This needs props. A toy cat, a box, a Geiger counter, something to go over the camera and make it black.

We start with a cat. We put the cat in a box. We take a Geiger counter, and we hook it in the box so if the Geiger counter detects a decay in ﬁve minutes, it opens a sleeping potion in the box. So, classically, there are two possibilities, either we open the box to ﬁnd a very annoyed cat, or we open the box to ﬁnd a sleeping cat. Of course, the box needs to be sound proof, and otherwise completely insulated from us, or we ﬁgure out if the cat is asleep or awake before we open the box.

What really happens, so far as we can tell with Quantum Mechanics? Well, since a nuclear decay is a quantum event that has two ways to go, it does both. So inside the box, the cat is both asleep and awake. If the box is completely insulated, then we on the outside are looking at a box with a cat that is both asleep and awake, at the same time.

Now, what happens when we look inside the box? At that point, our own quantum reality branches, and there are now two of us, one who is entangled on the branch where the cat is asleep, and one who is entangled on the branch where the cat is awake. So now there are two Joshes, who were more or less exactly the same, up until I open the box, and one sees an awake cat, and one sees an asleep cat.

The world is not always as it seems. If we walk
around outside, it mostly looks like the world is
ﬂat, not a sphere. It doesn’t feel like we are
traveling hundreds of miles per hour around the
Earth when we are sitting still, but we are, as
the Earth spins, and the Earth itself travels
at about 67,000 miles per hour around the
Sun.^{1}
It doesn’t really feel like we are on a more or less
spherical ball going 67,000 miles per hour, but we
are.

There is another aspect to our world that
is not easily observable. It’s called quantum
mechanics.^{2}
As it was originally understood, for some physics, what
happened was random. If you set up the experiment
exactly alike, it would give diﬀerent results each
time you ran it. An example would be that a
uranium atom will randomly decay at some point,
but whether that is in one second or in a billion
years, cannot be predicted. Einstein did not like
this interpretation because it involved randomness
and said that “God does not play dice with the
universe.”^{3}
Quantum mechanics, as originally understood,
included randomness, unlike any other physics
theory.

Before I continue, I will note that there is
some controversy in how quantum mechanics is
interpreted, and also that quantum mechanics
and general relativity are in some cases
contradictory,^{4}
and so there must be a better theory out there than
what we have. We know that there is still more to
learn about physics.

Einstein died in 1955 and so unfortunately did
not live long enough to hear of a solution to the
randomness. In 1957 Hugh Everett III published his
Ph.D. thesis “On the Foundation of Quantum
Mechanics”. Everett showed that following the
Schrodinger equation of the wavefunction, it will
appear that the wave function has collapsed into a
random value, but what actually happens is that all
the possibilities continue and it only looks like some
possibilities have vanished causing a random
result.^{5}

Quantum-gravity theorist Bryce DeWitt originally complained to Hugh Everett that he agreed with the math but that it didn’t “feel like he was constantly splitting into parallel versions of himself.” Physicist Max Tegmark wrote:

Everett had responded with a question:
“Do you feel like you’re orbiting the
Sun at thirty kilometers per second?”
“Touché!” Bryce had exclaimed,
and conceded defeat on the spot.
Just as classical physics predicts
both that we’re zooming around
the Sun and that we won’t feel it,
Everett showed that collapse-free
quantum physics predicts both that
we’re splitting and that we won’t feel
it.^{6}

Later on, Bryce DeWitt named this the many
worlds interpretation of quantum mechanics.
Basically, every time that quantum “randomness”
would occur, instead, the world branches, and all
the possibilities happen. So there is no randomness,
the universe just splits into branches, one for each
possibility.^{7}
It feels random to us, because, say, we watch if an
atom decays in one half life 10 times. Most of the
time we will end up on a branch where sometimes it
did and sometimes it didn’t, and so it looks
random.

So, as an example, we have Casey at the bat. The
pitcher throws the ball, the ball ﬂies to home, and
Casey swings. In some quantum branches, Casey
hits the ball, and in others Casey misses. Where do
the quantum branches happen? I haven’t seen a
rigorous calculation of Casey at the bat, and there
are some unknowns that aﬀect the result such as
are protons eternal or do they have a half life? But
from a Heisenberg uncertainty standpoint, we can
predict where the ball will be with well under 1 mm
of uncertainty in location and 1 millisecond in time,
so to the accuracy needed to decide if the bat hits
the ball, the ball is eﬀectively deterministic.
From when the ball is thrown to when the ball
gets to the plate, Newtonian physics is a quite
accurate approximation and so both Quantum
mechanics and Newtonian physics would predict
the same path of the ball so far as hitting or
missing the ball are concerned. Where does most of
the uncertainty take place? In the two places
where a small change gets ampliﬁed. Human
brains are using chemistry, and at some point
what happens in them comes down to did a
certain neuron ﬁre or not? This depends on the
exact position of molecules in relation to other
molecules, and so is very dependent on small
changes in the positions, and can be diﬀerent
with high probability. So the quantum branches
split depending on which signals are ampliﬁed
and then are transmitted by the nerves to the
pitcher’s muscles and to Casey’s muscles. Of course
there is nothing magical about nerves, a photo
multiplier tube would have the same eﬀect of
ampliﬁcation.^{8}

