According to some scientists, there is no such thing as empty space. What we have instead is called “quantum foam.” We can’t see it, but we just might be able to sense it.
The guy who came up with the term "quantum foam" is John Wheeler. In the “shut up and calculate” era of post-World War II era, he pushed both students and the world at large to keep thinking about Einstein’s theory of relativity and its consequences – so you know he was cool. He also had the middle name of Archibald – so you know he knew a thing or two about cool names. And so it’s natural that he used term “quantum foam” to describe one of the more perplexing ideas of physics.
The idea comes from the attempts to merge relativistic gravity with quantum mechanics. Gravity, Einstein proved, was a bending of the fabric of spacetime. It also behaves like a field. Place a point far away from the Earth, and it still will be part of the Earth’s gravitational field, but it will be out where the tug of gravity is weak. Place it close to the Earth, and the tug is stronger, and it will fall. Other planets warp spacetime and create their own gravitational tugs. So space isn't gravity-free, but a vast array of different gravitational tugs through which particles move. Pretty much everywhere that anything is placed, there is a gravitational field that it moves through.
Quantum mechanics doesn't work quite the same way. It is looked at as more point particles and waves, without fields. Quantum field theory attempts to look at space as another field that point particles move through. This is significant because it allows space to also be a field that point particles spring from. Although the idea of particles suddenly appearing seems nonsensical, it is not an unheard-of idea. And it's backed up by experimental evidence.
Scientists have observed quantum tunneling. This happens when a particle goes through a barrier that it should not have the energy to penetrate. It would be something like slowly rolling a soccer ball at a thick wall and watching it suddenly pop out the other side. Particles that do it must be getting a vast quantity of energy from nowhere. Physicists believe that, over short period of time, particles can suddenly “borrow” energy and tunnel out. The shorter the period of time, the more energy they can borrow. On a quantum scale, this isn’t so weird. Due to the Heisenberg Uncertainty Principle, the closer an object’s position is fixed, the more its momentum can fluctuate into unknown territory. If the particle’s position is definitely close to the wall, it might have the energy to tunnel through. And if the particle is going with a certain momentum, and you’re certain of that, its position might not be what you think.
We know that particles do make use of quantum tunneling, which means, from a conventional point of view, that over short periods of time they must “borrow” energy from the universe. And Einstein proved that energy and mass are equivalent. If the universe can borrow energy, why not mass?
Borrowing implies that the energy will be returned. The auditor of this particular debt is the law of conservation of energy. This is something that has also been observed. We don’t see energy popping up out of nowhere. We don’t see mass popping into existence. But then again, we’re not looking at small enough objects, or short enough time spans. If, when things get below a certain distance, energy can briefly pop into being, then so can particles. Those particles can have all different momenta, if they’re in existence briefly enough. As the spaces over which they appear get smaller and the time periods get shorter, the energy in the particles can get bigger and bigger.
Einstein showed that spacetime is a physical thing, and that it can get bent and stretched with mass and energy. These huge fluctuations in mass and energy over tiny, tiny distances have to churn it up. Over short enough distances in space, there would be tiny black holes and tiny planets, each stretching spacetime the same way that regular black holes and planets do. So as we zoomed in on spacetime, it wouldn’t be a smooth stretch of fabric deformed gently by large planets, it would be churned up, in constant motion, because of these tiny, but massive, fluctuations. Instead of fabric, we have foam.
There are ideas on how to “see” this quantum foam. They vary in technique. Some ideas, such as the randomly appearing and disappearing particles, have already been established. If two plates are brought close to each other, particles should be popping into existence on either side of them. Particles are also waves, and the distance between the plates limits the number of waves possible. The combined force of the short and long waves to either side of the plates will outweigh the force of the shorter waves inside them, and they’ll be forced together. This demonstration was actually done in 1997.
Others reason that the foaminess of the universe will have more of an effect on short, highly energetic light waves, which will have to navigate around the foam, than long waves. Short light waves should therefore be delayed by all these tiny detours, and should travel more slowly – on a macroscopic level – than long waves. This has not been established.