Science Friday - Are There Things That We Know We Can’t Know?

Episode Date: May 5, 2025

In “Into the Unknown,” an astronomer explores the mysteries of the cosmos and the limits of what science can test.What is time? If the universe is expanding, what is it expanding into? What happened just before the Big Bang?Some of the most head-scratching ideas in physics strain the limits of what science can test. In her book Into the Unknown: The Quest to Understand the Mysteries of the Cosmos, astronomer Dr. Kelsey Johnson describes some of those concepts, and sketches out ways to try to wrap your brain around them. Johnson joins Host Ira Flatow to talk about the limits of scientific inquiry, and what mysteries lie at the limits of science.Transcripts for each 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.

This is Science Friday. I'm Ira Plato. Today on the podcast, how much do we really know about how the universe works? I'm not sure we really know anything with a capital K. If you follow this program over the years, you know that I love to tackle the big but really simple, tough questions of science, the things that really make your hair hurt when you try to wrap your mind around them. Like, what is time? or if the universe started from the Big Bang, what happened before that? And sometimes I wonder just what the limits of science are to answering these questions.

I mean, are there things that we humans just can't know? Things that science can't really answer with the research tools that we have? Well, that's what we're going to be talking about. And joining me is Dr. Kelsey Johnson. She's a professor of astronomy at the University of Virginia, former president of the American Astronomical Society, author of the book, Into the Unknown, the quest to understand the mysteries of the cosmos.

Welcome to Science Friday. Hi, Ira. It's really an honor to be here. It's an honor to have you. Do you like to delve into these questions yourself? Oh, my gosh. I think about them all the time, and they're what get me out of bed in the morning.

You know what? When I talk to astronomers and physicists, sometimes they say the actual quest is better than the answer. Is that true for you? You know, I think it is true because there's something that's so valuable about curiosity. And once we've satisfied our curiosity, we kind of want to move on and get our next hit of curiosity. So there is something really important about the quest itself. Let's talk about some of these things.

It feels like every week, especially in astronomy and astrophysics, we get some results that says, hey, rethink what you thought you knew. I mean, how much of anything do we actually know for sure in astronomy and astrophysics? Well, how much of anything do we know for sure, I think, brings us to talking about what it means to know anything at all. I happen to have what is probably an unpopular opinion that I'm not sure we really know anything with a capital K. You know, this is because as we think about science, and I'm a scientist, and I believe in the importance of empirical inquiry, science is really good at testing things that can be tested. And we try to test things we think.

we know over and over and with harder and harder tests. And the more we test something and the harder the tests are that it passes, the more we get confidence in it. And if we get enough confidence, we might say that we know it. But we never know when the next test might break the thing that we think we know. And that's actually the fun part of science. We don't know what dark energy is. We don't know what dark matter is. I mean, we don't know what 96% of the universe is. made of, do we? No, and I'm so glad you brought that up, right? Because I think most normal people who are well-adjusted go through life thinking we kind of understand the universe, but we don't understand more than 95% of it. So what do we know about anything if you don't know

what 95% is? I don't know. I don't know what we know. So how do we go about testing things that are untestable, or do we just put them off for some other time and date somewhere down the future? This, I think this is the heart of the question for me is, I think there's a tendency when things are not testable to be like they're not testable. It's not within the realm of science. Let's not even think about it. But to me, that becomes self-fulfilling. Because if we don't let ourselves think about it, how are we ever to be able to make progress toward understanding or testing it? And so I'm fully of the opinion that just because something might not be testable, we need to hold open. some flexibility for whether it could be testable in the future unless we are willing to claim, and I am not willing to claim this, if we're willing to argue that our current physical understanding, our mathematical frameworks, and our technological capabilities are the pinnacle of what is possible, then we could argue maybe things will never be testable. But I'm not willing to make that claim.

I don't know about you. Right. But there are also simple things that we've talked about over the years. like time. Simple things. I've had scientists come on and written books about time since there's no such thing as time. I mean, that's just, whoa. Time is one of these super slippery concepts that I would argue we have the most exposure to, right?

