Q: What is quantum immortality?

Physicist: A common refrain in quantum theory is “anything that can happen does”.  Shockingly, and with total disregard for our own human experience, this seems to be true.  When you consider what this means in the context of living and dying, a lot of weird, uncomfortable conclusions bubble up.  In particular: if a situation can end with you either alive or dead, and every result literally does happen, then you will always experience a result where you live.  No matter how ludicrously unlikely, if there’s a non-zero chance of surviving, then you always do.  From your point of view, the world around you does whatever crazy contortions it must to ensure your continued point of view.  “Quantum suicide” and “quantum immortality” get their name from this idea and that it may be impossible to die (from your own point of view), no matter how hard you try.

Quantum immortality is a philosophical thought experiment about what happens when you combine quantum many-state-ness with the anthropic principle and survivorship bias.  It’s worth underscoring that this is a thought experiment, not advice.  It’s an interesting idea, but I wouldn’t bet my life on it.

The anthropic principle says that whatever needed to happen in order for an observer to be observing, happened.  For example, because you’re reading this, then (among other things) you must have access to the internet, speak English, live in an environment capable of supporting life, and were born rather than not born.  For anyone/anything not reading this, none of those things may necessarily be true.

The anthropic principle helps explain why the refrigerator light seems to always be turned on.  It’s set up to be on whenever you’re in a situation to see it.

On it’s own, the anthropic principle is already pretty powerful.  It is the governing principle behind why “you are here” signs are always accurate.  It says that nobody ever regrets playing Russian Roulette, they only regret inviting their friends.  And it explains why the Earth is capable of supporting introspective critters such as ourselves, despite all of the incredibly unlikely things that had to go right for that to happen.  That last shouldn’t seem too surprising; if only one in a million planets can support life, where would you expect to find living things?

If every possible version of a given event does happen, then you will only (continue to) experience outcomes where you survive.

It’s the “quantum” that makes the quantum suicide thought experiment interesting.  You’ll probably find a planet capable of supporting life in the universe, because there are a lot of opportunities (based on the other solar systems we can see) and there’s evidently a non-zero chance.  What quantum theory does is change that “probably” to “definitely” if there’s ever a non-zero chance.

In classical physics (which is to say: when you just walk around and use your eyeballs), everything seems to be in a single state.  Your coat is on one hook, not many.  Your front door is open or closed, but not both.  If you lose your keys, they’re someplace specific, even if you don’t happen to know where.

In quantum physics on the other hand, when we assume that things are in only one state our predictions fail.  The most famous example of this is the double slit experiment, where coherent light is shined on a pair of slits (regular readers are no doubt sick of regularly reading about the vaunted double slit experiment).  Instead of seeing two bars of light on the far side of the slits, we instead see “beats”; many bars of light corresponding to interference between every possible path through the two slits.  The terrifying thing about the double slit experiment is that it continues to work even when the light intensity is turned down to just one photon at a time.  If we assume that each photon goes through only one slit, then we expect to see a build up of photons in just two bars.  The fact that we see many bars indicates that the photons go through both slits.

Even individually, when photons pass through the double slits they’ll hit a projection screen not in two bars, but in several, corresponding to wave interference between the two slits.  In order to predict where a photon is likely to land, you have to take into account every possible path that it can take.

Other than being clearly weird, and simple enough for first-year physicists to do the math themselves, there is nothing special about the double slit experiment.  The same “things can be in many states” idea applies across the board in quantum theory.  It is the back bone of chemistry, particle interactions of all kinds, freaking everything.

And photons aren’t special either.  You can do the double slit experiment with anything (as far as we know), it’s just that bigger things are harder to work with.  The largest things to successfully demonstrate going through both slits are molecules of C284H190F320N4S12.  That’s a modest 810 interconnected atoms!  Our inability to demonstrate the quantumness of macroscopic things seems to be an engineering barrier rather than some undiscovered physical law.  Every indication so far is that there’s no division between the “quantum world” and the “classical world”.  Instead (like every other physical law), quantum laws seem to apply universally.

Notice that the refrain is “anything that can happen does” and not “everything happens”.  Assuming the laws do apply on all scales, one of their more frustrating predictions is that the probability of observing yourself somewhere else is zero and the probability of observing two or more of yourselves is likewise zero (literally: wherever you go, there you are).  These situations are impossible.  Despite being in many states, none of your states directly interact with the others.  Other quantum versions of yourself are like the bottom half of a Muppet; something you feel like you should be able to see, but there’s a good reason you never do.

