What is the weak equivalency principle? How was antimatter discovered and why is it the perfect thing to put gravity to the test? What did the CERN experiment discover? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPT (AUTO-GENERATED)

On October 14th, 1971, during the final lunar EVA of the Apollo 15 mission, astronaut David Scott stood in front of the camera holding two objects. In one hand was a hammer. In the other was a feather. And he was about to conduct an experiment. A very simple one. He held the hammer and feather out, each the same height above the lunar surface. And then he let them go. The hammer and feather fell and struck the ground at the exact same time. Now, of course, if you were to repeat this experiment on the Earth, you would not get the same result. And that's because of air resistance. The hammer would go straight down while the feather would take its sweet time. But on the moon, no air, no problem, no difference. There was no difference between the feather and the hammer. And the origins of this experiment go all the way back to 1589 with Galileo himself. Now, his student popularized a story of him dropping balls of different weights from the famous Leaning Tower of Pisa. Galileo almost certainly didn't actually do that.

The old man generally preferred rolling balls down inclined planes to better control and measure his experiments. But the result is the same. His experiments disproved a long-held belief, and I mean long-held, like thousands of years long-held belief from the days of Aristotle that heavier objects fell faster because heavy meant I really like being on the ground, and if you're heavier, that means you like being on the ground even more, and if you like something more, you're going to work harder to do it. But this was wrong, and Galileo showed that. And so did David Scott a few hundred years later. All objects fall to the ground with the same rate. When Einstein developed his general theory of relativity, he started with this one assumption. One fact of the universe that he believed to be true. Today, we call this fact the weak equivalency principle, or WEP, when you're sick of typing weak equivalency principle over and over and over again. It's a very, very simple idea. All objects, regardless of their mass, or composition, or makeup, or history, or anything else, will fall at the exact same rate in a gravitational field.

It takes the idea of Galileo and expands it. It turns it from a strange observed property of nature into a bedrock foundational assumption of how the universe works. This, by the by, was one of Einstein's genius moments of which he had several. For centuries, physicists have been trying to figure out why all objects fall at the same rate. Einstein just said, let's assume that it does as a property of the universe and move on. And by moving on, he meant developing the entire theory of general relativity. So the WEP, the weak equivalency principle, is a cornerstone, the cornerstone of general relativity. No WEP, no general relativity, no fun. And in physics, we test our theories. We also have to test our assumptions. The weak equivalency principle is an assumption that makes general relativity possible. If you're wondering exactly how, I did a whole multi-part series on general relativity way back when, so go reference that. That starts with the weak equivalency principle and proceeds. So if we really want to poke at General relativity, which we really, really do because it has stubbornly refused to yield any cracks or mistakes in over a century, which is super annoying because even though Einstein was great, we'd all, we'd like to learn something new.

A good way to poke at general relativity is to poke at the weak equivalency principle. This means asking, do all things, and I mean all things. fall at the exact same rate. Cannonballs? Check. Hammers? Check. Feathers? Check. What about something wild? What about something that Galileo and Einstein never even thought of? Some kind of matter that they never even knew existed? What about antimatter? If we were to, very carefully, hold a hammer and an anti-hammer in our hands and let them drop, what would they do? Well, the weak equivalency principle says they would fall to the ground just the same. But the weak equivalency principle is an assumption about how nature works. We need to put it to the test. But why antimatter? There are all sorts of exotic particles and materials out there. Galileo and Einstein didn't have, I don't know, neutrinos and Diet Coke. So why should we focus on antimatter? Because antimatter comes to us from the most non-general relativity theory possible, quantum mechanics.

