Science Friday - Searching The Universe For Clues To The Ultra-Small

Episode Date: October 16, 2024

What exactly is … everything? What is space-time?At one extreme, you’ve got the weird rules of quantum physics that deal with subatomic particles. At the other extreme, you’ve got the vast expanses of space, such as spinning galaxies and black holes.By mapping the cosmic microwave background, surveying the distribution of galaxies around the sky, and listening for gravitational waves, researchers are studying the cosmos for clues to the quantum. They hope that by finding patterns in some of these large-scale structures, tiny irregularities involving quantum effects in the earliest days of the universe might be revealed.Charlie Wood, a staff writer covering physics for Quanta Magazine, has written about some of these space-time mysteries in a special issue. He joins Ira to discuss the nature of space-time and how scientists are trying to decode its physics.Transcript for this segment will be available after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Why are physicists using huge astronomy projects to hunt for clues to quantum physics? The equations just don't give you the answer, but the universe knows the answer. So we'd like to figure out what rules the universe is following. It's Wednesday, October 16th, and you're listening to Science Friday. I'm SciFri producer Charles Bergquist. On the one hand, you've got the weird rules of quantum physics that are easiest to understand when they're dealing with the ultra-small. On the other, you've got the vast expanses of space, spinning galaxies, black hole, things so big it's hard to get a handle on.

Can you use one to help investigate the other? Charlie Wood, a staff writer covering physics for Quantum Magazine, joins Ira to grapple with some of those big questions of space and time. Welcome to Science Friday. Hi, Ira. It's great to be here. Thanks for having me. Nice to have you, Charlie. Let me start with a macro view of physics, a really thought-provoking piece in which you say that fundamental physics has a problem.

Some call it a nightmare scenario. some say a crisis. Can you explain? In a nutshell, what that scenario, the crisis, the problem is. Yeah, the problem is that we're sort of running out of experimental clues to lead us forward as we ponder these questions about dark energy and dark matter and, you know, what kind of happens at the smallest sub-sub-sub-abomic scales. And for a lot of the previous century, there were some pretty concrete clues.

Things, you know, colliding particles would bounce that a weird. or have a strange ratio of energies. It was 1% off from what you expected. You know, a real target you could try to hit as you were building your theories. And, you know, the last few decades, you know, we've built these incredibly sophisticated machines. The large Hedron Collider in Europe is 27 kilometers around. And, you know, so we're really zooming in harder and harder than ever before. But we haven't really seen any surprises like that recently.

There are all these kind of hints and anomalies that pop up for a little while, but then they never seem to congeal into something specific to point the way forward. And, you know, how big of a clatter can you build? Maybe we'll build one that's 100 kilometers around. But, you know, it really seems like we've sort of plucked a lot of the lower hanging fruit from these experiments. And yet we still have these really compelling mysteries. So they're really looking for surprises, is what you're saying.

Oh, sure. I mean, we know they must be out there. The equations that we have, you talked about the big rules of general relativity that describe stars and black holes and planets and things. and then we have these rules of quantum mechanics, which do describe everything, but they're most obvious at the tiny levels. And, you know, these don't match too well.

And the clash becomes especially obvious when you get way, way, way down and scale to the Planck scale at 10 to the minus 35 meters. There, the equations just don't give you the answer, but the universe knows the answer. So we'd like to figure out what rules the universe is following. That's where I'm headed next. You say if the universe knows the answer,

you're right that the next physics, revolution might come from above. You write about some of these huge astronomical projects that are looking at things like galaxies to try to figure out quantum effects. How would one actually explain the other? Right. So I think, you know, the hope is that the universe had a beginning and that beginning was very violent and intense, far more intense than any event that we could create here in a particle collider on Earth. And so if we could see, you know, fingerprints left behind by the quantum effects during that event, then we could infer something about the rules that we can't kind of access directly in experiments. And so the idea here is that when the universe was being

