When the electron, the first subatomic particle to be identified, was discovered in 1897, it was thought to be a tiny speck with electric charge, moving around on a path governed by the forces of electricity, magnetism and gravity. This was just as one would expect for any small object, given the incredibly successful approach to physics that had been initiated by Galileo and Newton and carried onward into the 19th century.
But this view didn’t last long. Less than 15 years later, physicists learned that an atom has a tiny nucleus with positive electric charge and most of an atom’s mass. This made it clear that something was deeply wrong, because if Newton’s and Maxwell’s laws applied, then all the electrons in an atom should have spiraled into the nucleus in less than a second.
From 1913 to 1925, physicists struggled toward a new vision of the electron. They had great breakthroughs and initial successes in the late 1920s. But still, something was off. They did not really find what they were looking for until the end of the 1940s.
Most undergraduates in physics, philosophers who are interested in physics, and general readers mainly learn about quantum physics of the 1920s, that of Heisenberg, Born, Jordan and of Schrödinger. The methods developed at that time, often called “quantum mechanics” for historical reasons, represented the first attempt by physicists to make sense of the atomic, molecular, and subatomic world. Quantum mechanics is all you need to know if you just want to do chemistry, quantum computing, or most atomic physics. It forms the basis of many books about the applications of quantum physics, including those read by most non-experts. The strange puzzles of quantum physics, including the double-slit experiment that I reviewed recently, and many attempts to interpret or alter quantum physics, are often phrased using this 1920s-era approach.
What often seems to be forgotten is that 1920s quantum physics does not agree with data. It’s an approximation, and sometimes a very good one. But it is inconsistent with Einstein’s relativity principle, a cornerstone of the cosmos. This is in contrast to the math and concepts that replaced it, known as relativistic quantum field theory. Importantly, electrons in quantum field theory are very different from the electrons of the 1920s.
And so, when trying to make ultimate conceptual sense of the universe, we should always be careful to test our ideas using quantum field theory, not relying on the physics of the 1920s. Otherwise we risk developing an interpretation which is inconsistent with data, at a huge cost in wasted time. Meanwhile, when we do use the 1920s viewpoint, we should always remember its limitations, and question its implications.
Today a reader asked me “Out of the quantum fields which have mass, do any of them also have weight?” I thought other readers would be interested in my answer, so I’m putting it here. (Some of what is discussed below is covered in greater detail in my upcoming book.)
Before we start, we need to rephrase the question, because fields do not have mass.
Every book on science focuses attention on a little sliver of a vast, complex universe. In Waves in an Impossible Sea, I had intended to write mainly about the Higgs field, and the associated Higgs particle that was discovered in 2012 to great fanfare. I was planning to explain how the Higgs field does its job in the universe, and why it’s so important for the existence of life.
However, this plan had a problem. The Higgs field would be irrelevant were it not for quantum physics on the one hand and Einstein’s relativity on the other, and to comprehend the latter requires some understanding of Galileo’searlier concept of relativity. To show why the Higgs field can give mass (more precisely, rest mass) to certain types of particles requires combining all of these notions together. Each of these topics is daunting, worthy of multiple books, and I knew I couldn’t hope to cover them all in 100,000 words!
To my surprise, resolving this problem wasn’t as difficult as I expected, once I picked out a few crucial elements about each of these subjects that I felt everyone ought to know. Lining up those conceptual points carefully, I found I could give a non-technical yet accurate explanation of how elementary particles can get mass from a Higgs field. (A more mathematical explanation has been given previously on this website, in two series of articles here and here.)
Yet what surprised me even more was that the book’s main subject slowly changed as I wrote it. It became focused on the question of how ordinary life emerges from an extraordinary cosmos. Though a substantial section of the book is devoted to the Higgs field, it is situated in a much wider context than I originally imagined.
Once we clear away the hype (see the previous posts 1, 2, 3, 4), and realize that no one is doing anything as potentially dangerous as making real wormholes (ones you could actually fall into) in a lab, or studying how to send dogs across the galaxy, we are left with a question. Why bother to do wormhole research at all?
