josh1

## Main Question or Discussion Point

The question isn't whether a theory is testable now, it's whether it's testable in principle. For example, there's nothing in our current understanding of string theory that precludes it from being falsifiable. The main problem for string theory is the same problem that continues to torture the entire field, namely, that we simply don't understand the gravitational physics of the vacuum.

For string theory this means that we don't yet understand how to break supersymmetry in a way that reveals the kind of low energy phenomenology it can describe. Similarly, for the theories that go under the rubric of "quantum geometry", it's preventing the discovery of what kind of low energy gravitational behaviour it has so that we have nothing to compare it to either. Thus at present, neither theory produces numbers that we can check agains experiment.

There's another source of confusion here. It has to do with statements like, here's an experiment we can perform to see if spacetime is discrete and hence if something like LQG is correct. The confusion is that a theory doesn't have to describe spacetime in the explicitly discrete terms that LQG does to be consistent with an experimental signature indicating some kind of discretenes. Any theory with a fundamental length will also predict such a result, including string theory. Another example is the resolution of gravitational singularities of one kind or another. We already know that singularities must be resolved somehow and there are at this time a lot's of arguments about how this might be achieved being put forward based in string theory, LQG, and I'm sure other approaches.

Let me just make one final point. In the past string theory and LQG have been described as complementing each other in that LQG shows how a background independent quantum gravity theory can be constructed, and strings shows how the low energy limit might be obtained from a quantum gravity theory. But this is misleading in that we don't know for a certainty that nature has no trick when it comes to gravity that shows that background independence isn't needed. However there can be no doubt about the reality of the low energy world we live. The truth then is that LQG tries to solve a problem that may turn out to have no basis in nature, while string theory solves a problem that every quantum theory of gravity must, and that at this time

For string theory this means that we don't yet understand how to break supersymmetry in a way that reveals the kind of low energy phenomenology it can describe. Similarly, for the theories that go under the rubric of "quantum geometry", it's preventing the discovery of what kind of low energy gravitational behaviour it has so that we have nothing to compare it to either. Thus at present, neither theory produces numbers that we can check agains experiment.

There's another source of confusion here. It has to do with statements like, here's an experiment we can perform to see if spacetime is discrete and hence if something like LQG is correct. The confusion is that a theory doesn't have to describe spacetime in the explicitly discrete terms that LQG does to be consistent with an experimental signature indicating some kind of discretenes. Any theory with a fundamental length will also predict such a result, including string theory. Another example is the resolution of gravitational singularities of one kind or another. We already know that singularities must be resolved somehow and there are at this time a lot's of arguments about how this might be achieved being put forward based in string theory, LQG, and I'm sure other approaches.

Let me just make one final point. In the past string theory and LQG have been described as complementing each other in that LQG shows how a background independent quantum gravity theory can be constructed, and strings shows how the low energy limit might be obtained from a quantum gravity theory. But this is misleading in that we don't know for a certainty that nature has no trick when it comes to gravity that shows that background independence isn't needed. However there can be no doubt about the reality of the low energy world we live. The truth then is that LQG tries to solve a problem that may turn out to have no basis in nature, while string theory solves a problem that every quantum theory of gravity must, and that at this time

*only*it does. In fact, it's when one comes to understand how string theory does this against the background of so many failed attempts, that the inevitability of string theory will begin to really impose itself on you.
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