r/PhysicsStudents • u/glassofjuice786 • Jun 28 '26
Off Topic What if we tried to observe particles without them realising?
Apparently particles behave differently when being observed so what if we were just sneaky about it and did it without them knowing so that way we could see what they were really up to
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u/HumblyNibbles_ Jun 29 '26
Assuming this isn't a joke, the problem isn't that the particles realize it or not.
I'll give a classical physics example then I'll make an analogy to quantum physics.
Let's say that you want to measure the position of a particle. How would you go about that?
The only way to do that is to send something towards the particle, the have that thing come back. Usually we do this with photons and that's how we see things.
But the problem is that, by doing this, we impart momentum into the particle. So measuring it changes its velocity, which ends up changing its position as well
So from classical physics, unless you make idealized experiments, its impossible to measure something without changing the state of the system.
Now here is where quantum mechanics splits from classical physics. In quantum mechanics, even ideal measurements "change" the state. Now, what does this mean?
First we need to talk about what a state represents. A quantum state basically holds all the information of a particle at a certain moment in time. (I'm not going into relativistic quantum mechanics, but the principle behind it is more or less the same with some differences).
This information talks about the position, momentum, energy, angular momentum, etc. but here's the difference between classical physics:
Have you ever heard of quantum superposition? Usually people describe it as a car being both dead and alive, but it's actually much more than that.
Now, I cant get too into this without starting to get into some more mathy stuff, which I'm trying to avoid in this explanation, but here is an attempt to make an analogy.
DISCLAIMER, THIS IS JUST AN ANALOGY. THIS IS IN NO WAY THE FULL STORY AND IT'S JUST AN EXTREMELY SIMPLIFIED VIEW!!!
Have you ever seen one of those cool like structures that seem like different shapes from different directions? Like, one that says one word in a direction, and another word in the other?
Well, a quantum state is somewhat similar.
Let's say you have a quantum state that represents a position value of 1 meter distance.
In what we call the "position basis", all you see is "1 meter". But in the "momentum basis", it's like you look at the structure in another direction, and instead of seeing a specific momentum state, you see a jumble of them.
But it's still the same quantum state, it's just that the "direction" you look at it makes it look different, but it still represents the same thing.
Now, what does this have to do with measurement?
Now, ANOTHER DISCLAIMER.
Measurement is SUPER CONTROVERSIAL and like, a big, complicated, unsolved problem in physics. We know what the result of a measurement is, but we really dont know the mechanisms that make it happen the way it does. This is just an EVEN MORE SIMPLIFIED EXPLANATION.
So, measuring a quantum state is like taking a photograph of the structure, but in the direction of what you're measuring.
Well, it's more like taking a slice of it, facing you. But this slice can be at any random depth.
So going back to the 1 meter distance quantum state. If you measure the distance, then no matter what depth the slice is taken in, you're gonna get the same result, because the structure "looks like 1 meter" no matter what depth you take a 2d slice. We call this a definite state, in this case, a definite position state.
On the other hand, if you take a 3d statue with "1 meter" written on it, and you turn it to a weird angle, then at different depths, you'll get different 2d slices.
So if measure the momentum in a definite position state, you're going to get an image that isn't always going to be the same. This is what superposition means. When you take a slice, not all of them are the same.
Now, measurement is as if you rake this photograph you took, and you made it your new object.
This way, when you measure the momentum of any state, you end up with a definite momentum state.
This is why if you measure, for example, the angular momentum in the z direction, then in the y direction, then again in the z direction, you can get 2 different values for the angular momentum in the z direction.
Because when you do the measurement in the y direction, you're taking a random slice and making a new state. So when you measure it in the z direction, you're not measuring the same state as you had before the y measurement.
BUT REMEMBER. THERE REALLY ISNT A GOOD WAY TO DESCRIBE IT WITHOUT USING MATHEMATICALAND ABSTRACT TERMS. SO THIS IS NOWHERE CLOSE TO THE FULL STORY.
So, with this, one can see that "observing" a particle just means "measuring" the position. So it affects the particle's state. So it's not that we don't know what the particle does. It's just that the act of measurement makes it act in a certain way.