Note that if a robot was designed to hit the ball, it could be designed to be eﬀectively perfect, to hit the ball in all the quantum branches or at least make a strike less than a one in a million event. Alternatively a robot could also be designed so in some quantum branches it hit the ball and in others it missed the ball. Of course, this robot would have a much lower batting average, compared to one designed to minimize quantum randomness in its decisions. Human brain design, to the extent that they are quantumly random, is sort of a design ﬂaw since evolution is an imperfect designer. Good decisions should be dependent on the evidence and careful thought. Randomness in making a decision is a ﬂaw. In baseball, the decision if and where to swing the bat needs to be done at nearly the limit of the human speed of thought, so there probably is some quantum randomness in it. As I see it, close decisions where it could have gone either way would have more chances of being diﬀerent in diﬀerent quantum branches.

What to does this all mean? Well, I suppose you can
use a quantumly random number generator to run a
lottery and then you can guarantee that everyone will
win.^{9}
On a slightly more serious note, you can just
pretend that Quantum Mechanics doesn’t exist.
Make decisions like there is only one result, and as
long as you aren’t doing something like transistor
design you will never know the diﬀerence. However,
it is interesting to know that there are lots of other
you’s out there. If you want to be sure to try
both paths, you could make decisions with a
Geiger counter, but unfortunately, each you
only gets to remember the branch you are on
since you very quickly becoming entangled with
one branch, and we can’t ﬁnd out about the
others.

This all comes out feeling very normal, just like walking on Earth doesn’t feel like walking on a merry-go-round even tho’ both are spinning.

I think it is a little comforting and a little scary that somewhere out there, there is a Josh who is living a life where everything physically possible has gone wrong, also a Josh who is living a life where everything physically possible has gone right, and every shade in between. The universe is vast, it contains multitudes. In one sense this is sort of a “ground hog day” movie reincarnation, you get to live your same life over and over with all the possible variations. In another sense this mirrors a heaven and hell view, except that everyone is in their own personal hell, and their own personal heaven, and all the span in between. However, unlike Buddhist or Christian concepts of reincarnation and heaven and hell, there is no underlying justice driving the result, just random-seeming physics.

Carl Sagan called living all these possible lives the Haldane
consolation,^{10}
but I am not sure how much of a consolation this is.
I suppose the most consoling thing I can say about
it is that for any branch where a young person dies,
there is almost certainly a branch where they
live.^{11}

In philosophy club, I mentioned that I think there are two diﬀerent deﬁnitions of when someone dies. The ﬁrst deﬁnition is someone dies when they die in a single branch. The second deﬁnition is that someone dies when they die in the last branch.

I suspect that there are many branches where humanity or the vast majority of people do not exist. For example, there have been multiple chances for the United States and the Soviet Union to have an all-out nuclear war. The fact that we do not remember it does not mean it hasn’t happened, it just means that it hasn’t happened on this branch.

This might result in people ﬁnding themselves luckier
than would be physically expected for near death
experiences,^{12}
since all the branches happen, so if there is a
branch where you live, you will perceive it as
just a near miss, even if it was only a one in a
million chance that you could survive. Humanity
as a whole will ﬁnd ourselves luckier than we
really are since we can look back and see that
we have survived every single time, when in
reality the past might be littered with multiple
times where on other branches humanity went
extinct.

I personally ﬁnd it fascinating that all the quantum possibilities happen. This is both hopefuland terrifying. There are both branches where things are wonderful, and branches where things are horrible.

I was asked if I only believe in multiple worlds because I am afraid of death. I don’t think so; I think that multiple worlds makes sense scientiﬁcally; I don’t think multiple worlds is just wishful thinking. What it does do for me, is even when I am most pessimistic, and think that there is almost no chance that humanity survives, that anything I care about will survive, I can be pretty sure that there will be a few branches where humanity or their descendants survive. So it does give me some hope.

^{1}https://www.scientificamerican.com/article/how-fast-is-the-earth-mov/

^{2}One good introduction is The Feynman Lectures on
Physics Volume III. Available as a book and online at:
https://www.feynmanlectures.caltech.edu/III_toc.html

^{3}In a letter to Max Born in 1926

^{4}The problem is that gravity is not renormalizable. See
Quantum Field Theory, 2003, by A. Zee, Chapter III.2 for
the mathematical details

^{5}The Theory of the Universal Wave Function
http://ucispace.lib.uci.edu/handle/10575/1302

^{6}Max Tegmark, Our Mathematical Universe, pg
190-191

^{7}The formal name for this is decoherence. The
mathematics of this can be found in multiple places,
including The Emergent Multiverse by David Wallace,
chapter 3

^{8}As Max Tegmark said: “a single photon bouncing oﬀ
of an object had the same eﬀect as if a person observed it. I
realized that quantum observation isn’t about consciousness,
but simply about the transfer of information.” (Our
Mathematical Universe, by Max Tegmark, pg 199)

^{9}Just not necessarily in the branch you end up
in.

^{10}The Demon-Haunted World, pg 206, by Carl
Sagan

^{11}Certainly for any normal accident or crime there
would be branches where the tragedy did not happen.

^{12}Such as a deadly car accident that nearly happens.