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Published online 28 October 2009 | Nature | doi:10.1038/news.2009.1044
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An intergalactic race in space and time
A burst of γ-rays lets scientists test quantum theories of gravity.
Astronomers have used a high-energy burst of light from a distant galaxy to test the fabric of space and time. The work is the best test yet of attempts to create a 'theory of everything'.
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General relativity and quantum mechanics – I don’t understand what the problem is? Why can’t there be both? The first suggests post explosive order the second suggests ‘debris’ or pre- or post-explosive matter. Which came first if either? Why can’t you logically work it out that way round? If you have a yard stick why can’t you figure out everything in-between?
Dear Tina, your intuition is completely correct. Quantum mechanics and general relativity (including its special relativistic limit which also includes the Lorentz invariance) can be correct at the same time – they can be interpolated – without changing their principles in any way. And that's what string theory is doing.
It satisfies all the postulates of quantum mechanics exactly, and it also satisfies the Lorentz symmetry, the equivalence principle, and all other key conditions underlying general relativity – also exactly. It has to. Any sensible theory had to satisfy both sets of conditions.
String theory is the right interpolation (you rightfully demanded) between the two limits that are a priori independent, as you correctly say, and anyone who studies the interpolation carefully – and who knows how to do such things – will eventually find this fact, too.
The incorrect notion that quantum gravity has to violate some principles of relativity has been promoted by some people who are not good physicists in any sense. Good physicists have always known that this opinion was untrue or at least completely unjustified by the evidence, and the newest Fermi's empirical results remove any realistic doubts from this fact.
String theory has not been demonstrated experimentally to be in perfect harmony with quantum mechanics or any other theory; it's "postulated" to be in perfect harmony with quantum mechanics and is "postualted" to be the theory that links gravity to quanta, but it can never be experimentally demonstrated owing to Human Kind's technological limitations.
I should add, however, that a description of elementray particles as "points" is quite unbelievable and the vibrational energies depicted as "strings" does sound more realistic.
Just a simple point: quantum mechanics and string theory are two theories and whether two theories are in perfect harmony with one another is a theoretical question, not an experimental question. And the answer to this theoretical question has been demonstrated: string theory is in perfect harmony with quantum mechanics.
It's yet to be seen whether and when string theory will be experimentally demonstrated. "Naive", brute-force methods to test the theory seem to be manifestly undoable in practice but it is not true that "naive", brute-force methods are inevitably the only ones that exist.
The test appears to assume that the short- and long-wavelength gamma rays started out at the same time. Is the production process well enough understood to be reasonably certain of this?