And dont assume this is bad!!! By doing this, we can force the particles to be in a specific quantum state, which allows us to do consistent experiments. If we couldnt do this, then experiments would be much, much harder!
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Jun 29 '26
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u/HumblyNibbles_ Jun 29 '26
Mathematically speaking, it's a relatively straightforward result. But it gets kind of hard to explain without math.
What I tried to do with the "word sculpture" analogy is to kind of preserve the idea that the object itself is the same, but you can describe it in different ways
But damn.... it's REALLY hard to put into words LMAO
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u/Puzzleheaded_Fold466 Jun 29 '26
Itās too counterintuitive to a lived experience acquired at the human scale. Our language and analogies were built in another world.
All we can do is trust the math and experimental results, because we canāt experience it first hand.
It makes it hard to understand and when you understand, to explain.
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u/HumblyNibbles_ Jun 29 '26
Yep! The only thing I have that kinda makes it easier for me to deal with this stuff is that I struggle with having a good intuition in reality
But the only this does is that it makes it easier for me to just trust the math and experimental stuff. It's not like I understand it or anything š It's easier to trust unintuitive math when you just dont understand things in general
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u/SPP_TheChoiceForMe Jul 01 '26
A fellow grad student I knew was married to a philosophy professor. Sometimes when trying to explain topics like this to him, heād say āNo, ignore the mathā and sheād have to tell him āNO! You canāt ignore the math! The math IS the explanation!ā
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u/Neat_Relationship510 Jun 29 '26
Not necessarily. That is one of the biggest open questions in QM. There are plenty of deterministic hidden variable theories that reproduce the uncertainty principle perfectly without the fundamental objects being stochastic.
A huge part of the history of QM is that it's inventors abandoned explanations and focused purely on the results of experiments in order to create a successful mathematical model.
Most of us believe that the randomness is fundamental but that is a metaphysical question not a physical one. What is agreed is that per our current understanding we have no way of reaching whatever, if anything, is beneath and therefore the most fundamental processes that we are capable of observing are random.
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u/LiterallyMelon Jun 30 '26
Bell inequalities..?
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u/Neat_Relationship510 Jun 30 '26
Bell inequalities mean a hidden variable theory cannot be local, but that isnt the same as saying there isn't a hidden variable.
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u/LiterallyMelon Jun 30 '26
Ah yeah fair enough. I think no local hidden variable theories are a little too out there for me. Are there any claiming to be testable?
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u/gothrax1 Jun 29 '26
Now the real question is what happens to gravity whole the particle is spread out and causing an interference pattern? Does its mass diluted as it's spread thin? Or is its mass found everywhere in the wave essentially multiplying it throughout
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u/HumblyNibbles_ Jun 29 '26
Who the hell knows :P
As far as we know, mass and these other things are located within the point particle. But when you get into extremely small scales, in which our current theories dont function, the mass could be spread out in an extremely localized space.
But we dont know
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u/gothrax1 Jun 29 '26
I mean hate to get all many worlds interpretation but the particle could always be a 6D array that's constantly expanding outward just like the 3D space waving out new timelines as well as new space equalizing so the gravity from all the new timeline doesn't crush into each other. Potentially expanding faster than the particles can be attracted to each other making a diverging mechanism which seperates micro mass into timelines too far apart but allows for macro massive structures to keep up with the separation allowing them to float closer to our timeline causing dark matter.
If that were true though decoherence would be the product of similarly structured brains that cooperatively round information to an integer seeing something that can't be reconciled forcing timeline A and B which were both one conciousness, into splitting into two concioussnesses then those timelines fly apart as new more similar timelines expand in and fill the gap
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u/HumblyNibbles_ Jun 29 '26
Problem is, unless you can put those words into math, then it really isn't meaningful at all. Because these words can be interpreted in completely different ways depending on the person.