The answer is that it has nothing to do with actually making wormholes… at least, not in the sense of science fiction portals that you and I could use to travel from here to some faraway place across the universe. It has to do with potentially gaining new insight into the quantum physics of gravity, space and time.
How Do We Study Black Holes, and Why?
Why do scientists do research on black holes? There are at least two very different reasons.
Large black holes can be observed in nature. These black holes, which astronomers and gravitational wave experimenters study, are well-described by non-quantum physics — “classical” physics, where the future is (in principle) truly predictable from the past.
Small black holes are a window into quantum gravity — the unknown quantum physics of spacetime, where space itself is governed by the uncertainty principle, meaning that the very shape of spacetime can’t be precisely specified. This is relevant for black holes far too small for us to discover using astronomy, yet far too difficult for us to produce experimentally. They are important because they pose conceptual problems and puzzles for quantum gravity. Theoretical physicists think about black holes, and study their math, in hopes of uncovering quantum gravity’s secrets.
To gain more insight into their workings, scientists also simulate black holes on computers, and study analogues to black holes in laboratories.
Why Wormholes?
In contrast to black holes, there may be no wormholes worthy of the name anywhere in our universe. Though recent research clearly shows that there’s no principle that forbids wormholes from existing, it also shows it’s unlikely that large wormholes can be produced or can endure in our universe. While black holes are a generic outcome of the collapse of a huge star, wormholes are relatively delicate, and difficult to create and maintain.
But wormholes may be even more interesting than black holes for the problems of quantum gravity. This was only appreciated, slowly at first, over the past 10 years.
It’s hard to define the quantum state of a black hole. [In quantum physics, objects don’t just have locations and motions; roughly speaking, they have “states”, in which they have a combination of many locations and motions all at once.] The basic obstacle is entropy, a measure of missing information. The air in your room has entropy, because although you may know its temperature and pressure, you do not know where every atom of air is; that’s missing information. It turns out that a black hole has entropy too, which means that our usual description of a black hole is intrinsically missing some crucial information. That prevents us from knowing precisely what its state is.
But surprisingly, in some circumstances the quantum state of a wormhole can be sharply defined — in which case its entropy is zero. (Such a wormhole is not missing any information. But if you take either half of this wormhole and ignore the other half, you find a black hole. That black hole has entropy precisely because you’re ignoring all the information included in the other half of the wormhole!) To obtain and understand such a wormhole involves giving it two apparently different but actually interchangeable descriptions, one in terms of space-time and gravity, where the wormhole’s geometric shape is clear, and one in terms of what one might call a gravity-independent auxiliary quantum system, in which its quantum state is precisely defined.
The Power of Duality: A Rosetta Stone for Quantum Gravity
One physical object, two quantum descriptions — one with gravity, one without; the first with more space dimensions than the latter. It’s like being able to read the same text in two completely different languages. It’s an example of what physicists often call “a duality.” (I’ve gone into more detail about this in recent posts here and here.)
This is the message of what goes by the mantra of “ER=EPR”, referring to two famous and apparently unrelated papers from 1935 by Einstein and Rosen, with the second having Podolsky as a co-author. ER=EPR asserts that two apparently different things,
a tangible bridge across curved, extra-dimensional space between two regions, and
a less tangible bridge, established with quantum entanglement between objects in the same two regions, without any use of gravity,
are literally the same thing.
An conceptual illustration of the proposal that two perfectly entangled quantum systems (EPR) are equivalent to a wormhole connecting the locations of those systems (ER), and represent two languages for describing exactly the same thing. The wormhole is empty; the slabs shown merely indicate how distances are shrinking as one proceeds to the wormhole’s midpoint. Not shown is that the wormhole changes shape over time; for the situation in this picture, this wormhole is “non-traversable”, because there’s insufficient time to cross from one side of the bridge to the other before it shrinks down to nothing.
Discovering that spacetime is related to quantum entanglement, and that ER and EPR involve the same issues, is somewhat like discovering that two poorly understood and partially readable texts in completely different languages are actually two translations of exactly the same document. It’s a Rosetta stone. Parts of the document can easily be read in one language, other parts in the second language; and putting them together, we find we can read more and more.
