Observation in Quantum Mechanics
In quantum mechanics, the act of observation or measurement is more than just visually seeing or detecting something. It involves an interaction between the quantum system (such as a photon in the double-slit experiment) and a measurement apparatus. This interaction collapses the wave function, which describes the probabilistic distribution of possible states (like positions or momenta) that the particle can occupy.
Double-Slit Experiment
In the double-slit experiment:

• Wave-like Behavior: When light (or electrons) passes through two slits and then hits a screen, it creates an interference pattern characteristic of waves. This pattern arises due to the wave nature of the particles interfering with themselves.
• Particle-like Behavior: If the path of each particle (which slit it passes through) is known or can be deduced through some means (even if the means are not human visual observation), the interference pattern disappears, and the particles behave more like localized particles (like bullets), not waves.

Role of Observation
The crucial point is that any interaction that allows us to determine or infer through which slit the particle passed constitutes an observation in quantum mechanics. This interaction disrupts the coherent superposition of states (particle going through both slits simultaneously in a wave-like manner), collapsing the wave function and thereby affecting the outcome observed on the screen.
Types of Observation:

1. Direct Observation: This can involve visually seeing which slit the particle passes through using detectors or other means. This is the most straightforward form of observation.
2. Indirect Observation: Even if humans don’t directly observe the particles, if the experimental setup allows for determining through which slit the particle went (e.g., by recording its momentum change, which can indirectly indicate the slit), the wave function collapses.

Quantum Mechanical Definition of Observation
Observation in quantum mechanics refers to any interaction that leads to the collapse of the wave function. This collapse occurs because observing the particle’s path disturbs its quantum state irreversibly, destroying the interference pattern.
Conclusion
Therefore, in the double-slit experiment, the term “observation” encompasses any interaction that allows one to ascertain the path of the particle, not just human visual observation. The crucial aspect is the disruption of the superposition state, leading to the absence of interference patterns on the screen. This highlights the dual nature of light (and other particles) as both wave-like and particle-like, depending on whether their wave function remains intact (undisturbed by observation) or collapses (due to observation).

For further reading, exploring the principles of quantum measurement theory and the concept of wave function collapse would provide deeper insights into how observation fundamentally shapes our understanding of quantum behavior.

I’ve been deliberately avoiding this question. By asking me to answer, you either honor me or hate me. I can’t tell. I respect you anyways, brother. I hope you have been paying attention.

The double slit experiment.

For every system, there is an extent to its expanse. (+, -) The further you get from the origin, the closer you come to the boundary.

For every system, there is an extent to the scale of its properties. (×, ÷) The further you get from the origin, the closer you come to the boundary.

Human language is such a clumsy thing. There is danger in defining words too specifically. So, I will not define observation. In order to receive information from a system, part of the system must be removed.

What we call quantum is not the building block of the universe. Quantum is the boundary. The boundary is where the system breaks down. When you remove information from the system, it breaks down further. There is a limit.

So, can visual observation by a conscious observer cause the state of light to shift from particle to wave or vice versa?

Waves exist, but a wave is not a thing. A wave is an event within a thing. Light is not a wave. You can try to reason this out with words, but once again, human language is such a clumsy thing.

Picture a wave in your mind instead. Chances are your mental image is a squiggly black line on a white background. Let’s just go with that, but the wave can be in any form, longitudinal or transverse. It doesn’t matter. In your mental picture, something is waving. With the squiggly black line on the white background, that something is the line. But we say light is a wave. Okay, picture a wave in your mind without the black line, where there is nothing waving, just the wave itself. You will eventually find that a wave is not a thing. A wave is an event within a thing.

In order to receive information from a system, part of the system must be removed. What happens when a wave breaks? Energy is conserved and has to go somewhere. In something that is not solid, the energy scatters. In the ocean, water is separated from the body of water in a spray of droplets.

But the double slit can be done with single particles, so where is the wave? What is waving? How can a single particle interfere with itself? It can’t.

You need a background medium through which the particle moves, or is part of, that is capable of forming into a current or flow that travels with the particle. The best candidate is a field of some sort.

