I invited a group of cosmologists, experimentalists, theorists, and particle physicists and cosmologists. Stephen Hawking came. We had three Nobel laureates: Gerard ’t Hooft, David Gross, and Frank Wilczek; well-known cosmologists and physicists, such as Jim Peebles at Princeton, Alan Guth at MIT, Kip Thorne at Caltech, Lisa Randall at Harvard; experimentalists, such as Barry Barish of LIGO, the gravitational-wave observatory. We had observational cosmologists, people looking at the cosmic microwave background; we had Maria Spiropulu, from CERN, who’s working on the Large Hadron Collider—which a decade ago people wouldn’t have thought was a probe of gravity, but now, due to recent work in the possibility of extra dimensions, it might be.
I wanted to have a series of sessions where we would, each of us, try and speak somewhat provocatively about what each of us was thinking about, what the key issues are, and then have a lot of open time for discussion. So the meeting was devoted with a lot of open time for discussion, a lot of individual time for discussion, as well as some fun things, like going down in a submarine, which we did. It was a delightful event, where we defied gravity by having buoyancy, I guess.
I came away from this meeting realizing that the search for gravitational waves may be the next frontier. For a long time, I pooh-poohed it in my mind, because clearly it’s going to be a long time before we could ever detect them, if they’re there. And it wasn’t clear to me what we’d learn, except that they exist. But one of the key worries I have as a cosmologist right now is that we have these ideas, and these parameters, and every experiment is consistent with this picture, and yet nothing points to the fundamental physics beneath it.
It’s been very frustrating for particle physicists—and some people might say it’s led to sensory deprivation which has resulted in the hallucination otherwise known as string theory. And that could be true. But in cosmology what we’re having now is this cockamamie universe. We’ve discovered a tremendous amount. We’ve discovered that the universe is flat, which most of us theorists thought we knew in advance, because it’s the only beautiful universe. But why is it flat? It’s full of not just dark matter but this crazy stuff called dark energy that no one understands. This was an amazing discovery, in 1998 or so.
What’s happened since then is that every single experiment agrees with this picture without adding insight into where it comes from. Similarly, all the data are consistent with ideas from inflation and everything is consistent with the simplest predictions of that, but not in a way you can necessarily falsify. Everything is consistent with this dark energy that looks like a cosmological constant—which tells us nothing.
It’s a little subtle, but I’ll try and explain it.
We’ve got this weird antigravity in the universe which is making the expansion of the universe accelerate. Now, if you plug in the equations of general relativity, the only thing that can antigravitate is the energy of nothing. Now, this has been a problem in physics since I’ve been a graduate student. It was such a severe problem that we never talked about it. When you apply quantum mechanics and special relativity, empty space inevitably has energy. The problem is, [it has] way too much energy. It has 120 orders of magnitude more energy than is contained in everything we see!
Now, that’s the worst prediction in all of physics. You might ask, “If that’s such a bad prediction, then how do we know empty space can have energy?” The answer is, “We know empty space isn’t empty, because it’s full of these virtual particles that pop in and out of existence, and we know that because if you try and calculate the energy level in a hydrogen atom and you don’t include those virtual particles, you get a wrong answer.” One of the greatest developments in physics in the 20th century was to realize that when you incorporate special relativity into quantum mechanics, you have virtual particles that can pop in and out of existence and they change the nature of a hydrogen atom—because a hydrogen atom isn’t just a proton and electron. That’s the wrong picture, because every now and then you have an electron-positron pair that pops into existence. And the electron is going to want to hang around near the proton. Because it’s oppositely charged, the positron is going to be pushed out to the outskirts of the atom, and while they’re there they change the charged distribution in the atom in a very small but calculable way. Feynman and others calculated that effect, which allows us to get agreement between theory and observation at the level of nine decimal places. It’s the best prediction in all of science. There’s no other place in science where from fundamental principles you can calculate a number and compare it to an experiment at nine decimal places.
But then when we ask, “If they’re there, how much should they contribute to the energy in the universe?” we come up with the worst prediction in physics. It says that if empty space has so much energy we shouldn’t be here. And physicists like me, theoretical physicists, knew they had the answer. They didn’t know how to get there. It reminds me of the Sidney Harris cartoon where you’ve got this big equation and the answer, and the middle step says, “And then a miracle occurs.” And then one scientist says to the other, “I think you have to be a little more specific at this step right here.”
The answer had to be zero: The energy of empty space had to be precisely zero.
Why? Because you’ve got these virtual particles that are apparently contributing huge amounts of energy, you can imagine in physics how underlying symmetries in nature can produce exact cancellations—that happens all the time. Symmetries produce two numbers that are exactly equal and opposite, because somewhere there’s an underlying mathematical symmetry of equations. So you can understand how symmetries could somehow cause an exact cancellation of the energy of empty space. But what you couldn’t understand was how to cancel a number to 120 decimal places and leave something finite left over. You can’t take two numbers that are very large and expect them to almost exactly cancel, leaving something that’s 120 orders of magnitude smaller left over. And that’s what would be required to have an energy that was comparable with the observational upper limits on the energy of empty space.
We knew the answer: There was a symmetry, and the number had to be exactly zero. Well, what have we discovered? There appears to be this energy of empty space that isn’t zero! This flies in the face of all conventional wisdom in theoretical particle physics. It’s the most profound shift in thinking, perhaps the most profound puzzle, in the latter half of the 20th century. And it may be so for the first half of the 21st century, or maybe all the way to the 22nd century. Because, unfortunately, I happen to think we won’t be able to rely on experiment to resolve this problem. When we look out at the universe, if this dark energy is something that isn’t quite an energy of empty space but just something that’s pretending to be that, we might measure that it’s changing over time. Then we would know that the actual energy of empty space is really zero and this is some cockamamie thing that’s pretending to be energy of empty space. Many people have hoped they’d see that, because then you wouldn’t need quantum gravity, which is a theory we don’t yet have, to understand this apparent dark energy.
Indeed, one of the biggest failures of string theory’s many failures, I think, is that it never successfully addressed this cosmological-constant problem. You’d think, if you had a theory of quantum gravity, it would explain precisely what the energy of empty space should be. And we don’t have any other theory that addresses that problem, either. But if this thing really isn’t vacuum energy—if it’s something else—then you might be able to find out what it is and learn and do physics without having to understand quantum gravity.
The problem is, when we actually look out, every measure we’ve made right now is completely consistent with a constant energy in the universe over cosmological time. And that’s consistent with the cosmological constant—with vacuum energy. So if you make the measurement that it’s consistent with that, you learn nothing. Because it doesn’t tell you that it’s vacuum energy, because there could be other things that could mimic it. The
only observation that would tell you—give you positive information—is if you could measure that it was changing over time. Then you’d know it wasn’t vacuum energy.
And if we keep measuring this quantity better and better and better, it’s quite possible that we’ll find out that it looks more and more like a vacuum energy, and we’re going to learn nothing. The only way to resolve this problem will be to have a theory, and theories are a lot harder to come by than experiments. Good ideas are few and far between. What we’re really going to need is a good idea, and it may require an understanding of quantum gravity or it may require that you throw up your hands, which is what we’re learning that a lot of people are willing to do.
In the Virgin Islands we had a session on the anthropic principle, and what’s surprising is how many physicists have said, “You know, maybe the answer is an anthropic one.” Twenty years ago, if you’d asked physicists if they hoped that one day we’d have a theory that tells us why the universe is the way it is, you’d have heard a resounding “Yes.” They would all say, “That’s why I got into physics.” They might paraphrase Einstein, who said—while referring to God, but not really meaning God—that the question that really interested him was: Did God have any choice in the creation of the universe? What he really meant by that was, Is there only one consistent set of laws that works? If you changed one, if you twiddled one aspect of physical reality, would it all fall apart? Or are there lots of possible viable physical realities?
Twenty years ago, most physicists would have said, on the basis of four hundred and fifty years of science, that they believed that there’s only one allowed law of nature that works—that ultimately we might discover fundamental symmetries and mathematical principles that cause nature to be the way it is, because it’s always worked that way.
So, that’s the way science has worked. But now—because of this energy of empty space that’s so inexplicable that if it really is an energy of empty space, the value of that number is so ridiculous that it’s driven people to think that maybe, maybe, it’s an accident of our environment, that physics is an environmental science, that certain fundamental constants in nature may just be accidents and there may be many different universes in which the laws of physics are different, and the reasons those constants have the values they have in our universe might be because we’re there to observe them. . . .
This is not intelligent design. It’s the opposite of intelligent design. It’s a kind of cosmic natural selection. The qualities we have exist because we can survive in this environment. That’s natural selection, right? If we couldn’t survive, we wouldn’t be around. Well, it’s the same with the universe. We live in a universe—in this universe. We’ve evolved in this universe because this universe is conducive to life. There may be other universes that aren’t conducive to life, and lo and behold, there isn’t life in them. That’s the kind of cosmic natural selection.
We’re allowed to presume anything. The key question is, Is it a scientific question to presume there are other universes? That’s something we were looking at in the meeting as well. I wrote a piece where I argued that it’s a disservice to evolutionary theory to call string theory a theory, for example. Because it’s clearly not a theory in the same sense that evolutionary theory is, or that quantum electrodynamics is, because those are robust theories that make rigorous predictions that can be falsified. And string theory is just a formalism now, which one day might be a theory. And when I’m lecturing, talking about science, and people say to me, “Evolution is just a theory,” I say, “In science, ‘theory’ means a different thing,” and they say, “What do you mean? Look at string theory, how can you falsify that? It’s no worse than intelligent design.”
I do think there are huge differences between string theory and intelligent design. People who are doing string theory are earnest scientists who are trying to come up with ideas that are viable. People who are doing intelligent design aren’t doing any of that. But the question is, “Is it falsifiable?” And do we do a disservice to real theories by calling hypotheses, or formalisms, “theories”? Is a multiverse—in one form or another—science?
In my sarcastic moments, I’ve argued that the reason some string theorists have latched onto the landscape idea so much is that since string theory doesn’t make any predictions, it’s good to have a universe where you can’t make any predictions. But less sarcastically, if you try and do science with that idea. . . . You can try and do real science, and calculate probabilities, but whatever you do, you find that all you get is suggestive arguments. Because if you don’t have an underlying theory, you never know.
I say, “Well, what’s the probability of our universe having a vacuum energy, if it’s allowed to vary over different universes?” Then I come up with some result that’s interesting—and Steven Weinberg was one of the first people to point out that if the value of the energy of empty space was much greater than it is, then galaxies wouldn’t have formed, and astronomers wouldn’t have formed. So that gave rise to the anthropic argument that, well, maybe that’s why it [the vacuum energy] is what it is—it can’t be much more.
But the problem is, you don’t know if that’s the only quantity that’s varying. Maybe there are other quantities that are varying. Whatever you’re doing is always a kind of ad-hoc suggestive thing, at best. You can debate it, but it doesn’t lead very far. It’s not clear to me that the landscape idea will be anything but impotent. Ultimately it might lead to interesting suggestions about things, but real progress will occur when we actually have new ideas. If string theory is the right direction—and I’m willing to argue that it might be, even if there’s no evidence that it is right now—then a new idea that tells us a fundamental principle for how to turn that formalism to a theory will give us a direction that will turn into something fruitful. Right now, we’re floundering. We’re floundering in a lot of different areas.
As a theorist, when I go to meetings I often get much more out of the experimental talks, because I often know what’s going on in theory—or at least I like to think I do. I was profoundly affected by the experimental talks. In principle, we’re now able to be sensitive to gravitational waves that might change a meter stick that’s 3 kilometers long by a length equal to less than the size of an atom! It’s just amazing that we have the technology to do this. While that isn’t actually detecting any gravitational waves, there’s no technological obstructions to going to the advanced stage. Gravitational waves may, indeed, allow us a probe that might take us beyond our current state of having observations that don’t lead anywhere. I was very impressed with these findings.
At the same time, we had a talk from Eric Adelberger at the University of Washington, who’s been trying to measure Newton’s law on small scales. You might think, “Who would want to measure Newton’s law on small scales?” But one of the suggestions for extra dimensions is that on small scales, gravity has a different behavior. There has been some tantalizing evidence, which went through the rumor mills, that suggested that in these experiments in Seattle they were seeing evidence for deviations from Newton’s theory. And Adelberger talked about some beautiful experiments. As a theorist, I’m just always amazed that they can even do these experiments. And he gave some new results. There are some tentative new results—which of course are not a surprise to me—that suggest that there’s as yet no evidence for a deviation from Newton’s theory.
Many of the papers in particle physics over the last five to seven years have been involved with the idea of extra dimensions of one sort or another. And while it’s a fascinating idea, I have to say it’s looking to me like it’s not yet leading anywhere. The experimental evidence against it is combining with what I see as a theoretical diffusion—a breaking off into lots of parts. That’s happened with string theory. I can see it happening with extra-dimensional arguments. We’re seeing that the developments from this idea, which has captured the imaginations of many physicists, hasn’t been compelling.
Right
now, it’s clear that what we really need is some good new ideas. Fundamental physics is at kind of a crossroads. The observations have just told us that the universe is crazy but haven’t told us what direction the universe is crazy in. The theories have been incredibly complex and elaborate but haven’t yet made any compelling inroads. That can either be viewed as depressing or exciting. For young physicists it’s exciting in the sense that it means that the field is ripe for something new.
The great hope for particle physics, which may be a great hope for quantum gravity, is the next large particle accelerator. We’ve gone thirty years without a fundamentally new accelerator that can probe a totally new regime of the subatomic world. We would have had it, if our legislators had not been so myopic. It’s amazing to think that if they hadn’t killed the Superconducting Super Collider it would have already been running for ten years. The Large Hadron Collider is going to come online next year, and one of two things could happen: It could either reveal a fascinating new window on the universe and a whole new set of phenomena that will validate or refute the current prevailing ideas in theoretical particle physics—supersymmetry etc.—or it might see absolutely nothing. I’m not sure which I’m rooting for. But it is at least a hope, finally, that we may get an empirical handle that will at least constrain the wild speculation that theorists like me might make.
Such a handle comes out of the impact of the recent cosmic-microwave-background studies on inflation theory. I read in the New York Times that Alan Guth was smiling, and Alan Guth was sitting next to me at the conference when I handed him the article. He was smiling, but he always smiles, so I didn’t know what to make of it, but I think the results that came out of the cosmic microwave background studies were twofold. Indeed, as the Times suggested, they validate the notions of inflation. But I think that’s just journalists searching for a story.
The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos Page 23