born, there were these quantum fluctuations. Maybe you could think of these as virtual particles popping into and out of existence. And then as the universe expanded rapidly during its first fraction of a nanosecond, those particles got ripped apart before they could annihilate and push back into nothingness. And then they left these kind of dense spots that we can see in the cosmic microwave background, and we can also see them in the distribution of galaxies in the sky. So if we could just measure the distribution of galaxies precisely enough, just map out millions and millions and billions of galaxies, then maybe we could suss out exactly what sort of fluctuations led to putting them where they are. Can those big events happening

out in space, I'm talking about black holes, things like that, where you have these huge amounts of energy being given off and colliding, could they produce the kinds of particles we're looking for here on Earth in the colliders we can't make strong enough? Sure. I mean, cosmic rays, for example, are far more energetic than anything we can make in the LHC. If you remember, there was a little bit of concern when the LHC has switched on about whether it might make, you know, micro black holes that could cause problems for us.

Right. You know, that was kind of the logical argument for why we didn't need to be too worried about that as heavier stuff hits us all the time. And we've been just fine for billions of years. So there's definitely a lot of higher energy events out there. Yeah, you've written that, speaking of, high-energy events that one of the most promising avenues for new discoveries is the detection of these ripples in space time that we know is gravitational waves. Tell us more about that idea.

Gravitational waves are just so exciting to me. You know, I was saying earlier we've sort of picked a lot of the low-hanging fruit in physics with particle colliders and, you know, even starting to reach limits and, you know, how big of a telescope you can build. There certainly can't go infinitely large there too. Gravitational waves are this whole other tree. So 2015, when Lago was finally able to detect the collision of these two black holes, that was like the moment when Marconi picked up the first radio waves. And that kind of opened our eyes to the fact that there are all types of light, long light, short light, microwaves, x-rays, gamma rays, now we know about. And so we spent the last 200 years building tons of different cameras and microscopes up to Hubble

and James Webb Space Telescope and just learned so much about every physical process in the universe that shakes the electromagnetic field and shines. So now Ligo is this first step towards learning everything we possibly can about phenomena that shake the gravitational field. As you said, this fabric of space time itself. And so Lago found kind of the first most obvious source, which are these black holes colliding and neutron stars colliding. Those produce waves that are hundreds of kilometers long. And now there's tons of other projects in the work trying to find waves of different sizes. I mentioned in that article Lisa is this, trio of satellites that will hopefully launch in the next decade or two.

And they're going to look for waves that are millions to billions of kilometers long. Those would certainly come from mergers between supermassive black holes, which would occur when galaxies merge. But there could possibly be weirder stuff, too, things like defects in space time. During the universe's birth, it may have gone through a phase transition, like water freezing into ice. And, you know, that leaves cracks, for example. Or if you think of like a magnet with kind of one-size.

all the atoms point up and the other side, all the magnets point down between those two zones. There's a place where the particles switch from pointing one way to the other. You know, if spacetime had some kind of similar feature or defect, those might be vibrating and sending out waves that Lisa could detect. But, you know, we're really not going to know what we're going to see until we build these machines and go look. And do we have valid theories? I mean, valid, I guess in scientific terms, means you can test them out, right?

That would explain what the universe is all about. Certainly, we have lots of good ideas about what could come next at the next level. And certainly that's the problem now is sort of getting direct clues to guide us. But something that theorists mentioned to me in our conversations, especially when we're talking about things like quantum gravity, is that, you know, we don't have any theories that fully describe quantum gravity. Check all the boxes. Don't contradict themselves.

Match all of our current evidence. We have some partial theories, string theory is exciting. You know, there's loop quantum gravity. There's some ideas. nothing really, really satisfies all the criteria. So that suggests that the next theory, if we can find it, is very rare and very special. And so, you know, kind of step one is to just find one theory that's fully consistent and works at all these different scales.

