# Does QM violate the causality principle?

#### HamzahA

Hello,

As written, I am wondering if the causality principle gets violated in QM. Do virtual particles violate the causality principles? Do instantaneous events violate the causality principle?

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#### dipole

I have never heard of the "causality principle", you need to define what you are refering to.

However, Quantum Mechanics is a stochastic theory, not a causal one. The wavefunction of a particle can be thought of as being causal, or deterministic, but the actual measurment of the state of a particle is not.

#### craigi

There is a principle in special relativity that states that cause must precede effect for all inertial observers. This is also implicit in classical mechanics, but we simply call it causality.

QM doesn't violate causality, but it does cause complications for trying to find a simple intepretation of quantum mechanics. The EPR paradox challenges the notion of causality, for example, but since no meaningful information can be transmitted via the measurement of entangled particles, causality is preserved.

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#### James MC

It's been shown that if there is wave-function collapse, and if that collapse localises wave-functions, then this would violate causality as it would generate instantaneous infinite wave-function spread. This is Hegerfeldt’s theorem.
http://arxiv.org/pdf/quant-ph/0111060v2.pdf
Whether wave-function collapse occurs is a matter of "interpretation", or more precisely, a matter of how we ought to physically describe measurement processes themselves, and no one really knows how to do this.

#### bhobba

Mentor
In general all this stuff like causality being violated etc etc is very interpretation dependant.

QM as far as the formalism goes doesn't really say things like that.

If you really want to understand its conceptual core check out:
http://www.scottaaronson.com/democritus/lec9.html

Thanks
Bill

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