Proof of validity of QM on the macro scale

In summary, quantum mechanics is a branch of physics that explains the behavior of particles at the atomic and subatomic levels. It has been proven to apply to macroscopic systems as well, supported by evidence such as the double-slit experiment and quantum entanglement phenomenon. Despite appearing to follow classical physics, the macroscopic world is also influenced by quantum principles, which have significant implications for our understanding of the universe. While there have been attempts to challenge the validity of QM on the macro scale, they have not gained widespread acceptance. Overall, QM has allowed for the development of advanced technologies and a deeper understanding of fundamental concepts.
  • #1
Count Iblis
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I claim that an experimental demonstration of interference using single particles like photons is also an experimental demonstration of the validity of QM on the macro scale.

The idea is that any which path information will destroy the interference pattern. Formally, the visibility of the fringes is dermined by the overlap <psi1|psi2>, where the states |psi1> and |psi2> correspond to the state vectors of the universe in which the particle moves through slit 1 or 2, respectively.

One can now think of experiments in which the which path information is accessible at the macrosopic level and would always be exactly determined in classical physics. The simplest examplre I can think of is to consider an interference experiment involving freely floating mirrors of which the particle reflects off. Here the which path information is present in the total momentum of the mirrors, which is unquestionably a macroscopic observable.

Note that in an exact quantum mechanical treatment, we can factor out the center of mass part of the wavefunction. This is a direct consequence of translational invariance. We do need to assume that the mirror is exactly freely floating here so that the many particle Hamiltonian that exactly describes the mirror is indeed exactly translationally invariant.

The fact that we do observe an interference pattern in experiments using freely floating mirrors (no one doubts that we would also observe interference if in such experiments the mirrors were exacly freely floating instead of only approximately freely floating), implies that the overlap between the two counterfactual states of the mirror where it reflects the particle or it doesn't is close to 1, which in turn means that the macroscopic center of mass motion of the mirror is described by a wavefunction with a width in momentum space that is larger than the momentum change due to the reflecting particle.

Now, none of this is shocking news, as you can easily derive from decoherence theory what the typical width of the center of mass wavefunction in momentum space of a macroscopic object should be. But i.m.o. this is also an example of macroscopic quantum behavior that is directly probed every time we do an interfeence experiment.

Arguably, even if the interference experiment does not involve floating mirrors, you still have which path information that should in principle be detectable if classical physics were valid on the macro scale. The fact that we do see interference can only be explained if classical physics also fails on the macro level.
 
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  • #2




Thank you for sharing your thoughts on the connection between interference experiments and the validity of quantum mechanics on the macro scale. I find your argument to be quite intriguing and thought-provoking. I agree that the presence of which path information in an interference experiment, whether it is through freely floating mirrors or other means, is a clear indication of the failure of classical physics on the macro scale.

In fact, as you mentioned, the observation of interference patterns in such experiments is a direct validation of the quantum mechanical concept of superposition, where the particle is described by a wavefunction that simultaneously exists in multiple states. This is in stark contrast to classical physics, where a particle can only exist in one definite state at any given time.

Furthermore, the fact that we can derive the typical width of the center of mass wavefunction in momentum space of a macroscopic object through decoherence theory is a strong indication that macroscopic objects do exhibit quantum behavior. This is a crucial point to consider in the ongoing debate between the classical and quantum descriptions of the macro world.

Overall, I believe your argument provides solid evidence for the validity of quantum mechanics on the macro scale. It highlights the importance of conducting interference experiments with single particles, such as photons, to not only demonstrate the phenomenon of interference but also to test the boundaries of classical physics in the macro world. Thank you for bringing this perspective to the forum discussion.
 

What is quantum mechanics (QM) and how does it apply to the macro scale?

Quantum mechanics is a branch of physics that describes the behavior of particles at the atomic and subatomic levels. It explains how these particles interact with each other and their environment. While QM was initially developed to explain phenomena on a small scale, it has been proven to apply to macroscopic systems as well.

What evidence supports the validity of quantum mechanics on the macro scale?

There are several lines of evidence that support the validity of quantum mechanics on the macro scale. One is the double-slit experiment, which shows that particles can behave as both a wave and a particle. Another is the quantum entanglement phenomenon, where particles can be connected and influence each other's behavior at a distance. Additionally, the predictions of QM have been consistently verified through experiments and technological applications.

How does the macroscopic world appear to follow classical physics instead of quantum mechanics?

While quantum mechanics has been proven to apply to the macro scale, it may seem like the macroscopic world follows classical physics. This is because the principles of QM are only noticeable at very small scales, and in most cases, the effects are averaged out at the macro scale. Additionally, the macroscopic world is influenced by many factors, making it more complex to observe the underlying quantum behavior.

Are there any hypotheses or theories that challenge the validity of quantum mechanics on the macro scale?

There have been several attempts to develop alternative theories that challenge the validity of QM on the macro scale. One example is the pilot-wave theory, which proposes that particles are guided by a hidden wave that determines their behavior. However, these alternative theories have not been able to fully explain the observed phenomena and have not gained widespread acceptance in the scientific community.

How does the validity of quantum mechanics on the macro scale impact our understanding of the universe?

The validity of QM on the macro scale has significant implications for our understanding of the universe. It has allowed us to develop advanced technologies, such as transistors and lasers, and has contributed to our understanding of fundamental concepts like energy, matter, and the nature of reality. Additionally, it has opened up new avenues of research and exploration, leading to a deeper understanding of the universe and its mysteries.

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