# B Thought experiment -- A radioactive decay detector and randomness

#### dRic2

Gold Member
Take one radioactive element and put a detector all around it so that you can immediately detect whenever it will undergo radioactive decay. Have a clock connected to the detector to note the "exact" instant at which the atom decays. Let's say that after 3min after the clock started counting the atom decays.

Now (here comes the silly parte :D) imagine to go back in time and watch the same atom again. Will the outcome of the experiment be the same (the atoms decays exactly after 3min)? What do you think?

I know this is a kind of stupid experiment since we can't go back in time in the first place. Nevertheless this is puzzling me a lot.

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#### DrClaude

Mentor
Quantum mechanics is fundamentally probabilist. There is nothing in the theory that will tell you when a random event will be observed.

If you don't like that, you'll need a new theory beyond quantum mechanics, with elements not included in the current theory.

#### phinds

Gold Member
Now (here comes the silly parte :D) imagine to go back in time and watch the same atom again. Will the outcome of the experiment be the same (the atoms decays exactly after 3min)? What do you think?
It's not a matter of what we think, it's a matter of how the universe works. As Dr. Claude has pointed out, the way it works is probabilistic, so the answer to your thought experiment is that no, the chance of it being the same is near zero.

My point is, you should not be approaching science with questions like "what do you think?" but rather with questions like "what is the reality of the situation?"

#### dRic2

Gold Member
Okay. I just put "what do you think" to sound friendly for no reason.

Btw I do not have an opinion on QM... I do not know why you assumed I dislike it so much ahahah

The question came out thinking about the role of probability in QM. If a have a set of n particles undergoing decay then it is easy to talk about probability... here it is different and I thought that going back in time should not affect the result of the experiment. I was wrong. It seems very strange to me that something could act differently if I go back in time... That is all. But I guess this answer is to be expected otherwise it will imply that there is a certain way to reproduce the experiment (ie going back in time) and this is not possible because it violates the law of probability underlying QM.

This is what I was thinking. Can I say it is correct ?

#### DrClaude

Mentor
From the point of view of QM, going back in time an re-performing the experiment will give the same evolution of the wave function, so nothing has changed. But the theory is silent as to when the decay will be observed.

#### ZapperZ

Staff Emeritus
2018 Award
Take one radioactive element and put a detector all around it so that you can immediately detect whenever it will undergo radioactive decay. Have a clock connected to the detector to note the "exact" instant at which the atom decays. Let's say that after 3min after the clock started counting the atom decays.

Now (here comes the silly parte :D) imagine to go back in time and watch the same atom again. Will the outcome of the experiment be the same (the atoms decays exactly after 3min)? What do you think?

I know this is a kind of stupid experiment since we can't go back in time in the first place. Nevertheless this is puzzling me a lot.
How is this different than having multiple setup with the identical element and observing them?

Zz.

#### dRic2

Gold Member
How is this different than having multiple setup with the identical element and observing them?

Zz.
You are sure that they are identical. My point is that if A happens and you go back in time and this time B happens instead of A, then you are sure that there is nothing "hidden".

#### DrChinese

Gold Member
You are sure that they are identical. My point is that if A happens and you go back in time and this time B happens instead of A, then you are sure that there is nothing "hidden".
ZapperZ' point is that his version can be done, and there is no known difference in the initial setup. Theory and experiment match: there is no correlation between decay times of different particles that begin in identical states..

On the other hand, your version cannot be tested (since we can't time travel). Clearly, you can simply reject the obvious conclusion of ZapperZ's version because it does not meet your impossible standard. But the prediction would be: no correlation between one and the other.

It should be pointed out that the existence of "hidden" variables in the quantum world is rejected by most. All attempts to find them have been met with abject experimental failure and significant theoretical rejection.

#### dRic2

Gold Member
I didn't want to propose an experiment which is clearly impossible. What I wanted to say is that, given the postulates of QM, if I were to attempt such an experiment, what would I find ?

Re-reading my answer to ZZ's post I think I spoke (wrote) without thinking too much. I'm not saying that ZZ's experiment is not good as mine, I wanted to say that I'm shocked that they would be the same (if mine was possible).

We know that particles are identical, but it is more shocking to think that the same "object" could behave differently in same condition and same "time".

#### DrChinese

Gold Member
We know that particles are identical, but it is more shocking to think that the same "object" could behave differently in same condition and same "time".
To be "shocked", you'd have to believe in determinism. Please note that in some interpretations of QM, there is determinism.

