Understanding Quantum Mechanics #6: It’s not just a theory for small things.

[This is a transcript of the video embedded below. Some parts of the text may not make sense without the graphics in the video.]

One of the most common misunderstandings about quantum mechanics that I encounter is that quantum mechanics is about small things and short distances. It’s about atomic spectral lines, electrons going through double slits, nuclear decay, and so on. There’s a realm of big things where stuff behaves like we’re used to, and then there’s a realm of small things, where quantum weirdness happens. It’s an understandable misunderstanding because we do not experience quantum effects in daily life. But it’s wrong and in this video I will explain why. Quantum mechanics applies to everything, regardless of size.

The best example of a big quantum thing is the sun. The sun shines thanks to nuclear fusion, which relies on quantum tunneling. You have to fuse two nuclei together even though they repel each other because they are both positively charged. Without tunneling, this would not work. And the sun certainly is not small.

Ah, you may say, that doesn’t count because the fusion itself only happens on short distances. It’s just that the sun contains a lot of matter so it’s big.

Ok. Here is another example. All that matter around you, air, walls, table, what have you, is only there because of quantum mechanics. Without quantum mechanics, atoms would not exist. Indeed, this was one of the major reasons for the invention of quantum mechanics in the first place.

You see, without quantum mechanics, an electron circling around the atomic nucleus would emit electromagnetic radiation, lose energy, and fall into the nucleus very quickly. So, atoms would be unstable. Quantum mechanics explains why this does not happen. It’s because the electrons are not particles that are localized at a specific point, they are instead described by wave-functions which merely tell you the probability for the electron to be at a particular point. And for atoms this probability distribution is focused on shells around the nucleus. These shells correspond to different energy levels and are also called the “orbitals” of the electron, but I find that somewhat misleading. It’s not like the electron is actually orbiting as in going around in a loop.

I get frequently asked why this is not a problem for the orbits of planets in the solar system. Why don’t the planets emit radiation and fall into the sun? The answer is: They do! But in the case of the solar system, the force which acts is not the electromagnetic force, as in the case of the atom, but the gravitational force. Correspondingly, the radiation that’s emitted when planets go around the sun is not electromagnetic radiation, but gravitational radiation, which means gravitational waves. These carry away energy. And this indeed causes planets to lose energy which gradually shrinks the radius of their orbits.

However, the gravitational force is much, much weaker than the electromagnetic force, so this effect is extremely small and it does not noticeably affect planetary orbits. The effect can become large enough to be observable if you have a system of two stars that circle each other at short distance. In this case the energy loss from gravitational radiation will cause the stars to spiral into each other. Indeed, this is how gravitational waves were first indirectly confirmed, for which a Nobel Prize was handed out in 1993.

But this brings up another question, doesn’t it. Why aren’t the orbits of planets quantized like the orbits of electrons around the atomic nucleus? Again the answer is: they are! It’s just that for such large objects the shells are so close together that the gaps between them are unmeasureably small and the wave-function of the planets is very well localized. So it is an excellent approximation to treat the planets as balls – or indeed points – moving on curves. For the electron in an atom, on the other hand, this approximation is terribly bad.

So, all the matter around us is evidence that quantum mechanics works because it’s necessary to make atoms stable. Does that finally convince you that quantum mechanics isn’t just about small things? Ah, you may say, but all this normal matter does not look like a quantum thing.

Well, then how about lasers? Lasers work by pumping energy into a crystal or gas that makes the electrons mostly populate unstable energy levels. This is called “population inversion.” If one of the electrons drops down to a stable state, that emits a photon which causes another electron to drop, and so on. This process is called “stimulated emission”. Lasers then amplify this signal by putting mirrors around the crystal or gas. The light that is emitted in this way is coherent and very strongly focused. And that’s thanks to quantum mechanics because if the atomic energy levels were not quantized this would not work.

Nah, you say, this still doesn’t count because it is not weird. Isn’t quantum theory supposed to be weird?

Ok, so you want weird. Enter Zeilinger. Anton Zeilinger is famous for, well, for many things actually. He’s been on the hotlist for a NobelPrize for some while. But one of his most famous experiments is showing that entanglement between photons persists for more than one-hundred kilometers. Zeilinger and his group did this experiment between two of the Canary Islands in 2008. They produced pairs of entangled photons on La Palma, sent one of each pair to Tenerife, which is one-hundred-forty-four kilometers away, and let the other photon do circles in an optical fibre on La Palma. When they measured the polarization on both photons, they could unambiguously demonstrate that they were still entangled.

So, quantum mechanics is most definitely not a theory for short distances. It’s just that the weird stuff that’s typical for quantum mechanics – entanglement and quantum uncertainty and the ability of particles to act like waves – are under normal circumstances really really tiny for big and warm objects. I am here using the words “big” and “warm” the way physicists do, so “warm” means anything more than a few degrees above absolute zero and “big” means anything exceeding the size of a molecule. As I explained in the previous video in this series, it’s decoherence that ruins quantum effects for big and warm objects just because they frequently interact with other things, air or radiation.

But if you control the environment of an object very closely, if you keep it cool and in an ultra-high vacuum, you can slow down decoherence. This way, physicists have been able to demonstrate quantum behavior for big molecules. The record holder is presently a molecule made of about 2000 atoms or about 40,000 protons, neutrons and electrons.

An entirely different type of “large” quantum states are Bose Einstein condensates. These are clouds of atoms cooled to very low temperature, where they combine to one coherent state that has quantum effects throughout. For Bose Einstein Condensates, the record is presently at a few hundred million atoms.

Now, you may still think that’s small, and I can’t blame you for it. But the relevant point is that there is no limit in size or weight or distance where quantum effects suddenly stop. In principle, everything has quantum effects, even you. It’s just that those effects are so small you don’t notice.

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