Title: The Strangest Idea in Science: Quantum Immortality
Author: Cool Worlds

Transcript:
- It's easy to think
that quantum mechanics
is just a theory about tiny
particles, nothing more.
But what have I told you
that it might predict
that you can never truly die?
Quantum immortality.
Quantum theory tells us that
the world behaves in ways
that defy classical determinism,
famously described by Einstein
as "God playing dice."
Left to their own particles
don't have definitive positions
or velocities, they are everywhere
and nowhere all at once.
In truth, quantum mechanics
is perhaps best understood
as a theory of superpositions.
For example, an isolated
electron does not have
a sharp spin-up or spin-down state,
rather, it is quite
literally in a superposition
of both at once.
It's only when we look at it
that the electron seems to choose.
This behavior is most clearly illustrated
with the famous double-slit experiment.
Gradually shoot just one
electron at a time against a wall
with two small holes in it and you'll get
an interference pattern on the other side.
Somehow, even a single
electron acts as if it travels
through both slits at once producing
that interference pattern.
Indeed, the electron acts just like a wave
with phase maxima constructively
interfering to create peaks
and minimal combining into troughs.
But if we put a detector
on one of those holes,
thereby allowing us to track
which hole it went through,
the interference pattern disappears
and we just are left with two smudges.
They now seem to act like particles.
The consequences of this simple experiment
are perhaps the most
profound in all of physics.
It deeply uproot our
common sense intuition
and it reveals quite
clearly that the universe
is not what it seems.
If you feel confused
or perplexed by this, then
well, you're in good company.
For even Richard Feynman
who helped develop quantum
electrodynamics famously stated,
I think I can safely say
that nobody understands quantum mechanics.
And so physicists and philosophers
have argued for a century
about what quantum mechanics really means
for the nature of reality.
One thing they do largely
agree on is that prior
to any measurement, the
nature and evolution
of these superposition states
can be described like a wave.
Indeed, we call this the wave
function, first written down
by Erwin Schrödinger a hundred years ago.
The real question is,
how does this quantum wave-like
state suddenly transition
into the more familiar classical
particle-like behavior,
post-measurement.
For decades, the dominant view
has been the Copenhagen interpretation.
That is that these
superposition states persist
until they are observed, at
which point they collapse down
into a single sharp
state with a probability
given by the Born rule.
That is a probability given by the square
of the wave function.
This certainly produces exquisite
agreement to experiment.
Indeed, it is arguably the
most precisely tested theory
in all of science, verified
to within parts per trillion precision,
but it doesn't explain ontologically
what's really going on.
First off, it's important to say
that most physicists reject the idea
that wave function collapse
in any way involves human consciousness.
Instead, these measurements
could simply be an interaction
with the environment
such as a stray photon
near one of the slits.
But then here's the problem.
Surely, that photon then
also becomes mixed up
in the superposition, a combination
of striking the electron
and not striking it.
Certainly we know that superpositions
are not limited to individual particles.
Entire ensembles can be
in a superposition too,
in which case, we describe them
as being entangled to another.
In fact, that is the basis
behind quantum computing,
teleportation and cryptography.
So as our electrons from
earlier proliferate out
into the wider world, then surely too,
shouldn't this superposition state,
this march of entanglement spread out too,
eventually even encompassing
human observers.
But of course, the problem
is in our everyday life,
we do not experience any kind
of superposition effects.
So surely something is missing here.
This weirdness is most vividly elucidated
with the infamous
Schrödinger cat experiment.
Imagine a cat in a box
with a bottle of poison
that will crack open at a certain time
with a 50/50 probability
governed by the outcome
of a quantum measurement,
say whether an electron
is spin up or down.
Now after the critical moment passes,
the electron can be described as being
in a superposition of both states.
And since the measurement
device is entangled
with this quantum system,
then so too must be the cat existing
in a bizarre superposition
of being both dead and alive.
Wave function collapse via
the Copenhagen interpretation
at first seems to resolve this paradox,
but it comes with many problems.
For example, why is it that the observer,
be it a straight photon or the cat,
or indeed we who open the box are allowed
to be considered somehow
distinct classical entities,
whereas the electron has
to be considered quantum?
Surely, everything deep
down is truly quantum.
There's also no mechanism
for this mysterious unitary
and frankly, ad hoc
wave function collapse.
