Last week I explained what I argue to be the greatest theoretical challenge facing fundamental physics today; that the very concept of the spacetime continuum is flawed and in need of revision. This week I want to explain what I think is the very greatest challenge coming from the experimental and observational side. Science thrives on a dialogue between theory and experiment and when you put all this together you arrive, as I see it, at the most exciting time for theoretical physics for a century, perhaps even since the 17th century in terms of the expected level of shake-up.
The experiments and observations that I refer to do not relate to the Large Hardon Collider. While that should be interesting especially if they don’t find the Higgs particle … well the LHC is now broken for a few months and that gives us a chance to see what else is going on. What is going on is the possibility of testing physics at the Planck scale, i.e. at energies 10 million billion times greater than the LHC could ever produce. It’s a brand new field, hitherto considered by physicists completely impossible, called ‘quantum gravity phenomenology’.
Don’t worry, we won’t actually be producing energies that high on Earth in the near future, we will be turning to cosmology. But the energies available if we knew quantum gravity could be rather high. If you watch the SciFi Channel series Stargate Atlantis, the portal device is powered by a ‘zero point module’ that taps into the vacuum energy of completely empty space. I think it was Arthur C. Clarke who first brought this into fiction, but it was based on theoretical ideas at the time. One can give a simplistic estimate of this vacuum energy based on cutting off particle wavelengths at the ‘minimum wavelength’ of 10³³ cm and the size of the Universe. I do this in On Space and Time and it comes out naively as about 1094 grams of mass-energy per cubic centimetre of empty space. To put this in perspective, this is about 1088 (i.e.10,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000) times the energy consumption of the world in a year, in each cubic centimetre! You can think roughly of a kind of ’sea’ with the same density as Planck-scale quantum-black hole objects (as featured in my first post two weeks ago) perhaps making up the foam-like structure of spacetime which, at a distance, we see roughly as a continuum. But please don’t think take this too literally. This is more like a ‘sea’ of quantum fluctuations and may well be a theoretical artefact of the way we think about quantum mechanics.