When a star is large enough and collapses upon itself, it becomes so dense that it becomes a black hole, a supermassive super-dense object from which no light can escape once it passes a critical threshold known as its event horizon.

The fact that nothing can escape once it goes into the black hole can appear very troubling to a physicist, because that seems to contradict the second law of thermodynamics, which states that in an isolated system that isn’t stable, entropy will increase over time. A black hole would violate this law because information about the parts of objects falling into a black hole apparently would be lost, including information about the objects’ entropy.

This problem was addressed by Stephen Hawking, who described Hawking radiation. Particles and their anti-particles spontaneously arise throughout all of space all of the time, and then quickly annihilate each other. These are called virtual particles because they continually appear and disappear like phantoms. Hawking showed that near the event horizon of a black hole, one of these particles could get sucked in and the other could escape, remaining a real particle permanently. Such a particle would be a sort of radiation unique to black holes, and would be drawing upon the well of energy in the space-time of the event horizon, such that the black hole would eventually evaporate from this radiation.

Then this phenomenon was described even more deeply by Gerardus ‘t Hooft, who devised the Holographic principle. He showed how a particle going into a black hole gave a boost to the gravity of the black hole as it got close to the even horizon, generating a little bump in the event horizon that uniquely described the particle that went in. And these bumps completely determined the characteristics of Hawking radiation particles being emitted. So the information going into the black hole, including information describing the entropy of the particles, was not being destroyed. It was encoded entirely in the shape of the black hole’s event horizon. The event horizon was in fact a hologram describing all that the black hole contained.

This revelation, that to be influenced by forces from any 3-dimensional volume is in fact better described as being influenced by the information encoded in the volume’s surface, was tremendously useful, and it has suggested answers to more and more fundamental questions. Most importantly, it was applied to the event horizon of our own universe, the curtain of light that appears in every direction, 13.75 billion light years distant. The holographic theory of the universe holds that this vast bumpy curtain encodes all the information that the universe contains. From different places in the universe, the event horizon’s bumps look different, and indeed are different than over there, encoding the information for this particular spherical volume of space-time instead of that one.

This has some significant implications. First of all, it lowers the maximum amount of entropy that the universe can contain. A planck length, about 10-35 meters, is the shortest distance that in quantum physics could be said to have any meaning. All the matter and energy of the universe can contain some maximum number of degrees of freedom in its configuration, some maximum entropy, physically limited by classical quantum physics to one bit of information per planck-length cube of volume. But in the holographic principle, in a black hole for example, those bits are encoded on a surface, not in the volume. There would necessarily be a lot less room for information. And in an event horizon like that of our universe, things would be expected to be noticeably more blurry than the scale of the planck length.

In fact, we would predict that the smallest observable length to have any concrete meaning would be in the area of 10-22 meters. And that, it just so happens, is how far the GEO600 graviton detector is able to see before encountering white noise.

Now this isn’t proof of a holographic universe, because it’s just noise that this instrument is detecting, minute perturbations in the distances between lasers. It could be noise from some other source, or just faulty equipment. But there’s another experiment on the way to test it. Another unit, planck-time, is the time it takes for light to travel one planck length. It is 10-43 seconds. Craig Hogan, a particle astrophysicist at Fermilab in Illinois, is building the most accurate clock ever. He aims to reach the barrier with his clock below which time should not be able to be measured in a holographic universe.

Hogan is building two quantum logic clocks, one on top of the other. If indeed the nature of the universe is holographic, then his two laser beams which are split apart and then put back together will not quite be in step with each other, and he will be able to detect their interference. He may even be able to peer just deep enough with his Holometer to get some measurement of the pixelation of the universe, if it is there.

Now aside from these attempts at direct measurement, there is another sort of evidence that I find very compelling. And that is the resolution of the humorously named “fine-tuning problem.” Basically the combined weight from the mass of the virtual particles from the expected vacuum energy of the universe is so very much that it ought to collapse in on itself. Even the vacuum energy of a small region of space should be enough to collapse the whole universe. The expected amount of vacuum energy is so stupendously, colossally wrong that this very very big problem is given the tongue-in-cheek name “the fine-tuning problem.”

Using quantum field theory, we have a very high estimate for the number of degrees of freedom for all energy in the universe. Where L is the diameter of the universe and ℓ is the planck length, the number of degrees of freedom is (L/ℓ)3. But in the holographic theory of the universe, we would arrive at one planck energy quantum per volume of size L2ℓ. This just happens to be the upper limit for the universe before it would collapse.

And so we are able to make the reasonable conjecture about the nature of dark energy that we couldn’t make in quantum field theory. Dark energy is the vacuum energy of space-time.