Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: pittsburghjoe on 14/06/2019 23:31:03

Title: Do Matter Waves collapse when bumping into any large object?
Post by: pittsburghjoe on 14/06/2019 23:31:03
I heard a single photon trying to measure a Matter Wave can cause its wave to collapse. Do Matter Waves only collapse when something is trying to observe it or will any object do?
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: Colin2B on 15/06/2019 09:20:32
Do Matter Waves only collapse when something is trying to observe it or will any object do?
Firstly, make sure you donít confuse observer with a person or consciousness, it is just a term to indicate a measurement.
Wave function collapse is a strange term that is often misunderstood. It really means that something stops behaving in one way and changes state to behave in another. So it depends what the something is and what causes it to change state.
Take an electron. When it is moving in a crt tube it can show behaviours of both particle and wave, so in one way of modelling it we can say it has a wave function. When it hits the phosphor of the screen it stops travelling through the vacuum (changes state) and gives some of its energy to the phosphor causing a fluorescence. Both the electron and the phosphor are now in different states, the electron being part of the phosphor lattice.
Note: what I have described is a very simplistic description of what is happening.
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: pittsburghjoe on 15/06/2019 15:37:12
Let me ask in a different way. If a free atom were to bump against any large object, would its state change?
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: chiralSPO on 15/06/2019 18:46:22
Let me ask in a different way. If a free atom were to bump against any large object, would its state change?
probably
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: chiralSPO on 15/06/2019 18:49:32
It is increasingly difficult to prevent wave function collapse for systems of increasing complexity, over longer distances and times. Essentially any interaction with anything else (massive or not) can disrupt superpositions. This is the primary roadblock to quantum computing, and why IBM quantum computers are mostly (by mass, volume, and $$$) refrigerators capable of keeping the actual computer bit REALLY cold.
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: pittsburghjoe on 15/06/2019 18:54:17
So all the "atom in a box" analogies are pointless :/
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: chiralSPO on 15/06/2019 19:55:16
So all the "atom in a box" analogies are pointless :/

Not pointless, just vastly oversimplified. They are good learning tools, and can be used as a starting point to make very crude predictions about real-world phenomena (like UV-vis spectra of flat, highly conjugated molecules, or crystallin nanoparticles), but are by no means an accurate representation of any real-world phenomenon.

If I may, I suspect that some of the apparent contradictions you see in QM has to do with the fact that none of the examples that show up in introductory textbooks/classes/videos are depictions of real-world scenarios. "Imagine the entire universe only contains two point particles, each with mass m, connected by a massless perfect spring, with spring constant k, and average length d"

is a far cry from

 "Imagine a hydrogen molecule as a system of two protons, each with mass, charge, and magnetic moment, and two electrons, each with their own mass, charge, and magnetic moment. These four 'particles' are all interacting with each other as if they are more like 'waves' in that the distance between the protons ends up being on the order of the wavelengths of the electrons, so the whole thing is a big interference pattern, but the whole ensemble is in constant motion--the distance between the protons is constantly oscillating, but like an oscillator with a spring constant that depends on the intereference pattern of the electrons, which is itself influenced by the distance between the protons.... oh, and this ensemble is constantly bombarded with photons that can change the wavelengths of the electrons, or make the molecule vibrate faster, or make it rotate, each of which would change everything about the whole molecule except the mass and charge involved (so go back to the top), and it can change rotation, vibration, and electronic wavelength all simultaneously... oh, and then the molecule can change again by emitting a photon, which is a purely stochastic event... oh and the whole system occasionally bumps into other complex systems, exchanging energy, momentum, angular momentum, and sometimes even particles..."

And even this last description is a VAST oversimplification.
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: evan_au on 16/06/2019 03:44:13
We talk about carefully-crafted quantum superpositions collapsing when there is contact with some outside object (in a quantum computer, increased "coherence time" is an important goal).

Would be fair to say that the computer qubits now share a quantum superposition with whatever random object they interacted with (and so end up in a randomised quantum state themselves)?
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: alancalverd on 16/06/2019 09:08:38
"Imagine a hydrogen molecule as a system of two protons, each with mass, charge, and magnetic moment, and two electrons, each with their own mass, charge, and magnetic moment. These four 'particles' are all interacting with each other as if they are more like 'waves' in that the distance between the protons ends up being on the order of the wavelengths of the electrons, so the whole thing is a big interference pattern, but the whole ensemble is in constant motion--the distance between the protons is constantly oscillating, but like an oscillator with a spring constant that depends on the intereference pattern of the electrons, which is itself influenced by the distance between the protons.... oh, and this ensemble is constantly bombarded with photons that can change the wavelengths of the electrons, or make the molecule vibrate faster, or make it rotate, each of which would change everything about the whole molecule except the mass and charge involved (so go back to the top), and it can change rotation, vibration, and electronic wavelength all simultaneously... oh, and then the molecule can change again by emitting a photon, which is a purely stochastic event... oh and the whole system occasionally bumps into other complex systems, exchanging energy, momentum, angular momentum, and sometimes even particles..."

Except that nothing is actually oscillating, because moving charges emit electromagnetic waves and lose energy.....Fact is that quantum mechanics is the best model we have for what actually happens inasmuch as it sums up what we see and to a very good degree predicts what we might see, but it's still only a model based on observation. Collapsing wave functions are no more "real" than dying fairies, but they have some predictive value, just as ballet has entertainment value, once you understand the implicit conventions - you are watching athletes in tights, not tiny mammals with magical powers.
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: evan_au on 16/06/2019 22:29:09
Quote from: alancalverd
Fact is that quantum mechanics is the best model we have for what actually happens
Quote from: George Box
All models are wrong, but some are useful
See: https://en.wikipedia.org/wiki/All_models_are_wrong
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: alancalverd on 17/06/2019 10:37:07
Stafford Beer: "A dead mouse is the perfect model of a live mouse, but not for very long."
Title: Re: Do Matter Waves collapse when bumping into any large object?
Post by: geordief on 17/06/2019 10:44:54
Is it possible to say as an approximation how the  complexity of the mathematics of quantum mechanics  compares with that of GR?

I have heard that GR is number crunched to produce prediction and is extremely lengthy. Does the same apply to QM?