# The quantum-relativity connection

Quantum physics and general relativity are fundamental theories of physics that govern how things behave on microscopic scale, and planetary scale (respectively), but they couldn’t be any more different. The big question is, what connects these theories? If they are supposed to describe physics, then they must be connected in some fundamental way. There’s been a lot of research in the past decade or so that attempts to find that connection. As an outsider to the field of physics, what I didn’t know before reading a *Nature* article on the subject was that this research has been fruitful (however, it’s still conjecture, and not applicable to our universe). I’ll give a very layman overview of what I understood from the theory.

(Disclaimer: I know very little physics, so if you’re educated on the subject then I recommend you read the article, because here I’ve stripped down many concepts that I wasn’t familiar with, and I could have some things quite wrong.)

The theory that attempts to connect quantum physics and general relativity says that quantum entanglement knits together the space-time fabric of the universe. This theory is put forward by Mark Van Raamsdonk, building off the ideas of Juan Malcadena. Malcadena discovered that two stripped-down models of the universe, one governed by Einstein’s relativity and the other governed by quantum theory, are actually equivalent. The relativity universe is basically a 3-D volume of air, and the quantum universe is just a 2-D surface that encloses the air (think of a balloon). So, the first model is referred to as the **bulk** and the second one is the **boundary**. They’re equivalent (crazy!) and there’s actually a mapping between certain types of bulk universe to certain types of boundary universe.

By the way, I’ll try to explain quantum entanglement. In quantum physics, strangely, the *act of observing* a particle causes it to change it’s behaviour. Let’s say we cared about whether a particle was spinning upwards or downwards. Quantum theory suggests that before we observe, or measure the particle, it is spinning both up **and** down at the same time. The up and down states are said to be in a “superposition”. When we observe the particle, that superposition collapses and we see that it’s spinning just up or down. Entanglement takes this further by introducing a second particle. There are some phenomena where two particles get “entangled”, and what this means is that when we observe one of those particles, thereby collapsing its superposition, we actually collapse the superposition of the other particle as well. We can even set up the entanglement such that if we measure the first particle to be spinning up, then once we measure, the second particle **must** be spinning downwards. The craziest thing about this concept is that it still holds true, even if the particles are on opposite ends of the universe. And the behaviour of the other particle is always the same and instantaneous – so if the other particle is 1 light-years away, they are somehow transporting information *faster than the speed of light*. This stuff is so weird.

So anyways, Van Raamsdonk looked at an empty bulk universe and mapped it to the corresponding boundary universe. Then he looked at entanglement, and asked, if the boundary universe had no entanglement, what would happen to the bulk universe? Turns out that as you reduce entanglement in the boundary universe, the bulk universe stretches out like a piece of gum, and when entanglement is zero, the bulk universe is split into two parts. Therefore: entanglement is necessary for space to exist!

Another attractive sidenote to this theory is that entanglement explains wormholes too. A wormhole is a “tunnel” in the space-time between two black holes somewhere in the universe. This theory leads physicists to believe that these wormholes are really just entanglement happening on a massive scale, because they look fundamentally the same otherwise!

The theory is not just very exciting, but it has also resonated in other fields of physics. The nature of the entanglement reminded researcher Brian Swingle of his work in tensor networks while studying condensed-matter physics. It also recalls quantum error-correcting codes, which are used in the theory of quantum computation. What I’m most excited about however, is when Leonard Susskind introduces the notion of computational complexity, and suggests that it may be the key to understanding how the boundary universe changes over time, which is not addressed in the rest of the theory.

In short: I was shocked to learn of all of the fascinating discoveries that have been made to connect the seemingly disjoint theories of physics! Next on my reading list is Theoretical physics: Complexity on the horizon which promises to expound on the computational complexity linkages. Very exciting stuff!