Brian Greene mounted a lengthy defense of string theory today in the NY Times. He maintains that string theory is the grand finale of physics, the logical bridge between the contradictions of quantum physics and general relativity. Central to this argument is the concept that physics itself has been steadily "converging". String theory, in other words, is just Newton, Einstein and Bohr taken to their logical conclusion:
For nearly 300 years, science has been on a path of consolidation. In the 17th century, Isaac Newton discovered laws of motion that apply equally to a planet moving through space and to an apple falling earthward, revealing that the physics of the heavens and the earth are one. Two hundred years later, Michael Faraday and James Clerk Maxwell showed that electric currents produce magnetic fields, and moving magnets can produce electric currents, establishing that these two forces are as united as Midas' touch and gold. And in the 20th century, Einstein's work proved that space, time and gravity are so entwined that you can't speak sensibly about one without the others.
This striking pattern of convergence, linking concepts once thought unrelated, inspired Einstein to dream of the next and possibly final move: merging gravity and electromagnetism into a single, overarching theory of nature's forces.
If that's the history of physics you subscribe to, then I understand why string theory is so necessary: it's the only theory that can, at least in theory, tie together the divergent strands of the quantum world and everything else. But I think Green is wrong. In fact, I think the exact opposite history of physics is more plausible. In this history, the strands of physics have been steadily diverging and multiplying, and not converging into some uber-equation.
I'll begin at the beginning of the 20th century, when many physicists thought they had the universe solved. Some obscure details remained, but the basic structure of the cosmos was understood. Out of this naivete, relativity theory emerged, shattering classical notions about the relationship of time and space. Then came Heisenberg's Uncertainty Principle, which showed that we can't know everything about a single particle, let alone everything about all the particles in the universe. (One of the ironies of modern science is that some of its most profound discoveries are actually about the limits of knowledge.) Then came quantum physics. Soon light was both a particle and a wave. Everything was indeterminate. String theorists, in their attempts to reconcile these gaping schisms of theory, spread rumors of 11 dimensions, maybe more.
My point is that physics has not progressed towards any unified understanding of everything. Rather, it has exploded any hope of it. The greater the science, the greater the mystery. A final theory is getting farther and farther away. It is not, as Greene argues, somehow inevitable. The universe never promised to tell us all of its secrets.
This doesn't mean that physics hasn't learned anything new in the last 100 years. Quite the opposite. I think the ultimate testament to the progress of physics is its steady descent into confusion and contradiction. We should be suspicious whenever our theories get too neat, for if we know anything it is that reality is messy.
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> I think the ultimate testament to the progress of physics is its steady descent into confusion and contradiction.
This an extremely poor characterization of the status of physics. The ultimate testament to the progress of physics is the fact that the only parts that are confused and contradictory lie beyond the reach of current experimental technology. Our theories for the phenomena that are within reach have been profoundly successful. Where have you been?
Of course, physical theories for "the phenomena that are within reach have been profoundly successful." Absolutely true. But it's also true that there are many more phenomena in physics that are out of reach now than there were 100 years ago, or 50 years ago. Contradictions that didn't exist at the turn of the century now define the discipline. I believe that this is a sign of a healthy scientific field. In fact, the same process is at work in biology. Fifty years ago, there was the Central Dogma: DNA made RNA made Protein. Now that neat theory has become much more confusing: we have microRNA's, methylation, prions, RNA inherited via sperm, etc., etc.
I believe that this is the basic pattern of scientific progress. Every answer generates many new questions. It would be great if these questions all converged into one BIG QUESTION. But they don't, at least not yet.
One of the things I have never really understood is the basis for the conviction that there must, in fact, exist some single theory that explains all of physics. This may just be my own ignorance, as I am not a physicist. Can anyone explain to me the justification for this conviction?
Good question. I'm a biologist, and we never talk about unified or final theories. Why do physicists get to have all the fun?
One problem is, even if physics is about convergence and a "theory of everything" is a necessary next step, how do we know string theory is what they're converging to? String theory is under a lot of criticism right now, but I'm not sure it's from people who think we don't need or want a theory of everything-- they're from people who are questioning whether string theory is a good theory of everything.
I don't know if I'm in any place to legitimately criticize, but this seems to me like the big problem with string theory defenders-- they often resort to reasons why we wish string theory would turn out to be right, not reasons to think string theory will turn out to be right. The article does a great job convincing us that string theory would be useful and convenient and necessary for the progress of science, and so it might be persuasive for an argument that string theory is worth trying to explore further. But I'm not sure it does much to convince us that that exploration will have a payoff.
Meh.
Interesting blog. But what about chemistry? Aren't there some scientific fields that haven't gotten more "contradictory" over time?
> But it's also true that there are many more phenomena in physics that are out of reach now than there were 100 years ago, or 50 years ago.
Uh, I don't think so. This would mean that people 100/50 years ago had more powerful technology then we do now. I think you mean the opposite of what you said?
> Contradictions that didn't exist at the turn of the century now define the discipline.
