Connectedness
You haven’t heard from me in a while because I was down with the flu followed by a serious wound infection. It was serious, because the blood-flow through my left body-half is slow due to my braintumor – which makes it difficult for my immune system to fight bacteria. Luckily, a week of antibiotics was enough to fight off the infection.
As I was recovering, I had to save my energy, so there wasn’t much I could do besides lying in bed and thinking. At such moments (when the pain is not too bad), I often marvel at the connectedness of everything I see around me.
At one point I was looking out my window, at a clear blue winter sky, and I was reminded of Carlo Rovelli’s article “Space is blue and birds fly in it“. Moments like this remind me that abstract concepts like Einstein’s space and time not only exist in theoretical physics, but they are everywhere around us – the clear blue winter sky is actually the expanding deep space of Rubin, Hubble and Lemaitre – the space Einstein once believed to be characterized by the cosmological constant.
Einstein’s greatest mistake
Einstein introduced the cosmological constant to save his idea of a static universe: a universe that doesn’t change in size. If gravitation pulls all mass and energy together (just as the sun and the earth pull at each other), there has to be something that prevents the universe from collapsing in on itself. That something, Einstein believed, is what the cosmological constant, $\Lambda$, describes.
But when the idea of a big bang got hold of the scientific community – the idea that the universe is not static but expands, he called the idea of a cosmological constant “his greatest mistake”.
Later, when it was discovered that the universe’s expansion is accelerating (a discovery that is eyed with suspicion by some, as you can read here), Einstein’s cosmological constant was reintroduced to explain this acceleration (“dark energy is the name that astronomers gave to the mysterious something that is causing the universe to expand at an accelerated rate” [nasa.gov/dark-energy])
$\Lambda$ in quantum field theory
Besides Einstein’s general relativity theory, the cosmological constant pops up in an adjacent field of physics: quantum field theory. According to the uncertainty principle, what we conceive of as empty space (vacuum) is actually a seething mass of virtual particles appearing and disappearing everywhere all the time. The energy related to this virtual sea of particles is described by… drum roll please…the cosmological constant!
Experimental tests: the Casimir force
The quantum vacuum isn’t just a theoretical fantasy: there is a way to experimentally measure this vacuum energy. The prediction is that when two parallel metal plates are very close to each other there will be a force due to the quantum fluctuations that pushes the plates together – the Casimir force.
One evening when I was thinking about these things, sitting in a comfortable chair with a glass of whiskey, I sent an email to Nobel laureate Gerard ‘t Hooft (whom I know well because he wrote the preface to my book) in which I asked him whether he believed that the quantum vacuum is real.
Gerard ‘t Hooft: vacuum energy is real
The time was 11:00 PM, but he emailed me back within 20 minutes: “Yes I believe there is energy in the vacuum, but whether you should call it a quantum vacuum is a matter of taste (Gerard believes, like Einstein once did, that quantum theory yields an incomplete description of reality – we should keep searching for a more fundamental sub-quantum theory).
Physics can’t tell us anything with certainty, so I don’t know whether the vacuum of space is a quantum vacuum – we don’t even know for certain what space is… but I can’t help feeling amazed that vacuum energy ($\Lambda$) – whatever it is or however it works, pops up in theories as diverse as general relativity and quantum field theory.
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