I’ve been thinking lately about quantum physics, a topic that seems to attract all sorts of crazy speculation and intense controversy, which seems inevitable. Quantum mechanics challenges our deepest held most cherished beliefs about how reality works. If you study the quantum world and you don’t come away deeply unsettled, then you simply haven’t properly engaged with it. (I originally wrote “understood” in the previous sentence instead of “engaged”, but the ghost of Richard Feymann reminded me that if you think you understand quantum mechanics, you don’t understand quantum mechanics.)
At the heart of the issue are facts such as that quantum particles operate as waves until someone “looks” at them, or more precisely, “measures” them, then they instantly begin behaving like particles with definite positions. There are other quantum properties, such as spin, which show similar dualities. Quantum objects in their pre-measurement states are referred to as being in a superposition. That superposition appears to instantly disappear when the measurement happens, with the object “choosing” a particular path, position, or state.
How do we know that the quantum objects are in this superposition before we look at them? Because in their superposition states, the spread out parts interfere with each other. This is evident in the famous double slit experiment, where single particles shot through the slits one at a time, interfere with themselves to produce the interference pattern that waves normally produce. If you’re not familiar with this experiment and its crazy implications, check out this video:
So, what’s going on here? What happens when the superposition disappears? The mathematics of quantum theory are reportedly rock solid. From a straight calculation standpoint, physicists know what to do. Which leads many of them to decry any attempt to further explain what’s happening. The phrase, “shut up and calculate,” is often exclaimed to pesky students who want to understand what is happening. This seems to be the oldest and most widely accepted attitude toward quantum mechanics in physics.
From what I understand, the original Copenhagen Interpretation was very much an instrumental view of quantum physics. It decried any attempt to explore beyond the observations and mathematics as hopeless speculation. (I say “original” because there are a plethora of views under the Copenhagen label, and many of them make ontological assertions that the original formulation seemed to avoid, such as insisting that there is no other reality than what is described.)
Under this view, the wave of the quantum object evolves under the wave function, a mathematical construct. When a measurement is attempted, the wave function “collapses”, which is just a fancy way of saying it disappears. The superposition becomes a definite state.
What exactly causes the collapse? What does “measurement” or “observation” mean in this context? It isn’t interaction with just another quantum object. Molecules have been held in quantum superposition, including, as a new recent experiment demonstrates, ones with thousands of atoms. For a molecule to hold together, chemical bonds have to form, and for the individual atoms to hold together, the components have to exchange bosons (photons, gluons, etc) with each other. All this happens and apparently fails to cause a collapse in otherwise isolated systems.
One proposal thrown out decades ago, which has long been a favorite of New Age spiritualists and similarly minded people, is that maybe consciousness causes the collapse. In other words, maybe it doesn’t happen until we look at it. However, most physicists don’t give this notion much weight. And the difficulties of engineering a quantum computer, which require that a superposition be maintained to get their processing benefits, seems to show (to the great annoyance of engineers) that systems with no interaction with consciousness still experience collapse.
What appears to cause the collapse is interaction with the environment. But what exactly is “the environment”? For an atom in a molecule, the environment would be the rest of the molecule, but an isolated molecule seems capable of maintaining its superposition. How complex or vast does the interacting system need to be to cause the collapse? The Copenhagen Interpretation merely says a macroscopic object, such as a measuring apparatus, but that’s an imprecise term. At what point do we leave the microscopic realm and enter the classical macroscopic realm? Experiments that succeed at isolating ever larger macromolecules seem able to preserve the quantum superposition.
If we move beyond the Copenhagen Interpretation, we encounter propositions that maybe the collapse doesn’t really happen. The oldest of these is the deBroglie-Bohm Interpretation. In it, there is always a particle that is guided by a pilot wave. The pilot wave appears to disappear on measurement, but what’s really happening is that the wave decoheres, loses its coherence into the environment, causing the particle to behave like a freestanding particle.
The problem is that this interpretation is explicitly non-local in that destroying any part of the wave causes the whole thing to cease any effect on the particle. Non-locality, essentially action at a distance, is considered anathema in physics. (Although it’s often asserted that quantum entanglement makes it unavoidable.)
The most controversial proposition is that maybe the collapse never happens and that the superposition continues, spreading to other systems. The elegance of this interpretation is that it essentially allows the system to continue evolving according to the Schrödinger equation, the central equation in the mathematics of quantum mechanics. From an Occam’s razor standpoint, this looks promising.
Well, except for a pesky detail. We don’t observe the surrounding environment going into a superposition. After a measurement, the measuring apparatus and lab setup seem just as singular as they always have. But this is sloppy thinking. Under this proposition, the measuring apparatus and lab have gone into superposition. We don’t observe it because we ourselves have gone into superposition.
In other words, there’s a version of the measuring apparatus that measures the particle going one way, and a version that measures it going the other way. There’s a version of the scientist that sees the measurement one way, and another version of the scientist that sees it the other way. When they call their colleague to tell them about the results, the colleague goes into superposition. When they publish their results, the journal goes into superposition. When we read the paper, we go into superposition. The superposition spreads ever farther out into spacetime.
We don’t see interference between the branches of superpositions because the waves have decohered, lost their phase with each other. Brian Greene in The Hidden Reality points out that it may be possible in principle to measure some remnant interference from the decohered waves, but it would be extremely difficult. Another physicist compared it to trying to measure the effects of Jupiter’s gravity on a satellite orbiting the Earth: possible in principle but beyond the precision of our current instruments.
Until that becomes possible, we have to consider each path as its own separate causal framework. Each quantum event expands the overall wave function of the universe, making each one its own separate branch of causality, in essence, its own separate universe or world, which is why this proposition is generally known as the Many Worlds Interpretation.
Which interpretation is reality? Obviously there’s a lot more of them than I mentioned here, so this post is unavoidably narrow in its consideration. To me, the (instrumental) Copenhagen Interpretation has the benefit of being epistemically humble. Years ago, I was attracted to the deBroglie-Bohm Interpretation, but it has a lot of problems and is not well regarded by most physicists.
The Many Worlds Interpretation seems absurd, but we need to remember that the interpretation itself isn’t so much absurd, but its implications. Criticizing the interpretation because of those implications, as this Quanta Magazine piece does, seems unproductive, akin to criticizing general relativity because we don’t like the relativity of simultaneity, or evolution because we don’t like what it says about humanity’s place in nature.
With every experiment that increases the maximally observed size of quantum objects, the more likely it seems to me that the whole universe is essentially quantum, and the more inevitable this interpretation seems.
Now, it may be possible that Hugh Everett III, the originator of this interpretation, was right that the wave function never collapses, but that some other factor prevents the unseen parts of the post-measurement wave from actually being real. Referred to as the unreal version of the interpretation, this seems to be the position of a lot of physicists. Since we have no present way of testing the proposition as Brian Greene suggested, we can’t know.
From a scientific perspective then, it seems like the most responsible position is agnosticism. But from an emotional perspective, I have to admit that the elegance of spreading superpositions are appealing to me, even if I’m very aware that there’s no way to test the implications.
What do you think? Am I missing anything? Are there actual physics problems with the Many Worlds Interpretation that should disqualify it? Or other interpretations that we should be considering?