Quantum mechanics was developed independently from of our considerations of gravity. And to this day, we have no way of reconciling the two theories. General relativity is our description of the force of gravity, and it's through quantum mechanics we have a description of every other force. It's in general relativity that we have this picture of space-time, this smooth and harmonious fabric. And in quantum mechanics, we have a completely different picture of the subatomic structure of reality. It's this quantum foam that's constantly buzzing in and out of existence. These two theories do not agree with each other. We do not have a quantum theory of gravity. We do not have a quantum description of gravity. And so it's like, if you want to get as far away from general relativity as possible, you go over into quantum mechanics. And because of that, it seems like a good way to really put the screws to Einstein and stress test the weak equivalency principle. And if we're over here in quantum land asking, hey, what weird stuff does quantum mechanics predict that Einstein didn't know about that Galileo didn't know about? Boom.

Antimatter. In 1928, we had a successful theory of quantum mechanics due to the work of Schrodinger and Heisenberg and all the rest. We could develop, we could describe the behavior of atoms, we could describe the behavior of subatomic particles. We knew we were uncovering all the weird mysteries of the quantum world, like superposition, like wave-particle duality, like all probabilities of experiment, all the good stuff. but it did have one weakness. At 1928, as successful and powerful as quantum mechanics was, it did have one weakness. It could describe the very small, but it could not describe what small stuff did when it moved really fast, like close to the speed of light fast. In order to describe objects moving really fast, you need special relativity. You can't use normal Newtonian physics, Galilean physics. You have to upgrade yourself to special relativity. You need to think equals mc squared. You need to think spacetime. You need to think time dilation and length contraction.

All that good stuff. The theories developed by Schrodinger and Heisenberg were ignorant of special relativity. They were based on Newtonian principles, Newtonian mechanics. And everyone's like, well, we know that small subatomic particles can go really fast. We're doing it in our experiment. And we don't have a full description of how they should behave, what they should do. We need to bring in special relativity into this. And we need not just a quantum description of nature. We need a special relativity quantum version of nature. And the guy to do it was Paul Adrian Maurice Dirac. Now, Dirac was an odd doc, that's for sure. And it merits an episode to call his own. There's lots and lots of great stories about him, which would make for an entertaining tale. So please feel free to ask any time more about Dirac. But what we care about here is not all the fun stories about his life and how he was kind of an odd guy. What we care about is that Dirac was able to discover an equation that could combine quantum mechanics with the respective spacetime demanded by special relativity.

Schrodinger tried this for himself, gave up. Heisenberg tried it for himself, gave up. Dirac tried it for himself, did not give up, and figured it out. He figured out how to describe subatomic particles moving really, really quickly. He described how he was able to write down a single equation that captured all of the goodness of quantum mechanics, the wave functions, the probabilities, all of that, and also included a correct description according to special relativity. But as soon as he wrote down this equation, he noticed that there was something funny about it. When you go to calculate, you're like, okay, you're going to apply this equation and you've got some subatomic system and you've got a little electron here and it's going really fast. You ask, hey, equations of science, what is the energy of my electron when it's moving really, really fast? This is a very normal and standard question we like to ask in physics. Like, hey, how much energy do you have, pal? Well, his equations generated two solutions for the energy of an electron.

One solution was positive and one was negative. We encounter this all the time in physics and in math. Like if I ask you, what is the square root of four? There are two answers. There's two and then there's negative two. You square two, you get four. You square negative two, you also get four. When we calculate trajectories of if I throw a baseball to you, there ends up being a square root. There's one solution that is the normal parabolic path that you would expect of a ball moving from me to you. And then there's the complete mirror opposite of it. There's a negative solution of it. And most of the time when this crops up in physics, we're like, well, the negative solution, yes, it's a solution to the math, but it's unphysical. When I throw a ball to you, it's not going to swing low to graze the ground and then shoot back up into your hand. That doesn't make any sense. I don't encounter that in the world. So I just ignore that part. Interact's first instinct was to ignore the negative energy solutions, but something about it really bothered him.