Similarly, the math of a wormhole (ER) looks completely different from the math of two quantum-entangled non-gravitational systems (EPR). But in particular cases, Juan Maldacena and Lenny Susskind argued, they are two languages describing the same object. We can combine these two partial views of this single object to learn more and more about it.
Moreover, because we’re using math, not text, we can go a step further. Even in regimes where we cannot “read the document” in either language, we can use computers to explore. Scientists can try to simulate the math of the entangled auxiliary quantum systems on a computer, ideally a quantum computer so that it keeps track of all quantum effects, to learn more about the wormhole’s behavior in regimes where we have no idea how it works — in regions where the quantum uncertainty principle affects space and time.
Even more remarkable would be to actually make — not merely simulate — this entangled pair of auxiliary quantum systems. Then we would be closer to making a wormhole, with laws of nature different from ours and with its own gravity, that connects on to our world. But that’s a long ways off, and not the story for the present.
From ER=EPR to Traversable Wormholes
A further breakthrough, beyond the original ER=EPR idea, came with the work of Gao, Jafferis and Wall (see also here and here) in which it was demonstrated for the first time that “traversable wormholes” — ones that can truly serve as bridges across which objects can be transported — do make physical sense. Astonishingly, they are related by duality to an important and exciting research area in quantum information, called “quantum teleportation.” That’s the process by which, using two entangled quantum systems, quantum information can be brought to one of the systems, destroyed in that system, and recreated in that second system some distance away. Again, don’t expect anyone to be teleporting your dog, but simple information and ultra-microscopic objects might be transportable.
Be warned though; the teleportation only works if additional non-quantum information is traded between the two systems. In the wormhole language, that means you can only get through the wormhole if information is also passed outside the wormhole from the departure region to the arrival region. This makes it impossible to go someplace that you haven’t already been sending messages to, and to use any such wormhole as a shortcut — i.e., to get to your destination faster than could a near-light-speed spacecraft traveling outside the wormhole. Not only do portals to ultra-distant places remain science fiction, they now seem even more likely to stay that way.
Still, with these caveats, there’s still something amazing here: we can now imagine using the Rosetta stone of duality to simulate a traversable wormhole, and learn how it works in quantum gravity. That would be fantastic!
The Dream of Simulating Quantum Gravity
This is a dream, yet to be fulfilled. Computers are nowhere near being able to handle the questions we’d like to answer about the gravity we live with in our “four-dimensional space-time” (our familiar three space dimensions, plus one more for time). But by simplifying the problem in several steps (see the last figure of this post), we can at least hope to answer some early questions in a much simpler sort of wormhole in a simpler sort of gravity. This is what I’d prefer to call an artificially-simulated cartoon wormhole— rather than a “baby” wormhole, because unlike a baby, it isn’t a small version of an adult, nor has it any hope of growing into one. It’s more like a stick figure. It’s in two-dimensional space-time — one space and one time. That’s a big simplification — there’s nothing like normal gravity there! [Worse, we don’t have an exact duality in that case; the auxiliary quantum system we need isn’t really the same “text” as the wormhole. These are two systems, not one, with a limited but useful overlap.]
But cartoons aren’t to be mocked. Don’t underestimate them; cartoons are a powerful tool for educators everywhere, and subversive political cartoons have helped take down governments. For decades, famous physicists — Schwinger, ‘t Hooft, Gross and Neveu, Kogut and Susskind, and many more — have studied cartoon versions of real physics, especially ones in which our four space-time dimensions are replaced with just two. They’ve often learned interesting lessons from doing so, sometimes even profound ones.
[Note: Stick figure physics also can be a very good description of real stick-figure systems, for example a one-dimensional chain of atoms inside a material.]
I hasten to caution you that this technique does not always work. Not all of the lessons learned from stick-figure physics turn out to apply to the corresponding real-world problem. But this method has had enough success that we should take cartoon studies seriously.