And so not surprisingly,
debate has raised for decades
about what quantum theory really means.
- I sincerely hope that
this is the last time
that I find myself here.
- You just don't get it, do you, Jean-Luc?
The trial never ends.
- In 1957, Princeton graduate
student, Hugh Everett proposed
a radical alternative interpretation,
but simply the wave function
never collapses, ever.
Collapse implies that the other states
in the super position simply disappear
and only one persists.
Everett suggested that
all possible outcomes
of a quantum measurement occur evolving
in separate non-interacting
branches of reality.
This so-called many-worlds interpretation
was for a long time largely ignored
by the physics community,
but in the last decade or so,
it has risen to being a serious contender,
championed by the likes of
Sean Carroll, David Deutsch
and Max Tegmark.
You know, Everett's idea
is really pretty simple.
He just takes the Schrödinger
equation completely seriously.
There is no collapse in that equation,
and so that means that the
electron, the box, the cat,
you, me, indeed, the entire
universe are described
by one gigantic wave function,
often dubbed the universal wavefunction.
In place of collapse,
the many-worlds interpretation explains
the classical appearance
of our macroscopic world
by a concept known as decoherence.
So the question is this,
if the entire universe
is in a superposition, then
why don't branches interfere
with each other like they do
in the double-slit experiment?
The answer lies in phase coherence.
In simple systems,
quantum waves stay in-sync
and that allows them to interfere.
But in complex systems, interactions
with countless particles
scramble their phases.
The result, a clean split between worlds
known as decoherence.
Subtly, this branching of
realities is not an abrupt event,
it evolves as the decoherence spreads,
exponentially suppressing interference
between different quantum branches.
After decoherence, the
universes can no longer have
any cause or influence on one another.
So no, sadly, you can't
make a Council of Ricks.
Yet, the number of universes
here would be stupendous
because this branching
would've been going on
since the Big Bang itself
and across the entire cosmos,
you can even create new universes yourself
using the Universe Splitter app.
This is not sponsored, where
you can base your decisions
based off a quantum measurement
made over in Switzerland.
So in this branch of reality,
I'm posting this video at 2:00 PM
but in another, I posted it at
1:00 PM both really happened.
But if this whole idea
of splitting bothers you,
well, you're not alone.
Theoretical physicist, Bryce DeWitt
is said to have visited
Everett and praised him
for the elegant mathematics of his idea,
but told him that he was
bothered by the gut feeling
that he didn't feel like
he was constantly splitting
into power or versions of himself.
Everett is said to have Cooley responded,
"Do you feel like you're orbiting the sun
at 30 kilometers per second?"
"Touche," DeWitt said,
conceding on the spot.
- You think that's air
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- In the quantum multiverse,
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Now, back to the video.
(light music)
Amongst physicists, the
many-worlds interpretation
has grown in popularity thanks
to its rigorous treatment
of the Schrödinger equation.
But as the idea has gained traction,
many have realized that
it implies a bizarre
and radical consequence.
The idea can be best thought of as a game
of quantum Russian roulette
and was popularized by
Max Tegmark in his book,
"Our Mathematical Universe."
He describes a quantum machine gun
which can fire up to once per second
depending on the outcome
of a quantum measurement.
Specifically, each time
the gun is triggered,
it places a particle in a superposition
where it's equally likely
to be in two states at once,
for example, spin up and spin down
and then it measures that particle.
If spin down the gun
fires and kills you faster
than human perception.
Else, it just makes an audible click
and it repeats this every second.
Now, according to Everett,
there will be two parallel universes
after the first second,
one where you are dead
and the other where you're alive.
In other words, there is
exactly one copy of you
that has perceptions and
a stream of consciousness
both before and after the experiment.
Now since you can't
experience not being alive,
then the prediction is that
you will hear an audible click
and survive this measurement
with 100% certainty.
And of course, what's really bizarre
is that you could sit
there and experience dozens
of clicks in a row somehow
surviving time after time.
After 40 sequential clicks,
the Copenhagen interpretation
says that there's
a one in a trillion chance
that you'd still be alive,
but many worlds says that there
will be to 100% confidence
a version of you that survives
this extraordinary sequence of events
and that you would thus
be able to conclude
that Everett's idea was right all along.