I'd like you to actually state the contradictions you are talking about. The SM and GR define the discipline. They are both entirely consistent and non-contradictory by themselves. We know they are mutually inconsistent, but only in exotic regimes: black holes and early-time cosmology. If you examine physics in its entirety, it is the opposite of contradictory, we have gaps or contradictions only where we can't reach. You seem to think that string theory is all of physics, and this is very naive. You might want to ask your fellow blogger Chad Orzel what he thinks of this opinion.
> It would be great if these questions all converged into one BIG QUESTION.
Actually, it would be great if they gave one BIG ANSWER. I agree that new answers often lead to new questions, but the new questions always apply to more and more specialized areas. The sum of our knowledge always increases. We may become more aware of what we don't know, but ignorance and awareness of ignorance are not the same thing.
> One of the things I have never really understood is the basis for the conviction that there must, in fact, exist some single theory that explains all of physics.
If you don't have one theory, you have many. If that set of theories is mutually inconsistent or incomplete (and GR and the standard model are inconsistent in certain regimes), then this reflects a deficit in our understanding (if they're complete and consistent, then they can be considered different descriptions of the same theory). The desire for a single theory is predicated on the assumption that there is a single reality.
> String theory is under a lot of criticism right now, but I'm not sure it's from people who think we don't need or want a theory of everything-- they're from people who are questioning whether string theory is a good theory of everything.
Exactly.
I meant what I wrote. Do you think Raymond Poincare - who declared physics solved in 1903 - worried about the EPR paradox? Or quantum entanglement? Or quantum indeterminacy? Or the existence of 11 dimensions? These are physical phenomena that remain more than a little confusing, and not only for us laymen. Poincare didn't have the technology to see such things, and his physics was much simpler as a result. While our powerful technology allows us to detect strange new aspects of reality, detection and understanding are not the same thing. And I agree that the sum of knowledge always increases (that's the wonder of science). But I don't believe that the increasing sum of knowledge necessarily takes us closer to a unified theory. Reality certainly seems unified and singular, but then reality also seems as if it only consists of three, perhaps four, dimensions. Modern physics has taught me that my intuitions about, well, everything are rather wrong. Maybe the search for a unified reality is just another one of our erroneous instincts.
> Do you think Raymond Poincare - who declared physics solved in 1903 - worried about the EPR paradox? Or quantum entanglement? Or quantum indeterminacy? Or the existence of 11 dimensions? These are physical phenomena that remain more than a little confusing, and not only for us laymen.
Whether Poincare worried about entanglement is beside the point you were trying to make. Quantum eraser experiments now allow us to observe entanglement, and in Poincare's time they did not exist. Just because Poincare didn't know about entanglement doesn't mean the phenomenon did not, in fact, exist. And your own personal struggle with the concepts is not really relevant to the fact that physicists now have a complete theory describing quantum phenomena. If gravity didn't exist and neutrinos had no mass, we would be sitting pretty with a unified theory already.
> Modern physics has taught me that my intuitions about, well, everything are rather wrong.
Just because some of your intuitions have misled you doesn't mean all of them are "wrong". Without intuition, there is no science.
"If you don't have one theory, you have many. If that set of theories is mutually inconsistent or incomplete (and GR and the standard model are inconsistent in certain regimes), then this reflects a deficit in our understanding (if they're complete and consistent, then they can be considered different descriptions of the same theory). The desire for a single theory is predicated on the assumption that there is a single reality."
I don't understand the justification for concluding that if our best description of reality as of now is a set of theories that are inconsistent in different regimes, that this necessarily implies a "deficit in our understanding". Isn't it possible that there is no single consistent theory that explains reality in all regimes?
It is possible to construct relatively simple logical systems in which no complete consistent description of the entire system exists. What guarantees that physical reality is not like this? Isn't it possible that we exist in a single reality this is, nonetheless, indescribable by a single complete consistent theory?
But Patrick, the fact that Poincare did not know about quantum entanglement and thus thought he was close to a final physics proves Jonah's point. What strange phenomena do we not know about? Brian Green may think he is getting close or closer to a final theory, but just imagine what new facts we will have to explain in the future?
> It is possible to construct relatively simple logical systems in which no complete consistent description of the entire system exists.
I haven't studied logic, so I'd appreciate some specifics regarding the systems you're talking about. I suspect incompleteness/inconsistency is a defect in the construction, in the same way naive set theory results in Russell's paradox. But my answer to your question is that physics relies on a theory to make predictions about experiments. If two theories are inconsistent, they will make conflicting predictions. The experiment will either falsify one or the other, or both. Inconsistency is, in principle, resolvable through experimentation. We know that certain information is forever inaccessible, so theories can never be complete in the strictest sense, but we can strive for maximality, by inventing descriptions such as the wave function.
> What strange phenomena do we not know about?
Possibly many, but you're missing the point. The number of extant phenomena is independent of our understanding or knowledge of them. Jonah's point was the number of phenomena out of our reach is larger now than it was 100 years ago. Since the number within reach has grown considerably, this is clearly false. What he possible meant was the number of phenomena that we know of is larger, but that is not what he said. Both you and Jonah are enumerating phenomena conditioned on the state of current knowledge, and perhaps you subscribe to the viewpoint that reality doesn't exist until we observe it, but that sort of stuff is not taken seriously by any physicist I've ever met.