Lecture
On Quantum Physics

 

Physics claims to be the science that studies physical matter, so if we want to understand the nature of physical matter, physics would clearly be one place to look. A very quick and dirty (and grossly oversimplified) historical survey of what scientific physics has told us about physical matter during the past several centuries could be divided into three main periods: classical physics, molecular/atomic physics, and quantum physics. Let's look briefly at what each period has told us about the nature of physical matter.

Classical physics is probably best represented by the work of Isaac Newton -- especially his Mathematical Principles of Natural Philosophy. Classical physics has, for our purposes, two simple principles, viz., that physical matter exists in three separate states, solid, liquid, and gas; and that physical matter in these three forms operates according to certain immutable physical laws. That will be our too-simple summary of the basic principles of classical physics on the question of physical matter.

The question to ask yourself now is whether classical physics' description of how physical matter exists accords well with your common-sense perception of matter. I think it does. This is how physical matter actually looks and feels to us in our ordinary, un-critically-examined daily experience.

Molecular/atomic physics, the next major stage in the historical development of physics, tells us something very odd, however. It tells us that matter is not actually the way it appears to us when we're using just our ordinary, uncritical senses. According to this physics, matter is actually made up of extremely tiny particles called molecules, and those molecules are themselves made up of even smaller bits called atoms. Furthermore, according to this physics, even those tiny atoms are made up of yet smaller bits of matter called sub-atomic particles -- protons, neutrons, electrons, and so on.

What we ought to notice as most odd, though, is that the spaces between these subatomic particles are said to be enormous. An average atom, as you probably know from your physics or chemistry courses, is said to look a little like a miniature solar system. A central nucleus (itself made up of subatomic particles) is surrounded by negatively charged electrons that whirl about it in orbitals at huge distances from the nucleus. To give just a rough idea of how far away from the nucleus these orbitals might be -- and thus of how much empty space there is in an average atom -- if the nucleus were the size of a baseball, the first orbital might be several yards away from it, the next orbital might be a few hundred yards away from that, and the next orbital a few thousand yards away from that. The spaces between these circulating electrons, therefore, are very large, so it would be accurate to say that the average atom, like the average solar system, is made up mostly of space. It has very little actual matter in it.

In fact, the proportion of matter (or stuff) to space in the average atom is very small. One physicist estimated that in the average atom (an "average atom" is clearly only a hypothetical construct) the proportion of stuff to space is approximately the same as if one baseball were suspended in the air in the middle of a large baseball stadium like the Seattle Kingdome. One baseball in the midst of all that empty space would be approximately the same proportion of stuff to space that we would find in an average atom. So this means that an atom, like a solar system, is mostly made up of empty space with a relatively few very tiny particles circulating at great distances from each other.

That's what atoms are said to be, according to this physics, and molecules are just made up of bunches of atoms bound together by certain forces. So any given chunk of matter, like your table, for example, or your computer, or your elbow is made up of nothing but gazillions of atoms all hanging together with each other. So if each atom is almost all space, so is each molecule, and then so is the table and your elbow and your chair and the roof and floor of the room you're in right now. All made up mostly of empty space. Made up, in fact, almost entirely of space.

That's what molecular/atomic physics teaches us about what matter is really like. It basically says that yes, matter does exist, but there really isn't very much of it in any given physical object, and what you think of as matter is really almost all empty space. Matter is nothing like what your senses tell you it is. It's almost all empty space.

So let's ask now whether this account of what matter is truly like fits well with your normal sensory experience of the world. I think we'd have to acknowledge that no, this account of what physical matter is like does not fit well with my what my ordinary senses tell me. Our ordinary senses, according to this physics, do not actually give us an accurate picture of what matter is truly like. Our senses actually deceive us a bit here, probably because they are simply not fine enough.

According to this kind of physics, then, physical matter does exist, but there is not nearly as much of it as you might have thought there was. The chair you're sitting on, which you may have thought was solid matter, is actually almost all empty space, and so is your own physical body that's sitting there in the chair. And so is your shirt and the coffee mug and the mountains and the moon. All mostly empty space.

And then along comes quantum physics, a very strange bird indeed.

Quantum physics tells us that all those little things that we used to call subatomic particles are not actually truly particles at all.

What we used to call subatomic particles are not exactly particles, or "things," at all. They are not so much like little things, or little bits of stuff, as they are like little energies. We might perhaps refer to them as little packets of energy, except that that term might still give the impression that what we used to call subatomic particles were still particles, or at least were packages. To call them packets still might make it sound like they are little bundles or parcels or items of some sort, and that is precisely what quantum physics wants to avoid saying. So what we used to call subatomic particles are now instead to be called little amounts of energy. Just amounts of energy. The Latin word for "amount" is quantum (plural: quanta), so we will now refer to these... these... well, what we used to call subatomic particles, as quanta of energy, or amounts of energy.

So if we are going to speak accurately here we will not strictly refer to these subatomic "particles" as "particles" any more, because that would imply that they are things, stuff, matter. We will instead refer to them as quanta of energy. And quantum physics, at least when it is speaking strictly and precisely, does not want to imply that these quanta are "things," in the sense of material particles. And that is why today's physics and chemistry textbooks seldom draw atoms any more as little miniature solar systems. They now instead more often represent atoms as clouds of energies, concentrated in a more dense nuclear center and again concentrated in more and less dense orbital regions at some distances from the center.

Another question that we then need to ask about these little quanta of energy is whether they are even "existents" or not, namely, whether they are even something that exists. Quantum physics hesitates to refer to these little quanta as existents, and even refuses to say that they actually exist at some place at some time. Quantum physics prefers instead to say that these little quanta are more like probabilities than they are like actualities. They are said instead to have a tendency to exist at some place at some time. Current representations of atoms as clouds that are more and less dense in different atomic regions is an attempt to show that these little quanta -- what we used to call subatomic particles -- have only a given probability of existing in certain regions at certain times. In other words, they should not be conceived of as existents at all, but should rather be thought of as tendencies to exist. They should not be thought of as stuff at all, but only as probabilities, or tendencies, to exist.

This kind of physics does not seem to fit well with our normal perception of things at all. If this account of "physical" matter is actually true (as the quantum physicists say it is), and if our ordinary senses give us quite a different picture of how things are, then we are left with the question of which account of the physical universe we are going to believe, that of the physicists or that of our common sense experience.

In any case, we have, with quantum physics, left behind the over-simple concept of physical matter as just simply existing, and have instead come to see physical matter as made up only of quanta of energy with tendencies to exist. (And what kind of a thing is a tendency, anyhow?) If this account is true, then our images of physical matter may be little more than a kind of sensory illusion.

As Sir James Jeans, a prominent physicist earlier in the 20th century, expressed this idea:

The more we learn about the nature of the physical universe, the less it looks like a great machine and the more it looks like a great thought.

That is, the more that physics learns, the more it sees that the cosmos is not made up of simple physical matter like a great piece of ironworks all fit together with physical levers and material gears and wheels. The more we learn about it, the more the physical universe begins to look instead like a great cosmos made of thought-stuff. Or as Shakespeare (not a physicist) has said so aptly: "We are such stuff as dreams are made on; and our little life is rounded with a sleep" (The Tempest , IV,i,148).

So does physical stuff actually exist or not? Quantum physics suggests that physical matter, at least in the normal sense that we usually think of it, does not exist at all. There may be quanta and energies and tendencies and probabilities, but, odd as it might sound to naïve common sense, physical matter as we normally think of it does not seem to exist at all.