We're constantly exposed to the flow of time. But we have the least understanding of. And a lot of, I think, really great thinkers have done a really great, amount of thinking about time over the human history. And, you know, I don't want to cast any shade on them because clearly brilliant humans have thought about this. But modern physics actually starts to give us a little bit more insight into what time might be and why it might behave the way it does. But that doesn't mean we know this for sure. And part of the reason is that we are, of course, embedded in time. And so at least as of right now, doesn't appear to be

possible for us to get outside of time to measure it and observe it and see how it behaves. But time is one of these slippery concepts that I wish we all spent more time thinking about. There you go. You've very nicely worked that in. One of the big topics that you tackle in your book is the beginning of the universe and the big bang. And why is that so troublesome to understand? Okay. You went straight to the heart of this. Where the universe came from, I don't know how you feel about this, but for me, all of the big, beautiful existential questions that I think about the universe, where the universe came from is like the top of that list. Right.

And this brings us straight to the boundary of science and philosophy and theology, and all three of these get wrapped up. But we hit the limits of what we can investigate with empirical inquiry. Now, part of the problem here is that philosophical piece because even if I could tell you, and I would win a Nobel Prize if I could do this, and that would be really cool, but if I could tell you where the universe came from, you know, and there are a number of hypotheses on the table, and I don't know that we want to go through all of them right now. But let's say I could tell you where the universe came from, and I'm just going to make something up. I'm going to say some extraterrestrial, hyperdimensional, super-intelligent being snapped their fingers, and there was. the universe. I've seen that movie. Have you? Good. I mean, I feel like this is a really good plot device. I mean, Steven Spielberg should be all over this. But then, you know, I think for most thinking people, your next question might be, wait a second. All you did was kick the can down the road because where did that hyperdimensional super intelligent being that snapped their fingers come from? And it's very hard to get out of what is called infinite regression because we humans and science really

like there to be causes for effects. And when it comes to where the universe came from, I think often because we find ourselves with infinite regression and there's no obvious way out of it, this is often where many people, including some renowned theologians, conclude that there must be a higher power. And I'm not going to dispute that, right? That's not something that God did it. God did it. And maybe that's right. I don't know. But here's the thing. Even if, God did it or a higher dimensional super intelligent alien civilization did it, that's not nothing. And so we still haven't gotten back to how you get something from nothing. So it's really a philosophical quagmire. And there are other fundamental concepts that we take for granted, like

gravity. We've had a lot of fun explaining gravity over the last hundred years, haven't we? Oh, have we explained it? Because that would be a good memo for me to get. Well, you have the, Einstein had his way of explaining it, right? That it, that masses warp space, so you sort of fall into a warp piece of space. That's right. And that gravity is actually sort of the ether of the universe, right? Yeah. It's what makes up the universe.

And then you have the quantum physics, people say, no, there are particles of gravity, right? Gravity should be quantized like everything else. So you've got this disagreement going on for decades. Yeah. And I don't know that there's any end in sight. Gravity, you know, if we're going to talk about the known forces, and I want to put some emphasis on that word known, right? I think we need to acknowledge that there might be things we don't know about. But when we think about the known forces, gravity is really the odd duck in the room for a variety of reasons, but one of the reasons that

gravity is so odd and I think is at odds with our normal daily experience because we, you know, I don't know about you, but right now I'm sitting in a chair and the gravity of the earth has pulling me down, and I don't particularly feel any other forces tugging on me. That being said, of the known forces, gravity is ridiculously weak, incredibly weak compared to the other forces. You mean it takes such a big body like the Earth to create enough gravity to hold you down? Exactly. Or the sun or something like that. That's right. And we don't understand why gravity is that weak. So yes, Einstein took a good crack at this with general relativity, and there's, you know, I'm on board with general relativity and I use it every day.

And I think there's a lot of merit there. But general relativity itself depends on what we call this fabric of space time. And as an astrophysicist, we think about and we talk about and we utilize this fabric of space time all of the time in our work. But we don't really understand what that fabric is and what it is that might be warped by objects that have a property called mass and why the fabric of space time behaves the way it does, or why time behaves differently from space to begin with. So even Einstein's explanation for gravity is only sort of one level down. And what about the quantum people who say they've got, you know, we'll find those quantum particles that describe gravity. We need to find gravitons. Gravitons. Gravitons. And, you know, that there,

I would put money on getting a Nobel Prize, too. But we haven't, we haven't detected. to gravitons. And there are, you know, for fun, I just have to throw this in. There are hypotheses on the table that one of the reasons gravity behaves as strangely as it does, even though we're so used to it in our normal lives, is that gravitons might not be tethered to our fabric of space and time. And of all of the- Whoa, whoa, whoa.

Okay. All right, I'm pausing. there exists a hyperdimensional construct in which our familiar four dimensions of space and time exist. So we might imagine there are other dimensionalities outside of those we perceive. There are hypotheses on the table that gravitons can leave our comfy dimensions of space and time and go off into this hyperdimensional space that we don't appear to otherwise have access to. Now, I don't want to even suggest we haven't tested that. So it is sitting on the boundary of what is possible currently with empirical inquiry.

