I am wondering whether it is true to ask whether determinism is still completely viable at macroscopic scales given that the constituent particles behave according to QM when the dimensions get small enough?

I have commonly heard that QM is valid at the extremely small scale, whereas classical physics support larger scales, but what about macroscopic devices that behave because of quantum effects, like the tunnel diode or macroscopic observations that require QM to explain like the photoelectric effect and blackbody radiation?

  • $\begingroup$ Depends on what you mean by "determinism". Quantum mechanics is fully deterministic, only its physically measurable consequences are not. More generally, quantum mechanics is, as far as we can tell, valid on all scales, you are simply not used to thinking about it that way. We can measure coherent photons that have come from sources millions of lightyears away. That's quantum mechanics at work across cosmic distances. $\endgroup$ – CuriousOne Oct 9 '14 at 14:58

Since it is possible to build a macroscopic device whose state changes according to something that happens at the quantum level, the obvious answer is "Yes". By macroscopic determinism we usually mean that quantum effects are below a level that is measureable. Not that they don't exist. For example, quite large molecules such as fullerenes can show quantum interference effects, as can micromachined devices such as vibrating mirrors. So far there is no known upper mass limit for these effects.


You are labouring under some misconceptions.

Quantum mechanics is deterministic. It describes each system in terms of observables that evolve deterministically according to an equation of motion. However, these observables describe multiple versions of that object that can interact with one another in quantum interference phenomena. Objects interact with one another and the environment in such a way that they form layers each of which approximately obeys the laws of classical physics. The whole set of layers is called the multiverse:


The applicability of classical equations of motion on a macroscopic scale is a consequence of quantum mechanics. Distinctively quantum mechanical effects are more difficult to detect in macroscopic objects as a result of interaction with the environment, not because quantum mechanics ceases to apply to macroscopic systems:


Since multiple versions of a system can interact to produce a given outcome there is, in general, no such thing as which of those versions produced any given outcome. So while it is possible to predict how the multiverse as a whole will evolve it is not possible to predict what you will see because there is no single fact of the matter about that.

Explaining the results of experiments often requires explaining the behaviour of macroscopic objects using quantum mechanics. For example, the explanation of the EPR experiments requires describing "classical" systems in terms of their quantum observables:


  • $\begingroup$ Say, when was the last time a physicist has detected a multiverse? Would that be never? $\endgroup$ – CuriousOne Oct 9 '14 at 14:52
  • $\begingroup$ When was the last time a physicist detected the core of the sun? Never. So the core of the sun doesn't exist, right? The stuff that exists is what's required to explain how the world works. Quantum mechanics explains how the world works in terms of the multiverse, so the multiverse exists. $\endgroup$ – alanf Oct 9 '14 at 15:20
  • $\begingroup$ The last time physicists detected the core of the sun was the last time a solar neutrino experiment identified a solar neutrino. I would say that was rather recent, could have been in the last few minutes, or so (I don't know which facilities are active, right now). So where is my parallel universe detector? $\endgroup$ – CuriousOne Oct 9 '14 at 15:25
  • $\begingroup$ The thing that arrived at the detector was a neutrino, not the core of the sun. That measurement is a detection of the core of the sun in the sense that the core of the sun is needed to explain the outcome of the experiment. Similarly, the multiverse is needed to explain the results of lots of experiments, e.g. - single particle interference and EPR experiments. $\endgroup$ – alanf Oct 9 '14 at 15:49

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