And then, you know, step two will be, if you can find more than one of those theories, you'll need an experiment to tell those apart. But certainly the theorists I talk to don't feel like they're wasting their time today, but they're still just working for that one first theory that works. The quantum gravity part. the quantum gravity part. Why is that so hard? Why does it not fit together?

Oh, man. They're just, you know, quantum theory and general relativity, just two kind of completely different languages. I mean, and there are technical problems. There are conceptual problems. But for just a taste, kind of, you know, quantum theory says that everything is slightly random, slight fluctuation.

If you measure something twice, you won't get precisely the same result each time. General relativity, on the other hand, describes gravity as, the fabric of space time and bends in this fabric, create what we experience is the force of gravity. So then, you know, if we kind of believe both of these stories, then you're led to ask what happens if the fabric of space time itself fluctuates.

Well, okay, distances might get a little bit longer, a little bit shorter. That's fine. But then what are you measuring your distance relative to? You know, in quantum field theory, you always have a fixed backdrop, that whenever something fluctuates, you can always measure it relative to that. But in quantum gravity, you really lose that because you're, stage itself is changing. So something people frequently say is in quantum gravity,

you have no place to stand. And that's a real challenge. So like gravity is sort of geographical, as sort of a way we see the universe as being made out of objects, gravity itself sort of bending and flexing, and quantum is sort of digital. Would that be fair? And it's hard to unite the two?

Yeah, I think that's one way of looking at it. I mean, there's so many differences between them. Another difference is that, yeah, quantum is, you know, this idea of fluctuations has a little bit of randomness built into it where you can't kind of perfectly predict what you're going to get in any given circumstance, whereas gravity and general relativity is a classical theory where you, you know, the space time fabric only has one shape and you're always going to get one way that the gravitational event plays out. And so, yeah, reconciling those two languages is incredibly challenging.

Do we know that spacetime is even a fundamental part? of reality? I mean, or is it something that comes through because the math works well? Well, that's a good question. Certainly, that's something people have been thinking hard about over the last, gosh, 50 plus years. And there is certainly a maybe a strong to call a consensus, but a suspicion, a widespread suspicion among people who work on this kind of thing, that maybe it isn't, in fact, a fundamental kind of ingredient of our world. Maybe it's something that kind of comes out of the fundamental. mental ingredients. And the reason why people might think this is, very roughly, just the

difficulties that we've had in merging these two theories, you know, we know how to take the electromagnetic field, you know, light and quantize that into photons. And you can do the same thing with the gravitational field and quantize it into particles called gravitons. And they're mass lists and they have spin two. We kind of know roughly what they should be like, although they're far too weak to detect directly. But, you know, this theory works well for a while in most everyday circumstances, but breaks down in the center of black holes and the birth of the universe. And so, you know, perhaps this is a clue that spacetime itself changes dramatically in character when you get down to that scale. And so, again, this is something that we've seen before.

You know, we used to think that liquids and gases might be continuous, but gases have waves in them. I'm talking to you through sound waves. And you can quantize a sound wave. You actually get a phonon of what they're called. And those play a big role in how we think about supercontivity, for example. But if you really zoom in, the idea of a phonon or a sound wave eventually breaks down and you get molecules and atoms and quantum mechanics and wave functions and everything just completely changes. And so sound waves come out of that.

So a lot of people are thinking that spacetime very well may emerge in a similar character. Do we think possibly that dark energy and dark matter, the 95% of what the universe is made of that we can't see, could they be contributing to some part? particles that are missing? Yeah, absolutely. I mean, I think the consensus view on dark matter is that it literally is a missing particle. You know, we have electrons and they interact via the electromagnetic force, and we have neutrinos,

