To quote eminent scientist Tyler Durden: "On a long enough timeline, the survival rate for everyone drops to zero."

Actually… not necessarily.

If the quantum multiverse is real there may be a version of you that lives forever.

In the last couple of episodes we’ve been delving into a key mystery of quantum mechanics - why don’t we have quantum magical powers?

Or, more scientifically, what causes the divide between the weird behavior of quantum world and our large-scale, macroscopic world?

In particular, what causes quantum systems to transition from simultaneously existing in many states at the same time, to having only one clear observable state at the moment of measurement?

This is the measurement problem.

We’ve been exploring decoherence as a mechanism for this quantum-classical transition - and we’re not done yet.

But today I want to take a step back to think about and acknowledge something very important: by itself, decoherence does NOT solve the measurement problem.

It only explains why separate branches of the wavefunction - separate “alternate histories” - stop being able to interact with each other.

In order to understand what happens to those different branches, and to understand why we find ourselves in one of them, we need to embrace one of the interpretations of quantum mechanics.

For example the Copenhagen interpretation, which says that the wavefunction branches that we don’t observe somehow vanish at the moment of measurement.

Or the Many Worlds interpretation, which states that those other branches are just as valid as ours - implying that reality may split and multiply in all possible ways.

In that case we only see one branch because we live in that branch, and the others are rendered inaccessible by decoherence.

The problem with these interpretations is that, on the surface, they seem untestable.

We can’t ever peer into these other realities, so how do we know they exist?

Today I’m going to offer a test.

Admittedly NOT a very useful test - but one that is very fun to think about.

We’ll call this test quantum immortality.

It’s based on the famous Schrodinger’s cat thought experiment.

As a refresher: a cat is in an opaque box with a vial of deadly poison, which is released on the radioactive decay of an atom.

There’s a totally random 50-50 chance of the radioactive decay over a certain period of time - that means the quantum wavefunction of the atom splits equally - the atom is simultaneously decayed and not decayed until we observe it.

So then surely the cat’s wavefunction splits too - into dead and alive.

According to Copenhagen, one of these results becomes “real” when the physicist opens the box, while the other branch of the wavefunction vanishes.

But in Many Worlds both branches continue forever - and the physicist’s wavefunction also splits into two - I guess into guilty and relieved.

Neither version of the physicist knows about the other - and it seems like there’s no way for them to confirm the existence of the other branch of the wavefunction.

But there is one test - a rather morbid one.

The physicist could crawl into the box instead of the cat.

And instead of a vial of poison attached to one radioactive atom, connect the vial to many atoms - so that the poison is released if any of them decay.

Let’s be specific - there are 100 polonium-212 atoms, with half-life 300 microseconds - so each atom has a 50% chance of decaying in that time.

After 300 microseconds the connection between poison and polonium is cut, and the experiment is over.

So what are the chances of the physicist surviving?

Pretty bad I guess - that chance that any one atom does NOT decay in that 300 microseconds is 50-50, but the chance that none of the 100 atoms decay is basically zero.

It’s .5^100.

If we run this experiment over and over, every 300 microseconds, we need to do it for nearly a million times the entire age of the universe in order for the physicist to be likely to survive once.

But the point is they can’t repeat the experiment - at least according the the Copenhagen interpretation, which tells us that there’s ultimately only a single result from each quantum event, so a single result from the “Schrodinger’s physicist” experiment.

According to Copenhagen, all branches of the wavefunction besides “definitely dead” get cut off with ruthless efficiency almost all the time.

But that’s not true in Many Worlds - according to which all branches of the wavefunction persist.

So even after trying this experiment even once, there’ll be a branch of the wavefunction where the physicist opens the box and crawls out, to the amazement of their lab assistant, and hopefully their relief - although this physicist is clearly pretty wack, so who knows.

The physicist now has very good evidence that Many Worlds is right because the chance survival under any other interpretation of quantum mechanics is basically nil.

Many Worlds, on the other hand, guarantees their survival in at least one branch of the quantum wavefunction.

