This section has some further musings on the many worlds interpretation. One question is why it matters. What is wrong with postulating a fairy-tale collapse mechanism that makes people feel unique? The alternate realities are fundamentally unobservable, so in normal terms they do truly not exist. For all practical purposes, the wave function really does collapse.
The main reason is of course because people are curious. We would also want to understand what nature is really all about, even if we may not like the answer very much.
But there is also a more practical side. An understanding of nature can help guess what is likely to happen under circumstances that are not well known. And clearly, there is a difference in thinking. The Everett model is a universe following the established equations of physics in which observers only observe a very narrow and evolving part of a much larger reality. The Copenhagen model is a single universe run by gnomes that allow microscopic deviations from a unique reality following the equations of physics, but kindly eliminate anything bigger.
One major difference is what is considered to be real. In
Everett's theory, for an observer reality is not the complete wave
function but a small selection of it. That becomes a philosophical
point when considering vacuum energy.
According to
quantum field theory, even empty space still contains half a photon of
electromagnetic energy at each frequency, {A.23.4}. That
is much like a harmonic oscillator still has half a quantum of kinetic
and potential energy left in its ground state. The electric and
magnetic fields have quantum uncertainty. If you
measure
the electric or magnetic field in vacuum, you
will get a nonzero value. The same applies to other fields of
particles. Unfortunately, if you sum these energies over all
frequencies, you get infinity. Even if the frequencies are assumed to
be limited to scales about which there is solid knowledge, there is
still an enormous amount of energy here. Its gravitational effect
should be gigantic, it should dwarf anything else.
Somehow that does not happen. Now, in Everett’s interpretation a particle only becomes real for a universe when a state is established in which there is no doubt that the particle exists. That obviously greatly limits the vacuum energy that affects that universe. The existence of other particles might be firmly established in other universes, but these will then affect those other universes. In the Copenhagen interpretation, however, there are no other universes, and therefore no good reason to exclude any vacuum energy from affecting the gravity of the only universe there is.
Then there is the arrow of time. It is observed that time has
directionality. So why does time only go one way, from early to late?
You might argue that early” and “late
are just words. But they are not. They are given meaning by the
second law of thermodynamics. This law says that a measurable
definition of disorder in the observed universe, called entropy,
always increases with time. The law applies to macroscopic systems.
However, macroscopic systems consist of particles that satisfy
microscopic mechanics. And the Schrödinger equation has no particular
preference for the time
The conventional view postulates ad-hoc asymmetries that “just happened” to be that way. Why would that happen and why would it be in the same direction everywhere in an infinite space-time and an infinity of possible universes therein?
Then the conventional view adds evolution equations that magnify that
asymmetry using small perturbation theory. That sounds reasonable
until you examine those evolution equations more closely, chapter
11.10. The mechanism that provides the increasing asymmetry
is, you guessed it, exactly that poorly defined collapse mechanism.
Collapse is simply stated to apply for times greater than the
measurement
time. Obviously that produces asymmetry
in time. But why could the collapse not apply for times less than the
collapse time instead?
Now stand back from the details and take a look at the larger philosophical question. The well established equations of nature have no particular preference for either direction of time. True, the direction of time is correlated with matter versus antimatter, and with mirror symmetry. But that still does not make either direction of time any better than the other. According to the laws of physics that have been solidly established, there does not seem to be any big reason for nature to prefer one direction of time above the other.
According to Everett's theory, there is no reason to assume that it does. The many-worlds interpretation allows the wave function to describe both universes that are observed to evolve towards one direction of time and universes that are observed to evolve in the other direction.
That is not a trivial observation. The problem of the observed time asymmetry for a symmetric physics has now been removed. It has been replaced by the question why forward evolving systems appear to correlate with forward evolving systems, and backward evolving systems with backward evolving ones. While that is not a trivial question either, it is not implausible.
Perhaps, if we spend more time on listening to what nature is really telling us, rather than make up stories for what we want to believe, we would now understand those processes a lot more clearly.