At first glance,
there is not the slightest doubt: to us, the universe looks three dimensional.
But one of the most fruitful theories of theoretical physics in the last two
decades is challenging this assumption. The holographic principle asserts that
a mathematical description of the universe actually requires one fewer
dimension than it seems. What we perceive as three dimensional may just be the
image of two dimensional processes on a huge cosmic horizon. Up until now, this
principle has only been studied in exotic spaces with negative curvature. This
is interesting from a theoretical point of view, but such spaces are quite
different from the space in our own universe. Results obtained by scientists at
TU Wien (Vienna) now suggest that the holographic principle even holds in a
flat spacetime.
If quantum
gravity in a flat space allows for a holographic description by a standard
quantum theory, then there must be physical quantities. Especially one key
feature of quantum mechanics (quantum entanglement) has to appear in the
gravitational theory. When quantum particles are entangled, they cannot be
described individually (they form a single quantum object). The measure for the
amount of entanglement in a quantum system is called ‘entropy of entanglement’.
Researchers showed that this entropy of entanglement takes the same value in
flat quantum gravity and in a low dimension quantum field theory. This however,
does not yet prove that we are indeed living in a hologram but apparently there
is growing evidence for the validity of the correspondence principle in our own
universe.
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