1

How can macroscopic objects in real world have always-true cause-effect relationships when underlying quantum world is probabilistic? How does it not ever produce results different than what is predicted by Newtonian physics, except for borderline cases?

Qmechanic
  • 201,751
Aleksandar
  • 35
  • 4
    What about superconductivity, would you call it borderline? – anna v Dec 16 '21 at 06:54
  • 2
    What about lasers ? There is also no explanation for them in Newtonian physics. Also borderline ? – Alfred Dec 16 '21 at 07:06
  • 1
    Yet another example,- define Bose–Einstein condensate or macroscopic entangled systems within Newton mechanics :-D – Agnius Vasiliauskas Dec 16 '21 at 08:04
  • I see the technological advancements that quantum physics enabled of which superconductivity and lasers are examples. Also many others. Maybe better way to concretize my misunderstanding is to ask why if we conduct many experiments where identical lightbulbs are shining light through identical glass, there is always 96% of light passing through the glass and not a single in practically infinitely many cases where that percentage is e.g. 95% ? – Aleksandar Dec 16 '21 at 08:13
  • Does this answer your question? Where does the irreversiblity came from if all the fundamental interaction are reversible? – Roger V. Dec 16 '21 at 08:16
  • @Aleksandar You gave nice point to extrapolate a good argument from it :-) So ask- why 96% of light gets passed through that particular glass ? It is exactly because there's 96% chances that particular photon will pass a glass and 4% chances - that it will not. The biggest surprise is that you can't know in advance which photon will pass, and which - will not , after shooting it from source. This conundrum can be better analyzed only by using Quantum Mechanics, Newtonian mechanics passes out here completely, cause it can't explain phenomenon in details. – Agnius Vasiliauskas Dec 16 '21 at 08:30
  • @Agnius Vasiliauskas What I can't wrap my head around is if there is 96% chance that a single photon will pass, why can we always say with certainty(100%) that 96% of all the photons will pass? – Aleksandar Dec 16 '21 at 08:57
  • 1
    @Aleksandar Why you say with 100% certainty, that $1~kg$ of mass when pushed with $1~N$ force will go at $1~m/s^2$ acceleration ? It's probably because Newton law has firm roots in theoretical and experimental physics, right ? Same goes for certainty of materials reflectivity/transmittivity in Quantum Mechanics framework. Though, nobody has said that all physics law's are 100% certain. There's always a room for improvement in physical laws and there's always measurement(s) and calculation(s) errors in practice. – Agnius Vasiliauskas Dec 16 '21 at 09:08
  • 1
    " 96% of all the photons will pass" we do not say so. The 96% is for a classical electromagnetic wave . Photons are quantum mechanical entities – anna v Dec 16 '21 at 09:09
  • 1
    statistically, if you find the number of photons per second dividing the energy of the classical wave with $hν$, it is just a statistical number , not characterizing the individual quantum mechanical photon, – anna v Dec 16 '21 at 09:20
  • @anna v I picked " 96% of all the photons will pass" from Feynman video on youtube. – Aleksandar Dec 16 '21 at 09:22
  • from conservation of energy yes, but not the probability an individual photon will have to go through (x,y,z) at time t, which is what is measured in experiments with individual photons. In your set up, an individual photon can be scattered back, adding to the 4% less energy of the classical beam, 0 probability to pass through fell at it at that (x,y,z,t) – anna v Dec 16 '21 at 09:29
  • @anna v what do you mean by 'just a statistical number' and not 'individual photon'? Sorry, not familiar with quantum formulae. Just trying to develop way to roughly conceptualize how things work. I will also accept the answers such as 'that is not the question that should be asked' as Feynman said. I want to find as many answers that actually could be answered in intuitive way. – Aleksandar Dec 16 '21 at 09:40
  • I explain in my previous comment. statistics are classical physics – anna v Dec 16 '21 at 10:18

3 Answers3

3

Expectation values (of position, for example) in QM track the classical analogues. For large objects, position is localized and so is momentum (given the relative smallness of planck’s constant) - which keeps the position localized. In turn, this means that the expectation values of position closely match what we actually observe.

2

A simple way to understand this is to realize that a macroscopic object consists of trillions of individual quantum systems whose QM properties average out into Newtonian behavior as the number of particles in the system is increased.

The only exceptions to this rule are lasers, superconductors and condensates like liquid helium. In each of these special cases, those quantum properties get writ large for us by arranging for most of the quantum particles to not average themselves out but to instead (roughly speaking) all settle into the same state.

niels nielsen
  • 92,630
0

Statistical mechanics is a field that gives some good perspective to this. It's quite common that the math works out where you have an equation to describe your system, and there is a variable $N$ in your equation for the number of particles. If $N$ is a small value, the equation and your system still look very quantum. But once $N$ is large values, or you take the limit of the equation as $N$ approaches infinity (called the thermodynamic limit), you then see the system's macroscopic laws. This article might be of interest: https://arxiv.org/abs/1402.7172

Urb
  • 2,608
Chylomicron
  • 75