But there are a number of really beloved physical theories, including string theory, that suggests that this is what gravitons might do. Well, but you can't test it, right? I mean, strength theory has been around 40 years or so. Isn't strength theory closer to a religion than the science? Because you really can't test it? I would agree with you on that, actually. I would agree with you.

And where the boundary is between belief and science lives on this really hairy edge of what is and isn't empirically testable or what might be empirically testable in the future. Well, so if you talk about this multidimensional universe, we don't have the tools to find it, do we? Not yet. Although it's very interesting. There are some experiments that have been attempted over recent years. I won't say they've been very high-powered experiments, but sort of, you know, initial attempts that might have given us signatures of there being other dimensions. You know, one of the ways this could have manifested itself is if the Large Hadron Collider had created a micro black hole. You may recall that back in the day...

Oh, they were afraid of that. Exactly. Well, some people were afraid of it. Some of us were like, that'd be really cool. But if it had created a micro black hole, that would have been a smoking gun that there are these extra compactified dimensions. And so there are other experiments like that that can happen at both, you know, the small scale and really the large scale that could give us a hint, you know, sort of a hint at the existence. We have to take a quick break, but don't go away. More on this when we come back.

We know one plus one equals two seem self-evident, but proving it is actually an entirely different. issue. And so much of science is derived, or what we think of how nature works, is derived by mathematics. You're talking about string theory, for example, about how many extra dimensions there have to be. The math says it could be. But I mean, how are we so sure that math used by all physicists really does explain how nature works? I think that's a great question. And we put a lot of reliance on math because as far as we can tell, you know, we started this conversation by talking about what we can know and what it means for something to be known. And I would argue that mathematical principles are as close as we can come to knowing something in terms of their

their logical internal consistency and what appears to be a lack of reliance on anything else in the universe. Now, that being said, and this is this is, this is really a fun thing I do when I'm teaching this in class and I'm challenging my students to argue that they know certain things. Some smarty pants in class will always come to, well, I know that one plus one equals two. And I'll be very excited for them. And they're feeling very proud of themselves because how could one plus one not equal to? Right? That just seems so inherently self-evident that it has to be true. But here's the thing. There's this book called Principia Mathematica, written by Whitehead and Russell, in which they set out to prove 1 plus 1 equals

2. And it took them over 300 pages. And even in that proof, they had to start with fundamental assumptions or axioms that themselves are not provable. And so even things as simple as saying we know 1 plus 1 equals 2 seem self-evident. And along with everyone else probably listening, I'm willing to take that as a fact. But proving it is a way. actually an entirely different issue. So even math may have limits. You mentioned that questions like these keep you up at night. They do. All of these, you know, the big, beautiful questions that I think sit at the center of, for me, what it means to be human in the universe, I find myself compelled to be in the space of existential curiosity. And I happen to think it's a great

place to be. So you're okay with not knowing. I am okay. with not knowing. I mean, I would like to know. There are things I would love to know in my time on earth in this human body. But I think what I really value is curiosity. And I think often we think of curiosity as a means to an ends, as opposed to curiosity for curiosity's sake. And can we allow ourselves to be curious about something even if we might not know the answer? Even if humans may never know the answer, I would argue that curiosity still has value. You know, we humans, as far as we know right now, are the only sentient beings in the cosmos, and this could change tomorrow, right? We could detect extraterrestrial life any day now. But as of right now, we are the only

sentient beings in the universe that are capable of trying to understand the universe that we're literally made of. In many ways, we are the universe made physically aware. We are the universe made physically aware of itself. And to me, this comes with, I would argue, an ethical imperative. Because if we don't learn about the universe we are made from, who will? Well, I would love to sit in on your course someday, because it sounds quite interesting. You'd be welcome anytime. Thank you for your book, and thank you for taking time to talk with us today. You're right in my wheelhouse about subjects I love to talk about. Wonderful. Such a joy to be here. Dr. Kelsey Johnson, Professor Astronomy at the University of Virginia and former president of the American Astronomical Society.

And author of the book, Get This Book, Into the Unknown, The Quest to Understand the Mysteries of the Cosmos. That's about all the time we have. For now, a lot of people help make this show happen. D. Peter Smith. Praise of Gucci. Kathleen Davis. Santiago Flores. I'm Ira Flato. Thanks for listening.