“The physics community is mostly divided into two groups,” Johannes Kofler tells *PhysOrg.com*. “One group believes that quantum theory is underlying the classical world, and that classical physics comes from the quantum. The other group thinks that quantum physics has to be altered. It forbids that quantum mechanics works on a macro level in the classical world by postulating additional laws.”

Kofler belongs to the former group. He and Časlav Brukner, both from the University of Vienna and from the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences, have developed a novel theoretical approach to understanding the transition from quantum to classical physics.

Their work has been published in *Physical Review Letters* with the title, “Classical World Arising out of Quantum Physics under the Restriction of Coarse-Grained Measurements.”

“Our motivation is to understand how the classical world comes out of quantum physics,” Kofler says. “The established approach in research is decoherence where one has to take into account the complexity of systems and interactions with environment.” It is interaction with the environment that brings decoherence into play, destroying quantum coherences and making it impossible to observe quantum phenomena. “We believe we found a process complementary to decoherence which can explain the quantum-to-classical transition.”

Instead of referring to the environment of a system, or even to the change quantum laws, Kofler and Brukner created a theoretical framework that stresses the use of measurement apparatuses. It is their restricted accuracy which limits the observability of quantum phenomena.

“We took a rotating spin as a model system,” Kofler expounds via email. “There is a condition which all classical theories have to obey, called the Leggett-Garg inequality, but which can be violated by quantum mechanics.”

Kofler and Brukner demonstrated that the time evolution of a quantum system, no matter how macroscopic the system is, cannot be treated in a classical sense. “Just because something is big doesn’t mean it can be described by classical physics.” Then referring back to the case of spin, he continues: “Arbitrarily large spins can still have a quantum time evolution and violate the Leggett-Garg inequality.”

Next, the two realized that coarse-grained measurements are used in realistic conditions, such as situations that we are confronted with every day, as the resolution of the apparatuses usually is limited. “If you are bound to restrict yourself to coarse-grained measurements of the spin,” Kofler explains, “you get the classical Newtonian laws of motion.”

“Start with a spin system of macroscopic size and the Schrödinger equation that produces the quantum time evolution. Restrict the precision of your measurements and you can see the Newtonian physics emerging.” Kofler explains that measurements in quantum mechanics generally change the system. “But under our coarse measurements this change is such that a classical description is possible.”

Kofler admits that so far this work is not fully finished. “It’s really fair to say that classical physics out of quantum theory has not been entirely achieved yet by anybody,” he says.

But he and Brukner remain optimistic that at some point we will be able to get a *complete* understanding of how our well-behaved classical world emerges from the strangeness of quantum physics.

*Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.*

**Explore further:**
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## Koen

So it is the strange and unproven assumption of a particle's non-existing intrinsic potential energy that is responsible for well known "quantum strangeness".

Yeah, quantum physics is the physics revolution of a strange and unnatural assumption, while the more natural assumption was ignored.

## MongHTan,PhD

I think all matters, if broken to the quantum elemental levels, have intrinsic properties; and with the interactivity of these elemental properties, that an infinite order of all matters appears in the Universe, on Earth and beyond, that as far as we can observe and fathom today: be it by quantum or classical physics.

As such, IMO, both the quantum and classical physics could belong to a spectrum of Physics, Chemistry, and Biology that may be collectively called Cosmology today! Thank you.

## LearmSceince

Also you don't make it clear that you are talking about your own invention, not actual science. that could be confusing or misleading to newcomers.

## LearmSceince

## atulK

Heres something from a layman:

has anyone given a thought in this direction:

whereever you stand (not literally) there will always be three theories (grand unification is to be unasked, If i would put it rightly). Three theories being, quantum (level below you) classical (your level) and general relativistic (grand level) And if at all you want theory of everything - then take this trio in a fractal manner at any level.

## fredrick

you mean to say that, there should be three seperate theories each describing their own scale, and there can be no theory unifying them all? I, uh, don't know what a "taking something in a fractal manner" is, so if I've got my interpretation wrong its because of that.

*If* that is what you were trying to say, then yes, there have been plenty of people who have thought like that. As I'm sure you can appreciate, though, if there was no unification it would be immensely unsatisfying.

The main argument against having three seperate theories (or *two*, as is usually argued) is when things start to overlap. As above, quantum mechanics doesn't only affect the world at a quantum scale - it can have effects at our scale as well (or at the scale of a cat, hint hint). Similarly, General Relativity overlaps with quantum mechanics when we get to lots of matter condensed into a very small area (ie, black holes and big bangs). Can we really have seperate theories for seperate scales, when in fact those theories don't always apply to those scales?

## atulK

well... what i mean by taking it in a fractal way is - the "EXPERTS" need to figure that out. At present it is just my inner voice that this could be the way. Also, I think you are treating scales very classically. In theory of everything even scales will be included ... so overlap should not be a problem (or singularity)

is there a way to communicate one on one - or form a subgroup? so that we dont bug others or deviate from main thread too much.

Atul Kumthekar

## fredrick

hmm... there are scales now, and they work well. But, unlike what a lot of people will try to tell you, they aren't scales based on size alone. If General Relativity is to be used for any particular scale, it is not 'very big', but 'very heavy'. Newtonian dynamics generally works for more scales than just 'around about human size', just it doesn't work very well for 'very fast' and 'very (very) small'.

Anyway, all i'm trying to say is that there is not a continuum of theories used for different scales of size. Neither is there a continuum of theories used for different scales of any quantity. The two big theories we have describe different things, different qualities of the universe - not different scales (their usual application to different scales is a result of their application in describing different forces). A theory of everything would combine these descriptions of different things, and as such, would work at all different scales... the only need for a scale system in a TOE would be a general rule of thumb for which classical 'force' is predominant, but a rule of thumb only (given the overlap mentioned above)

## atulK

thank you for such wonderful comments. I am delighted to get such aspects on what really is 'scale'. Size and Numbers. hmmm...

>Anyway, all i'm trying to say is that there is

>not a continuum of theories used for different

>scales of size. Neither is there a continuum of

>theories used for different scales of any

>quantity. The two big theories we have describe

>different things, different qualities of the

>universe - not different ...

Have not understood these statements to its full extent.

With scale of numbers i had this interesting question thrown to people around me

The question was (with apologies for digression)

Are these two experiments equivalent (the term may need precise definition too)

1. tossing a coin 'n' number of times (of course

one after the other assuming this has meaning

on "classical" time axis, but stretch of imagination permitted by all means!- SIPBAM)

2. toassing 'n' coins simultaneousely (again

treating simultaneity in classical way -

SIPBAM !)

tried to understand this paper related to this thread topic:

http://homepage.u...0403.pdf

But my naive understanding of scales for a theory was that the theory fails when it predicts answers beyond measurability within tolerance - there is no such thing as 'exact' in any vocabulary i guess.

But i understand its not as simple as this.

## earls

## Savgargin

## Ragtime

Nov 24, 2007