# Multiple universes, measurement and laws of nature

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1. Nov 13, 2015

### entropy1

Considering the multiple universe view, if a measurement (or something else) makes visible which universe we are in, is it then also possible we find ourselves in one with (slightly) different laws of nature?

2. Nov 13, 2015

### phinds

Not sure what you mean. We are in the universe we are in. There is zero evidence of the existence of other universes (the "multiverse") but if such do exist it is believed that there can be no causal connection between our universe and any other.

3. Nov 13, 2015

### entropy1

Maybe I am using the wrong terminology, and do I have to say: "Many worlds". I was inspired by this:
I don't know much at all about this. I have heard though things along the line as that 'quantum uncertainty' (what is that?) allows for virtually an infinite amount of universes. So I was thinking: could there be universes among them that are looking slightly or even entirely different with respect to laws of nature, and would these 'quantum uncertainties' allow for navigating towards these universes? I guess a different way to phrase this would be: 'do the laws of nature have to be fixed?'. May be far fetched, but I am incredibly curious!

Last edited: Nov 13, 2015
4. Nov 13, 2015

### phinds

I think the many worlds interpretations of QM are utter blather so I'm not the person to help you with this.

5. Nov 13, 2015

### entropy1

I take no position in this. I am interested in the way QT sets boundaries on different approaches of conceptualising the phenomena in QM (preferably in a subjectivistic mathematical way).

6. Nov 13, 2015

### Staff: Mentor

No. The "multiple universes" of the MWI are much less interesting than that (and the world would be a better place if decades back we had thought to use the less exciting term "multiple outcomes interpretation").

All the post-measurement "universes" have to obey the same laws of physics because the only thing that's different between them is which way the wave function collapsed. The possible outcomes of the collapse were already in the pre-measurement wavefunction and obeying the laws of physics before the measurement.

Let's say I prepare an electron into the state that is spin-up along the z axis: $|z_+\rangle$. Send it through a Stern-Gerlach device oriented along the z axis, and of course it is deflected upwards with 100% probability, there's only one possible outcome and no split. So now I send it towards a Stern-Gerlach device oriented horizontally, along the x-axis. I take advantage of the identity $$|z_+\rangle=\frac{\sqrt{2}}{2}(|x_+\rangle+|x_-\rangle)$$ which makes it easy to calculate that half the time the measurement will be spin-up along the x axis and half the time it will be spin-down. Either way the laws of physics stay the same - I just did a measurement and got a result, no reason to expect the laws of physics to start changing under me.

A collapse interpretation says that the wavefunction collapsed so that I end up in a universe (the same one I started in, and the only one) in which the post-measurement wavefunction is either $|x_+\rangle$ or $|x_-\rangle$. MWI says that I end up in one of two post-measurement universes, differing only in that in one of them the post-measurement wavefunction is $|x_+\rangle$ and in the other it is $|x_-\rangle$.