In effect then, you would be able to prove
that the many-worlds
interpretation was correct,
but it would be as
strangely private proof.
For any the trillion universes,
everyone else would just
watch you die wondering
why you would do such a thing.
They would not see this proof
and thus this experiment fails
to convince the rest of the world.
Although Hugh Everett never
wrote about this idea,
Eugene Shikhovtsev biography states,
"Everett firmly believed
that his many world's theory
guaranteed him immortality."
His consciousness he argued,
is bound at each branching
to follow whatever path
that does not lead to death
and so on, ad infinitum.
Sadly, on our branch at least,
Everett had a difficult life.
After widespread rejection,
he became withdrawn,
failed to get an academic job,
drank and smoked too much,
and eventually died of a
heart attack at age 51.
Now Tegmark is very clear
that this is not an experiment
that he recommends conducting
for yourself at home.
And I want to emphasize
that point here too.
It would be incredibly stupid,
foolish and idiotic to engage
in this experiment at home
and I think it'll become
increasingly clear
as to why I think that's
right the rest of this video.
But hypothetically, Tegmark argues
that there are three criteria needed
to make this Macabre
experiment really work.
Criteria one, the random
decisions must be quantum.
Two, it must kill you
or make you unconscious
on a time scale shorter than
that which you can perceive.
And three, it must really kill you,
not just very badly injure you.
Satisfy those three criteria
and the argument goes
that you would somehow miraculously
survive time after time.
- Yes, I am invincible!
- In a 2001 lecture, philosopher
David Lewis expanded upon
this game of quantum Russian
roulette to broad immortality.
He reasoned that the entire universe
is ultimately governed by quantum events
and thus, all causes of death
satisfied Tegmark's first
and third criteria.
For example, if a car is
about to hit you on the road,
there is a stupendously small probability
that the vehicle will
quantum tunnel straight
through you without causing any harm.
And according to many worlds,
a version of you must
survive such encounters.
Indeed, if David Lewis is
correct, then eventually,
each of us will one day
gain personal evidence
from many worlds for we
will seem to always escape
life-threatening car crashes,
survive health scares
and eventually, outlive
our friends and our family.
Tegmark isn't in alignment
with David Lewis's position
though via his second criterion
because most causes of death
are not faster than our
perception timescale.
They are more typically
a slow deterioration
of our mentor faculties,
something that we could
gradually experience.
But even this point, Lewis
seems to have anticipated
with a frankly, dark and
terrible prediction for us all.
For Lewis concedes that, in general,
you will indeed deteriorate,
but all you need to do
is survive each moment,
not necessarily come out the
side particularly healthy.
Over time, you would accumulate
evermore deterioration
like the Struldbruggs
from Gulliver's travels
or the protagonist of death becomes her.
Lewis writes, "As you
survive deadly danger over
and over again, you should also expect
to suffer repeated harms.
You should expect to lose your loved ones,
your eyes and limbs, your
mental powers and your health.
Eternal life on such
terms amounts to a life
of eternal torment."
Is this the fate that awaits us all?
- Eowyn, my body is broken.
You have to let me go.
- Since quantum
immortality is so personal,
a good question to ask is
whether our own experiences
of the world are consistent
with its predictions.
So let's revisit the first half
of Tegmark's second criterion
that you must die or lose consciousness.
It's already non-trivial for Tegmark
to equate those two states,
but I suppose the reasoning here is that,
in neither state can
you perceive anything.
Physicist Anthony Aguirre
latches onto this point
in his book "Cosmological Koans,"
considering what it means
when we are anesthetized
or even asleep experiences
that we can all attest to.
One might argue that we should
always live on the branch
where we stayed awake
all night, every night
because the branches where we
sleep are imperceptible to us.
But of course, even asleep
there is a reduced form
of consciousness such as dreaming,
yet more, those reduced
states of consciousness
are not permanent.
Skip forward eight hours in both branches
and you would find a
conscious version of you.
Another line of anthropic attack
is to try and invoke the
mediocrity principle.
If we really live forever
in increasing in decrepit states,
then there will be far,
far more years of you alive
in that state than the relative
youth that we all enjoy now.
In fact, in the story of your existence,
it would be incredibly
unlikely that you'd happen
to inhabit a moment during your,
let's call it normal-aged life
rather than Struldbrugg years.
