Are There Things That We Know We Can’t Know?

IRA FLATOW: This is Science Friday. I’m Ira Flatow. 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.

KELSEY JOHNSON: Hi, Ira. It’s really an honor to be here.

IRA FLATOW: It’s an honor to have you. Do you like to delve into these questions yourself?

KELSEY JOHNSON: Oh, my gosh. I think about them all the time, and they’re what get me out of bed in the morning.

IRA FLATOW: When I talk to astronomers and physicists, sometimes they say the actual quest is better than the answer. Is that true for you?

KELSEY JOHNSON: 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.

IRA FLATOW: Well, 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?

KELSEY JOHNSON: 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. 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.

IRA FLATOW: 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 know?

KELSEY JOHNSON: No, I’m so glad you brought that up, because I think most normal people who are well-adjusted go through life thinking we understand the universe. But we don’t understand more than 95% of it.

IRA FLATOW: So what do we know about anything if you don’t know what 95% is?

KELSEY JOHNSON: I don’t know. I don’t know what we know.

IRA FLATOW: 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 in the future?

KELSEY JOHNSON: 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.

IRA FLATOW: Right. But there are also simple things that we’ve talked about over the years, like time.

KELSEY JOHNSON: Simple things.

IRA FLATOW: I’ve had scientists come on. And have written books about time, says there’s no such thing as time. I mean, that’s just– whoa.

KELSEY JOHNSON: Time is one of these super slippery concepts that, I would argue, we have the most exposure to. 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 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.

IRA FLATOW: There you go. You 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?

KELSEY JOHNSON: 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 the top of that list. 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, 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 a universe.

IRA FLATOW: I’ve seen that movie.

KELSEY JOHNSON: 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, 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. I’m not going to dispute that. That’s not something that–

IRA FLATOW: God did it.

KELSEY JOHNSON: 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.

IRA FLATOW: 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 100 years, haven’t we?

KELSEY JOHNSON: Oh, have we explained it? Because that would be a good memo for me to get.

IRA FLATOW: [LAUGHING] Well, Einstein had his way of explaining it, that masses warps space. So you fall into a warped piece of space.

KELSEY JOHNSON: That’s right.

IRA FLATOW: And that gravity is actually the ether of the universe. It’s what makes up the universe. And then you have the quantum physics people say, no, there are particles of gravity. Gravity should be quantized like everything else. So you’ve got this disagreement going on for decades.

KELSEY JOHNSON: Yeah. And I don’t know that there’s any end in sight. Gravity, if we’re going to talk about the known forces– and I want to put some emphasis on that word, “known.” 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 I don’t know about you, but right now, I’m sitting in a chair and the gravity of the Earth is 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.

IRA FLATOW: You mean it takes such a big body, like the Earth, to create enough gravity to hold you down–

KELSEY JOHNSON: Exactly.

IRA FLATOW: –or the sun, or something like that.

KELSEY JOHNSON: 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 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 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 one level down.

IRA FLATOW: And what about the quantum people who say that, well, we’ll find those quantum particles, that defy gravity?

KELSEY JOHNSON: Exactly. We need to find gravitons.

IRA FLATOW: Gravitons.

KELSEY JOHNSON: Gravitons. And that, there, I would put money on getting a Nobel Prize, too. But we haven’t detected gravitons. And there are, 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–

IRA FLATOW: Whoa, whoa, whoa, whoa.

KELSEY JOHNSON: All right, I’m pausing, Ira. If 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 suggest that this is what gravitons might do.

IRA FLATOW: Well, but you can’t test it. I mean, string theory has been around 40 years or so. Isn’t string theory closer to a religion than a science, because you really can’t test it?

KELSEY JOHNSON: 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.

IRA FLATOW: Well, so if you talk about this multi-dimensional universe, we don’t have the tools to find it, do we?

KELSEY JOHNSON: 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 initial attempts that might have given us signatures of there being other dimensions. 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–

IRA FLATOW: They were afraid of that.

KELSEY JOHNSON: 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 the small scale and really the large scale that could give us a hint, a hint at the existence.

IRA FLATOW: 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?

KELSEY JOHNSON: I think that’s a great question. And we put a lot of reliance on math because, as far as we can tell, 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 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 really a fun thing I do when I’m teaching this in class and I’m challenging my students to argue that they certain things. Some smarty pants in class will always come to, well, I know that 1 plus 1 equals 2. And I’ll be very excited for them. And they’re feeling very proud of themselves because how could 1 plus 1 not equal 2? 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 seems self-evident, and along with everyone else probably listening, I’m willing to take that as a fact. But proving it is actually an entirely different issue. So even math may have limits.

IRA FLATOW: You mentioned that questions like these keep you up at night.

KELSEY JOHNSON: They do. All of these, 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 this space of existential curiosity. And I happen to think it’s a great place to be.

IRA FLATOW: So you’re OK with not knowing.

KELSEY JOHNSON: I am OK 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.

We, humans, as far as we know right now, are the only sentient beings in the cosmos. And this could change tomorrow. 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 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?

IRA FLATOW: Well, I would love to sit in on your course someday, because it sounds quite interesting.

KELSEY JOHNSON: You’d be welcome anytime.

IRA FLATOW: 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.

KELSEY JOHNSON: Wonderful. Such a joy to be here.

IRA FLATOW: Dr. Kelsey Johnson, Professor of 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.

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