What's wrong with quantum immortality?

Quantum immortality is the concept of everlasting life through the power of quantum mechanics. Whole books and elaborate theories have been constructed around this fanciful idea. It is an example of quantum mysticism, a trend that really got started in the early ‘70’s when New Age combined with quantum theory. It continues to this day with books by Deepak Chopra, TV series like What the Bleep do we know? and others who aim to try to under-gird mystical, New Age ideas with the hard science of quantum physics.

We cannot rule out these ideas scientifically, but that’s not the point. None of them has a firm grounding in science or philosophy. They are purely speculative, fodder for New Age themed science fiction.

Quantum immortality is based on a thought experiment, introduced in the 1980s, called quantum Russian roulette which is a variation of Schroedinger’s cat, except that now the experimenter is inside the box instead of a cat.

The idea is to prove that the Many Worlds Interpretation (MWI) is correct through subjective experience.

In the standard Schroedinger’s cat experiment, a cat is placed in a box where there is some quantum mechanism that with 50% probability, activates a vial of poison inside the box. The poison, once activated, has 100% chance of killing the cat. Because of the quantum nature of the trigger for the mechanism, the whole interior of the box, cat and all, enters what is called a quantum superposition where the cat is both dead and alive at the same time.

An example of a quantum trigger would be a laser beam that is capable of firing single photons aimed at a semi-reflective mirror where each photon has a 50% chance of passing through the mirror. Behind the mirror is a detector which can detect individual photons. If you fire a single photon now, once it interacts with the mirror it will enter a state of superposition where its wavefunction will have both passed through the mirror and been deflected away from it.

Because everything is in a completely sealed box (which for practical purposes would have to be in a vacuum chamber at near absolute zero but we’ll ignore that for now), the cat is also in superposition until you look at the outcome.

Once you look at the outcome, you know which direction the photon went and the superposition collapses.

At least, that is what the standard interpretation, called Copenhagen, says. If MWI is true, however, what happens is that the world “splits” into two and the dead cat’s world continues one way and the live cat’s world continues another, never to meet again.

There are also version of the MWI where the world doesn’t split at all. Lev Vaidman has proposed that the apparent splitting of worlds is just how our minds interpret measurements and that the worlds remain firmly connected. Rather, the wavefunction simply changes shape in a process called decoherence so that the worlds now appear separate to us.

However this comes about, the critical result of MWI with respect to quantum Russian roulette is that in one world the cat is alive.

Now, suppose Alice is the experimenter here and in the time honored tradition of self-experimenters such as Sir Isaac Newton, who stuck a bodkin (a sharp, needlelike thing like awl) into his eye socket, J. S. and J. B. S. Haldane, a father/son pair, who tested poisonous gases and rapid decompression from diving on themselves, and several researchers who lived in caves for months on end to study the effects of cutting off all natural light on themselves, Alice decides to climb into the box and test the hypothesis of the Many Worlds Interpretation.

Bob meanwhile will remain outside the box to determine the results.

Alice and Bob have ensured that the experiment has three important components, according to Max Tegmark:

1. The trigger for death is quantum, not deterministic, not a random number generator.

2. The death is virtually instantaneous so that the person inside the box doesn’t register the outcome of the experiment.

3. The trigger is 100% fatal.

Alice climbs into the specially quantum sealed box. Suppose it is some kind of pod like you see on science fiction TV shows where people can go into cryofreeze to travel to other stars. What is death but a trip to another world?

Australian Philip Nitschke, whose name aptly sounds kind of like Nietzsche, invented something like this precisely designed to kill people who want to die. It is supposed to be tested in Switzerland. Like Swiss watches, I’m sure it works perfectly every time.

Once Alice is sealed inside, there is no turning back. The quantum mechanism fires.

For Bob, Alice has a 50/50 chance of coming out dead.

Bob has a secret only known to Alice, however, he is a psychopath who doesn’t mind killing people. Sometimes you have to recruit such people in the name of science.

If Alice comes out dead, so be it.

If she comes out alive, however, she goes right back in for another go.

While from Bob’s perspective, Alice does in fact die, from Alice’s perspective, she does not because her life, her soul as it were, sticks with the world where Alice survives.

That means that no matter how many times Alice passes through the death pod, it never triggers for her. She always lives.

The slippery slope of this argument, however, is that this must apply to all life and death situations. There is never a 100% chance of a quantum outcome, so there must always be a quantum chance, no matter how tiny, that in a life or death situation, you survive.

This possibility has enticed some people to conclude that we are immortal. We cannot die.

The thought experiment itself isn’t an example of quantum mysticism. All it says is that, if Alice, any copy of Alice, finds that the quantum death pod can’t kill her after 40 tries, then she must conclude that MWI, in all probability, is correct. After all, the probability that she would still be alive after that many attempts is one in a trillion.

Of course, there would also be a copy of Bob who observes this outcome as well, so he would conclude as much too.

Unfortunately, only 1 in a trillion worlds would know that because Alice is dead in all the others. Although it would confirm it for the experimenter, it would be her own subjective knowledge.

In addition, there is no indication that, because Alice dies instantaneously that her conscious mind will continue on with the living. Likely Alice’s mind would simply split like Bob’s and she would experience death with the usual probability, not immortality.

This is a direct consequence of the Born-Vaidman rule:

Probability Postulate
An observer should set his subjective probability of the outcome of a quantum experiment in proportion to the total measure of existence of all worlds with that outcome.

As well as this Behavior Principle:

We care about all our successive worlds in proportion to their measures of existence.

Probability amplitudes or “measures” as they are properly called indicate the total amount of probability assigned to some outcome.

You can think of probability measures as being like time intervals. If I were to offer you a prize if you did a similar quantum experiment 40 times and the consequence is that every time you lose you spend, say, a millisecond unconscious, in addition to your normal need for sleep, you shouldn’t play because you would, on average, spend about 32 years asleep. We already spend about that much time sleeping, so you would be unconscious for about 2/3 of your life.

If there is no splitting of worlds and the apparent splitting is just a psychological or neuro-physiological phenomenon, however, then it is possible that consciousness would remain with the living version of you (all the ones that do survive in fact). Your consciousness is “shared” across minds rather than being split. However, your probability amplitude of being in a living state would be very, very small, so you would be greatly diminished. You would fade from existence.

The real issue with the thought experiment is that it makes dying instantaneously a special case that is different from all other ways of carrying out quantum experiments. If the result were that Alice had to pay money each time she lost and won money if she won in the end, then she should play the same way she would if it were classical gambling. Why is the fact that the experiment is killing her make any difference in her decision making?

Nevertheless, quantum mystics latch onto this idea that we can survive anything. This is pure quackery.