I think that this does make
for a compelling counter argument,
but it is the same kind of reasoning used
by the Doomsday argument
and the Boltzmann brain discussions
which can all be challenged on
the issue of reference class.
Perhaps your future
state is very different
from this current state and thus, in fact,
you do occupy a mediocre position
of your current reference
class that is being youthful.
Despite our best efforts,
these concerns can't
deliver a knockout blow
to quantum immortality.
But if we revisit Lewis's argument,
we can discover a questionable assumption
that challenges the entire idea,
an assumption about the
way the universe works.
- If there's nothing wrong with me,
maybe there's something
wrong with the universe.
- A central line of reasoning
in David Lewis's case
for immortality is what he calls
"the corrected intensity rule,"
and it really speaks more broadly
to a topic of constant
tension in discussions
of the many-worlds interpretation.
Let's say a life-threatening
quantum triggered event occurs
with four distinct and
equally likely outcomes,
in which you perish in three of them.
Now in the Copenhagen interpretation,
we would simply assign a 25% probability
to each outcome occurring
using the Born rule
and indeed only one will occur.
But in many worlds, all four
happen with 100% probability.
Of course, if everything
happens to 100% probability,
then the theory really
lacks any predictive power
and thus, Everettians
get around this by saying
that each reality occurs
with a 25% measure
of existence or a 25% intensity,
again, taken from the Born rule.
Now your consciousness can
surely only inhabit one reality
at a time and thus, you might reason
there is a 75% chance of you
dying in this grim scenario,
even in the many-worlds interpretation.
But Lewis's trick here is
to discard all of the cases
where you die because, and I
quote, "death is oblivion."
The experience of being dead
should never be expected
to any degree at all because
there is no such experience.
This act of discarding
branches where you die
and then renormalization
is what Lewis calls
the corrected intensity rule
and it is pivotal to the
case for quantum immortality,
but many thinkers take issue
with this slight of hand.
For example, philosopher
David Papineau writes,
"It is by no means obvious why Everettians
should modify their
intensity rule in this way.
For it seems perfectly
open for them to apply
the unmodified intensity rule
in life-or-death situations,
just as elsewhere."
Many-world's champion, David Deutsch
also pushes back here
writing in his book that,
"This way of applying
probabilities does not follow
directly from quantum theory
as the usual one does.
It requires an additional assumption,
namely, that when making decisions,
one should ignore the histories
in which the decision maker is absent.
My guess is that this
assumption is false."
In other words, when the
universe makes its split,
there's by no means a guarantee
that you will take the
path in which you survive.
- I didn't think it would end this way.
- End? Oh, the journey doesn't end here.
Death is just another path,
one that we all must take.
- I think Papineau and
Deutsche make great points.
It is indeed wholly unclear
that we can discount
death branches in the way
that Lewis prescribes,
but neither of them can
falsify Lewis's claim either.
Sure, he's added an extra assumption here,
but that doesn't mean that it's wrong
and it stems from a reason perspective.
All the quantum weirdness
kicking around makes it difficult
to know what's real and what's not.
So let's instead try to think about
more conventional analogies.
So leaving aside generalizing
mortality just for the moment,
let's come back to the more pointed case
of the quantum Russian roulette example
that Tegmark described earlier.
A thought-provoking
analogy to this experiment
is to imagine a kind of
Star Trek Star transporter
that beams you up
and then rematerializes
you somewhere else.
Before the beam out, there's one of you
and after the beam out,
there's one of you.
Certainly in Star Trek,
most characters don't seem
bothered by this process,
so let's assume that it's perfectly safe
and not a death machine then.
But imagine instead, if it
created two versions of you
by some kind of transport malfunction,
again strictly in terms
of your own survival,
this really shouldn't bother
you, you will persist.
Although the ethics of cloning you,
probably our point of concern.
This splitting is exactly analogous
to what happens in many worlds.
Your consciousness, you,
whatever you want to call it,
can only occupy one body,
and yet both versions of you would insist
that they were real.
So which version of you really is you?
Now as a final twist, consider that one
of those two rematerializations fails
and leaves behind a gory
mess on the transporter pad.
Now you might reasonably be
disturbed at the prospect
of stepping on that transporter pad
because there'd be a 50% chance
that you would die, right?
