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Einstein, eggs, and why science needs philosophy.
Albert Einstein won the Nobel Prize in 1921, and Niels Bohr in 1922. They both won for their contributions to the emerging field of quantum mechanics, a subject they argued about for the rest of their lives. The theory’s predictions about how the universe behaves on the smallest scale are all spot on, so it seemed to be correct. But the big question was, “What does it mean?”
Einstein rejected the emerging consensus about what the theory said about the universe. It is not that the theory is wrong, but incomplete. There is more yet to be discovered, and when found, the usual way of understanding the world would be preserved.
Bohr, on the other hand, accepted the new theory as complete. The problem was that it was just a bunch of symbols, and determining what those symbols tell us about the underlying reality is not the job of the scientist. With quantum mechanics, scientists had to hand things over to the philosophers.
What is the line between science and philosophy? The story of quantum mechanics helps us see where it may lay.
Mechanics is the physics of how things move; think billiard balls or crash test dummies in colliding cars. In the 1680s, Isaac Newton’s three laws of motion seemed to complete the field. In 1905, Einstein’s relativity theory revised mechanics by reimagining space, time, and energy. Like Newton’s, his new laws were deterministic. Plug in the way the universe is at any time, and they will spit out exactly the way it was or will be at any other time.
But in the 1930s, probing atoms caused a new theory to emerge that further challenged our basic concepts. Consider the solar system model of the atom we learned in school with electrons orbiting a nucleus. It turns out that picture can’t be right. The electron has a negative charge, and a moving electric charge creates a magnetic field. That uses energy. Thus, the electron would constantly slow down and thereby spiral into the nucleus, collapsing the atom. Every atom in the universe should simultaneously disintegrate. Matter should not be possible.
But it is. How?
The answer won Bohr his Nobel prize. It turns out that electrons can only move in tracks located at specific distances around the nucleus. If the atom absorbs energy, say, by shining a light on it, the energized electrons will jump from one track to another. But electrons can only exist in those tracks. So, the electron would move from track to track without ever having been anywhere in between. That seems impossible. That is not how things move.
But it is. So, we needed a new theory of how things move, a new mechanics in which energy is absorbed and emitted in packets. Those packets had a smallest size, and all energy had to come in multiples of it. We always thought that energy was like butter. You can get any amount of butter you want. No matter how small the pat, you could always get half as much.
It turns out that energy is like eggs and not butter.
If you go to the store to buy eggs, on the other hand, you can’t buy half an egg. You could buy one, twelve, eighteen; any whole number, but it has to be a whole number. It turns out that energy is like eggs and not butter.
These eggs of energy were called “quanta” because they came in specific quantities. So, we needed a new theory of motion that explained how these quanta interact with matter, a mechanics for quanta, a quantum mechanics. Bohr worked on that with a collection of the greatest young minds in science.
The result was a strange theory governed by an equation discovered by Irwin Schrödinger, rejecting Newton’s and Einstein’s determinism. If you plug in everything about the world at a moment, what came out of Schrödinger’s equation was not a normal description in which each observable property, like position or velocity, has a definite value. Rather, the equation produces a list of every possible value coupled with a probability. We call this a state of “superposition,” that is, when a thing is in a state of superposition for an observable property, it simultaneously has every single value…to some degree.
This would be like going into Schrödinger’s Diner for breakfast. You would think the waitress would hand you a menu, and you would select a single item, say, scrambled eggs and toast with butter, which would then be placed in front of you to eat. But at this restaurant, what actually ends up on the table is every menu item at once, but none of them completely there.
If we take the observable property of position and use Schrödinger’s equation to determine where something is, we should get a specific place. But the equation actually determines that an object would sort of be every place it could possibly be at the same time.
This, of course, makes no sense to us because when we look at a thing, it is never in superposition. It always has one specific value; that is, it is always in a single particular place. But, and this is the weird part, we can never predict what place it will be. The best we can do is calculate the probability it will be there.
Regarding our metaphorical diner, we never get the weird combination of breakfast foods. We always end up with a particular menu item. But because this is a quantum restaurant, we are incapable of knowing which one it is going to be before it arrives.
