I understand that by ‘observation’ we mean the interacting of a measurement device with the experiment, but, the example of the double slit experiment is “macro-logical”, ie. we can also in a way, SEE it without a device, but what about the ones which are very small in size and can only be seen with sensitive intruments?
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It’s synonymous with measuring. The gist being that to measure something, you kind of have to interact with it. E.g. if you measure the location of an electron by bouncing a photon off of it, that photon will affect the velocity of that electron.
A very crude parallel might be that to measure the air pressure in a tire, you end up letting some air away from the tire.
“The observer effect is often misunderstood.
Electrons don’t “know” they’re being watched. When we measure them, we have to use light (photons), which disturbs their wave-like nature and makes them behave like particles.”
Not mine, author not me. Stolen from a post I have since lost.
Sidenote: The point of the double-slit experiment is that if you measure which slit the particle actually went through, the interference pattern on the wall disappears. Measuring the particle mid-travel changed its destination.
>but, the example of the double slit experiment is “macro-logical”
No we cant. The measuring that happens in the double slit experiment is about through which slit the particle goes, we dont “measure” the pattern at that point it doesnt matter anymore what you do.
An observation is in the end an interaction. To measure something you need an interaction and thus a transfer of energy. It doesnt matter if the interaction is through a photon or an electron or something else.
It has nothing to do with human eyes. As proven by the fact that we even know the double slit experiment exists. People looked and saw the interference pattern on the detector.
By measurement, what it really means is that something happens at the scale of the system that determines something about the system.
In the double slit experiment, you can use polarizing filters to do this. When you shine the light in, make it polarized, vertically, for example. Then, at the slits, put a vertically polarized filter on one slit, and horizontally on the other. The vertically polarized light we are shining should only pass through the vertically polarized filter. Using this set up, or realistically one more complicated but along the same lines, you can determine which slit it went through.
So the way quantum systems work is, there is a “wave of probabilities” that exists before anything is measured. That wave of probability will “snap” to a certain actual value when measured. The definition of a measurement, then, is basically anything that forces the system to actually “make a decision” about location, velocity, or any other of a multitude of properties.
So you sneaking a look at the double slit experiment doesn’t count as a measurement because it doesn’t force the system into one state or another. Putting polarized filters on the slits though, that forces a particular state because horizontally polarized light will not be able to pass through the vertically polarized filter.
In order to really get it, I think, you need to have some idea of what is going on before and after a measurement, what changes etc. that allows you to get an intuitive grasp on what exactly a measurement is.
Basically, “observing” means to “know its state,” as in measure, detect, or any other means of pulling information out of the system. Until you get information from the system, quantum mechanics says that quanta can be in multiple states simultaneously. This is called superposition. Once the state has been “observed,” it collapses from a superposition into a single state. I’m sure someone more knowledgeable than I might be able to explain why this happens.
The double slit experiment shows light to behave like a wave (think like radio waves), but other experiments have shown light to behave like a particle. It is said light behaves like both waves and particles. Performing the double slit experiment forces light to behave in a single way, like collapsing a superposition.
It means the particle in question was measured in some way.
Usually by hitting it with another particle.
It sounds a lot less confusing when you say Particle A changed its behavior when we hit it with Particle B.
There IS some weird stuff going on, but it’s not that its properties changed once measured. It’s the WAY that its properties change. Namely it seems to cause it to change from a wave into a particle when you do this.
Until it’s interacted with, it moves as a wave and does ‘wave-stuff’, but after that it starts behaving like it’s a single particle.
There’s also evidence that it will start acting like a particle if it’s GOING TO interact with another particle under some circumstances. More specifically if you set up the experiment to determine which slit the particle goes through AFTER the particle has already hit the back wall, it still acts like a particle.
“Observation” basically means any interaction with the system that can yield information about the state of the system. Imagine that you prepare photons with a distribution of polarizations. If you pass them through a polarization filter, you are effectively measuring the polarizations, which is an observation of the system.
Quantum states are extremely fragile. Any interaction with the environment destroys them (“the wave function collapses”). That is what is meant by measurement: any interaction with the surrounding environment. In the double-slit experiment when the photons hit the screen behind the slits, that’s a measurement. When those photons hit an air molecule on their way to the screen, that’s a measurement.
It isn’t just eyes. No matter what tools or technologies we use, all known methods of observation require interaction with the particle in ways that change its position and/or momentum, or ways that change mass, energy, waveform, or other properties. There are several pairs of properties that are interrelated of the two, so the more accurately we know one, the less accurately we know the other.
For the position/momentum pair, anything to increase knowledge of the position will decrease knowledge of the momentum. That is, any measurement to better find exactly where an electron’s energy is located will change where it is going, alternatively, any measurement to better find how the energy is moving will nudge it to somewhere else.