This is why exploration of one-dimensional wormholes, and of some sort of auxiliary quantum problem to which they might be approximately related, may be worthwhile. And this is why it’s important to learn to simulate these auxiliary quantum systems on quantum computers, as was done in the paper that generated all the hype, based on proposals made in this paper and this one. Even if we can’t hope soon to understand how three-dimensional quantum space emerges from quantum entanglement, we can perhaps hope to learn more about one-dimensional quantum space, using quantum computer simulation. Maybe what we learn there would already teach us a deep and universal truth about quantum gravity, or at least suggest new ways to think about its subtleties.
The experiment done in the recent paper is a baby step in this direction. Others have attempted something along similar lines, but this is the first experiment that seems to have focused on the truly wormhole-like regime, and found some evidence for what was expected already of wormholes (from direct calculation and from classical computers…I’ll write about those details in a future post.) That seems like a real step forward. But let’s keep things in perspective. No new knowledge was created by this experiment; its achievements were technical and technological. It’s not a conceptual breakthrough. (I’m not alone in this view; Lenny Susskind, Dan Harlow and Scott Aaronson all expressed the same opinion in the New York Times and elsewhere.)
But nevertheless, this experiment represents a little arrow that points to a possible big future… not a future of a new Elon Musk, building wormholes for fun and profit, but one of a future Einstein, comprehending the quantum nature of spacetime itself.
Fermilab’s main building at sunset. [Credit: M. Strassler]
This has been an exceptional few days, and I’ve had no time to breathe, much less blog. In pre-covid days, visits to the laboratories at CERN or Fermilab were always jam-packed with meetings, both planned and spontaneous, and with professional talks by experts visiting the labs. But many things changed during the pandemic. The vitality of labs like Fermilab and CERN depends on their many visitors, and so it is going to take time for them to recover from the isolation and work-from-home culture that covid-19 imposed on them.
My visit, organized by the LHC Physics Center [LPC], the US organizing center for the CMS experiment at the Large Hadron Collider [LHC], is my first trip to Fermilab since before 2020. I feared finding a skeleton crew, with many people working from home, and far fewer people traveling to Fermilab from other institutions. There is some truth in it; the place is a quieter than it was pre-2020. But nevertheless, the quality of the Fermilab staff and the visitors passing through has not declined. It is fair to say that in every meeting I’ve had and every presentation I have attended — and yesterday I started at 7:30 and ended at 4 without a single break — I have learned something new and important.
Today I’ll just give you a flavor of what I’ve learned; each one of these topics deserves a blog post all its own.
One Fermilab postdoc explained a new and very powerful technique for looking for long-lived particles at CMS, using parts of the CMS detector in a novel, creative way. Because it’s possible that the Higgs boson (or top quark, Z boson, W boson, bottom quark, or some unknown particle) can sometimes decay to a long-lived particle, which travels a macroscopic distance before decaying to a spray of other particles, this is an important scientific target. It’s one the existing LHC experiments weren’t really designed to study, but with a wide range of creative developments, they’ve developed an impressive range of techniques for doing so.
Another has a strategy for looking for certain decays of the Higgs boson that would be extremely difficult to find using standard techniques. Specifically, decays in which only hadrons are produced are very difficult to observe; hadrons are so abundant in collisions at the LHC that this is a signal drowned in background. But there is a possible way around this if the Higgs boson is kicked hard enough sideways in the collision.
A third is digging very deep into the challenging subject of low-energy muons and electrons. Particles with energy below 5 GeV become increasingly difficult to observe, for a whole host of reasons. But again, there can be decays of the Higgs boson (or other known particles) which would predominantly show themselves in these low-energy, difficult-to-identify muons or electrons. So this is a frontier where new ground needs to be broken.
A visiting expert taught me more about the technical meaning of “intrinsic charm”, which was widely over-reported as meaning that “there are charm quarks in the proton”. Understanding precisely what this means is quite subtle, even for a theoretical physics expert, and I’m still not in a position where I can explain it to you properly — though I did discuss it a closely related issue carefully. Moreover, he questions whether the story is actually correct — it depends on a claim of statistical errors being small enough, but he has doubts, and some evidence to support his doubts. (The same doubts, incidentally, potentially affect whether the difference of the W boson from the Standard Model prediction is really as significant as has been claimed.) In my opinion, it is not yet certain that there really is “intrinsic” charm in the proton. You can definitely expect another blog post about this!
