 You may recall from our video on the Higgs boson that empty space is not actually empty. It is filled with matter and energy fields. We model the waves in these fields as quantum harmonic oscillators. And given the Heisenberg uncertainty principle, the zero point energy for any wave in the field must be greater than zero. In the 1940s, a physicist named Hendrik Kazemir proposed that this zero point energy was real and for the electromagnetic field it could be measured. If you place two parallel low mass conducting surfaces close to each other, the fluctuations in the quantum field will be limited in between the surfaces but not outside the surfaces. This will create a negative pressure on the surfaces and push them together. Negative pressure is called tension. This effect is called the Kazemir effect. It wasn't until the 1990s that instruments sensitive enough to register the very small amount of force involved were available. Here we see that the force on one square centimeter plates placed one micron apart is equivalent to one one hundredths the weight of an average mosquito. The Kazemir effect shows that the vacuum energy is real, it's small, and it has negative pressure. But this is only for the electromagnetic force field. It's only one of a number of fields filling empty space. We do not know how many there are, so we can't predict the total amount of vacuum energy.