If this wasn't like throwing a baseball, this wasn't like taking the square root of four. We're talking about energy states of fundamental particles, right? when they're moving close to the speed of light. And quantum mechanics had taught us already that weird stuff happens all the time. So should we automatically reject the negative energy solutions because that doesn't make any sense? Dirac thought about this a lot, and that's what he did, was think about a lot of things a lot of the time. And he became really worried about this negative energy solution. How can something have a negative energy and where does it stop? Like positive energy stops at zero. I can have as high an energy as I want, but there is a limit. There is a floor. There's a ground floor energy to what I can have. But how deep does negative energy go? While I can have as high positive energy as I want, does that mean I can have as deep and low negative energy as I want? But then where does it stop? Is there anything physical here? Is the math trying to tell me something about the universe? Dirac being Dirac, which is to say a super genius, turned this into an advantage.

He contributed to Patreon. That's patreon.com slash P-M-S-U-T-T-E-R. That's my name. Where you get to support this show and keep it going. I truly, truly appreciate all of your contributions. That's patreon.com slash P-M-Sutter. Now, he envisioned something called the Dirac Sea, where he envisioned, yeah, we have all these particles floating around doing their thing, and they all have positive energy states. Some of them are a little bit energetic. Some of them are really, really energetic. And then underneath this is an infinitely deep ocean of particles occupying negative energy states. We don't normally get to see all these particles. We only see the particles that have positive energy. So everywhere you look, you're actually immersed in this infinitely deep sea of negative energy states, all occupied by particles. There's electrons going all the way down to infinity, but we don't get to see them. We only get to see the positive energy stuff. But then he asked, well, what would happen? Let's say if I if I combed around this sea and I and I saw one particle, one electron with this deep negative energy state.

What if I gave it a bunch of energy? What if I kicked it? You know, what if I found it in this negative sea and I give it a good flick? What would it do? Well, it now would have positive energy. I would see a particle running around. I get to see particles with positive energy. But what would it leave behind? It would leave behind a hole in this infinitely deep sea. The space, the position that this particle used to occupy was now a hole. But this hole would have momentum. This hole would have energy. And a hole in a sea of electrons would look like a positively charged particle. So if I gave this electron that had this negative energy state and I gave it a kick, what I would see is two particles. I would see an electron and I would see the hole it would leave behind. It would look like an anti-electron, an anti-particle. It would be antimatter. Nowadays, we don't use this visualization idea of the Dirac C to explain antimatter, but I still really, really bring it up all the time because it's such a great visual metaphor for what's going on where we can envision antimatter as holes in a negative energy C. To my knowledge, this is the first particle predicted to exist prior to its discovery from pure mathematics.

Dirac had a job to do, which was to reconcile quantum mechanics with special relativity. In doing so, he crafted an equation that did it. This equation naturally predicted the existence of matter that had opposite charge. Antimatter. A few years later, antimatter would be discovered in the laboratory, and now it's a normal part of our understanding of the cosmos. It's the mirror to every particle. It's the reflection. If I have an electron, it has a mirror, the anti-electron, also called the positron. I give you a top quark. There's an anti-top quark. I give you three quarks to make a proton. That's a positively charged bundle of particles. I can reverse it. I can mirror that. I can give you the complete opposite. Two anti-ups and an anti-down give you an anti-proton. Every single particle has a mirror of opposite charge. Okay. Great. That's nice, I suppose. We've got all this antimatter. Here's our question. Does the antimatter, this newfound particle, this kind of particle, this entire category of particles that we never knew existed, does it obey the same laws of gravity as everyone else? It's strange.

It's born from quantum mechanics. It was totally unexpected. So if something is going to do the trick of finally putting a crack in the weak equivalency principle, it's hard to find a better candidate than antimatter. Does it obey gravity the same way as everybody else? Weak equivalency principle says, hey, gravity is gravity all the same everywhere in the universe. Everyone obeys me. Everyone behaves exactly the same. You're not special. But Einstein didn't know about antimatter. Maybe it follows special rules. One tiny small problem for testing this. Actually, several large problems all on top of each other. One, antimatter makes a big bada-boom. You take a bit of matter, you take a bit of antimatter, you bring them close together, they annihilate in a flash of pure energy. 100% pure matter-to-energy conversion, which is a lot. I forget how much. Like, a few tons of antimatter could completely obliterate the entire Earth, as in unbind every single molecule, so we're just a drifting cloud of gas.

