What is ‘space’? Is it empty, or is it full of stuff that provides a medium through which photons, electromagnetic waves and everything else can move?
That’s a question scientists have yet to find an answer to, but a new theory suggests spacetime itself might be a ‘liquid superfluid’.
And, if proven true, it could force us to take another look at the Standard Model of physics.
The idea of spacetime being a fluid, known as ‘superfluid vacuum theory’ (SVT), isn’t entirely new – it was suggested as long as half a century ago.
But researchers Stefano Liberati, a professor at the International School for Advanced Studies (SISSA) and Luca Maccione, a research scientist at the Ludwig-Maximilian University in Munich, are the first to tackle the question of the viscosity of such a liquid.
That is, they worked out how ‘thick’ the liquid would be – and they found it to be almost zero.
In their paper Astrophysical Constraints on Planck Scale Dissipative Phenomena, they tackle the question of spacetime being a fluid.
They did this by creating models attempting to fuse gravity and quantum mechanics into ‘quantum gravity’.
One of the biggest problems in understanding the universe has been trying to figure out how things move through space.
Consider how waves move through water, for example – the wave propagates through the water, using it as a ‘medium’ through which to move.
As far as we know, a transfer of energy of this sort requires a medium, like how sound passes through air or heat through metal.
How, then do electromagnetic waves, photons and so on move through space, where there is believed to be nothing?
The supposed existence of a medium in space is more commonly known as an ether, but proving or disproving its existence has been a struggle.
In Liberati and Maccione’s research, they suggest that this ether is in fact a superfluid.
They say to us, the spacetime just appears as one ‘classical’ object, something that is whole.
But instead we should consider it as being merely the ‘visible’ aspect of a fluid.
Consider, in the same way, how we perceive water – to us it appears to be a flowing liquid, but in actuality it is a mass of H2O molecules.
Spacetime, the researchers say, is made of its own H20 molecules – although what these would be is unknown.
A key point of evidence for their theory revolves around the four fundamental forces of the universe – electromagnetism, weak interaction, strong interaction and gravity.
Quantum mechanics is able to explain all of these except for gravity.
The researchers say that a plausible model for quantum gravity would be the superfluidity of the cosmos.
But to model it as a fluid requires us to understand its viscosity, how thick it is, which they estimate to be close to zero.
‘If spacetime is a kind of fluid, then we must also take into account its viscosity and other dissipative effects, which had never been considered in detail,’ explains Liberati.
The reason for a viscosity of almost is that this would allow photons and other particles to travel with ease.
‘We can see photons travelling from astrophysical objects located millions of light years away,’ continues Liberati.
‘If spacetime is a fluid, then according to our calculations it must necessarily be a superfluid.
‘This means that its viscosity value is extremely low, close to zero.
‘We also predicted other weaker dissipative effects, which we might be able to see with future astrophysical observations.
‘Should this happen, we would have a strong clue to support the emergent models of spacetime.
‘With modern astrophysics technology the time has come to bring quantum gravity from a merely speculative view point to a more phenomenological one.
‘One cannot imagine a more exciting time to be working on gravity.’