This page answers some common questions about the many-worlds interpretation of quantum mechanics.
If our universe were a two-dimensional sheet of paper, a three-dimensional space could contain an unlimited stack of such sheets. In the same way, adding just one more dimension would provide plenty of “room” for countless worlds. A natural candidate for that extra dimension is time.
Worlds that exist at different moments can also be regarded as part of this multitude. We usually imagine parallel worlds as worlds that coexist at the same instant, but the world ten minutes from now can also be thought of as another world.
Time, like distance, can be measured. Even so, I believe that time does not “exist” in exactly the same way that distance does. Different worlds exist at different moments, and we experience the flow of time as a transition from one world to the next.
Even if the universe continues expanding forever, time cannot be meaningfully measured unless entropy continues to increase. Once entropy reaches its maximum, the phenomenon we call the flow of time may disappear.
Some people argue that the number of worlds must be infinite. I suspect that it is finite. If an electron’s position could vary by an arbitrarily small amount, then each tiny difference could correspond to a different world, producing an infinite number of worlds. However, if there are minimum units of length and time, positions cannot differ below those limits, so the total number of distinct worlds would be finite.
A common concern is that energy would increase whenever worlds branch. However, when we discuss energy conservation, we do not add together the energies of the universe at different times. In the same way, we do not need to add together the energies of different worlds. Energy is conserved within each individual world.
Many people believe that other worlds can never be observed. I remain hopeful that a way to detect them may one day be found.
I feel that the path-integral formulation and the many-worlds interpretation are closely related. One difficulty is that quantum mechanics does not single out a unique basis. As a result, the state corresponding to each world depends on the basis we choose. If no particular basis is preferred, the state of a given world becomes ambiguous.
I suspect that a unique basis can in fact be chosen. Particle position is a natural candidate, and for spin, one might treat “positions” in an internal space as the relevant basis.
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