The problem with this test is, of course, that 10^30 physicists need to die in other timelines for one to crawl out of the box feeling smug.

And essentially no one besides that one lucky physicist can ever know.

Even in the rare survival timeline, everyone else will probably assume the experimental apparatus broke.

This thought experiment is sometimes called quantum immortality.

You can imagine that any process leading to mortality is ultimately a sequence of quantum events - so there are timelines in which those incremental steps towards death never happen.

Keep that in mind before you try crawling into Schrodinger’s box yourself - you’re already testing Many Worlds by just existing.

Hugh Everett, who first came up with the many Worlds interpretation, actually believed in this sort of quantum immortality.

He smoked 3 packs a day, drank heavily, didn’t exercise, and died of a heart attack at age 51.

At least in THIS timeline.

Max Tegmark makes a good point regarding quantum immortality - which is that death is an incremental process, not a single quantum event.

So the closer you are, the fewer Many Worlds timelines include your survival.

And even if some insanely rare branches of your wavefunction keep you alive beyond your years, I’d advise you to quit smoking and do some crunches anyway - from where you stand now, your thread of consciousness is going to have to experience every single one of the bad future timelines.

Might as well try to make more of them good.

Also, Many Worlds might be wrong - I say live as though this is your one quantum timeline While we’re discussing dubious methods for predicting survival times - I think it’s time to answer the Doomsday Challenge question.

This was from our Doomsday Argument episode a couple of months back - sorry to be slow - I wanted to check my results by making sure humanity survived at least this long.

So far so good.

Anyway, if you want to try the challenge you should pause here because spoilers.

But to refresh your memory real quick, the argument goes like this: we are typical members of the human race, which means we should find ourselves at a typical point the span of humanity’s existence.

So more likely near the middle of our species existence rather than right at the start or right at the end.

Based on this, if we infer that there are likely to be too many more humans born after us than were born before us then we can guess how much longer humanity has left.

So the challenge question was this: if you are the 100 billionth person born, what are the chances that humanity will last until the year 3000?

Assume population doubles every hundred years.

The key is to figure out how many people will have been born by the year 3000, and then compare your birth rank to that number.

If population is doubling every hundred years there’ll be around 7.7 trillion people by then.

But that’s not the total people who existed up until then.

For that you need to include the ones that have died also.

Now, people used different approaches - but they all amount to integrating an exponential birth rate over the next 1000 years.

Depending on some assumptions like the average lifespan you get that something like 17-18 trillion people will have lived by the year 3000.

That would place you in the first .6% of people who ever lived - and the Doomsday Argument argues that, therefore, there's less than a 1% chance that so many people will ever exist - less that 1% that we’ll make it to the year 3000.

Now, some people used much more sophisticated probability arguments including Bayesian analysis or thinking in “observer years” rather than observer lifetimes.

But none of that helped - the probability for humanity's survival to the year 3000 remained at about a percent or lower.

Others pointed out that probably the global population will level off at 10 billion or so people, so it’ll take a lot, lot longer to get to several trillion humans.

That would push the 99% doomsday date to something like 10,000 years from now if we extend our average lifespan to 100 years.

And the smaller our population is, the longer humanity lasts - according to the Doomsday argument.

This has nothing to do with sustainability - it’s just this weird statistical argument.

Which I guess means if the population was only 100 people we’d last for trillions of years … Hmmm, that does make the Doomsday Argument seem even more absurd.

Anyway, if your name appears you’re one of the winners of the challenge question and you get your pick from the space time merch store.

Congrats - we’ll shoot you an email to sort out your prize.

We tried to make sure all correct answers got a prize in at least one quantum timeline - so if you didn’t see your name here, please congratulate the other you who did.

And to all of you - thanks for joining me on this wavefunction branch.

I have a feeling it’s going to be a hell of a ride, even if we’re not a quantum-immortal in branch.

So let’s stick it to all those alternate realities, and make this the best of infinitely many diverging histories of space time.