Quantum Suicide: Why is only experimenter convinced?

1. May 15, 2015

greypilgrim

Hi,

Tegmark argues that after $n$ repetitions of the Quantum Suicide experiment, the experimenter can rule out collapse interpretations of QM with certainty $1-0.5^n$ where, if I understand him correctly, $0.5^n$ is the probability that a collapse interpretation is correct but he was just on a streak of luck as the gun didn't fire in every of the $n$ repetitions.

He argues that this could only convince (with $n$ large enough) the experimentalist, but not the watching assistant or anybody else. Why is that? If the experimentalist is alive after $n$ repetitions, essentially he and his assistant are both in a lab and just heard $n$ clicks, but no gunshot. Why does it make such a big difference that the gun is pointed at the experimentalist and not the assistant?

Another thing I don't quite understand: Tegmark requires the trigger mechanism to be such that "the timescale between the quantum bit generation and the actual firing is much shorter than that characteristic of human perception, say $10^{-2}$ seconds".
Why do we need that?

Also: Of how much relevance is human consciousness in this experiment? A major problem of Copenhagen is the question of what systems can cause a collapse, and some authors think that only a conscious being can. MWI in general doesn't need such anthropocentric assumptions, but what about this particular experiment?

2. May 15, 2015

Staff: Mentor

I'm sorry, but a thread that starts with Tegmark's somewhat idiosyncratic gloss on the MWI isn't likely to go anywhere good.

Anyone who believes that they have a good answer to GreyPilgrim's question, PM me and we'll consider reopening the thread.

3. May 15, 2015

Staff: Mentor

Open again after some offline discussion

4. May 15, 2015

craigi

This thought experiment does allow us, in principle, to determine between the MWI and other interpretations and where the discussion would lead, if it were allowed, is to other ways that the MWI might be proven in the not too distant future (in the next century), such as work by Deutsch.

In the world where he survives every trigger pull, he was lucky compared to the other versions of experimenters who died, in the other worlds. But after n iterations, he could only be in the world where he survives, because he, as a consciouss entity, only exists in one world. To contemplate whether he survived requires that he is still alive. This is just classical probability.

What is the probability that he will survive n binary life or death events? 0.5^n

What is the probability that he has survived n binary life or death events, given that he's around to ask the question? 1

As n approaches infinity, the experimenter would be forced to conclude that both life and death outcomes are realised, and in turn conclude that the MWI is correct and that other contrary interpretations are incorrect. In the world where the experiment survives, he would be with an assistant who would draw the same conclusion as himself. However, there would be an overwhelming number of versions of assistants in other worlds, who wouldn't be able to draw this conclusion. So an assistant observing this experiment, wouldn't be able to draw any conclusion about the validity of the MWI.

The only reason for this, is that if the experimenter becomes aware that he is about to die, then will not be able to draw the conclusion that MWI is true. However, there would still be a version of the experimenter who would conclude that the MWI is true, even though an overwhelming number of versions experience their own deaths.

The relevance of consciousness is, nothing more than a requirement to understand whether the MWI is correct, after performing the experiment.

I think it goes without saying, don't try this at home!

It's worth noting, in case you missed it. That under the other interpretations, with the exception of MWI derivatives (such as the Relational and the Cosmological interpretations), as n approaches infinity, the experimenter has a probaility tending towards 1 of simply ceasing to exist.

5. May 15, 2015

Staff: Mentor

The quantum suicide experiment is, to be blunt, rubbish, as easily seen by the fact its argument is symmetric. It says you never experience death because in some universe you are alive. Of course you can just as well argue you never experience life because in some universe you are dead. What it relies on is something philosophers know well - namely a certain intuitive feeling we have about death and the status of experiencing it. You will find a detailed discussion about it in David Wallaces book:
https://www.amazon.com/The-Emergent-Multiverse-according-Interpretation/dp/0198707541

That isn't an issue at all for modern Copenhagen because these days an observation is considered to have occurred once decoherence has happened which is an unambiguously defined process. Our modern understanding of decoherence has morphed the measurement problem somewhat - its now not what causes collapse - its why we get any outcomes at all, which at a technical level is how a an improper mixed state becomes a proper one:
https://www.amazon.com/The-Emergent-Multiverse-according-Interpretation/dp/0198707541

Conciousness has nothing at all to do with QM, except in some very fringe interpretations. Despite Tegmarks attempt to show otherwise it has nothing to do with Quantum Suicide either.

Thanks
Bill

Last edited by a moderator: May 7, 2017
6. May 16, 2015

stevendaryl

Staff Emeritus
Well, no, because you can't "experience" death. That's the asymmetry. (Of course, you can experience the intense pain immediately prior to death, but presumably you can arrange things so that death happens so quickly that there's no time to experience anything.)

The stuff about suicide is only partly about quantum mechanics. It's also about the nature of identity, which is an interesting subject, but it's more philosophy than physics. However, I don't think you can firmly come down on one side or the other of a philosophical argument without engaging in philosophy. The conclusion "it's rubbish" is as much a philosophical conclusion as "it sounds convincing to me".

7. May 16, 2015

ShayanJ

Well, such a thing is rubbish, as far as physics is concerned. In physics, we work with statements that can be decided whether from first principles, accepted theories or experimentally verified facts, not statements that are philosophical. So I'm with bhobba, its rubbish!

