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What is between your hands?
Air of course, but what if you take away the air?
All you have left then is space, something we use to tell here from there.
But what is space?
It’s such a simple question, but no one has a good answer.
Einstein taught us that space must be joined with time to space-time.
From Einstein we also learned that space-time curves in the vicinity of masses which gives
rise to what we call gravity.
Einstein’s theory is called general relativity.
In general relativity, space-time is smooth like a rubber sheet.
Unfortunately, we know that this theory is only approximately correct.
The problem is that the masses which curve space-time have quantum properties.
This means they obey the uncertainty principle and can be in two places at once.
The curvature they cause, then, should have quantum properties too.
But in general relativity that isn’t so.
We therefore know that general relativity is incomplete.
It works well when the quantum effects of space-time are small, which is almost always
the case.
But when the quantum effects of space-time become large we need a better theory, a theory
of quantum gravity.
We don’t yet know the theory of quantum gravity, and therefore we don’t really know
what space, and time, are.
We have several candidate theories for quantum gravity, but none of them is generally accepted.
Nevertheless, from the existing approaches to quantum gravity we have some speculations
for what might happen with space and time:
First, we expect that in quantum gravity, space-time will fluctuate wildly even in the
absence of matter.
Second, space-time could be full with microscopic black holes or even weirder things like wormholes
or baby universes.
Third, since this is a quantum theory, it could do all these things at the same time!
Fourth In most approaches to quantum gravity, space-time
is not fundamental but made of something else.
That might be strings, loops, qbits or some variant of space-time atoms.
Fifth: In some of these cases space-time has properties
like a solid or fluid, it could be elastic or drag on stuff.
Sixth: Space-time might affect how light travels
through it.
It might not be entirely transparent, or light of different colors might travel at different
speeds.
Seventh:
Space-time fluctuations might destroy the ability of light from distant sources to create
interference patters.
Eighth:
In regions of strong curvature, time might turn into space.
This could happen for example inside of black holes or at the big bang.
Ninth: Space-time could be non-locally connected
with tiny shortcuts spanning throughout the universe.
Tenth: It might be that to combine quantum theory
with gravity, we do not have to update gravity but quantum theory.
And if that is so, the consequences could be far-reaching because quantum theory underlies
all electronic devices.
If has to be changed, this might open entirely new possibilities.
Quantum gravity, therefore, is not such a remote theoretical idea as it seems.
We all travel through space-time every day.
Understanding it could change our lives.