This is not without precedent in science.  For example, Newton’s laws simultaneously predict that 1) the Earth must be spinning and hurtling through space (based on how the other planets and stars move in the sky) and 2) that you’d never notice (because you’re hurtling along with the Earth).  Arguably, this “it’s very weird, but it’s also really hard to notice” aspect of quantum mechanics is why it was discovered after bronze and the wheel.

The double slit experiment is so clean an easy to work with because you only have to worry about two states: the path that light can take through either slit.  In reality the slits have some non-zero physical size, so there are many different paths photons can take through each.  Those paths are all so similar that assuming they’re the same is good enough for an undergrad lab class.  But if you want to nail down the exact pattern you see projected on the screen, you have to account for every possible path precisely.  This is true whenever quantum theory applies (e.g., chemistry); counting the mostly likely quantum states buys you a decently accurate prediction, but the more states you take into account, the more accurate your prediction.

To perfectly predict quantum phenomena, we have to take into account every possible way an event can happen, even completely bizarre possibilities (such as this blue “non-classical path”).  The less likely a path, the smaller its impact, but every path counts.

Low-probability states don’t add much, but they demonstrably add more than zero, so they must be physically real.  A mountain and a pebble affect the world differently, but they’re “equally real”.  So, assuming QM laws apply universally, every possible outcome of an event is physically realized and the fact that you can only experience situations where you’re alive means that you’ll be funneled into those realities where you continue to live.

There’s nothing ultimately special about life or death (unless you’re alive, and then it’s suddenly super important), they’re just more interesting to consider than, say, a quantum number generator that accidentally gives you sequential numbers forever.  Quantum suicide makes some tricks a lot easier since the same quantumy arguments apply very broadly.  For example, if a situation can end in either stubbing your toe or not, both results will occur.  In some parallel histories, when you kick a brick barefoot you’ll miss.  Still.  What do you honestly think will happen if you try?

The central tenant of quantum immortality (that anything that can happen does) applies to everything and every combination of things, it’s just that we’re good at worrying about and keeping track of ourselves.  There’s a vanishingly small probability that a Beanie Baby™ will “survive” intact for trillions of years (when it will have nearly doubled in value), so in some tiny set of the many possible futures it will.  The fact that it doesn’t have a point of view means that it won’t be bothered one way or the other.

If there’s X chance that you’ll be walking around in a thousand years, then there’s a chance of about X2 that you and some other particular person will both be walking around.  In other words, in some tiny fraction of possible futures you get to persist, and in a substantially more tiny (but still not quite zero) fraction you and Duncan MacLeod are both alive.  Until the gathering at least, after which there can be only one.

Duncan MacLeod: immortal from the Scottish highlands or just an impossibly lucky cosplayer who accidentally hasn’t died?

So this gives us a way of testing out the completely insane idea of quantum immortality.  If a bunch of us are accidentally alive in a thousand years, let’s all meet up and compare notes.  We don’t need to bother agreeing on a meeting place or time, since quantum immortals should be used to relying on happenstance.

Quantum immortality is, almost by definition, a subjective experience.  Clearly it’s possible to observe other people passing away (condolences everyone), but if quantum immortality is real, you’ll find out on your own.  This has given rise to some clever science fiction, but not a lot of useful science fact.  Point is: planning to live forever has not traditionally been an effective way to spend not-forever.  I find myself alive and the improbable result of an endless string of nearly impossible coincidences, while on a world that may be unique in the universe.  On the other hand, that’s everybody’s story and, not for nothing, don’t risk your life over some silly idea.

The “weaving photon” picture and a short summary of the experiment can be found here.

The fridge picture is from here.

MacLeod’s photo is from here.

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20 Responses to Q: What is quantum immortality?

  1. craig says:

    Not sure if I’m on the same wavelength – but in John Gribbin’s book “In Search of Schrodinger’s Cat”, he cites John Wheeler’s idea that by observing the cosmic background radiation of the Big Bang, we may be creating the Big Bang and the universe. In the next sentence Gribbin says that Feynmam was even closer to the truth when he said that the two-hole experiment “contains the only mystery”.

