I'm learning the basics of quantum physics and I've read that all objects have a wave function, but the only reason nothing in the macro-world superposes is that they're constantly interacting with the environment, which acts like an observer, so the wave function is in a constant state of collapse.

If that is the case, then what about a rock in a vacuum? Since there is no environment in space there is no observer, so would an asteroid superpose and then experience wave function collapse when we see it through a telescope?

Sorry if this is a stupid question.

  • $\begingroup$ If you want to go further into the current physics many partical mathematical models ( an asteroid is made up of larger then 10^23 atoms/molecules by some order of magnitude depending on its mass) start reading on the density matrix formalism, that explains mathematically that even though each atom can be described by a wavefunction the ensemble due to the distances and large numbers reduces to a classical behavior en.wikipedia.org/wiki/Density_matrix and the answer of motl here physics.stackexchange.com/questions/98703/… $\endgroup$
    – anna v
    Jul 6 '17 at 4:19

It's not a stupid question. Our limited understanding of the quantum world has led to many such questions, some with surprising answers!

To your question, and to address another point made here, the vacuum is not, unfortunately, a simple emptiness. It is actually incredibly complicated with (sometimes virtual) particles popping into and out of existence all the time. Quantum field theory deals with this, and if you want to crack into it, try Peskin & Schroeder's book.

You do not need an observer for phenomena to occur, and an asteroid in empty space will not superpose with another (assuming there were actually 2). Quantum field theory is still an evolving theory that is sadly incomplete, but in this area (QED and QCD) it is fairly well developed and enough results from CERN, Fermilab and other experiments have shown a substantial increase in the understanding of the standard model, which describes interactions of ordinary matter.

More to the point, every particle in the asteroid is interacting with other particles in the same asteroid. You cannot correctly expand quantum theories to macro objects because of the incredible number of emergent properties that come from collections of particles. Also, there is no reason to believe that a collection of particles with behave like a single large version of its constituents. Condensed matter physics is an area that deals with many of these emergent properties. Phonons are a great example of emergent physics that don't exist at the fundamental level. Schroeder has an amazing book on thermodynamics.

I hope this helps answer your question and gives you some food for thought.


A rock in space still has photons hitting it from various sources, so it still interacts with its environment, which prevents any macroscopic superposition from happening.

If you eliminate the sources of photons (i.e. suppose the rock were in an empty universe, and at zero temperature), then you can't see the rock anymore, so there's no problem there either.

  • $\begingroup$ Didn't think of photons. Would it superpose (theoretically speaking) were there no photons hitting it? $\endgroup$ Jul 5 '17 at 17:40
  • $\begingroup$ If there were no photons hitting it, and there were no other methods of interaction, then yes. $\endgroup$ Jul 5 '17 at 17:50
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    $\begingroup$ In addition, if the rock is at a nonzero temperature it will give off blackbody radiation, which also will act as a source of decoherence. $\endgroup$
    – Rococo
    Jul 5 '17 at 18:31
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    $\begingroup$ No, don't think so. The particles inside the rock are not at zero temperature, and they are constantly interacting with each other. They are doing this in a great number. $\endgroup$
    – Bob Bee
    Jul 5 '17 at 22:55
  • $\begingroup$ @BobBee See addendum by Rococo. $\endgroup$ Jul 5 '17 at 22:57

Observation, measurement and interaction do not collapse the wave function. In quantum mechanics a system can be present in different versions. For example, an asteroid can be in two (or more) different locations. The two different versions of the asteroid can in principle interact by becoming different and then come back together in such a way that what happened in between affects the outcome. This is called quantum interference.

If two versions of a system can't undergo interference then you can't detect that there are two versions. Measurements, observations and many other interactions prevent interference. All that is required is that information about the different states of the system leak into other systems. As a result of this, some of the information needed for the interference is spread out across multiple systems. The original system can't undergo interference without that information. This explains the fact that you can't see two different versions of the same system and so eliminates the need to postulate collapse. This is a controversial position because many physicists have adopted bad philosophical ideas. See "The Fabric of Reality" chapter 2 and ""The Beginning of Infinity" by David Deutsch, chapters 11 and 12 for popular accounts.

In the case of an asteroid, interaction with photons prevents interference between different versions of the asteroid, see




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