This randomness is the roll of the dice in the famous Einstein quotation and why he thought the theory was incomplete. Objects must always really have a single position, and the theory just doesn’t have enough power to tell us which one. Thus, he considered the probabilities to be a weakness of quantum mechanics, not a feature of the universe.
However, there are experiments where the outcome only makes sense if the object, when unobserved, is actually in a state of superposition, that is, to literally have all the different values simultaneously. We have the bizarre situation in which Schrödinger’s equation, a seeming law of nature, only holds true for an object if we don’t look at it. But when we do, instantly, the law which gives us a superposed state for the object is violated, and the object immediately appears to have one and only one value for the measured property. We just cannot know with certainty which one.
What is going on? What does the underlying reality actually look like?
The move from the symbols of physical theory to a picture of the underlying reality requires an interpretation. For decades, scientists feuded over what the correct interpretation of quantum mechanics is.
John von Neumann argued that the correct interpretation is the one the theory literally describes. When a system is not observed, it is in every possible property state. The instant we observe it, however, that interaction causes the superposition to collapse into a single state, which is the result of the measurement.
The problem is that measuring is just bringing one physical system (the one being measured) into contact with another physical system (the measuring instrument). This creates a bigger physical system that should be subject to the same physical laws. What here would cause the collapse?
Nobel laureate Eugene Wigner noted that there is one element of the process that seems different. The measurement is conducted, and the results observed by a conscious physicist. Perhaps, he surmised, it is the interaction of the human mind with the physical system that causes the collapse from superposition into a single value.
Hugh Everett III had a different interpretation. Maybe there is no collapse. Perhaps the superposition is maintained. But the superposition now includes the physicist’s consciousness. The measurement makes the scientist’s mind a part of the system, and it also partakes of the superposition. The property only seems to have one value because the mind divides, each part connected to one possible value. Hence, measuring the universe creates a multiverse, each independent universe connected to a single possible value of the measured property.
David Bohm picked up on Einstein’s preferred interpretation with real particles on a giant undulating subspace which he called the “guiding field.” The waves of the guiding field move the particles around, and the distribution of particles shapes the guiding field.
The important upshot of this interpretation is that the particles are never superposed but always have a single value for their properties, and it is entirely deterministic. The probability is no different from that of ordinary physics. If we knew exactly how hard a coin was flipped and where the thumb pushed on it, we could calculate whether it would come up heads or tails. But we don’t have exact knowledge of these factors, so it seems random when it isn’t. The probability in quantum mechanics is, therefore, what Einstein suspected, simply the result of as-of-yet undetermined variables.
All three of these are different pictures, different possible universes governed by the laws of quantum mechanics. Which one is true?
Image: Neils Bohr, blog.4psa.com
Physicists developed these interpretations and fought vociferously over which one to accept. Bohr told them to pipe down. The job of the physicist, he contended, was to develop theories that predict the outcomes of experiments. When you have a theory that correctly tells us the outcome of every experiment you can throw at it, the way quantum mechanics did, then the scientists’ job was through. The physicists did their job, and it was time for them to go home.
But this leaves an important question unanswered. If this theory is true, what does the underlying reality look like? Does human consciousness collapse a probability wave? Do we live in a multiverse? Is there an interconnecting guiding field beneath everything? What is the universe really like?
These questions do not belong to physics, Bohr argued; that is philosophy. Not that philosophers are on their own in making this call. They cannot ignore the results of science. If quantum mechanics correctly predicts even the most unexpected phenomena, then there must be something to it. Quantum mechanics does have something to say about how the universe is.
But it does not say it out loud. Science constrains philosophy. Philosophers must understand the science and work from it. But the science is not sufficient to answer the question. That is the work the philosophers must do for us. To paraphrase Einstein, philosophy without science is blind, and science without philosophy is lame. Bohr is telling us that science must be the eyes of philosophy to tell us what appears to be out there. But without philosophy, science is lame; it cannot move forward to the understanding of what is really real.
So, what is the answer? Which of these interpretations accurately describes the underlying reality? That is something the philosophers today are wrestling with, as they should.
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