8. May 16, 2015

stevendaryl

Staff Emeritus
But "It's rubbish" is a philosophical conclusion, not a scientific one.

9. May 16, 2015

ShayanJ

OK, its not rubbish. Its something we don't like in physics so we put it out for garbage collectors to take. Note I'm not telling its wrong, I'm just telling its useless, gives no conclusion. We do thought experiments because with some reasoning, we can draw conclusions from them. But what is the conclusion of this experiment? It just says if you do this experiment, the probability of MWI being true increases with the amount of time the experimenter stays alive after starting. You can do no reasoning, you can draw no conclusion. So why is it called a thought experiment? Why is it in physics? I prefer to put it outside for others to get it, whether they are garbage collectors or philosophers!

10. May 16, 2015

stevendaryl

Staff Emeritus
I thought the point was that MWI is experimentally verifiable, if you're willing to risk your life.

11. May 16, 2015

ShayanJ

That's the point. Even if you risk your life, you can't verify it. For that, you need an ethnic cleansing of physicists!!!
Because the reasoning is, as we iterate more and the experimenter doesn't die, the probablity of MWI being true increases. So if the experimenter does the experiment for all of his lifetime and doesn't die, its very probable that MWI is true. But we know that its not going to be like that. There is a really low, near zero, probability for that to happen. So You get the first physicist, he dies after a week. The second one, after two days. The third one, after 10 days. The fourth one, after one month. And so on and so fourth. You will never get a conclusion. The experiment is inconclusive even in telling us how probable it is for MWI to be correct, let alone telling us its correct or not!

12. May 16, 2015

ShayanJ

its good to continue further. Imagine we get to the luckiest physicist of all time and s\he performs the experiment all of his\her lifetime. But it only proves to the experimenter himself\herself that MWI is probably right. Others won't understand this. So this lucky experimenter understands MWI is probably correct but nobody believes him\her because there is no evidence. So s\he dies and the remaining physicsits will continue the experiment until they run out of physicists. Then its up to the last physicist to be smart enough and forget about MWI. But if s\he's not, s\he'll do the experiment too and dies. And physics goes to extinction along with all of the physicists. R.I.P. Physics!!!

13. May 16, 2015

craigi

Of course, the experiment couldn't be performed to demonstrate to the scientific community that the MWI is correct. It was never intended as such. To dismiss it on those grounds is to misunderstand its purpose.

It's a stark illustration, or the differing implications between collapse and relative state intepretations.

Contrary to common belief this is not purely philosophical and doesn't represent a dead-end for physics. I think people are generally getting this so let me offer a next step in developing this.

Suppose I were to tell you that I were about to roll a 6-sided die, once only, and if it doesn't land on 6, I will instantaneously erase your memory. Presuming of course you believe that I have the technology, what are your expectations for observing a 6 given each of the collapse and MWI interpretations?

This isn't the physics part yet. You'll have to wait for that, but try not to get thrown off by dismissing it as another useless thought experiment.

Last edited: May 16, 2015
14. May 16, 2015

Staff: Mentor

That's the philosophical issue where it gets traction.

Replace it by a machine to see the silliness of the argument.

Thanks
Bill

15. May 16, 2015

Staff: Mentor

Like I said its based on a feeling we have about death that introduces an asymmetry. That feeling is on shaky philosophical ground as easily seen by replacing it with say a computer that passes the Turing test.

Its not really something we discuss on this forum but the philosophy of the situation is detailed in Wallace.

IMHO is a total load of BS.

Thanks
Bill

Last edited: May 16, 2015
16. May 16, 2015

Staff: Mentor

It doesn't prove that.

Any interpretation will explain that thought experiment.

Thanks
Bill

17. May 17, 2015

Quantumental

To make matters even worse, consider the fact that the metaphysics of MWI isn't worked out. MWI can be interpreted metaphysically as being about worlds splitting (fission picture) or diverging. In the divergent picture all the worlds exist from the dawn of time, but diverge as changes occur in the branches. So world A and B is identical until X happens in A and in world B Y happens. If you are on branch B, you always were on branch B, you just didn't know yet which you were on.

David Wallace as well as Simon Saunders propose the diverging view in the "Many Worlds?" book. Alastair Wilson has written several papers about this which can be found here: http://alastairwilson.org David Deutsch's 'fungible' model is also resembling this rather than the traditional "splitting" which we are often presented with.

So how does this relate to Quantum Suicide/Immortality? If you never split, only diverge, you are isolated on your branch and so if you die there, you die there just as you would in a classical one world model, you were never your doppelganger.

18. May 17, 2015

Nick666

What kind of interpretation ?

In the world where the scientist doesnt die even after one million clicks, its obvious not only for himself but for other people in that world that are witnessing the experiment, that the chance of what has happened is ultra-low. If it would be our world, what would you conclude about such an experiment ?

Last edited: May 17, 2015
19. May 17, 2015

Staff: Mentor

Did you see the word any?

That's the first issue isn't it - the proof there are multiple worlds.

If many worlds was true the conclusion I would draw is I happen to be in a world where he was alive and that world has low, but non zero, probability. So?

Thanks
Bill

20. May 17, 2015

atyy

Tegmark is wrong. To prove MWI, he must know that he died in the other world. Otherwise, his observations are consistent with a failure of quantum mechanics (or at least that he is using the wrong Hamiltonian).