Another visiting expert pointed out that in some limited but interesting cases, there could be very slowly-moving particles captured not only in the core of the Earth but also floating near its surface, a possible target for underground experiments that are sensitive to extremely low energy collisions of unknown particles with atoms.
Then there are the applications of machine learning in particle physics, which are increasingly being used in the complex environment of the LHC to make certain basic techniques of particle physics much more efficient. I heard about several very different examples, at least one of which (involving the identification of jets from bottom quarks) has already proven particularly successful.
A visiting CMS experimentalist pointed out to me that in a search through LHC data that she’d been involved in for many years, there are two surprising collisions observed with an extraordinary amount of energy, and very unusual (but similar) characteristics. It’s hard to quantify how unusual they are, but hopefully we will soon hear about a similar search at ATLAS, which could add or subtract weight from this observation. In any case, upcoming data from Run 3 will give us enough information, within a year or two, to see if this hint is actually of something real.
If these events aren’t a fluke and represent something real and new, then one of the local theorists at Fermilab is the fellow to talk to; back in 2018, when only one of these events had been observed, he and a couple of others thought through what the options are to explain where it might have come from. The options are unusual and would certainly be surprising to most theorists, but he convinced me that they’re not inconsistent with theoretical reasoning or with other data, so we should keep an open mind.
Yet another visiting theorist taught me about the possibility of non-linearities in quantum physics. Steven Weinberg tried to consider this possibility some time ago, but it turned out his approach violated causality; but now, inspired by old ideas of Joe Polchinski, there’s a new proposal to try this in another way. I’m grateful for that 45 minute conversation, at the end of which I felt pretty confident that I understood how the idea works. Now I can go off and think about it. When I understand its implications in some very simple settings (the only way I ever deeply understand anything), I’ll explain it to you.
Oh, and on top of this, I gave a talk on Tuesday, about powerful and sweeping strategies for searches in LHC data that haven’t yet been done, but ought to be, in my opinion. My ideas about this are 10-15 years old, but I have stronger arguments now that rely on Open Data. That of course led to a variety of follow-up conversations.
The visit’s not over; I’ve got one more day to try to drink from this fire-hose.
There’s been a lot of chatter lately about a claim that charm quarks are found in protons. While the evidence for this claim of “intrinsic charm” (a name that goes back decades) is by no means entirely convincing yet, it might in fact be true… sort of. But the whole idea sounds very confusing. A charm quark has a larger mass than a proton: about 1.2 GeV/c2 vs. 0.938 GeV/c2. On the face of it, suggesting there are charm quarks in protons sounds as crazy as suggesting that a football could have a lead brick inside it without you noticing any difference.
What’s really going on? It’s a long story, and subtle even for experts, so it’s not surprising that most articles about it for lay readers haven’t been entirely clear. At some point I’ll write a comprehensive explanation, but that will require a longer post (or series of posts), and I don’t want to launch into that until my conceptual understanding of important details is complete.
Feynman diagram suggesting a photon is sometimes an electron-positron pair.
But in the meantime, here’s a related question: how can a particle with zero mass (zero rest mass, to be precise) spend part of its time as a combination of objects that have positive mass? For instance, a photon [a particle of light, including both visible and invisible forms of light] has zero rest mass. [Note, however, that it has non-zero gravitational mass]. Meanwhile electrons and positrons [the anti-particles of electrons] both have positive rest mass. So what do people mean when they say “A photon can be an electron-positron pair part of the time”? This statement comes with a fancy “Feynman diagram”, in which the photon is shown as the wavy line, time is running left to right, and the loop represents an electron and a positron created from the photon.
(where e— stands for “electron” and e+ for “positron”) can be used to verify the electric charges of the quarks of nature. [In this post I’ll usually drop the word “electric” from “electric charge”.] Specifically, the ratio R, at different energies, is both sensitive to and consistent with the Standard Model of particle physics, not only confirming the quarks’ charges but also the fact that they come in three “colors”. (About colors, you can read recent posts here, here and here.)