and we know a lot about those, but all these particles interact according to the rules of the standard model, which we've figured out over the last number of decades. But, you know, we see evidence that galaxies are spinning in a certain way, and we see the fingerprints in the, you know, afterglow of the Big Bang that suggests that there's another huge source of mass out there that's gravitating and pulling things around that isn't shining and isn't interacting through any of the known. So that, that very likely seems to be a new type of particle. Then on the other hand, we have dark energy, which is certainly a clue that we're missing something. That's a little bit less of a direct clue. It may be not literally a particle in the

same sense that dark matter seems to be, but dark energy seems to be related to the energy of space. We discovered it when we observed that the universe was expanding at a faster and faster rate. And so we expect the fabric of, well, not the fabric, but the vacuum of space to have, we're talking about quantum filter here. We expect

the vacuum of space to have some energy to it from the quantum fluctuations, these virtual particles popping into that of existence. But it's hard to reconcile that expectation with the slow expanse that we're observing. It's kind of easy to create a theory where you don't really have much of a vacuum energy. And then it's easy to create a theory where you have a huge vacuum energy.

But we observe this really small number. And that's really hard to explain. The Nobel Prize winning physicist, Steven Weinberg described this as the bone in our throats. So there is a sense that this is a deep clue about how quantum mechanics is talking to gravity. But I don't think we're sure yet what to make it that clue. Yeah, I think if I remember when he said in an interview with us, as he said, that really dark energy should be, what, thousands of times more of it than we actually see? Yeah, I think the number that I've heard, and this comes from a very back of the envelope

calculation people do, but, you know, 10 to the 120. That's a lot more than I said. It's a huge number. You know, it's sometimes called the worst prediction in physics. But, yeah, it's certainly deep and mysterious and people, yeah, are trying to figure out what that is. Yeah, and if you have these theories, how do you go about testing them, right? Because in science, even with string theory, which has been around, what, 30, 40 years,

there's no way to test it, doesn't it mean you have to give up on these things sometimes? Yeah, well, you know, I think you'll hear different hopes and expectations from different people. Certainly the theorists will say they're just looking for a theory that works, and they don't have one yet. So, you know, they have plenty of work to do and going out and finding one that fully works. Just think theory is more of like a partial theory. We have a sense of kind of how parts of it work in the sense that there might be a unique equation that describes how strings behave. But that's a very long way from telling us anything about our world. So, you know, certainly not a complete theory or not fully, fully worked out.

But no, yeah, ultimately, you know, the experiment is the only arbiter of truth. And I think that's why some of these astrophysical efforts are so exciting. You know, looking at the precise, you know, locations of galaxies on the sky. We've measured what's called a two-point correlation function, which is kind of if you put a meter of a certain length on the sky, what are the odds that both ends of the meter stick will land on a galaxy or a dense spot? But, you know, there's a lot of questions of what would happen if you did that with a triangle or a rectangle. And those are harder quantum calculations. They're also more subtle, experimental.

Yeah, service to take out. So, you know, I think, you know, those efforts are certainly very exciting and will hopefully give us some clues in the coming years and decades. Well, Charlie, I want to thank you for taking time to be with us today for some really thought-provoking stuff. We'll all be talking about over a beer this weekend. Thanks for having me. All right. If I can add one more thing, if listeners are into this kind of stuff, they might enjoy checking out Quanta's newsletter, Fundamentals.

So each week, my colleagues and I take turns taking a step back from the news of the day, and we look at some broad themes that have kind of come up regularly in our reporting. So if you're interested, you can sign up at Quantum Magazine.org slash fundamentals. There you go. Thanks, Charlie. Charlie Wood, staff writer covering physics for Quantum Magazine. He's based in New York. And they have this special issue out wrangling with all these space-time ideas.

That's really worth the read. That's all the time we have for today. Lots of folks help make this show happen, including Kathleen Davis. Diana Plasker. Beth Rami. Danielle Johnson. Santiago Flores.

Tomorrow, how dubious medical clams can catch hold and spread. We're looking into health misinformation. I'm SciFri producer Charles Berkwist. Thanks for listening. We'll see you soon.