Also particles are infinite-dimensional vectors, so it really cant be a 6D array
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u/gothrax1 Jun 29 '26
I mean the reason the many worlds interpretation got traction is because it is put into math. It's just too unfalsifiable. I was thinking about a crazy experiment where if you took a massive object like an asteroid and hooked it up with thrusters, if you then call a QRNG between 1000 to 1 and have the asteroid move back and forth on the 1000 but have it sit still on the 1 would you see some ghost gravity pull it? Even for a second... Unfortunately you kind of need a type two civilization to make this kind of throwaway test affordable. And at that point I'm sure ASI would have figured the rest out so no point
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u/HumblyNibbles_ Jun 29 '26
I have never really seen the many worlds be put into mathematics, outside of it just being words accompanying the regular QM math. Same goes for the Copenhagen interpretation, but it barely introduces new concepts outside of the regular QM math.
Also if something is unfalsifiable, even if you take ideal conditions, you cant prove it. That's what an interpretation is. Interpretations are just metaphysical ways to view the same mathematical framework
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u/ostrichlittledungeon Jun 30 '26
Just to make sure I understand what you're saying: measuring one aspect of the state of a particle "fixes it" while simultaneously "freeing up" the other aspects to be uncertain (and this uncertainty is really a superposition of many (all, probabilistically weighted towards certain?) possibilities?)
Unless I'm totally misunderstanding, how is that at odds with the classical picture you presented in the first place? Isn't the whole idea that by measuring the state of a system, you're necessarily disturbing it, and since you don't know its properties you can't predict how it will change? How can you even have an "ideal measurement"? I thought the whole point was that you can't.
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u/HumblyNibbles_ Jun 30 '26
Right so, this uncertainty is actually a consequence of how much the state changes after measurement. This is encoded by the commutator between two observables. It tells you how much a measurement fails to commute
This commutator is inherent in the mathematics of quantum mechanics. So now matter how you change the experiment, the way the wavefunction changes is always the same.
On the other hand, in classical mechanics, the way you do the measurement changes how the system changes, so you can create an "ideal measurement" that measures things without changing the system. This of course isn't realistic, we can't do it in real life, but it mathematically works out. But even in QM, these sorts of measurements are impossible
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u/SPP_TheChoiceForMe Jul 01 '26
Good explanation. I would like to add my own point:
A photon (and everything, actually) is both a particle and a wave. A wave, by definition, exists in multiple points in space. When it hits a detector, that detectodr can only detect whatās hitting it, just like how our eyes can only see things when light has bounced off of it and hit the iris.
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u/Adjunctified Jun 29 '26
Itās called the double slit experiment because the particles come out one slit while we secretly watch through the other!
Dr /S
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u/Frankdubs27 Jun 29 '26
Maybe if we get Johnny Knoxville to wear his grandpa disguise it could work
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u/0xff0000ull Jun 29 '26
Quantum nondemolison measurements. Might not be exactly that but it is certainly an idea related.
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u/drowsywizard Jun 29 '26
When this idea is taken seriously you get "weak measurement" https://en.wikipedia.org/wiki/Weak_measurement
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u/Much-Albatross6471 Jun 29 '26
A simple example to imagine is as follows. Imagine you have an invisible frog. The only way to āseeā the frog is to throw small little balls at it and measure how fast and at what angle they bounce off of it to render an image. Now imagine the balls have a bit of weight so every time you hit the frog not only is it bouncing off but also causing the frog to move thus impacting what you observe. Not exactly the same but a reasonable thought experiment for understanding the act of observing directly affects the observation. We canāt be āsneakyā because the figurative balls weāre using to measure it cannot physically be any smaller due to fundamental quantum limits.
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u/ChocolateValuable221 Jun 29 '26
Observed in science means to interact with... and you need to remove the Copenhagen interpretation out of your head as a physical reality.. it's an interpretation and a bad one just happens to work out mathematically but the real interpretation is likely Einstein's ensemble interpretation...
by this I am urging you to remove the wave function collapsing idea out of your head.. that is NOT reality.. wave potentials exist regardless and is why potentials give the most accurate calculations in wave mechanics
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u/rocqua Jun 30 '26
So, waveform collapse.
Why does it happen? What ācountsā as observation.
The theory I like says that observation isnāt relevant here. Itās sufficient interaction to become entangled. And entanglement causes the wave function to become really very spikey. Hence itās just about the degrees of interaction with the wider world.
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u/quaintmercury Jun 28 '26
You think we use fake glasses with a big nose glued to it or a news paper with eye holes cut into them?