There are other pairs that scientist find interesting, but they suffer the same problem. You can’t measure both the spin and the axis, any measurement of the spin changes the axis, and any measurement of the axis changes the spin.
Measuring with light means there was impact or emission of a photon, the photon’s energy causes a change.
Measuring with magnetism means there was magnetic field interference of the particle, causing a change.
Measuring with physical measurements require an impact or other interaction, causing a change.
Accelerators by definition increase energy, causing a change.
Devices like cloud chambers, spark chambers, and bubble chambers use both magnetic and physical approaches. Electronic detectors use energy or impact to detect charge, radiation based detectors change energy. Some future technology may enable it, but all known methods can be proven to disrupt the properties of the pairs.
So far it is provable that any interaction that increases precision of one measure will decrease precision of the other measure.
A method that doesn’t cause a change would be a tremendous breakthrough in science. Within the laws as we understand them today, none can exist. If there is some breakthrough that changes our understanding of the fundamental forces, it is possible (but unlikely) that it could exist.
While we’re on the topic, why is light considered both a particles and a wave? Why can’t we consider they’re particles behaving as a wave due to emergent properties of the particles being clumped together?
Human eyes can’t even see at the scale we’re talking about. It’s the machines that observe the machines in a test.
The universe observes itself constantly.
We lack the natural vocabulary to express the ideas directly because our natural language evolved up here in meat space. That leaves us with terms like “at which point the wall of dirt wants to slide down the mountain”. This isn’t an expression of some actual desire, is just the metaphor for a battle stable pile of dirt that will come “rushing” down the hill it is “disturbed” with the slightest “provocation”.
Something is “observed” when it goes from being immaterial to having a concrete result.
Two particles whizzing around ignoring each other is a quantum state. If they smack into each other and bounce off that’s an observation. And now they’re whizzing around again again.
So a cause is observed whenever it produces an effect.
The mathematical definitions get a lot more stringent than that but that’s basically it.
Quantum mechanics deals with subatomic particles, which are very small. Small enough to be moved by photons or electrons used to measure them.
Here’s an analogy: take a pool table and cover most of it with a black sheet and try to find the balls on it by shooting cueballs and listening for collisions. When you hit, you can hear where that collision happens, but you also know that it can’t still be there since you just caused it to move.
IMO, an “observation” is the event where it triggers a state change in something else – the “observer.” Our “seeing,” yes, but also participating in a reaction, getting entangled, …
I want to study nocturnal creatures. I cannot see in the dark, so can only see them by shining my light at them to see them. They act spooked as fuck when flashed by a light. I have changed their behaviour by observing them.
Its not literally a human looking that changes it, it’s that out tools to measure stuff themselves change the behaviour of quauntum particles.
Observation is related to making a measurement, which in the Copenhagen interpretation collapses the wavefunction.
But there is no science or anything really around what causes a collapse or what a collapse is physically. There isn’t even any evidence that a collapse actually happens.
So there are lots of issues with the Copenhagen interpretation, especially around what an observation/measurement is and what a collapse is.
The wavefunction collapse postulate in the Copenhagen interpretation isn’t just unproved, it’s not even testable in theory.
If you drop the collapse postulate everything seems to work. If say the environment interacts with a photon, the environment would split into half left and half right. So part of the environment(you) sees the photon go left and a seperate part sees it go right. So since each of you is completely decohered(seperate) then it would look like a collapse, but it’s just wavefunction evolution all the way. Observation would just have a high level emergent propoerty rather than anything special happening low down.
I understand that measurement as it relates to the double slit experiment corresponds to measurement and the collapse of a quantum distribution.
Is there any concept of quantum measurement that can detect the distribution of quantum states?
Think about it, not in terms of actual observation, literally with your eyeballs, but simply measuring it or even something interacting with it..
I’ve always preferred the interpretation of like, a quantum system is “observed” when it interacts with another quantum system. A “quantum system” can be kind of anything, it’s just an arbitrary box drawn around anything made up of quantum particles (which is pretty much anything and everything). And this “interaction” is any exchange of information, whether it’s energy, matter, temperature, motion, etc. A system remains unobserved if and only if no information at all is passed to another system. Since humans are made up of quantum particles (as we exist in the universe and are made of matter), you could for example draw a box around a person as one system, and have the rest of the universe as another. That said because different parts of the universe are never truly isolated from each other (except in very specific circumstances), it’s really not possible for something to be “unobserved” in a practical sense.
As a theoretical example, in the Schrodinger’s Cat experiment, the interior of the box is “unobserved” if and only if it remains completely and totally isolated from the surrounding universe. Nothing can pass in or out of the box, no air, temperature, sound, or radiation. The “quantum systems” of the interior of the box and the rest of the universe remain separate, no information can pass from one to the other, therefore it is meaningless to make any definitive predictions about what has happened inside the box. All we can do is estimate probabilities of what might have happened, which leads to the superposition of the cat being both alive and dead. As soon as the box is cracked open, the systems begin to interact, information is exchanged from one to the other, the probabilities collapse and the cat is definitively either alive or dead.