So, if a conscious observer is knowingly or unknowingly removing information contained in such a field, then yes, a conscious observer could shift the behavior of quanta in a double slit experiment.

That leaves the quantum eraser experiment out. It is not time for that.

Best wishes, my friend.

The essential property of an observation is that some process has a macroscopic (non quantum-level) effect.

As long as a quantum-level process (say the motion of a photon) is initiating merely another quantum-level process (say the motion of a single electron), it makes sense to say that nothing has actually been observed yet, and the overall state is still progressing under the probabilistic rules of quantum mechanics. In some crude sense, anything remains possible.

But when such an effect is amplified to determine the combined state of quadrillions of quadrillions of … quadrillions atoms, any inherent randomness in that overall state is driven asymptotically to zero: based on an observation of the location of some photon, the battleship turns left or turns right, and the part of its state function accommodating the other option is driven effectively to zero.

But that of course is a continuum. When the state of only one particle is involved, it makes sense to say that no observation has been made. When a kilogram of matter has been altered, an observation clearly has been made. But when 100 atoms, or 10, or 3 have been altered, it’s kind of a gray area as to whether that is an observation or not — there’s no hard and fast line between the quantum and macroscopic realms. Quantum rules work at the smallest scale, and traditional physical laws work at macroscopic scales, but the area in-between is notoriously difficult to model, or even describe.

The easiest way is to just specify that an observation is an interaction. Any kind of interaction. Even a photon can be an “observer” and carry out an “observation” by interacting with something else. You run into no logical contradictions if you do this, although what you do find is that the variable properties of systems become variant.

Two observers may disagree as to when a particle took on a particular property. In the famous Wigner’s friend thought experiment, Wigner and his friend come into disagreement as to when the ψ wave “collapsed” and the particle being studied actually takes on a particular definite value. Although, they do not disagree as to what the value is, only when it acquired being.

Something that is variant is more like velocity. Velocity makes no sense if you do not specify a reference frame, i.e. a coordinate system. We can talk about the velocity of a train relative to a rock, relative to a car, but there is just no velocity of the train unto itself. Something that is invariant is identical in all possible reference frames. Two observers are not going to disagree as to whether or not the train is indeed a train, one’s not going to see it as a frog while the other sees it as a train. The train itself is invariant, but its velocity is variant.

If you were to state that all interactions qualify as observations, then you inevitably run into the conclusion that the variable properties of particles all depend upon a reference frame specified by ψ. Although, the specific values of particles do not depend upon reference frame, but when those values come into being depend upon reference frame.

You then do not need to posit any “intelligent creatures” or even “measuring devices.” You just have to take into account that it is meaningless to speak of the properties a system has without specifying some coordinate system given by ψ. The reason you “collapse” ψ after a measurement also then becomes rather obvious, because you change your relation to the system being observed so you have to update your coordinate system to account for it, kind of like taring a scale, and not because you “collapsed” some sort of physically existing ψ wave.

You have to do this because part of the coordinate system you can choose yourself, in the same way you can choose which reference frame you view the train from: either on the ground, in your car, or in the train itself, and in each you will measure a different value for the train’s velocity. However, what ultimately makes quantum mechanics unique is that part of the coordinate system you actually cannot choose yourself and will be randomly determined after an interaction, forcing you to have to constantly tare your scale, so to speak.

According to our interpretation, at the moment of measurement there is not a splitting of the world or consciousness, but a transition to this or that context in which a certain quantum correlation is already predetermined. Outside the context, a certain correlation is not predetermined, only the correlation itself is predetermined. Moving into one context or another corresponds to the choice of coordinate system (point of view); it is not a physical process. In that sense, the word “transition” isn’t exactly good. An observer simply discovers that he or she is in a certain context, within a certain point of view (in this case, unlike in classical physics, he or she cannot choose his or her context and cannot return to the original position). If the “coordinate system” is fixed, the correlated value of the physical quantity is fixed. So the quantum correlation is “coordinate”. It is coordinate both in the sense of the initial choice of the “coordinate system” and in the sense of the coordinate dependence of correlated physical quantities at a fixed choice of the initial coordinate system.

