Not exactly. Quantum physics applies no matter how you measure it. The double-slit experiment is an example of this: Photons moving through two slits will form a wave interference pattern on a detector plate, even though the detector doesn’t affect the position of the photons beforehand.
It’s more like: when you become aware of the results of a quantum measurement, you yourself become a part of the quantum system, and being a part of the system requires measurements to have real values. Whether you should interpret this as a wave-function collapse or branching into multiple parallel universes is up for debate though.
When you perform the measurement on which slit the particle passes through, then the measuring device is also part of the system and it affects it. The measurement reduces the degrees of freedom in the system so there are no longer two equivalent ways for the particle to pass through the slits (either A or B), but rather you now have a measured slit and an unmeasured slit. Since there are no longer multiple ways to achieve the same result, the is no longer interference due to equivalent probabilities.
Matt Stassler has a nice series of blog posts on this.
Honest question: what happens afterwards? When we’ve stopped observing, does it reassemble into it’s superpositive form? Are we depleting quantum states somehow?
Sorta! According to the Heisenberg Uncertainty Principle, there’s an upper limit to how much we can “know” about the given state of a quantum system. This isn’t an issue with our measurements, but a fundamental property of the universe itself. By measuring one aspect of a quantum system (for example, the momentum of a particle), we become less certain about other aspects of the system, even if we had already measured them before (such as the position of the same particle).
Though (as far as we know), we aren’t going to run out of quantum states or anything like that.
Maybe I’m too dense, but what happens with other quantum states that aren’t position/velocity based? I’m thinking things like when we collapse spin, e.g. in entangled particles.
I’ve heard that entangled particles are “one use”, I’d assume they can be restored and possibly re-entangled, but how?
The position-momentum uncertainty relationship is just a specific case of a more general relationship. There are other uncertainty relationships, such as between time and energy or between two (separate/orthogonal) components of angular velocity. The relationships basically state that whenever you measure one of the two values, you are required to add uncertainty to the other.
Unfortunately, this is kinda where my knowledge on the subject starts to hit its limits. As for spin, it has a lot of effects on the energy of the system it’s involved with, so I believe the energy-time or angular momentum exclusion principles would apply there.
You might also be thinking “why not have two entagled cloned particles, and measure the momentum of one and the position on the other?”. While you can duplicate particles, there are reasons why that doesn’t work that I don’t really remember tbh. I’m sure PBS Spacetime on Youtube has an episode on it somewhere though if you’re interested
It gets even more interesting: to interfere in the double slit experiment, the light has to take a longer path for some points and light is really good at finding the shortest path. And, since you can extend the double slit experiment to infinite slits with infinitely thin blockers between the slits, you can leave away the slits entirely and still have a valid version of that experiment and get interference. It’s just, that most interference is destructive.
Veritasium had a very interesting video about that recently and my extrapolation of this is that there is neither a collapse of wave functions nor multiple parallel universes.
My intuition says that the wave function is there after being “observed”. There is no multiple possible outcomes, just very visible ones and a lot of destructive interfered ones.
However what i just wrote is not science but me extrapolating from science so don’t take it for anything more than that. It somehow causes quantum physics to make intuitive sense for me so i like it. Nothing more than that.
My example is more in regards to wave/particle duality as it shows up in variations of the double slit experiment. Putting a detector at one of the slits is an active interaction, giving you the particle-like behavior rather than the interference pattern.
What I mean to say is that the detector is not what’s changing the particle; It’s the process of learning about an aspect of the quantum system that forces it into one state or another (at least from our own personal perspectives).
Not exactly. Quantum physics applies no matter how you measure it. The double-slit experiment is an example of this: Photons moving through two slits will form a wave interference pattern on a detector plate, even though the detector doesn’t affect the position of the photons beforehand.
It’s more like: when you become aware of the results of a quantum measurement, you yourself become a part of the quantum system, and being a part of the system requires measurements to have real values. Whether you should interpret this as a wave-function collapse or branching into multiple parallel universes is up for debate though.
When you perform the measurement on which slit the particle passes through, then the measuring device is also part of the system and it affects it. The measurement reduces the degrees of freedom in the system so there are no longer two equivalent ways for the particle to pass through the slits (either A or B), but rather you now have a measured slit and an unmeasured slit. Since there are no longer multiple ways to achieve the same result, the is no longer interference due to equivalent probabilities.
Matt Stassler has a nice series of blog posts on this.
Yes, but that’s semantics. Clearly the observation has some effect, but it’s not from any force we recognize.
Honest question: what happens afterwards? When we’ve stopped observing, does it reassemble into it’s superpositive form? Are we depleting quantum states somehow?
Sorta! According to the Heisenberg Uncertainty Principle, there’s an upper limit to how much we can “know” about the given state of a quantum system. This isn’t an issue with our measurements, but a fundamental property of the universe itself. By measuring one aspect of a quantum system (for example, the momentum of a particle), we become less certain about other aspects of the system, even if we had already measured them before (such as the position of the same particle).
Though (as far as we know), we aren’t going to run out of quantum states or anything like that.
Thank you for your answer!
Maybe I’m too dense, but what happens with other quantum states that aren’t position/velocity based? I’m thinking things like when we collapse spin, e.g. in entangled particles.
I’ve heard that entangled particles are “one use”, I’d assume they can be restored and possibly re-entangled, but how?
Good question! You are certainly not dense!
The position-momentum uncertainty relationship is just a specific case of a more general relationship. There are other uncertainty relationships, such as between time and energy or between two (separate/orthogonal) components of angular velocity. The relationships basically state that whenever you measure one of the two values, you are required to add uncertainty to the other.
Unfortunately, this is kinda where my knowledge on the subject starts to hit its limits. As for spin, it has a lot of effects on the energy of the system it’s involved with, so I believe the energy-time or angular momentum exclusion principles would apply there.
You might also be thinking “why not have two entagled cloned particles, and measure the momentum of one and the position on the other?”. While you can duplicate particles, there are reasons why that doesn’t work that I don’t really remember tbh. I’m sure PBS Spacetime on Youtube has an episode on it somewhere though if you’re interested
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It gets even more interesting: to interfere in the double slit experiment, the light has to take a longer path for some points and light is really good at finding the shortest path. And, since you can extend the double slit experiment to infinite slits with infinitely thin blockers between the slits, you can leave away the slits entirely and still have a valid version of that experiment and get interference. It’s just, that most interference is destructive.
Veritasium had a very interesting video about that recently and my extrapolation of this is that there is neither a collapse of wave functions nor multiple parallel universes.
My intuition says that the wave function is there after being “observed”. There is no multiple possible outcomes, just very visible ones and a lot of destructive interfered ones.
However what i just wrote is not science but me extrapolating from science so don’t take it for anything more than that. It somehow causes quantum physics to make intuitive sense for me so i like it. Nothing more than that.
My example is more in regards to wave/particle duality as it shows up in variations of the double slit experiment. Putting a detector at one of the slits is an active interaction, giving you the particle-like behavior rather than the interference pattern.
What I mean to say is that the detector is not what’s changing the particle; It’s the process of learning about an aspect of the quantum system that forces it into one state or another (at least from our own personal perspectives).
uh, I’m a total quantum layman, but I’m pretty sure its the detector.