We’ll begin with a completely different way of dealing with the measurement problem. As you’ll recall, the conventional interpretation of quantum physics says that something dramatic happens to wave functions when a measurement occurs. Let’s consider the simple case of a single quantum particle. Before a measurement, its wave function may be spread over a wide range of positions and speeds, or be in a superposition of numerous quantum states. But when we measure position, we get exactly one position. If we measure angular momentum, the wave function instantly collapses to a single state with a definite value.
The initial wave function of the particle depends on how the physical system was prepared. The wave function contains all the information required to determine the probability of getting any result of a specific measurement. But even for our single particle, there is no way to tell beforehand what the result of any measurement will be because there is always a range of possible results.
Why one result should become reality rather than any other is the mystery. There seems to be no logical reason that this is the case. But there is a way to make this problem disappear completely: suppose every possible result of a measurement actually happens. This is the idea behind the many worlds interpretation of quantum physics, first formulated by the American physicist Hugh Everett III in 1957.
DEFINITION
The many worlds interpretation of quantum physics says that all possible results of a measurement actually do occur. At the instant of a measurement, the universe splits into multiple copies of itself, with each distinct result becoming part of one new world. Each of these worlds then has the potential to split again when future measurements are made. Thus it leads to countless worlds existing simultaneously, each with its own space, time, history, and observers.
Take Schroedinger’s cat, for example. When the box is opened, the seemingly arbitrary collapse into a dead or alive cat-state is a logical problem for the Copenhagen interpretation. This problem disappears if we hypothesize that both outcomes really do happen, and that the universe “splits” into two as soon as the box is opened. In one universe, the cat is deceased, while it lives on in another. Otherwise the two universes are identical at the moment the box is opened.
According to the many worlds interpretation, the moment we peek inside Schroedinger’s box, the universe splits into two: one in which the cat is alive and one in which the cat is dead. The universes are otherwise exactly the same.
We don’t need a formal lab experiment in order to create new universes in this interpretation. Events that would correspond to wave function collapse happen all the time. If Everett’s interpretation is correct, then there are a countless number of worlds, so many that they are essentially infinite. Many of these alternate universes would be very similar to ours, differing in only minor ways (e.g., just a few electrons misplaced here and there due to recent events). But other branches would be very different, having diverged a long time ago. Many small differences would pile up over the years, each with compounding consequences.
An important part of this theory is that after the split, there is no interaction between the different realities. That explains why we don’t detect these alternate worlds, but it also makes it difficult to test the truth of this interpretation. We can’t go somewhere and find these alternate universes, because all of space and time gets copied, including you and us.
ATOM TRAP
Sometimes the term parallel dimensions is used synonymously with “many worlds.” This can be a source of confusion since the term dimension has a specific, mathematically-based meaning. For example, we encounter three spatial dimensions each day. As we will see when we talk about string theory and supersymmetry, there is a place for additional dimensions in theoretical physics, but it has nothing to do with the measurement problem or the many-worlds interpretation. Moreover, Everett’s mathematical formalism calls for perpendicular worlds rather than parallel. This technicality ensures that you can’t easily hop from one world to another.
This alternative to the Copenhagen interpretation has had many critics as well as supporters. It is a logically elegant solution to the measurement problem, but at what cost? The universe we see around us is truly vast, and has been around for billions of years. Countless zillions of radioactive nuclei have decayed in all that time. According to quantum physics, each one of them could have decayed at a different time. In the many worlds interpretation, not only did they each decay at different times, but each time a whole new world was spawned! The number of distinct, noncommunicating worlds would be truly mind-boggling. Is logical consistency and definiteness worth such extravagance?
Some scientists have suggested it might be possible to communicate between two of these worlds that have only recently split, using technology of the near future. But until and unless that is demonstrated, it is hard for some to regard many worlds as a true scientific theory. This hypothesis makes no predictions that can be validated in our own universe. So far as we now know, none of these alternate realities have any effect on any other, even though there are a huge number of them.
Another problem with the many worlds interpretation is that it trivializes the concept of probability, which is a very important feature of quantum physics on a practical level. The predictions of quantum physics are all quantitative probabilities, ranging from zero to one (100 percent) and this is perfectly consistent with what we actually observe. When identical quantum systems are prepared, the frequency with which we observe the various outcomes agrees exactly with what quantum physics predicts. In the many worlds interpretation, everything that might happen actually does happen, so all probabilities are trivially 100 percent.
On the other hand, the many worlds view has been a gold mine for science fiction writers. Many of us enjoy imagining all sorts of “what if” scenarios at great length. It is fascinating to imagine what the world would be like if Abraham Lincoln did not succumb to his assassin’s bullet, or if the Axis powers had been victorious in World War II. It’s that much more entertaining if such worlds might actually exist. What’s more, the branching of alternate realities would get rid of the main logical problem with time travel. If you went back in time and accidentally killed your grandfather when he was a boy, you simply create another version of reality, not a logical conundrum.
The many worlds interpretation neatly sidesteps the measurement problem in quantum physics, and so it doesn’t have any special problem with the role of conscious observers. If we create a whole new world every time a wave function collapses (along with replicate sets of observers) it doesn’t really matter what causes the collapse. However, most other interpretations of quantum physics do have to deal with the question of consciousness sooner or later, so we’ll explore.
Leave a Reply