Searching The Universe For Clues To The Ultra-Small

IRA FLATOW: This is Science Friday. I’m Ira Flatow. It’s time to revisit our quest to figure out just what the heck the universe is made out of, I mean, the nature of space and space-time itself. On the one hand, you’ve got the weird rules of quantum physics that deal with the ultra small. And on the other hand, you’ve got the vast expanses of space, spinning galaxies, black holes, things so big it’s hard to get a handle on. That’s the other side. Can you use one to help investigate the other?

Joining me now to help grapple with those questions is Charlie Wood. He’s a staff writer covering physics for Quanta Magazine based here in New York. Welcome to Science Friday.

CHARLIE WOOD: Hi, Ira. It’s great to be here. Thanks for having me.

IRA FLATOW: Nice to have you, Charlie. Now, 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?

CHARLIE WOOD: Yeah, the problem is that we’re running out of experimental clues to lead us forward as we ponder these questions about dark energy and dark matter and what happens at the smallest sub, sub, sub, subatomic scales. And for a lot of the previous century, there were some pretty concrete clues. Colliding particles would bounce at a weird angle or have a strange ratio of energies that was 1% off from what you expected. A real target you could try to hit as you were building your theories.

And over the last few decades, we’ve built these incredibly sophisticated machines. The Large Hadron Collider in Europe is 27 kilometers around. And 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 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 how big of a collider can you build? Maybe we’ll build one that’s 100 kilometers around. But it really seems like we’ve plucked a lot of the lower hanging fruit from these experiments, and yet we still have these really compelling mysteries.

IRA FLATOW: So they’re really looking for surprises is what you’re saying?

CHARLIE WOOD: 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 these don’t match too well. And the clash becomes especially obvious when you get way, way, way down in 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.

IRA FLATOW: That’s where I’m headed next. You say, if the universe knows the answer. You write 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?

CHARLIE WOOD: Right. So I think 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 fingerprints left behind by the quantum effects during that event, then we could infer something about the rules that we can’t 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 poof back into nothingness. And then they left these 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.

IRA FLATOW: 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?

CHARLIE WOOD: 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 switched on about whether it might make micro black holes that could cause problems for us.

IRA FLATOW: Right.

CHARLIE WOOD: Well, 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.

IRA FLATOW: 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 as gravitational waves. Tell us more about that idea.

CHARLIE WOOD: Gravitational waves are just so exciting to me. I was saying earlier, we’ve picked a lot of the low hanging fruit in physics with particle colliders and even starting to reach limits in 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 LIGO 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 opened our eyes to the fact that there are all types of light, long light, short light, microwaves, X-rays, gamma ray now we know about. And so we’ve 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 LIGO found 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 also possibly be weirder stuff, too. Things like defects in space-time during the universe’s birth that may have gone through a phase transition like water freezing into ice. And that leaves cracks, for example.

Or if you think of a magnet with one side, 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. If space-time had some kind of similar feature or defect, those might be vibrating and sending out waves that LISA could detect. But we’re really not going to know what we’re going to see until we build these machines and go look.

IRA FLATOW: 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?

CHARLIE WOOD: Certainly we have lots of good ideas about what could come next at the next level. And certainly that’s the problem now is 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 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. There’s loop quantum gravity. There’s some ideas, but 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 step one is to just find one theory that’s fully consistent and works all these different scales. And then 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 talked to don’t feel like they’re wasting their time today. They’re still just working for that one first theory that works.

IRA FLATOW: The quantum gravity part?

CHARLIE WOOD: Yeah, the quantum gravity part.

IRA FLATOW: Well, why is that so hard? Why does it not fit together?

CHARLIE WOOD: Oh, man. Quantum theory and relativity are just two completely different languages. I mean, and there are technical problems. There are conceptual problems. But for just a taste, a part of, quantum theory says that everything is slightly random, slight fluctuations. 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 as the force of gravity.

So then if we believe both of these stories, then you’re led to ask what happens if the fabric of space-time itself fluctuates? Well, OK, distances might get a little bit longer or a little bit shorter, that’s fine. But then what are you measuring your distance relative to? 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 your 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.

IRA FLATOW: So gravity is geographical, a way we see the universe as being made out of objects. That gravity itself bending and flexing and quantum is digital. Would that be fair? And it’s hard to unite the two?

CHARLIE WOOD: 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 this idea of fluctuations, it has a little bit of randomness built into it where you can’t perfectly predict what you’re going to get in any given circumstance. Whereas gravity and general relativity is a classical theory where 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.

IRA FLATOW: Do we know that space-time is even a fundamental part of reality? I mean, or is it something that comes through because the math works well?

CHARLIE WOOD: 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 strong to call it 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 ingredient of our world. Maybe it’s something that comes out of the fundamental ingredients.

And the reason why people might think this is, very roughly, just the difficulties that we’ve had in merging these two theories. We know how to take the electromagnetic field, 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 are massless. And they have spin too. We know roughly what they should be like, although they’re far too weak to detect directly. But 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 perhaps this is a clue that space-time itself changes dramatically in character when you get down to that scale. And so, again, this is something that we’ve seen before. 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 superconductivity, 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 space-time very well may emerge in a similar character.

IRA FLATOW: Do we think possibly that dark energy and dark matter, the 95% of what the universe is made out of that we can’t see, could they be contributing to some particles that are missing?

CHARLIE WOOD: Yeah, absolutely. I mean, I think the consensus view on dark matter is that it literally is a missing particle. 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 we see evidence that galaxies are spinning in a certain way and we see the fingerprints and 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 normal—so 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 field theory here– we expect the vacuum of space to have some energy to it from the quantum fluctuations, these virtual particles popping into that existence. But it’s hard to reconcile that expectation with the slow expanse that we’re observing.

It’s 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 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 of that clue.

IRA FLATOW: Yeah, I think if I remember when he said in an interview with us, he said that really, dark energy should be, what, thousands of times more of it than we actually see.

CHARLIE WOOD: Yeah, I think the number that I’ve heard, and this comes from a very back of the envelope calculation people do, but 10 to the 120.

[LAUGHTER]

IRA FLATOW: That’s a lot more than I said.

CHARLIE WOOD: It’s crazy. It’s a huge number. It’s sometimes called the worst prediction in physics. But yeah, it’s certainly deeply mysterious and people, yeah, are trying to figure out what that is.

IRA FLATOW: Yeah, and if you have these theories, how do you go about testing them? 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?

CHARLIE WOOD: Yeah. Well, 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 they have plenty of work to do in going out and finding one that fully works.

String theory is more of like a partial theory. We have a sense 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 it’s certainly not a complete theory, or not fully worked out. But no, yeah, ultimately, the experiment is the only arbiter of truth. And I think that’s why some of these astrophysical efforts are so exciting.

Looking at the precise locations of galaxies on the sky, we’ve measured what’s called the 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 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 surveys to take out. So I think those efforts are certainly very exciting and will hopefully give us some clues in the coming years to decades.

IRA FLATOW: 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.

CHARLIE WOOD: Thanks for having me, Ira. And if I can add one more thing?

IRA FLATOW: Sure.

CHARLIE WOOD: 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 come up regularly in our reporting. So if you’re interested, you can sign up at quantamagazine.org/fundamentals.

IRA FLATOW: There you go. Thanks, Charlie. Charlie Wood, staff writer covering physics for Quanta 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.

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