Sunday, May 10, 2015

...and glory to the quantum!

(part 2)

If you’re still not convinced that the wave-moose, I mean, the wave-particle duality is confusing and not even well-defined, consider this: there's not even a consensus on how much of each part makes up the quantum entity (a bit like Jesus). According to Bohr, it is strictly a wave or a particle, never both. According to de Broglie it is both (a particle guided by a wave), and according to other people (like Penrose), it is neither (the duality principle is just an illustration, not reality). And intermediate positions have also been expressed by pioneering physicists. Here, I’m going to expand on the neither-nor viewpoint, which so far only says what the object is not, and not what it is.

'It' is a quantum. The word 'quantum' is now much more commonly used as an adjective than as a noun. However, in its initial meaning it is a noun, and when we describe how this noun behaves, we are describing its… mechanics! This is the way in which I understand the term Quantum Mechanics, and it's my conjecture that this is how it was (etymologically) meant to be understood. In fact, in old papers I have seen authors talking about 'the quantum' in the same way that we nowadays talk about 'the particle'. Now, of course words have no intrinsic meaning hard-wired into them - it is us who load them with meaning. In that sense, there is no a priori reason why a 'quantum' is a better term than a 'particle' or a 'qauntum particle', but there is a pretty good a posteriori reason: the word 'particle' is already loaded with too much meaning - it inevitably evokes a picture of something round, solid, and localized in a tiny region of space, and none of these properties are necessarily properties of the quantum. 

So what is the quantum? It's hard to define it with no mathematics and in just one sentence, but let's say that it's the smallest amount of energy that can exist on its own. However, there are different types of quanta – an electron quantum, a photon quantum, etc., which we commonly refer to as different particles. A crucial difference between the former and the latter is that the quantum does not need to be localized at a particular point of space. It could be, but it doesn’t have to be. In fact, its natural state is not localized, but due to the fact that quanta interact with each other, it's hard to isolate them into this natural state. Thus, in reality - and in experiments - a quantum often appears localized, but only because there is an external force that confines it. Now, it is often said - by people I do respect as scientists, mind you - that the de-localized nature of quantum objects is completely non-intuitive: here are just two examples where I recently heard that statement. The argument is that it is non-intuitive because we never experience anything like it in our lives. But this is a poor argument: the fact that the Earth is round and rotating, that a bowling ball and a feather in vacuum fall in the exact same way, or that a body in motion will stay in motion unless an external force stops it, those are all aspects of nature that we never experience directly but can only infer from observations. Yet we never say that they are non-intuitive, in fact we never even question them and have accepted them as almost mundane.

I see a sort of a vicious circle around Quantum Mechanics: by now it's practically a cliche to say that it is non-intuitive, and this is repeated and perpetuated every time the theory is mentioned. However, there is nothing intrinsically non-intuitive in the theory, because intuition - just like the meaning of words - can evolve, and has evolved many times in the history of science. However, a prerequisite for QM to ever become intuitive is that we stop reiterating that it's not. 

In fact, I think there is a curious analogy to be found in the history of science. Insisting that objects are naturally localized into 'particles' is very similar to insisting that an object's natural state is at rest. There are forces acting all around us, so it looks as if everything eventually comes to a rest, if there's nothing pushing it. But if you remove the surrounding forces (as Newton realized and postulated), an object will continue moving indefinitely. Everyone accepts this today, although it would have sounded very non-intuitive to Aristotle and everyone else in his generation. In the same sense, we are used to objects being localized, because electrons interact with nuclei to form atoms that interact with other atoms to form molecules that interact to form, well, everything around us. What Quantum Mechanics simply tells us is, the localized nature of everything around us is not its natural state; it is due to interactions. Remove interactions, and the 'particle' spreads out and is no longer a 'particle' in the sense we would typically attach to that word.

We actually do observe this all the time: light is made out of quanta that interact weakly with other quanta - in most situations, in fact, negligibly. This is why we are used to thinking of light as waves - it is much closer to its natural (by which I mean non-interacting), de-localized state, since there are no forces confining it. When we do the double-slit experiment that I outlined in part 1 of this post, the photon quanta first propagate freely and look like waves, but then they interact strongly with the detector in the end - e.g. a photographic film in which they get absorbed - making them appear localized. This property looks particle-like, but really we always have the same object - a qauntum - but in the presence or in the absence of interactions. Eventually, the jump in intuition required to accept that fact is no bigger than the one that was needed to accept Newton's principle that an object will keep on moving if there's no interaction to stop it. 

The current intuition that stuff should ultimately be localized comes, of course, from trying to imagine everything as particles. Why this obsession? Why think of anything as a particle? Why do we say it's hard to imagine a baseball as a wave? Why don't we just imagine everything as a wave, or better: as energy! That's what Einstein tells us anyway! (E = mc2)The Earth is not a hard sphere! Neither are atoms nor nuclei nor anything. They are all a bunch of energy clustered together because it attracts itself. How do you define 'hard' anyway? How would you define the size of a particle if you want to stick to that notion? You can never actually touch it, you can only get to a given distance before the repulsion gets too strong. The Earth appears hard because, while it attracts us on the large scale, it's repulsive on the short scale (due to electrostatic repulsion between our atoms and Earth’s atoms). Most of us know all of those individual facts, yet strangely cling to the image of everything as made out of tiny matter-balls that we call particles.

There's nothing that's actually solid. Matter is energy (is quanta of energy). If anything appears solid to you, it's because the energy that constitutes it is repulsive to the energy that constitutes your hand.

Maybe right now this sounds hard to grasp, and you're thinking -'what's the use, if it's abstruse?' (no rhyme intended). Could one argue that better intuition is gained in the wave-particle picture than in this 'quantum' picture? My answer to that is another question: why is then 'non-intuitive' the most common adjective assigned to Quantum Mechanics? My argument is that this is largely because of trying to describe an object by starting from the notion of a classical particle, and then outlining all the aspects in which it's not like one. Isn't it better to just define the object through its properties, complicated as they might be? Isn't that the way to break the vicious 'non-intuitive' circle?

I want to discuss in more detail said properties of the quantum, and the way it compares to the wave-particle view, as well as the difference between the quantum and the wave function. But in the interest of me being terribly late with new posts, I'll leave this for a future part 3.  

No comments:

Post a Comment