Welcome to the world of the weird: Quantum Physics. So strange that it couldn't possibly be true, and yet it is...
"An informed list of the most profound scientific developments of the 20th century is likely to include general relativity, quantum mechanics, big bang cosmology, the unraveling of the genetic code, evolutionary biology, and perhaps a few other topics of the reader's choice. Among these, quantum mechanics is unique because of its profoundly radical quality. Quantum mechanics forced physicists to reshape their ideas of reality, to rethink the nature of things at the deepest level, and to revise their concepts of position and speed, as well as their notions of cause and effect...
Unlike general relativity, which grew out of a brilliant insight into the connection between gravity and geometry, or the deciphering of DNA, which unveiled a new world of biology, quantum mechanics did not spring from a single step. Rather, it was created in one of those rare concentrations of genius that occur from time to time in history. For 20 years after their introduction, quantum ideas were so confused that there was little basis for progress; then a small group of physicists created quantum mechanics in three tumultuous years. These scientists were troubled by what they were doing, and in some cases distressed by what they had done. The unique situation of this crucial yet elusive theory is perhaps best summarized by the following observation: Quantum theory is the most precisely tested and most successful theory in the history of science. Nevertheless, not only was quantum mechanics deeply disturbing to its founders, today-85 years after the theory was essentially cast in its current form-some of the luminaries of science remain dissatisfied with its foundations and its interpretation, even as they acknowledge its stunning power."
Quantum Mechanics has to be, for my money, the craziest thing humans have ever come up with. The implications are huge, but comprehension is very challenging precisely because our brains were not designed to think the way you need to think if you want to understand the way the universe works underneath the illusions and behind the curtain. The modern configuration of quantum physics was solidified between 1925 and 1928 by a host of brilliant and very young, unfettered minds: Pauli, Heisenberg, Born, Jordan, Schrödinger, Dirac, Fermi, Bohr. Ironically, having himself laid some of the early foundations for quantum theory, Albert Einstein never accepted it to the day he died. It was a rare case of old Al being just plain wrong. I can see why it is troubling to a great many people. Quantum theory raises a host of unsettling questions. It tugs at the very fabric of our notions of what reality should mean. It shows us a glimpse of a universe that is not neat and buttoned-up, but very messy, and very alien. I believe it's uncomfortable because all of our experience, everything we know, is of the "big world", where we only get to see the statistically normalized cumulative effects of what goes on at the subatomic level. Down there, effect doesn't always follow cause, and electrons can travel forwards or backwards in time with equal ease. (they flip their charges and are called positrons when they travel backwards in time). Quantum theory predicts (and measurements confirm) a universe that is far stranger than anyone had ever in their wildest dreams imagined.
I wanted to list a few of my favorite quantum oddities:
1. One Or The Other. The classical view of matter is that it is predictable. You can measure the position and momentum (or velocity) of matter (a car, say) using simple tools, and that measurement is very reliable. A car is where it is. And it's momentum is what it is. In the world of the very small though, that supposition breaks down spectacularly. Schrödinger showed that you can write an equation that describes every possible value of every observable quality of a given quantum system. This equation is known as the Wave Function or Wave Equation of a particle or system. And that sounds all peachy, one equation can tell you everything you need to know about a particular quantum system. Except there is a huge caveat in this devil's bargain and I'll explain it here. The probability of finding the POSITION of a particle (say, an electron) in a quantum system is given by the square of the magnitude of the function (stay with me, it's not that bad) - the POSITION of the particle is "spread out" over the 3-dimensional volume defined by the wave equation and the more spread out the volume is, the less probability you can locate it. Now, the MOMENTUM of the particle is given by the slope of the wave equation (really, stay with me). The steeper the slope, the less probability you can determine the MOMENTUM of a particle. What this all means is that to find the POSITION of a particle precisely, the wave function must be sharply peaked (to narrow down the volume it could be in). But a sharp peak requires a steep slope, and as described above, a steep slope means the spread in possible values for the MOMENTUM of the particle will be large. The opposite is also true. If you've stayed with me until now, here's the payoff: The Wave Function for a system will tell you the POSITION of a particle OR the MOMENTUM of a particle, but NOT both at the same time. Think about that one. There is NO WAY you can know those two properties of a quantum particle at the same time. If you know the position of an electron, you cannot know its momentum. And if you know the momentum, you cannot know its position. This rule is named for the first person to discover this implication of wave equations, it's called the Heisenberg Uncertainty Principle.
"The need to abandon a classical picture in which position and momentum can be determined with arbitrary accuracy, in favor of a blurred picture of probabilities, is at the heart of quantum mechanics."
2) No Sharing Allowed. When you apply the wave equation to the two electrons in a Helium atom, something very strange happens. The wave equation can tell you the positions of the two electrons, but it cannot determine which electron is which. If you were to interchange the particles, the Helium atom system should look identical. But because as stated above the position probability is based on a square, the implication is that the wave equation of the new exchanged system is either identical to or the negative of the original. If you stuff one electron into the quantum state another one occupies, the wave equation for the moved electron reverses sign. The implication of this is as follows: Two electrons in the same quantum state will have wave equations that are anti-symmetric (the negative of each other), and they will cancel out. Which collapses the probability of finding two electrons in the same quantum state down to precisely zero. This rule is called the Pauli Exclusion Principle. And it's a good thing this is true, as it accounts for the differences in size and properties between the different elemental atoms.
Ok, those are semi-strange, but a little abstract. How about something really weird?
3. Superposition. Ok, ready? A quantum system (like a Helium atom) can exist in any one of a number of stationary states, but also exists in a sum or superposition of ALL those states at the same time. What this means is that the system literally exists in every one of its possible states simultaneously. Multiple realities. Ok, that's strange enough, but just wait. When the system is observed (measured), it collapses down into one (and only one) of these quantum states. Which state it will collapse into varies with time and is always changing. So ponder that one: the quantum system exists in multiple realties at once, and does not commit to one reality until you ask it to. And which one it commits to depends on when you ask it.
3. Quantum Entanglement. This may well be the strangest and most thrilling result of quantum theory. It is possible to construct a system of two atoms in an "entangled" state in which the properties of the atoms are shared and locked together. This has been demonstrated many times. Whatever one atom does, the other one exactly mimics it and vice versa. Ok, strange enough, but now consider that separating the atoms does not break this entanglement - it knows no bounds - these atoms could be on opposite sides of the universe and the entanglement would remain solid. This behavior is unexplainable except through quantum theory, which predicts it. Lest you think this is all academic, the practical implications of entanglement are huge. It has already been used in quantum communications systems (as you can imagine...what one does, the other copies...) and a world of truly powerful quantum computer systems that take advantage of entanglement is just on the horizon.
Curiouser and curiouser.
Photo: "A representative radial wave function of two electrons scattered in the collision of an electron with a hydrogen atom."
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