#### DennisN

imagine to go back in time and watch the same atom again. Will the outcome of the experiment be the same (the atoms decays exactly after 3min)? What do you think?
The experiment can't be done, so whatever anyone think would happen would be guesswork at best and fantasy at worst.
I remember a previous thread here on PF with almost the same premise, and I will see if I can find it...

Edit:

Is the future undetermined or simply undeterminable?

Last edited:

#### dRic2

Gold Member
The experiment can't be done, so whatever anyone think would happen would be guesswork at best and fantasy at worst.
Although I already stated this in my original post, I don't totally agree. If you accept QM, even if you can't go back in time, you can say for sure that if you could the outcome would be the one that the other members said.

To be "shocked", you'd have to believe in determinism. Please note that in some interpretations of QM, there is determinism.
I never tried to interpret QM. Usually I stick to the famous "shut up and calculate". But once in a while I think about it and I find my self clueless and "shocked".

#### ZapperZ

Staff Emeritus
2018 Award
You are sure that they are identical.
And you are sure they are not?

Your second go-around after you reverse time is that you end up with the SAME, identical radioactive sample with the same, identical setup. How is this not the same with having multiple setup, each having the same radioactive sample? We can even make this simpler. Instead of a sample, let's just have ONE free, isolated neutron.

Are you saying that your neutron that had travel back in time and restarting the experiment is different than my neutron that didn't go through the time-backward travel? How are they different as they are about to go through the same process?

Zz.

#### DennisN

Although I already stated this in my original post, I don't totally agree. If you accept QM, even if you can't go back in time, you can say for sure that if you could the outcome would be the one that the other members said.
So what you are saying is "we can not test this, but we can say for sure that we know what will happen."?
That's not what I call science.

The problem here, as I see it, is when you hypothetically invoke the concept of time-travel, it derails the thought experiment. Because no-one knows what time-travel itself would imply.

#### PeterDonis

Mentor
imagine to go back in time and watch the same atom again.
You don't have to imagine that. Just imagine doing the same experiment many times with many identically prepared atoms. The times will vary randomly.

#### ZapperZ

Staff Emeritus
2018 Award
You don't have to imagine that. Just imagine doing the same experiment many times with many identically prepared atoms. The times will vary randomly.
But unfortunately, if you saw his reply to my suggestion to do this, he doesn't think that it is the same thing as his scenario.

Zz.

#### PeterDonis

Mentor
given the postulates of QM, if I were to attempt such an experiment, what would I find ?
This question is unanswerable, because given the postulates of QM, the experiment you want to attempt is impossible.

#### dRic2

Gold Member
And you are sure they are not?

Your second go-around after you reverse time is that you end up with the SAME, identical radioactive sample with the same, identical setup. How is this not the same with having multiple setup, each having the same radioactive sample? We can even make this simpler. Instead of a sample, let's just have ONE free, isolated neutron.

Are you saying that your neutron that had travel back in time and restarting the experiment is different than my neutron that didn't go through the time-backward travel? How are they different as they are about to go through the same process?

Zz.
You have two identical particles A and B. You wait 3 minutes: A decays, B doesn't. You might think that A and B are not identical after all: maybe there is something you do not see inside them. I've heard (I did not study it yet) that such problems are called "hidden variables theories" and that Bell's theorem rule them out. So, A and B must be identical and the different behaviour can only be explained in therms of probability. This being said, I do not think Bell's theorem is something so obvious to people thus my concern in considering the particles to be identical. If you go back in time and you stick with only one particle then you are sure (at least in my opinion) that you do not introduce any other variable by considering other particles (this was the whole point of my argument).

That being said, problem solved. Thank you all for the replies.

I just have one more curiosity
This question is unanswerable, because given the postulates of QM, the experiment you want to attempt is impossible.
I though time-travel are not forbidden by QM, but by Relativity. Did I miss something ?

#### PeterDonis

Mentor
I though time-travel are not forbidden by QM, but by Relativity.
It's not that time travel is absolutely forbidden by either; you can construct self-consistent scenarios in which time travel occurs. But such scenarios will have to obey the Novikov Self-Consistency Principle:

https://en.wikipedia.org/wiki/Novikov_self-consistency_principle

At least, that's the only consistent formulation of time travel scenarios that we know. And going back in time to re-do a measurement and having it come out a different way would violate that principle.

#### ZapperZ

Staff Emeritus
2018 Award
You have two identical particles A and B. You wait 3 minutes: A decays, B doesn't. You might think that A and B are not identical after all: maybe there is something you do not see inside them.
This is what we call "speculation".

Zz.

"Thought experiment -- A radioactive decay detector and randomness"

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