  2. Tommy Scott says:

    A common refrain in quantum theory is “anything that can happen does”. Shockingly, and with total disregard for our own human experience, this seems to be true. When you consider what this means in the context of living and dying, a lot of weird, uncomfortable conclusions bubble up. In particular: if a situation can end with you either alive or dead, and every result literally does happen, then you will always experience a result where you live. No matter how ludicrously unlikely, if there’s a non-zero chance of surviving, then you always do. From your point of view, the world around you does whatever crazy contortions it must to ensure your continued point of view. “Quantum suicide” and “quantum immortality” get their name from this idea and that it may be impossible to die (from your own point of view), no matter how hard you try.

    This is the biggest load of absolute nonsense I’ve ever heard. If this is the sort of worldview you believe in, it’s no wonder you’ve abandoned God and all rational thought.

  3. David Peters says:

    It is a thought experiment used to show the conclusion of a specific line of reasoning. I don’t understand why you mention god or wether you actually believe in one, but a god certainly has no place in this particular discussion. If you want to invoke one then you have come to the wrong thread.

  4. Tommy Scott says:

    “It is a thought experiment used to show the conclusion of a specific line of reasoning.”

    It is a load of total absurdity.

    “I don’t understand why you mention god or wether you actually believe in one, but a god certainly has no place in this particular discussion.”

    I mention God because the author of these physics blogs rejects God and this sort of ludicrous thinking is evidence of why one would reject all reason and logic and therefore God, the creator, as well.

    “If you want to invoke one then you have come to the wrong thread.”

    No, this is just the place to speak of God. To people who are so lost in their reasoning that they need a wake up call to reality.

  5. David says:

    Quantum theory is uninterpreted. This article talks, at times, as if we have an interpretation. But what we have instead is a list of possible ideas, all of which, without exception, have major problems. The many worlds interpretation is one of them, and it’s the one the writer of the article happens to think is true. He sometimes talks as if we know it to be true, and as if he knows all about what is behind quantum mechanics. This way of communicating can mislead and confuse (because what he says is confusing, so you have to go to him to get it explained, rather like the catholic church kept a monopoly on knowledge for centuries by keeping it weird and complicated). It can mislead and confuse non-scientists, students, and inquisitive people who are trying to get their heads around QM.

  6. The Physicist The Physicist says:

    @craig
    That sounds a little like opening a milk carton and finding that it’s gone bad being the cause of that milk going bad in the past. “Observer effects” in quantum mechanics (where your observation physically affects something else, rather than just your perception of that thing) make me cautious. They tend to get explained away by careful examination.
    That said, I haven’t read Gribbin’s book, so there may be some context I’m missing. Is it worth reading?

  7. The Physicist The Physicist says:

    @David
    Fair enough!
    The interpretation I use is called “Relational Quantum Theory“. We already have established laws that beautifully describe the interactions between sets of isolated quantum systems. RQM assumes that everything is a quantum system and describes “measurements” as an interaction between a given quantum system and another system that includes yourself.
    I like RQM because it doesn’t involve making up new laws, there’s no hierarchy (you treat everything the same way, mathematically), it smoothly unifies entanglement (interactions between isolated systems) and measurement (interactions between quantum systems and the world at large), and it doesn’t suffer from the usual batch of paradoxes that other interpretations often do (I assume these are the “major problems” you mentioned).
    My goal is to be as clear as possible (which is especially tricky in quantum information). If I’m not, please let me know.
    This stuff can be “weird and confusing”, but not because there’s some shady cabal of scientists trying to keep the public out. It’s just that a solid understanding takes a lot of work (in this case “work” = “linear algebra”) and there is a learning curve.
    If you’re looking for another source (and you should), almost any textbook with “quantum information” in the title is a good start. You can also look at chapters 1-4 here. There RQM shows up quietly under the guise of the “product state” (see chapter 3), where the states in question are the state of the quantum system and the state of a “pointer” (which can literally be a machine that points).

  8. ppnl says:

    Erm, yeabut what about quantum hell? Living for eternity does not preclude the possibility of living in eternal torment. Imagine living in a rotten cancerous body that by some freak of probability simply refuses to die.

  9. The Physicist The Physicist says:

    @ppnl
    That would not be ideal!

  10. Anders says:

    Would quantum immortality apply to my past self as well? I mean if quantum immortality is real would I have always existed from my own point of view going into the past?

  11. The Physicist The Physicist says:

    @Anders
    That is incredibly profound!
    I’m honestly not sure. Physical law is remarkably diplomatic between the past and future, but I don’t recall having existed forever, so evidently it doesn’t apply. This is well work arguing about!

