This group also has ties to other theorists in the Department, including those in non-relativistic quantum many-body theory, nuclear theory, and astrophysics. Current research within the group includes study of sources of gravitational radiation and radiation reaction in general relativity, applications of numerical relativity, calculations in relativistic astrophysics of massive black holes, beyond standard model of particle physics including dark matter , quantum gravity phenomenology, and foundations of quantum mechanics.
The theory group hosts a number of distinguished visitors each year. It is tempting to unify gravity as well.
Superstring theory is a good candidate to unify all interactions. Recently there are big developments in this theory. We study string duality, Dirichlet membranes, Dirichlet instantons, F theory, and M theory. Since quantum field theory is the basic tool in particle theory, we are very much interested in every aspect of quantum field theory.
It is worthwhile to investigate it and develop it for its own sake. We are attempting to construct a new quantum field theory based on noncommutative geometry. It is also important to apply it to various branches of physics. For instance, condensed matter physics is a good field to test it. There is a good chance to gain entirely new insights from a quantum-field-theoretical point of view. Indeed, we have already obtained very new results by applying it to the analysis of quantum Hall effects.
What holds true in that thought problem holds true for every object in the real world: The behavior of each part is inextricably related to that of every other part. It is also, Smolin thinks, a promising way to obtain bigger answers about how nature really works, across all scales. Smolin is keenly aware that he is pushing against the prevailing devotion to small-scale, quantum-style thinking.
Much as all of the parts of the universe are linked across space, they may also be linked across time, he suggests. His arguments led him to hypothesize that the laws of physics evolve over the history of the universe. Over the years, he has developed two detailed proposals for how this might happen. His theory of cosmological natural selection, which he hammered out in the s, envisions black holes as cosmic eggs that hatch new universes. More recently, he has developed a provocative hypothesis about the emergence of the laws of quantum mechanics, called the principle of precedence —and this one seems much more readily put to the test.
If you perform an experiment that has been performed before, you expect the outcome will be the same as in the past. Strike a match and it bursts into flame; strike another match the same way and … you get the idea. Smolin hypothesizes that those laws actually may emerge over time, as quantum systems copy the behavior of similar systems in the past.
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One possible way to catch emergence in the act is by running an experiment that has never been done before, so there is no past version that is, no precedent for it to copy. Such an experiment might involve the creation of a highly complex quantum system, containing many components that exist in a novel entangled state.
If the principle of precedence is correct, the initial response of the system will be essentially random. As the experiment is repeated, however, precedence builds up and the response should become predictable … in theory. Although precedence can play out at the atomic scale, its influence would be system-wide, cosmic.
Getting the two classes of physics theories to work together, though important, is not enough, either. What he wants to know—what we all want to know—is why the universe is the way it is. Why does time move forward and not backward?
How did we end up here, with these laws and this universe, not some others? Like Hogan, he is less concerned about the outcome of any one experiment than he is with the larger program of seeking fundamental truths. For Smolin, that means being able to tell a complete, coherent story about the universe; it means being able to predict experiments, but also to explain the unique properties that made atoms, planets, rainbows, and people.
Here again he draws inspiration from Einstein. The most likely way to get the big answers is to engage with the universe as a whole. If you wanted to pick a referee in the big-small debate, you could hardly do better than Sean Carroll, an expert in cosmology, field theory, and gravitational physics at Caltech. He knows his way around relativity, he knows his way around quantum mechanics, and he has a healthy sense of the absurd: He calls his personal blog Preposterous Universe. Right off the bat, Carroll awards most of the points to the quantum side.
That has been the prevailing view ever since the s, when Einstein tried and repeatedly failed to find flaws in the counterintuitive predictions of quantum theory.
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No matter how the theories shake out, the large scale is inescapably important, because it is the world we inhabit and observe. Taking a larger view, the real issue is not general relativity versus quantum field theory, Carroll explains, but classical dynamics versus quantum dynamics. Relativity, despite its perceived strangeness, is classical in how it regards cause and effect; quantum mechanics most definitely is not.
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Einstein was optimistic that some deeper discoveries would uncover a classical, deterministic reality hiding beneath quantum mechanics, but no such order has yet been found. The demonstrated reality of spooky action at a distance argues that such order does not exist.get link
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There are still huge gaps in what quantum theory can explain. In the case of time, then, the jury is still out. In essence, the universe as a whole is the answer, and the challenge to physicists is to find ways to make it pop out of their equations. Even if Hogan is right, his space-chunks have to average out to the smooth reality we experience every day.
Even if Smolin is wrong, there is an entire cosmos out there with unique properties that need to be explained—something that, for now at least, quantum physics alone cannot do. By pushing at the bounds of understanding, Hogan and Smolin are helping the field of physics make that connection. They are nudging it not just toward reconciliation between quantum mechanics and general relativity, but between idea and perception.
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