To keep the previous posts short, I didn’t give evidence that the data agrees only with the Standard Model; I’ll start today by doing that. But I did point out that the data doesn’t quite match the simple prediction. You can see that in the figure below, repeated from last time; it shows the data (black dots) lies close to the predictions (the solid lines) but generally lies a few percent above them. Why is this? The answer: we neglected a small but noticeable effect from the strong nuclear force. Not only does accounting for this effect fix the problem, it allows us to get a rough measure of the strength of the strong nuclear force. From these considerations we can learn several immensely important facts about nature, as we’ll see today and in the next post.
Figure 1: Data (black dots) showing R as a function of the collision energy 2Ee. Horizontal colored lines show the three predictions for R in the regions where the data is simple and 3, 4 or 5 of the quarks are produced. The minor jumpiness in the data is due to measurement imperfections.(more…)
A post for general readers who’ve heard of quarks; if you haven’t, you might find this article useful:
Yesterday I showed you that the usual argument that determines the electric charges of the various types of quarks uses circular reasoning and has a big loophole in it. (The up quark, for example, has charge 2/3, but the usual argument would actually allow it to have any charge!) But today I’m going to show you how this loophole can easily be closed — and we’ll need only addition, subtraction and fractions to close it.
Throughout this post I’ll shorten “electric charge” to just “charge”.
A Different Way to Check Quark Charges
Our approach will be to study the process in which an electron and a positron (the electron’s anti-particle) collide, disappear (“annihilate”), and are converted into one or another type of quark and the corresponding anti-quark; see Figure 1. The rate for this process to occur, and the rate of a similar one in which a muon and anti-muon are produced, are all we will need to know.
In an electron-positron collision, many things may happen. Among the possibilities, the electron and positron may be converted into two new particles. The new particles may have much more mass (specifically, rest mass) than the electron and positron do, if the collision is energetic enough. This is why physicists can use collisions of particles with small mass to discover unknown particles with large mass.
Figure 1: (Top) an electron and positron, each carrying energy Ee, collide head-on. (Bottom) from the collision with total energy 2Ee , a quark and anti-quark may emerge, as long as Ee is bigger than the quark’s rest mass M times c2.
In particular, for any quark of mass M, it is possible for an electron-positron collision to produce that quark and a corresponding anti-quark as long as the electron’s energy Ee is greater than the quark’s mass-energy Mc2. As Ee is gradually increased from low values, more and more types of quark/anti-quark pairs can be produced.
This turns out to be a particularly interesting observation in the range where 1 GeV < Ee < 10 GeV, i.e. when the total collision energy (2 Ee) is between 2 and 20 GeV. If Ee is any lower, the effects of the strong nuclear force make the production of quarks extremely complicated (as we’ll see in another post). But when the collision energy is above 2 GeV, things start to settle down, and become both simple and interesting.
A post for general readers who’ve heard of quarks; if you haven’t, try reading here:
The universe has six types of quarks, some of which are found in protons and neutrons, and thus throughout all ordinary material. For no good reasons, we call them up, down, strange, charm, bottom and top. Today and tomorrow I want to show you how we know their electric charges, even though we can’t measure them directly. The only math we’ll need is addition, subtraction, and fractions.
This also intersects with my most recent post in this series on the Standard Model, which explained how we know that each type of quark comes in three “colors”, or versions — each one a type of strong nuclear charge akin to electric charge.
Today we’ll review the usual lore that you can find in any book or on any website, but we’ll see that there’s a big loophole in the lore that we need to close. Tomorrow we’ll use a clever method to close that loophole and verify the lore is really true.
The Lore for Protons and Neutrons
Physicists usually define electric charge so that
the proton has electric charge +1
the electron has charge -1,
the neutron has charge 0 (i.e. electrically neutral, hence its name).
[Throughout the remainder of this post, I’ll abbreviate “electric charge” as simply “charge“.]
As for the six types of quarks, the lore is that their charges are [using notation that “Qu” means “electric charge of the u quark“]:
