More than a decade ago, British physicist Stephen Hawking said there was a 50-50 chance that a unified "theory of everything" would be discovered in 20 years' time. Now Hawking thinks the theory has been found.
In "The Grand Design," he and co-author Leonard Mlodinow explain why a concept called M-theory offers the only path they can see to understanding the universe's grand design. Hawking got a lot of click traffic earlier this week for his observation that God wasn't needed to explain the origin of the universe. But his claim that "M-theory is the unified theory Einstein was hoping to find" could be, if anything, more scientifically controversial.
"Stephen often overstates the case, and that's fine," said Lawrence Krauss, a theoretical physicist at Arizona State University who's coming out with his own book about the ultimate questions of physics next year. "That's by virtue of the fact that it's hard for him to go into detail because of his medical condition. Because of that, he makes brief, blunt statements. It's almost like the Bible. Whenever he says anything, people jump on it."
M-theory is a key jumping-off point for "The Grand Design." The string theorists who came up with the term have never agreed on exactly what the "M" stands for, although the words "membrane," "matrix," "mystery" and "magic" have all been floated as possibilities. My favorite explanation is that M-theory is the "mother of all theories."
String theory suggests that the fundamental constituents of reality are not pointlike particles (such as the concepts we have for protons, neutrons and electrons) but are more like tiny strings vibrating at different "frequencies." Such ideas can be used to make linkages between gravity and the other fundamental forces in physics, but only if you build 10 dimensions into the picture.
Theorists found that five different strains of string theory explained how the universe worked, from five seemingly irreconcilable perspectives. But if you added one more dimension to the picture, effectively turning the dimensional dial up to 11, everything made sense. The five perspectives could be seen merely as different ways of expressing the same super-theory. That's what's known as M-theory.
Hawking and Mlodinow may make it sound as if M-theory has to be the theory of everything, but Krauss says it's too early to declare "M-Mission Accomplished." One big issue is that M-theory makes more than one prediction about the nature of the universe. In fact, the number of predictions it makes is somewhere around 10 to the 500th power. That's a 1 followed by 500 zeroes.
"On the surface, that sounds like a bad thing," Krauss said. He has observed that this kind of string theory isn't so much a theory of everything as it is a theory of anything (or a theory of nothing). But most scientists have come around to the view that the multiplicity of M-theory's predictions is actually a virtue. Seen from this perspective, it may be that anything is possible when it comes to creating universes. We just happen to be in a universe where all the lottery numbers have added up to win what astrobiologist Paul Davies calls the "cosmic jackpot."
"Interestingly enough, what people are hanging onto is the lack of ability to make predictions," Krauss said. "It turns a wart into a beauty mark."
What Krauss finds exciting is that there could be ways to verify that something can come from nothing -- which is the point behind Hawking's claim that God isn't necessary to explain the universe's creation.
Physicists have noted that the positive energy contained in particles and the negative energy represented by gravitational attraction appear to balance out precisely. "Empirically, we can actually have evidence that the universe came from nothing. One of the key things is that the total energy of the universe is zero, which is only possible if the universe came from nothing. It could have been otherwise. It could have been not zero," Krauss said.
The concept of a zero-energy universe and getting something from nothing may sound crazy, but this article from Mercury magazine and this video of one of Krauss' lectures, both titled "A Universe From Nothing," show that the ideas has been percolating among scientists for years. Such ideas are central to "The Grand Design," as well as to the book that Krauss is currently in the midst of writing.
"This is very premature, because we still don't know what M-theory is," Krauss told me. "The interesting question for me, ultimately, more than this metaphysics, is whether we'll be able to empirically answer these questions. Science has gotten to the point where there's the hope that we'll be able to turn some of this metaphysics into physics."
Physicists Leonard Mlodinow and Stephen Hawking work together in Hawking's office in Cambridge, England.
Mlodinow agrees with Krauss that M-theory still has miles to go, but he says it may be as close as science can get to the fabled theory of everything. The Caltech physicist has collaborated with Hawking for years -- not only on "The Grand Design," but also on "A Briefer History of Time," a streamlined version of Hawking's classic work. Mlodinow has also done science writing as a solo act, as the author of "Feynman's Rainbow" and "The Drunkard's Walk."
During a telephone interview, Mlodinow told me that "The Grand Design" was truly a joint effort, in which he and Hawking traded, debated and restated each other's prose. "Everything was pretty much passed back and forth, so actually it would be hard to identify which one of us wrote what," he said. "In fact, at times where I've tried, I've gone back to my computer to see -- and sometimes I'm wrong."
Thus, Mlodinow is as good a source as Hawking for insights into the meaning of "The Grand Design." Here's an edited transcript of our Q&A:
Cosmic Log: In the past, Stephen has talked about the quest for a theory of everything. The book makes it sound as if it's not so much one theory of everything, but a series of theories for different model-based views of reality. Do you get a sense that it's going to be possible to come up with one unified theory of physics?
Leonard Mlodinow: Well, the book is about why the laws of nature are what they are, and where the universe came from. We do say in the book that we believe the unified theory is M-theory. So we not only believe that it's possible, but we believe that it's here.
Q: But M-theory is an array of different perspectives on reality, and one of the things about that approach is that one model works for one scale, or one sphere of physics, and perhaps another theory -- I don't know whether you'd call it a subtheory, or another perspective -- works for a different one.
A: M-theory is the most general quantum theory that would include gravity, using the constraints that we feel need to be employed -- for example, that it's finite and would make reasonable predictions. Whether it's a single theory or a network of theories is not yet known. I think Stephen feels that there's a good chance it's a network of theories, which is what we see today. Where they overlap, they agree. In other areas where they don't overlap, they make their own predictions. Stephen believes that's OK, and we shouldn't be disappointed if the final theory is a network of theories. According to model-dependent realism, all that is OK. It's just the way reality is. You can't ask which of the theories in that network is more "real."
Q: Do you have a slightly different point of view? Because it sounds as if you're presenting Stephen's view as distinct from your own.
A: No, I agree with Stephen. We debated this idea of model-dependent realism over quite a period of time. I'm saying that just because I'm assuming you were interested in Stephen's opinion more than mine. But I'm happy to jump in as well.
Q: In the latter part of the book, there's some discussion about how God does or does not play a role in the big questions about the universe....
A: Well, people have always wondered about the big questions: Where did the universe come from? Why is nature the way it is? At first we had mythology to answer that question. I suppose people just made up stories, and they became the myths. Or they evolved. Later we had the religions that we have today, and philosophy grew up. People used applied reason, intuition and some small amount of observation as well -- and came up with their own concepts on the answers to these questions.
"The Grand Design" delves into subjects ranging from M-theory to God's role.
A few hundred years ago we developed this thing called the scientific method, where we come up with theories phrased in mathematics, and we require that they not only describe what we're looking to describe but also make further predictions that can be tested. Then we do experiments, and if we find that the predictions are not right, if they're not verified, then we alter or discard the theory.
In the book, we argue that this is a better method. It's led to the modern society that we have today -- to vaccinations, computers, electricity, television, telephones, everything else. When you understand nature to that extent, you can apply it. Since you really understand what's going on, you can create all this technology, which you don't create based on mythology, philosophy and religious explanations.
As far as God goes, we describe our theory of where the universe came from, and why the laws of nature are as they are. And we show that with this theory, there's no need for a God to create the universe or to create the laws of physics as they are. All of this can come purely from physics, from science, from nature.
Q: There's always a question about "what happened before the big bang," or about the nature of time. Stephen dealt with that in "A Brief History of Time," and you helped with that vision through your work on "A Briefer History of Time." How does this book advance the ball?
