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Do we have evidence for new physics? Or not?
🕑 Added 2023-08-05 12:00:06 +0000 UTC[This is a transcript with links to references.]
I constantly hear physicists claim that there is something “wrong” with our understanding of the universe. It’s an argument they frequently use to justify new experiments, a better detector, a new telescope, a bigger collider.
At the same time, they’ll also tell you that, try as they might, they can’t find any evidence that disagrees with their best existing theories. You’ve seen the headlines: Einstein was right again. Yet another anomaly gone. So, what’s going on. Do we have evidence of new physics? Or not? That’s what we’ll talk about today.
In a recent article in the CERN courier, assistant professor Karri DiPetrillo argued for a larger particle collider. She wrote that we “know the Standard Model is not the complete picture of the universe. Experimental observations and theoretical concerns strongly suggest the existence of new particles at the multi-TeV scale.” TeV stands for Terra electron volt and is a measure of energy. The Multi TeV scale is what the new collider would test. There are no observations or concerns that suggest anything at the TeV scale.
Another example. In a recent press release, Jim Fast from Jefferson lab was quoted with the statement “We know there are some things wrong with the Standard Model”. He said this to explain why we need a new experiment to measure the weak charge of the electron. Makes you wonder how he knows that. Is it those observations or concerns that don’t exist?
It’s been going on like this for a long time. Already in 2018, the then director of Fermilab, Nigel Lockyer, told the BBC that we need a bigger collider because “Everybody believes there’s something there… From a simple calculation of the Higgs’ mass, there has to be new science.” That is, of course, also wrong. For starters, no one can calculate the Higgs’ mass, it’s a free parameter, it’s determined by measurement.
What DiPetrillo, Fast, and Lockyer have in common is that they’re all experimental particle physicists. Experimentalists in this field for the most part just repeat theoretical arguments. If you push them, they’ll admit that they don’t really believe what the theorists say. But if it brings them money, it seems, they’re willing to put their disbelief aside.
So let’s look at the most commonly used arguments according to which there “has to be new physics” that supposedly justifies all this investment. It’s a really long list and the heat death of the universe is only 10 to the 100 billion years away, so for today I’ve just picked the most common claims. That’s the mass of the Higgs boson, quantum gravity, dark matter and dark energy, neutrino masses, and the lack of antimatter in the universe. We’ll look at these one by one, starting with the mass of the Higgs boson.
In the foundations of physics, we currently use two different theories. One is Einstein’s General Relativity. Yes, that guy again. Einstein taught us that the effect we call gravity is really due to the curvature of space-time. The other theory we use in the foundations is a quantum field theory, imaginatively named the “Standard model of particle physics”. It collects the known elementary particles and the three forces that hold them together: the electromagnetic force and the strong and weak nuclear force.
The Higgs boson is one of those particles in the standard model, and its mass is a free parameter. This means you measure it, then you use it for further predictions. There’s nothing wrong with it. So why do particle physicists keep going on about it?
I believe it’s a mistake similar to the mistake that once made physicists think that black holes don’t exist. They’re confusing mathematics with physics.
The first description for black holes came from Karl Schwarzschild. In his equations, there’s a singularity at the *horizon of a black hole. That there’s a singularity doesn’t mean the maths is singularly incomprehensible, though there’s that, it means that some maths thing returns the value infiniy. This seemed to suggest that black holes are unphysical, a mathematical possibility that doesn’t exist in reality.
It turned out however, that this singularity at the horizon is an artifact of the particular mathematical description that Schwarzschild used, and that it has no physical counterpart. So there’s nothing physically wrong at the black hole horizon. This becomes apparent if you calculate what you can actually measure. For example, what you’d experience if you crossed a black hole horizon. The answer is nothing. The horizon isn’t a physical thing. Only problem with it is that you can only cross it one way.
What happens in Schwarzschild’s equations is that some mathematical quantities look weird, but once you ask what you can measure, everything is normal. That’s why we no longer think there’s something weird going on at the black hole horizon. There’s a singularity in the *middle of the black hole, but that’s a different story. I’ll get to that in a minute, if you let me, but first let me wrap up the thing with the Higgs mass.
The mathematics of the standard model says that quantum fluctuations make a large contribution to the mass of the Higgs boson. Physicists worry about that like they worried about the singularity in Schwarzschild’s equations. But like in Schwarzschild’s equations, the quantity they worry about can’t be measured. Only thing you can measure is the mass of the Higgs boson itself. They’re arguing about something that doesn’t exist. It’s like complaining that Harry Potter’s magic spells violate energy conservation. Maybe they do, but so what, they’re not real!
