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I am starting studying quantum mechanics and observed that classical physics ceases to be accurate at the microscopic world (atomic length-scales).

Here is my question, cast as a thought experiment regarding the change of the scales:

Imagine an astronomically large world, so large compared to our world that our world appears as a ‘microscopic world’ to the observers in that larger world (e.g., our planets will be, relatively, the size of electrons), then

  • Will their physics be different from our classical physics? That is, do we obey a different type of physical law as observed from their perspective?
    If the physical laws change when changing the scale of observation, is this change just due to the spatial size (length scale) of the observed phenomena?

  • If a scientist from the larger world discovers earth (which is of the size of an ‘electron’ for her) would we be behaving quantum-mechanically as observed by the “big” scientist?

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    $\begingroup$ I'm voting to close this question as off-topic because it's about what a hypothetical giant alien scientist would see when observing us. $\endgroup$ – Kyle Kanos Dec 4 '15 at 11:27
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat, since it seems that the discussion about whether this is on topic (and related issues) is ongoing. $\endgroup$ – David Z Dec 7 '15 at 17:42
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I suppose that the question can be understood as a “thought experiment” about the relevance of scales (energy, length or time scales) in the laws of physics as we understand them. In my view, the core question is:

“Does the descriptive laws of a system change as one changes the scale at which one observes/probes the physical system?”

This is a rightful and worthy question in physics — and physicists have indeed tried to answer that!
A similar question has been also asked and answered here: “Are the physical laws scale-dependent?”.

a1) If there is a large world so large, when their world is compared to us, our world becomes a microscopic world, ... so will their physics be different from classical physics?
a2) Is it due to the size ... that the physical laws have changed? If so, will the physical laws differ for the world where our planet is the size of an electron according to their scales? Will we have a different type of physical law as observed from their perspective?

This can be true (i.e., the physical laws for the “giants” could be different), regarding the fact that at different scales (energy, length or time scales), we observe different laws of physics. Physicists are aware of this fact that every theory has a range of validity. This can be understood in the most fundamental way, via the idea of renormalization group; see e.g., Anderson, P.W. “More is different”, Science 177, 4047 (1972) pp. 393–396 <PDF>.

More intuitively, one can see that at an atomic scale (length $\sim 10^{-9}$ m), non-relativistic quantum theory provides a good description of matter, while at macroscopic scales (length $\sim 1$ cm) classical mechanics is enough to describe the physics [see “length scales”]. All physical laws have intrinsic scales which determine the range of their validity; for example, non-relativistic classical physics becomes a good description when velocities are much smaller than the velocity of light, $c$.

Therefore, at those extremely large scales where the “big scientist” explores (which should be beyond astronomical scales), the laws of physics could certainly be different.

b) If a scientist from the large world discovered earth ... will we be behaving quantum mechanically as observed from the “big” scientist.

In the answer to part b), I have interpreted the part “will we be behaving quantum mechanically” as “behaving according to a more fundamental laws of physics” — which would not be necessarily quantum mechanics as we know it. I elaborate on this point in part c).

Generally, it is possible in principle (but not always in practice!) to obtain the laws of physics at a larger scale from the more fundamental laws at a smaller scale. So, ultimately, it could turn out that the laws of physics observed by the “big scientist” can be obtained by the physical laws which we, at a smaller scale, have discovered (for example, the standard model + general relativity). Hence, if the “big” scientist probes down to our scales, she observes the fundamental laws which we have obtained so far.

c) will [the “big” scientists] conclude that our planet obeys the laws of quantum mechanics?

First of all, if we accept our current knowledge of nature as true, then quantum mechanics is the fundamental law which governs us (as systems of many particles), the Earth, etc.

The giant scientists who observe us ‘from above’ will see that the classical laws of physics describes the macroscopic phenomena on Earth. If they delve deeper into microscopic (atomic) scales, they will observe that quantum laws provides a good description of the phenomena at that scale. Hence, the giants would observe the same behaviour and laws as we observe; the reality does not change.

However, it is possible that the “big” scientists observe different laws at their own (very large) scales; but when they observe the “small-scale” objects as our Earth or our bodies, they obtain the same laws as we have obtained.