But on the other hand, that dead version
of you never even materialized enough
to experience consciousness.
So surely, it's impossible
for you to become
that puddle of flesh.
And we can take this a step further
and say that the crew never even attempted
to rematerialize the second beam.
The malfunction was detected
and the file deleted inside the computer.
In this case, aren't we exactly equivalent
to the original scenario
of a one-to-one beam out,
in which case few objected?
Now in fairness, I promised
you a conventional analogy
and then started talking
about transporters.
So the fact we don't understand
how such a machine
would truly work perhaps
only adds more confusion to the mix here.
So consider instead a
classical analogy described
by Sean Carroll in his book,
"Something Deeply Hidden."
Okay, so Lewis's whole
argument is that you really
shouldn't be bothered about the branches
where you die because you
can't experience that.
But Carroll strongly disagrees with that.
After all, if someone snuck
it behind you with a gun
and instantly killed you, you
should not be okay with that.
The fact that you can't
experience being dead
is completely beside the point.
It is totally reasonable to not want
to be killed, regardless.
- The reason why we don't want to die
is not just that we will experience pain,
but that sort of prospectively right now,
the idea of being dead in
the future bothers me, right?
Like, if someone said, you know,
you're going to die in this, in that date
might be useful information,
but I'd be sad, right?
If that date was soon.
And I think the same thing is true
in the quantum and mortality experiment.
I don't buy the move that says, well,
in all the branches where
you're dead it doesn't matter
'cause you're dead, you
don't feel anything.
- Unfortunately, David
Lewis passed away soon
after giving that lecture,
which spawned this whole
quantum immortality discussion,
but I imagine he would've pushed back
against Carroll's scenario as
being a very different beast
because in that case,
you definitely do die.
Whereas, I think Lewis would maintain
that in the quantum picture,
your stream of consciousness
continues uninterrupted.
It's definitely reasonable to be disturbed
by the finality of Carroll's
classical experiment,
but Lewis's whole point is
that in the quantum version,
there is no finality.
It was Max Tegmark who really popularized
the quantum Russian roulette experiment,
but it's interesting to note
that he somewhat walked back
on the idea that you'd
really survive such a thing.
As we noted earlier, he
points out that most forms
of death fail his second criterion
of occurring on a timescale
faster than human perception.
But I think to me, it is deeply unclear
that this criterion would truly safeguard
your continued survival.
In particular, philosopher
Charles Sevens has argued
that the timescale is irrelevant.
After the quantum gun fires,
the universe has already split into two
even before the bullet reaches you.
Both versions are credibly
what you might call you
and thus, you will die in one of them.
I think that point alone makes
for a very compelling reason
as to why you should
never try this experiment
because in truth, no physical process
is truly instantaneous.
But Tegmark and others are also bothered
by the thorny issue of
how many worlds deals
with probabilities or
really measures of existence
between the different branches.
I think that this is the
issue more than any other,
the critics of the
many-worlds interpretation
often bring up.
How is it possible that
all of these outcomes occur
with 100% probability?
For example, after a large number
of Russian roulette experiments,
the number of worlds in which you survive
is astronomically small.
Shouldn't this bother you?
Physicists Lev Vaidman agrees writing
that the large measures of
the world with dead successes
is a good reason not to play.
But in truth, aren't we
all already low measure?
After all, since the Big Bang,
countless branchings have happened
and the number of worlds in
which you happen to be born
and be here would be an
incredibly small fraction.
And so by this logic,
you shouldn't even
really be here right now.
- We've all been dancing
around the basic issue.
Does data have a soul?
I don't know that he has.
I don't know that I have,
but I have got to give him the freedom
to explore that question himself.
- And so, we finally come to it.
This bizarre concept is as much
about the theory of the mind
as it is about quantum reality
because a latent issue
throughout this entire discussion
has been what does it really
mean to talk about you,
your consciousness, your
identity, your selfhood?
This is quite possibly the
most difficult question
that humanity has wrestled
with throughout the ages.
And as you might imagine,
there's no agreement.
My Columbia colleague, Brian
Greene thinks of it this way.
Each copy is you.
You just need to broaden your mind
beyond your parochial
idea of what you means.
Each of these individuals
has their own consciousness
and so each of them
believes he or she is you,
but the real you is their sum total.
