# Does true randomness exist in nature?

• A
Does randomness exist in nature?

We say every event must abide by the laws of nature, including QM probability/uncertainty. QM says outcomes are uncertain. Does uncertainty imply both randomness and probability? It seems that randomness is superfluous to the uncertainty principle, and it makes more sense to say only that uncertainty implies probability, and vice versa. Or are uncertainty and probability unrelated? According to Wiki they are:

Wiki: "One way to quantify the precision of the position and momentum is the standard deviation σ. Since is a probability density function for position, we calculate its standard deviation."

But there are other laws of nature we can use to test the viability of randomness. Here's a question/problem about randomness using conservation of momentum (CoM):

Particle A with mass Ma, moving at less than the speed of light in the X direction, hits particle B of mass Mb. The resulting velocity of particle B can be known using CoM.

If, instead, the resulting velocity of particle B was a distribution curve (uncertain), CoM would be violated.

Am I missing something here?

## Answers and Replies

The velocity of particle A is also uncertain.

• Truecrimson
entropy1
Gold Member
I understand that there is no way to determine randomness, since measurements of uncertainty are probabilistic by definition (axiom). I was told there is a deterministic version of QM, namely the Bohmian interpretation.

The velocity of particle A is also uncertain.
Does the uncertainty mean that the velocity is varying?

I understand that there is no way to determine randomness, since measurements of uncertainty are probabilistic by definition (axiom). I was told there is a deterministic version of QM, namely the Bohmian interpretation.
Right, I read about that recently. The Broglie-Bohm theory takes into account all known effects from across the universe. That makes a lot of sense to me.

ChrisVer
Gold Member
Particle A with mass Ma, moving at less than the speed of light in the X direction, hits particle B of mass Mb. The resulting velocity of particle B can be known using CoM.
with what certainty do you know the velocity of A? [I am talking in the macroworld]... Even in the macroworld, measuring those quantities just once will not prove you the CoM...as an example your experiment's result might be (on X axis alone) : $p^{A}_{init} =0.452,~p^{B}_{init}=0, ~~~~p^{A}_{fin} =0.312, ~p^{B}_{fin}=0.142$ (some momentum units).
In order to say whether CoM holds or not, you have to do several such measurements and at the end test it as an hypothesis [CoM is true vs CoM is false]...this will prove CoM within some certainty/confidence. If nothing's wrong with your setup, you'll be able to reject the CoM is false hypothesis.
Nothing is absolute in physics, but some things are so certain that it's almost impossible to not be true. In fact even giving you the momenta measured without some uncertainty would be - points in a lab report.

The rest [qm's uncertainty principle] is not telling you that your momentum will always have an uncertainty... it tells you that the position and momentum cannot be known simultaneously [or in other words by measuring the one you lose information on the others]... a particle that is in a momentum eigenstate will have a determined momentum with 0 uncertainty [theoretically speaking].

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with what certainty do you know the velocity of A? [I am talking in the macroworld]... Even in the macroworld, measuring those quantities just once will not prove you the CoM...as an example your experiment's result might be (on X axis alone) : $p^{A}_{init} =0.452,~p^{B}_{init}=0, ~~~~p^{A}_{fin} =0.312, ~p^{B}_{fin}=0.142$ (some momentum units).
In order to say whether CoM holds or not, you have to do several such measurements and at the end test it as an hypothesis [CoM is true vs CoM is false]...this will prove CoM within some certainty/confidence. If nothing's wrong with your setup, you'll be able to reject the CoM is false hypothesis.
Nothing is absolute in physics, but some things are so certain that it's almost impossible to not be true. In fact even giving you the momenta measured without some uncertainty would be - points in a lab report.

The rest [qm's uncertainty principle] is not telling you that your momentum will always have an uncertainty... it tells you that the position and momentum cannot be known simultaneously [or in other words by measuring the one you lose information on the others]... a particle that is in a momentum eigenstate will have a determined momentum with 0 uncertainty [theoretically speaking].

If we or any other life form didn't exist in the universe in order to do any measurements, could there exist a particle travelling with constant velocity for a very short time, say 10 Planck times? Or is everything vibrating and inconstant as in String Theory?

vanhees71
Science Advisor
Gold Member
According to QT there is "true" randomness in nature in the sense that for any system in any state there are always observables whose values are undetermined. Of course, now I expect a discussion rapidly getting over 100 postings all employing "interpretations" and other esoterics. But be warned, all that's no physics ;-).

• bhobba
bhobba
Mentor
Does the uncertainty mean that the velocity is varying?

Of course not.

In QM we have interpretations that are totally deterministic with exact velocities etc - eg BM. The reason we cant predict outcomes is in BM due to the certainty relations you cant know initial conditions.

The way to understand these fundamental issues (eg exactly what QM does and does not imply) is to study the theory and interpretations.

I highly recommend the following:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Thsnks
Bill

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bhobba
Mentor
According to QT there is "true" randomness in nature in the sense that for any system in any state there are always observables whose values are undetermined. Of course, now I expect a discussion rapidly getting over 100 postings all employing "interpretations" and other esoterics. But be warned, all that's no physics ;-).

QM indeed has 'true' randomness. All interpretations do is explain it differently.

Thanks
Bill