— Francois-Igor Pris, “Contextual Realism and Quantum Mechanics”

Intelligence has nothing to do with it. “Classical” has a lot to do with it. Let me explain.

A “classical” entity: a human, a cat, a brick, a video camera, has well-defined states. A brick is either on the table or on the floor, never both. A human is either moving or standing still, never both. A cat is either alive or dead, never both. (Putting aside pointless philosophical musings about cats being, after all, “quantum” since they consist of a very large but finite number of uncorrelated quantum degrees of freedom… true, but for all practical intents and purposes, bricks, cats, etc., are classical. Any quantum behavior is averaged out to the extent that it vanishes.)

A “quantum” entity is different. A quantum entity does not have a classically defined position or momentum. Rather, it has a state. That state, which may be described (depending on your mathematical preference) by a vector in an abstract space, or by a fancy wavefunction, can be used to calculate the probability of finding that quantum entity with a certain value for its position or momentum when it interacts with something classical. Let me explain how.

What the equations actually tell us is that the state of the quantum entity evolves such that it is constrained by its past, present, and future interactions, including interactions with classical things such as bricks, cats or humans. Quantum physics is manifestly nonlocal (this is the essence of Bell’s famous theorem) so it is subject to constraints that can be arbitrarily far in space or time, yet curiously, somewhat counterintuitively, it remains causal (it is not possible to arrange the future to induce an observable influence on the past).

A quantum entity is neither a particle nor a wave, it just displays behaviors of both under the right circumstances. The state of that quantum entity obeys a wave equation including the ability to exhibit interference patterns. But when a quantum entity interacts with a classical thing, the interaction may be localized, creating the appearance of something resembling a miniature cannonball, i.e., a particle.

Concerning the double-slit experiment, the quantum entity may be a photon or an electron (so let me just call it an electron from now on), which is confined by three things: a) the location and time of emission; b) the slits; and c) the location and time of detection. These are the constraints that determine the wavefunction that characterizes this electron’s state.

One possible interpretation, obviously inspired by the uncomfortable consequence of quantum nonlocality, namely that the location and time of detection is one of the constraints characterizing the system, is to take that constraint out of the picture initially. We then describe the state of an electron that is characterized by the location and time of emission, and the presence of the slits. The resulting wavefunction can be used as a probability amplitude, characterizing the probabilities of possible locations and times for the electron’s detection. When we actually do detect the electron, the picture changes: We now have a new wavefunction that is also constrained by the location and time of detection. This rather abrupt replacement of one wavefunction with another we call “wavefunction collapse”.

Now it is true that this interpretation requires an intelligent creature, such as a human who prefers this probabilistic interpretation (the Copenhagen interpretation). But apart from some (not generally accepted) extensions of the quantum theory that treat wavefunction collapse as a physical process, the collapse is only a clumsy mental aid. It is a philosophical musing, not physical reality. In reality, that electron was always governed by a wavefunction subject to nonlocal constraints (again I stress, this is the fundamental essence of Bell’s theorem.)

So to sum up, a quantum entity (photon, electron, etc.) is neither a particle nor a wave; its state is governed by a wave equation so it sometimes exhibits the consequences of wave-like behavior; its interactions may be highly localized, giving the impression of a particle; and intelligent observers have nothing to do with this unless we buy into the controversial concept of objective collapse (of the wavefunction), and perhaps not even then.

Despite some quantum woo-woo that you find out there, the “observation” in a QM experiment has nothing whatsoever to do with intelligence, or with consciousness. It refers to any interaction of a particle with its environment … which may or may not be an actual measurement apparatus.