That's a lot of energy. That's one problem. Very, very temperamental material here. Second problem, there's hardly any antimatter in the universe. So we have to make it for ourselves. We can't just go grab it. Third, gravity is really weak. Like, really, really weak. So testing the weak equivalency principle is a real pain in the neck. So we've got this really delicate material that tends to make a big bada-boom. We have to make it ourselves and we have to test this incredibly weak force. Well, the only way out is through, so let's tackle these problems. You know the old saying, what's the best way to eat an elephant? That's one bite at a time. So first, we need to turn off the other forces because they're just going to get in the way. We want a pure, clean look at how antimatter behaves in gravity. So we can't deal with a lot of electromagnetism. We can't deal with the strong nuclear force or the weak nuclear force. We have to reduce or eliminate those as much as possible so we can get a pure look at just gravity, which means we can't look at individual particles because the electromagnetic force on an individual particle is just way too strong.

If I just drop some positrons in a container, the electromagnetic interaction of the positrons with all the other particles in the container is going to be way huger than gravity. So I need neutral antimatter. How do you make neutral antimatter? Well, by making anti-atoms, of course. A hydrogen atom is electrically neutral. It has one proton, one electron, the charge is cancelled out, and the atom itself is electrically neutral. So I need the mirror version of that. I need anti-hydrogen. I need a positron, and then I need an anti-proton. So to do that, to make anti-hydrogen, anti-atoms, I need to take a bunch of protons, accelerate them to nearly the speed of light, slam them into a wall. That makes a shower of particles, including anti-protons. Once I get enough energy... I can pull some particles out of the infinite negative energy, see if that's the metaphor we want to use. I have the energy to do it now, and they start flying around. Then I can use magnetic fields to channel the antiprotons into a separate track, slow them down.

CERN, the big particle accelerator facility in Europe, does this on a daily basis. What comes next is a special experiment called the Alpha-G experiment, which ran in 2023. We've got a source of anti-protons over here. Then we have a radioactive source over here that is naturally making anti-electrons, aka positrons, through radioactive decay. Then I bring those positrons over to the anti-protons. They bind together to make anti-atoms of antimatter, anti-hydrogen, which is like Normal hydrogen, except all the charges are reversed, but overall it's electrically neutral. But we can't just hold antihydrogen in our hands because of the aforementioned big bada boom, so we need to trap the antiatoms, and we use what's called a penning trap, which is essentially a magnetic bottle. This works because while antihydrogen is electrically neutral, it still has a magnetic moment, which is a fancy term for looking like a bar magnet. So we can apply a magnetic force to keep the antiatoms held still. Now we need to chill everything out.

Literally, we use lasers to sap energy away from the antiatoms. We bring their temps down to almost absolute zero. Then we tweak the magnetic field to keep the atoms, about 100 or so of them at a time, perfectly balanced against gravity. So we've got these anti-atoms, these anti-hydrogen atoms suspended in midair using very precise magnetic fields to keep them there, using lasers to cool them off so that their temperatures are nearly absolute zero, so they're holding still for once. And then slowly, slowly, ever so slowly, we turn down the magnetic field strength so that the anti-atoms can start to drift and start to feel the force of gravity. We're using the magnetic fields initially to suspend them, to fight against gravity, to keep them exactly in one spot. And then we dial that down and let the antimatter respond to gravity. If we were doing this in the middle of an interstellar space with no gravity around, the anti-atoms would drift in all directions. On the Earth, the atoms are either going to drift down, meaning that they react to gravity normally, or they're going to drift up, meaning they respond to gravity differently.