Final Note:
One would understand the importance of the scales better if one compares the observations of us humans and tiny microbes$\ast$:
When we observe the water in a glass, we see it as a static system in mechanical and thermodynamic equilibrium (with no apparent changes) for which, for instance, the Archimedes’ principle is valid. But a small microbe (of the size $10^{-6}$ m), would ‘see’ the same water as a very complicated fluctuating system which is not (locally) at mechanical equilibrium at all! The microbe needs (at least) some kind of Navier-Stokes equations (elaborate fluid dynamics) to describe the behaviour of the fluid surrounding it. If we also probe (e.g., with a ‘microscope’) the water at the micro-metre scales (the scale where the microbe lives), we will observe the same behaviour as the microbe. If we probe further into smaller scales (like the atomic scales) — as physicists have done — we will see that at that scale, quantum laws prevail, and that the laws at larger scales can be ‘derived’ (in principle) from such fundamental quantum laws. This shows the striking effect and importance of scales in the observed physical laws.

I hope that the answer provides a better understanding of the issue. Beyond this, one needs introduce a more elaborate formalism (renormalization group) which seems not proper at the current level of discussion.


$^\ast$ In this analogy (which is, by the way, far from sci-fi!), humans play the role of the “big” scientists and the microbes play the role of tiny objects under observation.

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  • $\begingroup$ You make a very good point but I don't understand by what you meant when you said "it is possible in principle but not always in particle" $\endgroup$ – Socre Dec 4 '15 at 17:46
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    $\begingroup$ Socre, I said “possible in principle (but not always in practice!)”. That means that even if you have a very good fundamental theory (for instance, the Schrödinger equation), it is not possible in practice to derive the physical properties of all macroscopic bodies (composed of many particles) by solving the proper set of Schrödinger equations -- even the best supercomputers can't do that -- although, in principle, if you had a ‘godly’ computing power, you could solve those set of equations exactly and obtain any physical property thereof. $\endgroup$ – AlQuemist Dec 4 '15 at 18:12
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I am starting studying quantum mechanics and observed that classical physics ceases to be accurate at the microscopic world.

Yes, though there is continuity in physics, the classical fields emerge from the quantum mechanical fields smoothly as numbers of quanta and dimensions grow.

My question is if there is a large world so large, when their world is compared to us, our world becomes a microscopic world,( planets will be the size of electrons) so will their physics be different from classical physics?

Well, when we look a the universe in numbers, the atom is order 10^-9 meters, humans ~1 meter, star distances of order of 10^11 meters, already the scale you envisage exists within our capabilities of observation and measurements and galaxies are beyond this scale, ~10^20 meters. And yes the physics is different, it obeys special relativity ( which rules in the microcosm also) and General Relativity rules and regulations , a physics that needs a lot of mathematics to be understood. Still there is a continuity between classical physics and general relativity going from the planetary size framework to the cosmic one.

If a scientist from the large world discovered earth( electrons ) will we be behaving quantum mechanically as observed from the "big"scientist.

This is a science fiction question,because it is a moot point whether these huge conglomerates obeying General Relativity could develop a consciousness to the point of studying physics. Convoluting back to the human scale though, one can say with certainty that no, even these science fiction beings would not see the human scale as a quantum mechanical one. The quantum mechanical scale is fixed by the value of Planck's constant, which they would measure the same as we measure it, in their own units, and the Heisenberg uncertainty principle.

The statistical deterministic behavior of classical many particle systems and the probabilities derived from classical statistical mechanics should not be confused with the quantum mechanical indeterminancy.

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    $\begingroup$ I do not agree with the third part of the answer by anna v. Those giant scientists may even observe some laws beyond general relativity; furthermore, they could be able to fathom more deeply into the laws of nature, beyond the Planck length scale -- something more fundamental than our microscopic quantum theory. At any case, I suppose the essence of the posted question (apart from its sci-fi form) is about the relevance of scales in the laws of physics. $\endgroup$ – AlQuemist Dec 4 '15 at 15:46

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