  12. craig says:

    Physicist: In response to your question, Gribbin’s book gives a decent overview of the history and mysteries of QT but I felt that many important issues would have been less mysterious with better or more detailed explanations. For instance, alluding to my post above, I had real difficulty understanding his description of Wheeler’s delayed-choice thought experiment. He uses this to support the notion that “the whole universe can be thought of as a delayed-choice thought experiment in which the existence of observers who notice what is going on imparts tangible reality to the origin of everything.” But he then leaves this thought dangling without further ado – which is probably a problem with authors having to present the history and mysteries of quantum theory in less than 300 pages.
    Cheers

  13. David Peters says:

    Hmmm, interestingly I don’t recall being born either! What does that mean? 🙂

  14. David says:

    Sorry, I wasn’t around.

    RQM doesn’t have many worlds. A lot of the stuff in the article does. The bits about ‘quantum immortality’, which to me is far from solid in any context, only has any basis in some form of MW. Are you saying that what you call ‘quantum immortality’ (which to me is a bit of a nonsense, as we don’t know enough to pursue these interpretations that far) arises from RQM!?

    To me this includes an element of just messing with people’s ideas about their very long term future, which immoral organisations like the catholic church did as well, to get power over people.

  15. The Physicist The Physicist says:

    @David
    It does and it doesn’t.
    The Many Worlds Interpretation describes a situation where universes split (somehow). It’s a weak metaphor for what it’s like to be in many states (or at least, what it’s like to perceive the world from only one of those states).
    Admittedly, it can be difficult to tell the difference between RQM and MWI. Both involve assuming that people can be in many states. The difference is that MWI implies that the entire universe comes along for the ride when you “split”; it’s objective. While with RQM how the states of a system get divvied up depends on how they interact with another system; it’s subjective.
    Either way, this whole article is nothing more than a crazy thought experiment. Just an interesting thing to think about. Nobody’s starting a church. Please don’t waste time believing you’ll live forever.

  16. David says:

    RQM and MW assume people can be in many states in totally different ways. The article has the many worlds version, it’s that simple. I’m not saying you’re like the Pope by the way, sorry if it sounded like it.

  17. The Physicist The Physicist says:

    @David
    Fair enough.
    I don’t mean to convey that. How did I imply MWI?

  18. David says:

    The idea that ‘every result literally does happen’ is the many worlds interpretation, and not RQM. The first two lines of the article are very misleading:

    “A common refrain in quantum theory is “anything that can happen does”. Shockingly, and with total disregard for our own human experience, this seems to be true.”

    We have no idea if this is true. It only seems to be true if you happen to believe one particular interpretation, out of twenty or thirty that we have. And the important point is, a very large number of people think none of them are right – that is, that we’re in that situation that some physicists find it oddly hard to face up to, and admit to themselves and others: we don’t know the answer yet.

  19. The Physicist The Physicist says:

    @David
    That particular phrase does show up in MWI.
    But that things can be in many states, that you have to take into account every possible state, and that there’s no difference between which states will be “realized” or not before a measurement (other than the amplitude dictating the likelihood), are all standard notions across most interpretations (Bohm had a different tack, but his ideas aren’t generally applicable). In particular, they show up in both MWI and RQM. In RQM you don’t assign a greater “realness” to some of the states over others; that would involve introducing new dynamics.
    There are subtleties. In the double slit experiment I think it’s fair to say that each photon goes through both slits. MWI would say that, if you were the photon, the universe splits and each event occurs separately. In RQM you say instead that all of the operators are linear (as in, “f(x+y)=f(x)+f(y)”), and so you handle each state as though they were separate. From the photon’s point of view, there’s very little difference.
    Thanks for sticking around by the way. This is a useful conversation to have.

  20. David says:

    There’s a big difference between the different states being spread across many worlds, and the different states being divided up in other ways within this world. In RQM there’s only one world, and that’s a very different picture.

    You don’t get immortality in either picture, because in many worlds the different states don’t really have anything to do with each other. When it’s people, they’re effectively different people, who never overlap. Making them all one person is not accurate.

    I think this kind of speculation goes way beyond what we can reasonably think about, just anyway. And it makes the ‘immortality’ you talk about not even as good as what Woody Allen said he didn’t want – he said “I don’t want to achieve immortality through my work; I want to achieve immortality through not dying. I don’t want to live on in the hearts of my countrymen; I want to live on in my apartment.”

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