In contrast, Lev Vaidman takes
a more pragmatic view stating,
"there are many different
Levs in different worlds,
but it is meaningless to
say that there is another I.
There are, in other words,
beings identical to me
at the time of splitting
in each of the worlds
and all of us came from the same source,
which is me right now."
Philosopher David Wallace
pries open this view
a little deeper arguing the
sense of I, can only make sense
if identity is confined to a single branch
of the quantum multiverse.
Since it's not clear how this can happen,
Wallace concludes that we may
have inadvertently
demonstrated that many worlds
is not a conceit of multiple selves,
but rather, it is dismantling
the entire notion of selfhood.
It denies any real meaning to you.
You know, I can't help, but be reminded
of another debate raging in quantum theory
regarding the so-called black
hole information paradox.
You can watch our early video
to learn all about that,
but it's an example where
two seemingly sound theories,
quantum mechanics and
general relativity collide
and imply inconsistent results.
Here, quantum mechanics
collide with selfhood
and once again, the result seems to be
that there is something amiss
in our fundamental understanding.
Indeed, cosmologist David Aguirre argues
that the entire debacle
can be characterized
as a reductio ad absurdum
against our current understanding
of many worlds and the theory of the mind.
- For that one fraction of a second,
you are open to options
you had never considered.
That is the exploration that awaits you.
Not mapping stars and studying nebula,
but charting the unknown
possibilities of existence.
- On the 27th of October 1962,
at the height of the Cuban Missile crisis,
a Soviet submarine near
Cuba was surrounded
by American destroyers and so had to dive
to avoid detection.
Whilst unable to contact Moscow,
the Americans dropped
practice death charges
to try and signal the
submarine to surface,
but the Soviet captain
thought that these were real
and ordered his crew to launch
a 10 kiloton nuclear weapon
against the Americans.
His political officer agreed,
but the second officer,
Vasily Arkhipov did not
and managed to talk his
captain down from firing.
World War III did not
happen that day nor since,
because of course, if it did,
none of us would likely be here.
In the many-world's interpretation,
we could never live on a
branch where that happened
because we'd be dead.
Indeed, perhaps all of
history is a sequence
of improbable events that
somehow conspired to lead to you
because in branches where
they didn't, you aren't there.
- What happened?
- Instead of firing, they
suddenly disarmed and canceled.
- Fate, protects fools, little children
and ships named Enterprise.
- I think it's easy and
a bit lazy to dismiss
the immortality concept
because it feels wrong
or we just don't like it.
But it's harder to come up
with a fully rigorous dismissal
of it and hold on to the
many-worlds interpretation.
Ultimately, these kinds of
gedanken experiments help us
to see where we need to do more work,
where our understanding
hits the end of the road.
And like the information paradox,
I think that there is something
profound in this idea,
a lesson for us about who we really are
and the nature of reality.
Personally, even if many worlds is true,
I think Lewis's idea of
immortality of Struldbrugg's
is hard to swallow.
After all, many worlds doesn't
allow for anything to happen,
outcomes still emerge
from systems governed
by physical rules.
Yet more, at least to me,
the whole notion seems to run afoul
of the fact that it
would be much more likely
that I would currently
inhabit a much older state
than my present one.
But there is a silver lining to it all.
For in many worlds, even
if we eject the whole worry
about who is really you in
these multiple branches,
certainly in these other
branches, doppelgangers of you
and I would be there, would
be living their lives.
And even critics of the
quantum immortality argument
like Peter Lewis in his
book, "Quantum Ontology,"
agree with this logic, writing,
"Certainly the many world's theory
has the following consequence.
At any future time, there
is a branch containing
a living successor of you."
So maybe out there, somewhere,
Hugh Everett is still
alive at 94 years old,
perhaps surrounded by
his great-grandchildren
and his physics Nobel.
David Lewis has just popped over
for a friendly game of chess
and to continue their debate
on quantum immortality.
And amongst these other
branches, we might indulge
to imagine all of our hopes
and dreams transpiring,
aspirations for a better
world, a happier life.
And even though we could never
access those island universes
where we made different choices,
where our loved ones lived longer,
where history played out
differently, they do exist.
It proves to us that such
dreams are realizable.
And I think that can
be a tremendous source
of inspiration and comfort.
So until next time, stay
thoughtful and stay curious.
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