Of course, we actually have to figure out when the anti-atoms leave the chamber, but We do this by letting them hit a wall, a plate on either the top or the bottom. And when this happens, one of the anti-atoms touches one of the normal atoms and makes a one atom sized big bada boom, makes a little flash, and we can record the energy. So we intentionally allow the antimatter to annihilate on either a top plate or a bottom plate, and we record it. Now, the biggest source of noise in this experiment, the biggest source of Confusion is that there are cosmic rays constantly raining in from the wider universe, punching through our atmosphere, punching through our experimental apparatus, hitting the plates of our detectors. They're making little recordable flashes of their own. So as we're sitting here waiting for our atoms to drip on either one plate or the other, there's constantly cosmic rays, bing, bing, bing, hitting both plates. So we need to monitor everything for a long time. Record the average number of cosmic ray strikes on both the top and bottom plates.

Then wind our magnetic field down and see if there's extra flashes on one plate or the other. This is going to introduce a lot of uncertainty in the experiment. Remember, we're dealing with 100 individual atoms or so. With this constant flood of cosmic rays in the background, we're looking for more flashes on one plate or the other. Kind of a difficult experiment. Slight upgrade from Galileo dropping weights off the Leaning Tower of Pisa. But the end result is that this experiment, run over and over and over again, counted roughly 80% of the anti-atoms fell through the bottom end of the container. Given the statistics and the uncertainty involved and all the noise caused by the cosmic rays, this is huge, screaming validation that antimatter falls down. This validates the weak equivalency principle. As far as our experimental uncertainty can allow for this, antimatter behaves exactly the same as normal matter in the influence of a gravitational field. That's exactly what Einstein said was going to happen.

But we didn't know ahead of time. We had to test it. Einstein didn't know about antimatter when he developed general relativity. And so we got to make sure that this little surprising corner of the universe, thanks to Paul Dirac, does what it's supposed to do. But we're not done. You see, we've confirmed that antimatter does fall down, but we don't know yet how quickly antimatter falls down. Planned upgrades to this experiment aren't just going to measure which plate the antimatter strikes, but how long it takes for the antimatter to go from the center of the magnetic bottle to the bottom plate. Because if, yes, antimatter falls down, But we don't yet know how quickly it falls down. And if there's a tiny difference, even a 1%, a 0.1% difference in the rate of falling under gravity between matter and antimatter, that would be huge. That would be a gigantic crack in the weak equivalency principle. That would tell us that what we think is a foundational bedrock assumption of the way the universe works is flawed or incomplete.

This would be all we need to start moving beyond general relativity because we would finally find a flaw in that theory, a flaw in one of its assumptions, so that we could build a superior version of a theory of gravity, maybe a quantized theory of gravity. But we don't know yet because we haven't run the experiment. All we know is that antimatter falls down, just like everything else. I know that that may be a little bit of a letdown. that it might be the opposite of a climax. You might even call the end result of this episode, Panty. Oh, I do make me laugh sometimes. Thank you to Neil, you, Kenji, and at BrevitySC2 for the questions that led to today's episode. And thank you for all of your Patreon contributions. That's patreon.com slash pmsutter. Please keep those questions coming for more juicy episodes. And if you can't contribute and you don't have any questions you're curious about, then just tell a friend. about the show, keep spreading the word. That is the best thing that you can do.

And I can't thank you enough for all of your contributions. I would like to thank my top Patreon contributors this month. They are Justin G, Chris L, Aberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, Joshua, Scott M, Rob H, Scott M, Lewis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Eileen G, Steven W, Deb A, Michael J, Phillip L, and Stephen B. That's patreon.com slash pmsutter. Keep sending questions to askaspaceman at gmail.com or the website askaspaceman.com. And I will see you next time for more complete knowledge of time and space.

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