Another argument they sometimes make is that the Higgs-Boson is different from the other particles in the standard model. And that’s correct, but just because it’s different doesn’t mean there’s something wrong with it.
And in any case, the argument that there’s supposedly something wrong with the mass of the Higgs boson was what led to all those wrong predictions for new particles at the LHC, like supersymmetric particles and additional dimensions and gravitons and what not. So, whenever a particle physicist tries to tell you there’s something supposedly “weird” about the Higgs boson, keep in mind they already fooled us once.
Let’s then talk about something more interesting. As I said, we currently use two different theories in the foundations of physics, General Relativity and the Standard Model. Unfortunately, these two theories don’t cooperate. That’s because the particles in the standard model have quantum properties. For example, they can be in two places at once. But General Relativity is not a quantum theory. So the gravitational field of those particles can’t be in two places at once. This makes no sense, it’s just mathematical rubbish.
What we need to solve this problem is a theory that gives quantum properties to gravity or, since Einstein said that gravity is really an effect of space-time curvature, a theory that gives quantum properties to space and time itself. It’s usually just called “quantum gravity”.
The theory of quantum gravity is also believed to resolve the singularities that we have in General Relativity, at the Big Bang and inside black holes. It’s because in these cases space-time curvature is extremely large, and then the quantum fluctuations of the geometry should become important. And this is something we *know is wrong with our current theories, we’re missing a theory of quantum gravity.
But. Particle physicists usually don’t talk about this. It’s for a simple reason: Gravity is an extremely weak force compared to the other forces, electromatic and the two nuclear forces. You can’t measure it if you look at single particles, like those in the standard model. Their gravitational pull is just too tiny. We can only measure gravity for large objects because it’s different from the other forces in the standard model. It can’t be neutralized, so it adds up. If you form large chunks of matter, the nuclear forces become unnoticeable and the electromagnetic force usually becomes fairly weak, unless possibly you’re lying in an MRI machine which is why you really shouldn’t take guns anywhere near those things.
That’s why testing quantum effects of gravity isn’t something you can do with single particles. You’d take something bigger. I talked about possible experiments for quantum gravity in a previous video. For today I just want to say, yes, quantum gravity is an issue we need to resolve. But particle physics won’t help you with it.
Next point: Dark matter. If we take Einstein’s General Relativity and all the particles of the standard model, we run into a problem. It’s that these two together just don’t correctly describe many observations in astrophysics. There are two ways that physicists are trying to resolve the problem.
One is to add new particles, that’s the idea of dark matter. The problem with dark matter is that the hypothesis is so flexible it can be adapted to everything. Whenever an observation doesn’t fit to a prediction, astrophysicists change the dark matter model. One of the least understood problems in the universe is Elon Musk, but I wouldn’t be surprised if dark matter will soon be used to explain that too.
The other way to explain the mismatch between prediction and observation is to change general relativity. This is known as modified gravity. But… modified gravity can’t be tested with particle colliders or particle detectors. Indeed, it generally can’t be tested in the solar system, because the modifications only become noticeable far away from big, massive things like our sun.
That’s why particle physicists don’t like modified gravity. They can’t get any money out of it. They prefer the idea that the observations are explained by dark matter particles, because looking for particles is what they do. But not *any particles, no, specifically such particles that can be either produced in the next particle collider or at least measured in their detectors.
The problem is that even if dark matter is made of particles, which it may not, there’s no reason why it should be a type of particle that can be produced at the next bigger particle collider or be measured by the next bigger experiment.
If you point that out, they’ll tell you that, well, at least you can rule out *some ideas. Sure. But that’s like saying I don’t know where your car keys are, but if you give me 10 billion dollars I’ll check if they’re in my fridge. At least that way we can rule out *some ideas. Deal?
So, the observations usually attributed to dark matter are a real problem that requires solution somehow, but that solution doesn’t necessarily require changing the standard model, certainly not at the energies that a next bigger collider could test.
What’s with dark energy? The observations attributed to dark energy are well explained by a cosmological constant, that’s just a constant of nature, end of story. In terms of an explanation, a constant of nature is as good as it gets.
Some theorists try to argue that the constant is not constant but really a quantum field that’s made of particles which you can then look for with a particle collider, but that’s an unnecessarily contrived hypothesis of which, again, there are infinitely many, so testing them is an unpromising strategy.