Every single time this thread gets made, the top answers fail to convey the significance of the actual answer.
The only honest answer right now is that nobody knows what observation is. The current top comment defines it by synonymity with “measurement”, but this term is no more well-defined than “observation” with respect to fundamental physics, though it is, at least, correct – the problem underlying the question of “what observation is” is termed the “measurement problem”.
Where things get less correct, and the issue that needs constant correction in these threads is that observation/measurement is not synonymous with “interaction”. Of course, it is true that observing a quantum state will involve interacting with it, and interacting with the state in any way does inevitably change it. But observation/measurement have a special kind of effect that other interactions don’t have; namely, they cause wavefunction collapse.
What does this mean? You may be familiar with the concept of superposition, whereby a particle is described as being in “multiple places at once”. This distribution through space is the particle’s wavefunction. When the wavefunction is spread out over multiple possibilities, the particle is said to be in superposition.
You may also have heard of “entanglement”. Entanglement is the usual outcome when quantum systems interact. Suppose you have an electron, e1, in a superposition of spin up and spin down. You have another electron that you know is spin up. You collide these with each other in such a way that, whatever e1’s spin is, e2 will end up spinning the other way.
After the collision, e2 has entered superposition. Its spin has to be the opposite of e1’s, but since e1 is in superposition, this is not determinate – for this reason we say that e1 and e2 are both in superposition, and that they are entangled, because e2’s spin depends on e1’s.
Now we can see what is special about observation: when we observe particles, we interact with them, but the world does not seem to enter superposition. Instead, the wavefunction seems to collapse – it singles out and adopts just one of its many possible values.
The question here is, what fundamentally distinguishes observation, which collapses the wavefunction, from ordinary interaction which causes entanglement? Nobody knows the answer right now, but anybody who boils down the issue to “observation is just interaction” has entirely missed the point of the problem.
To be clear, there is an argument to be made that wavefunction collapse is just an artifact of our perspective, and if this is true then in fact observation really is just another kind of interaction that happens to seem special to us. But to jump straight to that without covering what the measurement problem is in the first place is not a real answer to your question.
Everything is composed of tiny particles that seem to behave randomly and it is very chaotic so we should think of their state as a probability of being one way or another. These particles are interacting and influencing each other creating entanglement: if we know the state of one particle then we can deduce the state of particles entangled with it based on rules about how they interact.
At a large scale, there are so many particles involved with their behaviors all linked together so the randomness goes away and we can know for certain the macro state. But at the scale of the smallest individual particle, we cannot know its exact state without measurement which involves knowing the state of other particles that have interacted with it (such as a photon bouncing off of it and then passing into your eye or some other detector) and then using that information to determine the state of the particle in question. Once we know the state, we no longer represent it probabilistically – we now have the actual value. (This is called “collapse of the wave function”)
We cannot know the exact state of an individual quantum particle on its own but can figure out its state based on interactions with other particles which is what measurement is.
Anything is possible when quantum particles exist in isolation but they bonk into each other in a deterministic way which causes a single reality to emerge.
One important thing to add: if you take Everett’s Many Worlds interpretation, then you can just do away with worrying about what exactly an “observation” is or what an “observer” is.
In this interpretation (which is my personal favorite), everything is just part of the entire wave function. So of course any “observer” is going to see one of the variations of what is being “observed”. It is not like there is some magical difference (apologies to those holding on to the Copenhagen Interpretation).
The main objection that is given most is that there is no way to tell. Which is the point. Why make up different categories of stuff and worry about “collapses” when you can just trust the formula?
In any case, the ELI5 level note here is that by going with Everett, you don’t need to worry about how it would behave if not observed. Everything is just on the wave function.
Specifically in the case of Quantum Mechanics ie the Double Slit Experiment, observation is literally bouncing an electron off the electron you’re trying to measure. People really overthink this, a lot. It has nothing to do with consciousness etc. A good metaphor would be an experiment like this:
You have a highschool gymnasium with two soccer goal nets on one side. All the lights are off, you can’t see anything. At some point someone is going to roll a soccer ball towards the nets and it will go in one. You also have a soccer ball and you can kick it once towards the other soccer ball, if it hits the other ball they’ll both go off into the second net but otherwise you have no idea what’s going on. “Observation” is just when your ball hits the other ball. Without that collision you have no idea where the first ball ends up.
That’s almost exactly what’s happening in the double slit experiment, “observation” is literally using a photon to hit the electron headed towards the diffraction grate. That’s it.