:46 · let’s call him Bob. Bob says, that’s nice, but we haven’t checked the model over here. And, he says,
11:53 · I could make this model more complicated so that the curve goes instead this way. Or that way. Or
11:59 · any other way. I’ll pick this one, call this my “prediction”, and hey, I’ll publish it in PRL.
12:06 · Why do I predict it? Because I can. Because you see, my model agrees with all the data,
12:12 · so this prediction could be correct, right?
12:15 · Right? And it’s falsifiable. Therefore, I am a good scientist” And all of Bob’s friends with
12:22 · all their different predictions say the same. They are all good scientists. Every single one
12:27 · of them. And as result of all that “good science,” we get any possible prediction.
12:32 · Then then do an experiment and the data come in and would you know it they agree with the
12:38 · standard model. And Bob and all his friends say: Oh well, no worries, we’ll update our prediction,
12:44 · now the deviations are in this range where we still haven’t measured it,
12:48 · we need bigger experiments. And also, I’ll write a new paper about it.
12:53 · What’s the problem with that procedure?
12:56 · The problem is that those models with all their different predictions are unnecessarily
13:01 · complicated. They should never have been put forward. They are not scientific hypotheses,
13:06 · they are made-up stories, like my friend the prince of Nigeria who will send you money
13:12 · tomorrow. Though, if you send me one hundred dollars today I’ll have another word with him.
13:17 · There are only two justifications for making a model more complicated. The first is if you
13:23 · already have data that requires it. We can call this an inconsistency with data. The second is
13:29 · if the model isn’t working properly: it makes several predictions that contradict each other,
13:34 · or no predictions at all. We can call this an internal inconsistency.
13:39 · And that’s what’s going wrong in particle physics. They have no justification for
13:45 · making the standard model more complicated. When they do it nevertheless, it isn’t working,
13:51 · because that’s just not how science works. If you change a good model,
13:56 · then that change should be an improvement, not a complication.
14:00 · I believe the reason they don’t notice what they’re doing is that they have invented all these
14:06 · pseudo-problems that the complicated models are supposed to fix. Like the absence of unification.
14:12 · Or some parameters being small. These aren’t real problems because they do not prevent them from
14:19 · making predictions with the standard model. They are just aesthetic misgivings. In fact,
14:25 · if you look at the list of unsolved problems in the foundations of physics on Wikipedia,
14:29 · most problems on the list are pseudo-problems. I have a list in which I distinguish real from
14:37 · pseudo-problems, to which I’ll leave a link in the info below.
14:40 · And there are a few real problems in the foundations of physics. But they are difficult
14:46 · to solve and particle physicists don’t seem to like working on them, but then I repeat myself.
14:52 · I’ve been giving many talks about this. It hasn’t made me friends among particle
14:57 · physicists. But it’s not like I am against particle physics. I like particle physics.
15:03 · That’s why I talk about those problems. It bothers me that they’re not making progress.
15:08 · There are some common replies that I get from particle physicists. The first is to just deny
15:15 · that anything is wrong. Because, hey, they are writing so many papers and holding so
15:21 · many conferences. Or they will argue that it sometimes just takes long time to find evidence
15:27 · for a new prediction. For example, it took more than 30 years from the hypothesis of neutrinos
15:32 · to its confirmation. It took half a century to directly detect gravitational waves and so on.
15:39 · But both of those objections are beside the point. The issue isn’t
15:44 · that it’s taking a long time. The issue is that particle physicists
15:48 · make all those wrong predictions. And that they think that’s business as usual.
15:54 · The next objection they bring up is normally that,
15:57 · yes, there are all those wrong predictions, but they don’t matter. The only thing that
16:03 · matters is that we haven’t tested the standard model in this or that range, and we should.
16:09 · The problem with this argument is that there are thousands of possible tests we could do in
16:15 · physics, and all of them cost money, sometimes a lot of money. We must decide which tests are the
16:22 · most promising ones, the ones most likely to lead to progress. That’s why we need good predictions
16:30 · for where something new can be found. And that’s why all those wrong predictions are a problem.
16:35 · Particle physicists know of course that predictions are important,
16:39 · because that’s why they always claim that some new experiment would be able to rule out this or that
16:45 · particle. Though they usually don’t mention that there wasn’t any reason to think those
16:50 · particles existed in the first place. Besides, in which other discipline of
16:56 · science do we excuse thousands of wrong predictions with saying it doesn’t matter?
17:01 · Another common reply I get from particle physicists is that it doesn’t matter that
17:07 · those models are all wrong because while they’re working on it, they might stumble
17:12 · over something else that’s interesting. And that’s possible, but it’s not a good strategy
17:18 · for knowledge discovery. As I’ve already said a few times, it does as a matter of fact not work.
17:25 · Also if that’s really the motivation for their work, then I think they should put
17:31 · this into their project proposals: Hey, I don’t actually think that those particles
17:36 · I’m going on about here exist but please give me money anyway because I’m smart and maybe while I
17:42 · write useless papers I’ll have a good idea about something else entirely. I’m sure that’ll fly.
17:48 · Another objection that particle physicists often bring up is that this guessing worked
17:53 · in the past. But if you look at past predictions in the foundations of physics
17:58 · which turned out to be correct, and that did not just confirm an existing theory,
18:03 · then it was those which made a necessary change to the theory. The Higgs boson, for example,
18:11 · is necessary to make the standard model work. Anti-particles, predicted by Dirac, are necessary
18:18 · to make quantum mechanics compatible with special relativity. Neutrinos were necessary to explain
18:25 · observations. Three generations of quarks are necessary to explain CP violation. And so on.
18:32 · That the physicists who made those predictions didn’t always know that doesn’t matter. The point
18:38 · is that we can learn from this. It tells us that a good strategy is to focus on necessary changes to
18:46 · a model, those that resolve an inconsistency with data, or an internal inconsistency.
18:51 · One final objection I want to mention usually doesn’t come from particle physicists,
18:56 · but from people in other fields who think that we need all those models
19:00 · to explain dark matter. But that’s mixing up two different things.
19:05 · We need either dark matter or a modification of gravity to explain observations in astrophysics
19:12 · and cosmology. But if it’s dark matter, then the only thing we need to explain observations
19:18 · is how the mass is distributed. Details about the particles, if they exist, are unnecessary.
19:24 · What particle physicists do is guessing these unnecessary details. They guess, for example,
19:31 · that those particles will be produced at some particle collider. Which then doesn’t happen.
19:36 · So what will happen to particle physicists? Well, if you extrapolate from their past
19:42 · behaviour to the future, then the best prediction for what will happen is:
19:46 · Nothing. They will continue doing the same thing they’ve been doing for the past 50
19:52 · years. It will continue to not work. Governments will realize that particle physics is eating up a
19:58 · lot of money for nothing in return, funding will collapse, people will leave, the end.
20:04 · A lot of people are intimidated by physics. Don’t be. It isn’t magic,
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