Let’s then talk about neutrinos, which are probably the “new physics” argument that causes the biggest confusion. Neutrinos are one of the particles in the standard model. There’s three of them. Neutrinos are, as the name suggest, electrically neutral. Because they have no electric charge, they interact very rarely and are hard to measure.
We’ve known for about 20 years that neutrinos have masses and there’s been a Nobel prize for that. Consequently if you look up the standard model on Wikipedia it’ll tell you that of course neutrinos have masses. But a lot of theoretical physicists would tell you that neutrino masses are physics “beyond the standard model”.
I think that’s bad terminology, but here’s the rationale. We know that neutrinos have masses, but we’re not sure how they get them. The issue is that other particles, like the electron, come in two versions, we call them left-handed and right-handed. And the mass of the electron comes about by an interaction between its left-handed and right-handed version and the Higgs-boson. But for neutrinos we’ve only ever observed the left-handed version, so we can’t do that.
There are three different options to give masses to neutrinos. The first option is to just postulate that right-handed neutrinos exist, we just haven’t yet observed them because they’re too heavy.
The second option is that neutrinos are different from electrons, in that their right-handed version is the same as the left handed one. This is called a Majorana particle. You can then take just two of the neutrinos we already know exist and couple them to the higgs and they get masses. If neutrinos were majorana particles that’d allow for a particular type of radioactive decay of neutrons, known as the neutrinoless double-beta decay. There is an experiment called KATRIN looking for this. So far it hasn’t seen anything, but it’s only just begun collecting data.
The maybe most popular option is a combination of the previous two in which case the right-handed neutrinos are too heavy to be produced at the next bigger particle collider.
The third option is that you create a mass from the left-handed neutrinos and the higgs without making them majorana by coupling it to a *pair of Higgses. For this you don’t need any new type of physics, but if you do this, it creates a mathematical problem in the Standard model, it would then break down at high energies. This could mean that at those high energies there’d be new particles popping up. But the energies for this are again well out of reach of the next bigger collider.
This means the issue with the neutrino masses is similar to that of quantum gravity. We know that there has to be some solution, some “new physics”, but the solution becomes necessary at energies so high they’re orders of magnitude away from even the next bigger collider. Though in both cases there are other experiments that could shed light on the situation.
Another reason that I often see particle physicists name to argue there’s new physics is the alleged puzzle that there’s more matter in the universe than antimatter. They believe that the universe should have been created with equal amounts of both, so that they completely annihilated, leaving behind nothing but radiation, which is clearly not what we see, unless you are radiation in which case, thanks for watching, don’t forget to subscribe.
This supposed problem is quickly solved. There’s no reason why the universe should have started out with equal amounts of matter and antimatter.
I think this argument sounds plausible to non-experts because they’ve heard that we know from Dirac something about the symmetry between matter and antimatter. And that’s right. But all that Dirac taught us is that antiparticles exist. His revelation says nothing about how *many* of these particles there are in the universe. I explained this in more detail in an earlier video.
So, when it comes to the matter-antimatter asymmetry, there’s nothing in need of explaining in the first place. Particle physicists also sometimes claim that observing violations of certain types of symmetry, known as CP symmetry, will help explain this. It won’t. No matter what they measure about CP symmetry, we’ll still need initial values for the amounts of different types of matter in the early universe because it’s how our current theories work.
In summary. The mass of the higgs boson, dark energy, and the matter-antimatter asymmetry, require no further explanation and certainly no new physics. But it’s correct that we know that new physics must exist. Two good theoretical arguments are that we are missing a theory of quantum gravity, and that we don’t know the origin of neutrino masses. We also have a good experimental reasons to believe in new physics, that’s the observations which are usually attributed to dark matter. Even if dark matter isn’t the correct explanation, we still need some explanation.
However, for none of this new physics do we have any reason to think it’ll show up in any of the detectors that are currently under constructions, or at a next larger collider. If you hear a physicist say otherwise, they’re making things up and think you don’t know any better. But luckily you’re watching Sabine’s channel so you’re not easily fooled.
Comments
I'd like to see an episode on sterile neutrinos, expaining what it takes for a particle to be its own anti-particle and if two particles are identical, why they should annihilate each other.
MOND, it seems to me, is a patch on Newton's gravitation theory that is, in turn, a (pre) patch on Einstein's (and Newton's is still widely used in astrodynamics, I do this myself). But now on to something I find more interesting: reading this articule by Sabine H. I finally understand why particle physicists, with rare exceptions, dislike her so much.