A: One of Stephen's big ideas in this book is called "top-down cosmology." It's the idea that we should trace the history of the universe from the present time backwards -- and that the universe has many histories because it's a quantum system. In "normal" physics, we work in a laboratory and we do experiments. We set up the experiment in an initial state, then we let it go for a while, then we do measurements on its final state -- and we check predictions. The theory tells us how the initial state should develop, and then we make predictions about the final state.
We can't do that with the universe as a whole. We don't set up the initial state. We don't have a laboratory where we can control what's going on. We can't repeat the experiment and take the data. Also, the universe -- since we believe in quantum theory now -- is a quantum system.
In normal cosmology, people start with the initial state as if it were a laboratory -- which it's not -- and they use classical ideas, meaning that there's one history of the universe which they trace forward. Stephen believes that we should start from our observations now, because that's all we can do, and trace it backwards, taking into account the fact that the universe has many histories and not just one.
Q: Right, there's a discussion in the book about how the past is as much affected by quantum mechanics as the future is. So there's uncertainty about the past -- which is counterintuitive. That must be a hard sell with normal people who say, well, I remember specifically what I had for dinner yesterday. We know for sure what happened in the past because of things ranging from human memory to the fossil record to the process of baryogenesis at the beginnings of the universe. So how can you say that there's a factor of uncertainty about past events?
A: Well, if you happened to have experienced all possible aspects of the universe for all of time, there would not be uncertainty. Quantum theory doesn't say that if you ate an egg, you might not have eaten the egg. Let's get that straight. What quantum theory says is that in between the times when we observe and measure, and interact in that way, these properties that we talk about have no meaning.
For instance, in classical theory, if you push a billiard ball down the table, and if no one is interacting with it or measuring it, it still has one path with a well-defined position at every time. Those properties exist. In quantum theory, if you push it and then no one interacts with it, you cannot in general say that it has a particular position and velocity at any time. In classical theory, we say that it has those properties, and when we measure it, we're just reading off those properties. In quantum theory, it's not correct to say that a measurement is merely reading off those properties. Rather, it doesn't have those properties when we don't measure it.
Now, if you had an egg yesterday, you interacted with the egg, and there's an egg there. When we look at the universe today, with top-down cosmology, we don't allow for the possibility that the moon is made of green cheese -- because we already know that the moon isn't made of green cheese. We put in all the data of all our observations, and that prunes down the number of different histories that have to be taken into account. But where observations haven't been made, we don't.
So the vagueness of the past is the vagueness of things unmeasured in the past.
Q: Does that imply then that there will be no way to answer that classic question, "What happened before the big bang"? Because the uncertainty goes to an indeterminately high level?
A: No, it's not that. As you go backward in time, quantum theory, combined with general relativity, tells you that if you go back early enough in the universe, time ceases to have the meaning that we assign to it today. It ceases to act as we know it. So it's not a well-posed question to say, "What happened at the beginning of time?" -- because time doesn't go back to the beginning.
According to general relativity, time and space exist under certain conditions. Quantum theory tells you that there are always fluctuations in empty space, and if you make the universe small enough, the fluctuations are great enough that the matter is squashed down enough that this affects the character of space and time itself. Time doesn't exist at that point. So the question doesn't make any sense.
Q: I know we're coming to end of our time -- speaking of that -- but do you hold out hope that humans will at some point understand the totality of the grand design? Or is the grand design something that our brains aren't just big enough to hold? Or is it something that is unknowable, because that's just the nature of the universe?
A: No, we believe that humans can understand it. That's the great triumph and the great miracle of the universe.
More about Stephen Hawking and the cosmos:
- Hawking says God's not needed. So?
- Video: NBC News' Brian Williams on "The Grand Design"
- Do we need to get off Earth by 2110?
- Hawking goes zero-G: 'Space, here I come'
- Aliens may pose risk to Earth, Hawking says
- Up close with Dr. Hawking
- Interactive: Beyond the Big Bang
- Interactive: The Symphony of Everything