Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [29]
1 See Chapter 7, “The Death of Space and Time.”
2 In fact, the quantum coins have to be created together, then separated, to show spooky action at a distance, which is another reason the tale of coins on different sides of the Universe shouldn’t be taken too seriously. As pointed out, it isn’t a well-thought-out story. It exists merely to convey one amazing truth and one amazing truth only—that quantum theory permits objects to influence each other instantaneously, even when on opposite sides of the Universe.
3 The information on the original particle, P, must be transmitted by ordinary means—that is, slower than the speed of light, the cosmos’s speed limit. So even if P and P* are far apart, the creation of P*—the perfect copy of P—is not instantaneous, despite the fact that communication between the entangled particles, A and P, is instantaneous.
4 It is worth emphasising that, even with entanglement, the most you can ever do is make a copy of an object at the expense of destroying the original. Making a copy and keeping the original is impossible.
6
IDENTICALNESS AND THE ROOTS OF DIVERSITY
HOW THE BEWILDERING VARIETY OF THE EVERYDAY WORLD STEMS FROM THE FACT THAT YOU CANNOT TATTOO AN ELECTRON
I woke up one morning and all of my stuff had been stolen, and replaced by exact duplicates.
Steven Wright
They came from far and wide to see it—the river that ran uphill. It flowed past the fishing port, climbed through the close-packed houses, before meandering up the sheep-strewn hillside to the craggy summit overlooking the town. Startled seagulls bobbed on it. Excited children ran beside it. And at picnic tables outside pubs all along the river’s lower reaches, daytrippers sat transfixed by this wonder of nature as beer crept steadily up the sides of their beer glasses and quietly emptied itself onto the ground.
Surely, there is no liquid that can defy gravity like this and run uphill? Remarkably, there is. It’s yet another consequence of quantum theory.
Atoms and their kin can do many impossible things before breakfast. For instance, they can be in two or more places at once, penetrate impenetrable barriers, and know about each other instantly even when on different sides of the Universe. They are also totally unpredictable, doing things for no reason at all—perhaps the most shocking and unsettling of all their characteristics.
All of these phenomena ultimately come down to the wave-particle character of electrons, photons, and their like. But the peculiar dual nature of microscopic objects is not the only thing that makes them profoundly different from everyday objects. There is something else: their indistinguishability. Every electron is identical to every other electron, every photon is identical to every other photon, and so on.
1
At first sight this may not seem a very remarkable property. But think of objects in the everyday world. Although two cars of the same model and colour appear the same, in reality they are not. A careful inspection would reveal that they differ slightly in the evenness of their paint, in the air pressure in their tires, and in a thousand other minor ways.
Contrast this with the world of the very small. Microscopic particles cannot be scratched or marked in any way. You cannot tattoo an electron! They are utterly indistinguishable.
2
The same is true of photons and all other denizens of the microscopic world. This indistinguishability is truly something new under the Sun. And it has remarkable consequences for both the microscopic world and the everyday world. In fact, it is fair to say that it is the reason the world we live in is possible.
THINGS YOU CAN’T TELL APART INTERFERE
Recall that all the bizarre behaviour in the microscopic world, such as an atom’s ability to be in many places at once, comes down to interference. In the double slit experiment, for example, it is the interference between the wave corresponding to a particle going through the left-hand slit and the wave corresponding to the particle