Rad Antonov
Happy Sunday to you as well Jeffery. My pleasure, enjoy!
Happy Sunday, Rad! Thanks for those links. I have the paperback set of the FLP, so at least I can read that offline.
Rad Antonov
https://www.feynmanlectures.caltech.edu/III_03.html This chapter in the FLP I think is a succinct and readily accessible answer to your first question. Forces are treated by including the potential for the force in the Schrödinger equation. You can turn Tracey’s example around and ask: does a particle in a spatial superposition “feel” a force potential? The answer is yes. It is called the Aharonov-Bohm effect and has recently been demonstrated even for a gravitational potential. https://par.nsf.gov/servlets/purl/10340035 Now, does a particle in a spatial superposition generate its own gravitational potential and if it does, how do we represent it? No idea. As to your second question, I find Hawking’s black hole pair production paper a lucid framing of the problem. https://projecteuclid.org/journalArticle/Download?urlId=cmp%2F1103899181
Edited after rethinking: Regarding superpositions: Tracey mentions similar by her bringing up the double slit experiment, so how technically does quantum theory account for forces and superposition? I don't recall ever reading about that. However, after reading your reply I did find this: https://iopscience.iop.org/article/10.1088/1367-2630/ab104a So, it appears that physicists have thought about extending GR. I didn't know that there was a consideration from that front before.
Rad Antonov
Is there room for MOND and dark matter in the universe?
Rad Antonov
Could it be that the very notion of space-time does not apply to particles in superposition? It’s only when a system is kicked into an eigenstate / wave function collapses that space-time is manifested. That would make gravity an emergent phenomenon. Nothing original here, ideas like this have been circulating.
Thomas: MOND is a classical theory, it doesn't say anything about quantum effects. Though there are some people (like Verlinde) who say that MOND-like laws are a consequence of quantum gravity.
The problem is that space-time, in Einstein's theory, doesn't have superpositions, whereas the particles that give rise to the curvature do.
Happy Friday, Tracey! The same question holds for other forces. I have no idea. I have never heard that aspect dealt with. I found this paper: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://dash.harvard.edu/bitstream/handle/1/27413728/single_photon_paper.pdf So now we can perform this experiment to study this specific issue!
Good morning (here in central Europe it is), Jeffery, I´m glad to hear that, I always liked GR, and wasn´t claimed since decades, that the discovery of gravitational waves would be the last puzzle piece, to prove it right? Good morning Tracey, right, QM and gravity again...would be nice, to bring gravity measurement down to a level of protons or bucket balls, at least...
One of the simplest situations to consider is a single particle going through a double slit and appearing to pass through both slits and self-interfere. Where exactly should gravity be assigned -- the left slit or the right slit? Or, consider if wave function collapse is an actual physical event, what happens to gravity in that scenario? This is a problem regardless of whether gravity is correctly defined by GR or MOND or anything else.
MOND doesn't predict well outside certain cases from what I read. Sabine doesn't seem worried about MOND overturning GR, so I would guess that while MOND does provide some interesting arguments, it isn't robust enough.
right Jeff, but what about MOND? If MOND-like theories are right, isn,t in this case the whole GR on the test bench? Much confusion in physics currently
About quantum gravity. If gravity is not a force but a curvature of space-time then what's the problem? Why are physicists looking for a 'force' when they should be looking for a curvature of space-time for particles? It seems that gravity isn't weak at quantum scale, but that the curvature is by nature incredibly small at those really, really, really, really, really tiny masses. Right?
The discussion about modified gravity here wets my appetite for further detailed explanations from Sabine on how MOND helps shed light on yet another area (sigh) which hints at the need for “new physics” …. or at least the need for new models of the universe. This area is based on recent observational data from the James Webb space telescope observations (JWST). JWST detected early galaxies to be much larger than expected. This is described in: https://www.nature.com/articles/s41586-023-05786-2 I think in a recent YouTube presentation Sabine mentioned that MOND might help reduce the degree of theory upheaval needed to account for these observations… which would comfort me because I have friend who claims the observations wonderfully support his unified theory which indicates the universe is NOT expanding but instead that time is circular. I am personally uncomfortable with the thought of repeating this post every N billion years … or worse still going bald again every N billion years…REGARDLESS of how novel, illuminating and unifying my friend’s math might be. Is it not simpler to avoid assuming that nature MUST be unified?
