A brief introduction to quantum mechanics and the interconnectedness of all things

I was privileged to give the eulogy at the funeral of my dear friend Nat Billington. To show that he had retained his sense of fun in the face of his terrible illness, he left instructions for some of the guests at the reception afterwards to give short speeches. This is the title he gave me, and approximately what I said.

I must confess that I didn’t know what Nat had in mind when I heard this title. Now that I’ve heard our other speakers, I think I can see what he was after.

Is anyone in the room a quantum physicist? That’s a relief.

This is going to require a brief history lesson, so please stick with me.

Once upon a time, we had no idea why or how things moved. We could see that things fell if we dropped them. We could see that birds could flap their wings and fly. We could see the sun, moon and stars move across the sky. We did not have a theoretical way to explain it.

Then, in the Seventeenth Century, Sir Isaac Newton developed his three laws of motion and his Law of Universal Gravitation, the building blocks for the framework of Newtonian Mechanics. Things accelerate in the direction of a force that’s applied to them, and carry on until another force makes them stop. This idea was simple and enormously powerful: it was the cornerstone of technological breakthroughs for three hundred years. It made sense at human scale and was consistent with how we experience the world. It also enabled you to win at billiards.

At the start of the Twentieth Century, Albert Einstein began to think about the speed of light and what happens when something travels at or close to it. His thinking led to his special and general theories of relativity, which connected motion and time. Einstein’s theory predicted that time would be altered in strange ways for anyone traveling close to the speed of light. That may seem esoteric and far removed from everyday life. Although we’re not conscious of the effects of relativity, we depend on them today in many ways. For example, the Global Positioning System that guided many of you here works by having a constellation of satellites containing very accurate clocks. If we listen to four of them and carefully measure the small differences between the times they report, we can work out how far away we are from each of them and, therefore, where we are on Earth. However, the accuracy required for this to work his phenomenal — so great, in fact, that we need to make adjustments for the imperceptible difference in the way the satellites experience time due to their motion around the Earth. If we did not do this, positions reported by GPS would drift by over 10km per day.

The seeds of quantum mechanics were sown at the around the same time, as physicists tried to explain other phenomena, such as the colour of light emitted by hot objects. They could make a calculation that fit the observations if they allowed the atoms in hot objects to have only a small number of different energy levels — that is, if they quantised the energy levels. This is counter-intuitive: at human scale, if we heat something up it simply gets a bit hotter; if we stop heating it up, it slowly cools down. We do not see objects jumping from one temperature to another with nothing in between. Yet, at the atomic scale, that is what this theory demanded.

There were many other observations that physicists were trying to understand. One of these needs me to describe an experiment carefully, so please try to form a picture in your head as I talk you through it.

Imagine, in a dark room, a light bulb. It casts a pool of light onto the wall. If I put a barrier in the way — let’s say a thick piece of card — light no longer reaches the wall. If I cut a thin slit in the barrier, light leaks through and I get a smaller pool of light on the wall.

With me so far?

Now, put another slit in the card. I see a second pool of light, right? It turns out that if I get the width of the slits and their spacing just right, I see something else: a pattern of bright and dark areas on the wall.

Why is this? It’s called an interference pattern. Put the bulb in the room to one side for a moment and imagine a smooth, calm pond. Drop a pebble into it on one side: waves ripple outwards. We see waves moving along the surface of the water, but if we look carefully at one spot in the water, all we see is that the water is moving up and down. Now drop a pebble into the other side of the pond and pretty soon the waves mingle. Now imagine we look at an area of the pond where the motion from both waves is synchronised. Both waves cause the water to rise together and to fall together. Then we’ll see an extra big wave at that point. Now imagine we look somewhere else where the rise from one wave coincides with the fall from the other. You can see that the motion cancels out, and in the midst of the apparently moving waves, we have created a patch of calm water that doesn’t move at all. This is called destructive interference.

Now go back to our patches of light and dark on the wall. Physicists explained this pattern in the same way. Light is a wave, they said, and the waves emerging from the slits are like the ripples on a pond. Where they strike the wall behind they are either synchronised and extra bright or out of synchronisation and, because of destructive interference, they cancel each other out and produce dark areas.

Where does quantum mechanics come into this? Physicists had also theorised that one of the effects of quantising energy is that light itself is quantised. That is, there is a minimum amount of light. This is called a photon.

When we were thinking of the light bulb emitting waves of light, it was easy to imagine the parts of the waves passing through the slits an interfering on the other side. If we replace the light bulb with a device that can emit one photon at a time, we would expect now to see that each photon either hits the barrier or goes through one or other of the slits. And this is indeed what happens if we put detectors just the other side of the slits—we can either see the photon lighting up the barrier or we detect it at one side or the other.

However, if we put back the original experiment and instead observe the photons striking the wall, we see an amazing result. If we add up the impacts over time we find the same pattern of dark and light areas emerges.

This is extraordinary. We only fired one photon at a time yet there is still interference. In the absence of any other source of light, we conclude, the single photon must have passed through both slits at once and interfered with itself.

How is this possible? According to quantum theory, a particle doesn’t have a single defined position. It has a probability function, which means that it has a varying likelihood of being in different places. Only by observing it—a destructive act—do you “collapse the probability function” and discover its actual location. Sounds weird? The physicist Niels Bohr said that “if quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.”

Newton helped us with billiards. Einstein helped us with GPS. And we rely on the results of quantum mechanics, too, in our daily lives, notably in the operation of the lasers in our DVD players and our networks.

Think again about that idea of a probability function for a minute. It means that there is a probability of finding a particle anywhere. It may be very small, but it isn’t zero. And like the photon interfering with itself, there is also a probability of any two particles interacting. It may be very, very small, but it isn’t zero. So in a real sense the universe is organised so that everything in it can potentially influence everything else. Researchers recently discovered the existence of “quantum biology”. It turns out that some of the mechanisms that make our cells work rely on particles moving across barriers that they are too big to pass through. They conclude that, at this tiny scale, the probability function for these particles makes them occasionally appear on the wrong side of the barrier. Our very lives depend on quantum mechanics.

This, I think, is what Nat wanted me to get across. All of us, the dead and the living, are connected. Not in a metaphorical way, but by the fundamental way in which the fabric of our universe is woven. It is measurably true that we all influence one another, and that the matter of which we are made carries the potential for anything.