To the first question, an “observation” in this context is something that determines whether a photon has gone through a particular slit or multiple slits. Such an observation changes the behavior of the photon by having it interact with some kind of detector or “observerer.” People keep saying that you would use a camera of some sort. These people haven’t thought it through. A camera has a low probability of detecting a photon for a lot of detailed reasons I won’t go into here, but cameras which detect the photon take the photon out of play. (It is absorbed.) The trick is to observe the photon without absorbing it. That takes some doing. But it can be done. And once it is done, it doesn’t matter whether any human know about it or takes it into account. The photon has been disturbed. It has been “observed” by a small particle of some kind.

The second question has to do with determining whether light is a particle or a wave. It isn’t so there is no determination to be made. We can define a new thing sort of like a particle, but not really, and call it something like a QP and say it is that. But we can’t say it is a particle or a wave with the meaning of the words as we know them.

The issue here is that nature behaves the way she does and she doesn’t care whether we understand it or not, nor what we think. Light is neither particle nor wave in any way that you think of macroscopic particles or waves.

You tend to think of waves having energy that you could sub-divide as many times as you wish and find the energy spread out over a large area.

By observing light carefully through many experiments, we find that it just doesn’t fit into either category fully. We go looking for small amount of light and it doesn’t seem to be spread out over the wave. It seems to pop up in little packets that appear large for the area affected. It is almost as if the energy in the wave concentrates down into a single electron.

Further, it doesn’t seem like a particle either, since it can go through two slits and interfere with itself.

So, it is either some new kind of particle that has a wave function to it, or it is some kind of a wave that can collapse down into an energy packet absorbed at a single electron. Our classical way of thinking about particles and waves doesn’t fit this.

So for a long time, we talked about the particle-wave duality. Then we embraced QED (quantum electro-dynamics) which said that light is made of particles, but that the particles have a probability wave function, and the particles somehow don’t behave like particles. They don’t take one path or the other and we can’t say they take both paths, or we refuse to. Yet mathematically, we have to say that the photon takes all paths with different probabilities and we integrate all the probabilities (with some hocus pocus to get around the fact that the integrals contain infinities.)

So these really don’t resemble any particles that you have any experience with.

And then we started to say, maybe they are none of the above. Maybe they are just excitations in the quantum field.

Let me repeat. Nature doesn’t care what you call it or think about it. It is just doing its thing. And photons do not act like any wave or particle that you know of. If you call them a particles or a waves, it is probably going to get in the way of understanding light.

It seems like you are asking, if we could take a smart pill, if we could say whether it is a wave or a particle. No, we know it is neither.

For some work that I do in lasers, I pretend light is a wave, because I know that the math will give me the right answer. But in other problems, I pretend light is particles, because I know the math will work. And in very low light, I treat the detection of the light as Poisson statistics.

If you insist that light be either a particle or a wave, you will never get an answer and you will never understand light.

In ye olde double-slit experiment, the term “observation” doth bear great significance, yet it is not as simple as the mere act of seeing with one’s eyes. In the realm of quantum mechanics, to observe doth mean to measure. ‘Tis the act of measuring which doth collapse the wave function, thus determining whether light doth behave as a particle or a wave.

When light passeth through the two slits unobserved, it doth travel as a wave, creating an interference pattern upon the screen, like ripples upon a pond. This pattern is the mark of a wave, with peaks and troughs intersecting, creating bands of light and dark.

However, when an observation, or measurement, is made at the slits to determine through which slit the light doth pass, this act of measurement doth collapse the wave function. The light then doth behave as a particle, and the interference pattern is no more, replaced by two bands corresponding to the slits.

The act of observation doth not require the gaze of a sentient creature but rather the interaction with a measuring device. ‘Tis the knowledge gained from the measurement which doth alter the outcome, not the act of looking with the eyes alone. The device which measureth the light’s passage doth cause the wave function to collapse, forcing the light to choose a definite path, thus behaving as a particle.

In summary, in the double-slit experiment, observation meaneth the act of measurement, which doth collapse the wave function and determine the light’s behavior. Mere visual observation by intelligent beings hath no effect unless it is accompanied by a measurement that doth gain information about the light’s path.

Forsooth, this experiment doth reveal the strange and wondrous nature of quantum mechanics, where the very act of measuring can change the fabric of reality itself.