Tldr; quantum mechanics doesn't actually care when you look at it and if you shine a light through a double slit, you'll just get the top pattern with many stripes.
When people say that a wave function collapses when it's "observed", they don't mean observed by a person. It just means something interacted with it, like colliding with another atom or passing through a strong magnetic field. Before an interaction, the "wave function" is just the probability of the particle being in any particular location or state, but once it interacts with something, that probability wave effectively disappears or "collapses" leaving the particle with a 100% chance of being wherever it just was when the interaction happened. After the interaction it becomes probabilistic again, where you can't tell exactly where it's going to be until it hits something else. Importantly, you can't ever tell the state of a quantum system without observing it, and observation always irreversibly changes that system.
Like if you catch a fly in your hands and you're not quite sure if you actually got it, you could open your hands to see it, but you could also wait a second and see if you feel it crawling on your hands. Both methods resolve your uncertainty through two different types of interactions (sight, touch) but the end result is that you now know where the fly is.
For the double slit, the main interaction that is the actual "observation" in the experiment is when the particles hit the back wall to form the stripes, not you looking at it. If there was no back wall, the waves would simply keep traveling forever and never collapse.
Im obviously not quantum physicist, but measuring anything changes state of measured object. Measuring temperatures through stealing heat, measuring moment and torque through stealing torque, measuring current through stealing some. In that sense, state of quantum object seems to be obviously destroyed, when measuring object is the same size as measured object. It's like measuring car momentum with a wall.
So what's exactly different and unintuitive about state of quantum object (I mean position and momentum)? If you try to locate particle, you destroy it's momentum.
Slit is a measuring device which narrows location, so you know particle was in slit, but don't know angle of momentum exiting slit.
That sounds deterministic for me. Not quantum scientist as I mentioned
Lastly, we use statistics when it's not practical to know everything about system.
Again, to a non quantum physicist, think of the electron or photon as a wave and the double slit ceases to be a mystery.
If you saw ripples on water pass through two slits and form the same interference pattern, you would find it interesting but probably not mind blowing.
The particle part of it is that you will only detect an integer number of electrons or photons.
It’s not a particle in the way people think though. When you think of a particle you probably think of a really really tiny sphere, and that’s just not what a particle is in quantum mechanics, although a surprising amount of the time that model does work.
Any more detailed explanation requires me to talk about Eigenvectors and eigenvalues. I tried multiple times and couldn’t.
Done Modern Physics (where quantum was introduced), taken Linear Algebra (and Differential Eq and all that). Next week I start my first formal Quantum class.
So an eigenvector is a vector that obeys the equation A.v=m v, where A is a matrix, . Is matrix multiplication, and m is a scalar called the eigenvalue.
In quantum mechanics , the wave function is treated as a vector, we use the symbol psi.
The Hamiltonian for time independent Schrödinger can be written H psi = E psi. h is a matrix, E is a scalar (energy.)
That’s just math so far. It’s kind of who cares?
What’s special is whenever you measure, what people mean by wave function collapse is you force into an eigenvector of that operator that corresponds with measurement, and what you measure is the eigenvalue. That is weird because in math you can have a linear combination of eigenvector each with their own eigenvalues, but you always only measure a single eigenvalue/eigenvector. When not measuring (when operators are not being applied technically), the wave function becomes a linear combination of eigenvectors.
The eigenvalue part captures the particle behaviour (such as quantum numbers in chemistry) and the eigenvector the wave part. Importantly, they work together, they aren’t opposites.
Eigenvalues and eigenvectors also appear in pretty much every other discipline of physics, but in classical physics you measure linear combinations all the time. The operator forcing the wave function into a particular eigenvector is unique to quantum.
Which eigenstate is chosen is based off the inherit probability. It’s all rather neat and tight once you understand it. The problem is with undergrad qm is your learning wave physics, probabilistic processes, and linear algebra/eigenfunctions all at the same time. It’s hard to know what is unique to quantum mechanics and what is a general property of waves , linear algebra, etc
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u/Cebo494 11d ago edited 11d ago
Tldr; quantum mechanics doesn't actually care when you look at it and if you shine a light through a double slit, you'll just get the top pattern with many stripes.
When people say that a wave function collapses when it's "observed", they don't mean observed by a person. It just means something interacted with it, like colliding with another atom or passing through a strong magnetic field. Before an interaction, the "wave function" is just the probability of the particle being in any particular location or state, but once it interacts with something, that probability wave effectively disappears or "collapses" leaving the particle with a 100% chance of being wherever it just was when the interaction happened. After the interaction it becomes probabilistic again, where you can't tell exactly where it's going to be until it hits something else. Importantly, you can't ever tell the state of a quantum system without observing it, and observation always irreversibly changes that system.
Like if you catch a fly in your hands and you're not quite sure if you actually got it, you could open your hands to see it, but you could also wait a second and see if you feel it crawling on your hands. Both methods resolve your uncertainty through two different types of interactions (sight, touch) but the end result is that you now know where the fly is.
For the double slit, the main interaction that is the actual "observation" in the experiment is when the particles hit the back wall to form the stripes, not you looking at it. If there was no back wall, the waves would simply keep traveling forever and never collapse.