Physics has predicted all kinds of mind-bending things with astounding accuracy. Gravitational waves were dreamt up a century before they could be felt, and we can say with a good degree of certainty what is going on inside the sun. So it is as embarrassing as it is stunning that physics’ greatest failure is describing the simplest possible thing: nothing at all. 


When we look to the skies, we see that everything, everywhere and in all is rushing away from us. The universe is expanding. We call the driver of this expansion dark energy. It is important to understand that dark energy (and its sibling dark matter) are not theories, they are observations. We see their effect everywhere through our telescopes, we just don’t know what it is. With no known mechanism for the universe’s expansion, physicists have looked to the quantum world for an explanation.


A core principle of quantum physics is uncertainty. At any given time, we cannot with certainty put a specific number to the energy of a system, particle, or point in space. Zero is a specific number, so this seems to say that even the vacuum of empty space is nonzero. Space bubbles with indeterminate quantum energy fluctuations. This is (very roughly) how quantum field theory views the world. The suggestion: is this fluctuating ‘vacuum energy’ the origin of dark energy? If this were the case it would be a tremendous triumph, quantum mechanics verified on the scale of the observable universe. Careful measurement by astronomers put the density of dark energy at 200 pico-ergs per cubic centimetre. A volume of empty space one thousand times bigger than Earth contains enough dark energy to boil a kettle. Predictions from quantum theory put the vacuum energy at around 10^110 ergs per cubic centimetre. 

Invoking theories such as supersymmetry, which are not yet fully confirmed can lower this prediction to around 10^45 ergs per cubic cm. This absolute theoretical lower bound still predicts the volume of a single hydrogen atom to have about as much energy as the yield of the Hiroshima bomb. 


Needless to say, physicists are completely embarrassed by this level of disagreement. No other discrepancy even close to this magnitude is available for comparison. As it stands, it is an open question. Figure out where all that energy is (or isn’t), and you’ll probably get a letter from Stockholm…

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