Many wrong or at least incomplete or naive answers here. It is not necessary to “disturb” a particle to “observe” it. Modern versions of the infamous double-slit experiment can generate which-path information without ever interacting with the particle, the particle is tagged at emission and by analyzing the outcome scientists can tell which path it went through (or if it went through both slits and interfered with itself) without any interaction from emission to detection. Some physical processes demand generating information, others do not. In the double slit experiment we can put a detector to generate which-path information or we can not do so. We the conscious beings are only involved in deciding which physical arrangement we set up. After that, we are not involved at all. If the experiment is run with a setup which generates which-path information, the experiment will generate such information, and it will be either slit A or slit B. If the experiment is run with a setup which does not generate which-path information, it will produce an interference between both paths. It does not matter if anybody looks at those results or not, it is the physical arrangement of the setup which determines what the result will be like. Of course if nobody looks at the result we can never prove what the result was, and if somebody does look at the result, some may say that the result was X because he looked at it. That’s not so, the result depends only on the physical arrangement of the system, whether it generates information or not, regardless of any potential observers. We can tell about the past history of the Earth before any sentient beings existed on it, because the processes through the planet’s history have generated a lot of information, even when there was nobody to look at that information.
Sending through photons of light one at a time doesn't produce two bands of light like we'd assume. Instead, the photons still produce the same interference pattern as before, as if they understand where all the other photons will land before they’ve ever been fired. This shouldn't be the case. The photon, when fired, should decide which slit it will go through and yet somehow appears almost to be splitting itself in half, traveling through both slits, and then interacting with itself before recombining. Light particles shouldn't continue to act as waves when fired individually. Once sent out from the laser, they deliver all of their energy to a single point unrelated to the landing spots of all the other photons. So why are we still seeing the interference pattern as if the photon is aware that there's two slits? It only gets weirder from here on out. Scientists have set up experiments attempting to detect which slit the photon has gone through. However, using detectors to define the photon's path destroys the interference pattern, leaving just the two normal bands of light we expected to see from regular particle behaviour. Observation of the photons will produce no interference pattern yes, exactly as if they’re aware that we're watching. And this elusion happens even in the most careful of detection experiments where scientists have tried their best to preserve the path of the photons (that is, the path of the waves since the photons act like particles at the beginning and end but wave-like during travel).
In a 1999 experiment, special crystals were placed in front of the double slit. The crystals took the incoming photon and split it into identical, entangled twins. One photon continues on to create the interference pattern, its entangled pair will go to a detector that will tell the scientists which slit the original photon went through. Because only one of the pairs is sent to the detectors, there is no decoherence for the original photon. And yet, amazingly, if the detectors told the scientists which slit the original photon had traversed, the interference pattern would again disappear. Our knowing which path the photon took caused it to behave differently than when we didn't know. Could our knowledge cause a photon to alter events that have already taken place? There was another instrument in this experiment as well. The quantum eraser was an extra set of detectors which again hid the path of the original photon from these detectors we couldn't know which slit was traversed. Here, once more, the interference pattern emerged. To anyone, hearing about these results would lead to the conclusion that our simple knowledge of light's path determines how it will behave or that observing the universe in some ways decides our reality. They are incredible implications, but ones which shouldn't be taken yet as fact. The realm of quantum mechanics gives us a stunningly accurate description of the world but it is also one with unbelievable phenomenon and information we still haven't acquired.
The first theories for why this interference pattern happened had to do with particles spreading out into waves while they were in motion, but this was later amended to say that the wave was a wave of probability. During their travel, the photons are a wave function of the possibility of all paths and this wave behaves just like any other to produce our interference pattern. When the particle is observed, and only then, a specific path is decided. This is according to Niels Bohr and Werner Heisenberg's Copenhagen interpretation of quantum mechanics. These possible paths are possible realities that interact, influencing the chances that some paths will manifest, and others will not. The interference pattern, in short, isn't the photon itself but the probability that the photon will arrive at that location. So while we cannot determine where any individual particle will land, we can assess the probability of it landing at any particular part of the interference pattern. This is calculated using the Schrodinger Equation. Photons aren't the only things that exhibit this phenomenon. Electrons, atoms, molecules, and even antimatter in one study have been shown to behave in exactly the same way.
What would happen when you fired a stream of electrons (something you could obtain by taking a radioactive source that underwent beta minus decay) at a double slit, with a screen behind it? What type of pattern would you see? Bizarrely enough, a source of electrons gave you an interference pattern! Somehow, these electrons must be interfering with the other electrons from the radioactive decays. So let's send them through one-at-a-time, and have a look at what appears on the screen:' So they did that experiment, and kept an eye out for what the pattern would look like after each and every electron that passed through. Somehow, each electron was interfering with itself as it passed through the slits! So this led physicists to the question of how this was happening; after all, if electrons are particles, they should be passing through one slit or the other, just like pebbles or bullets. So which one was it? They set up a "gate" (where you shine photons to interact with whatever passes through the slit) to find out which slit each electron passed through, and found, sure enough, that it was always one slit or the other. But when they looked at the pattern that emerged, they found the particle pattern, not the wave pattern.
In other words, it looked like the electron somehow knows whether you're looking at it or not! Or, as physicists sometimes frame it, the act of observation changes the outcome. This might seem peculiar, but this is actually what happens in pretty much all quantum systems set up like this: things evolve like they're in a wave-like superposition of all possible outcomes until you make the key "observation:' which forces the system to give you one real answer. Many particles can be created in an entangled state: where you know that, for example, one needs to have a positive spin and one needs a negative spin (e.g., +/-1/2 for electrons, +/-1 for photons, etc.), but you don't know which is which. In fact, until you make a measurement, you have to treat each particle like it's a superposition of the positive state and the negative state. But once you "observe" the property of one of them, you immediately know the corresponding property of the other. This is weird, because just like the electron passing through the slit, particles behave differently when they're in a superposition of states versus when they're forced to be in one "pure" state. You can, in theory, entangle two particles here, move the other one a light-year away, "observe the first one (and immediately know its spin), and you will immediately know the spin of the other one; you won't have to wait a year for the speed-of-light to transmit that signal.
Now, if that sounds spooky to you, that's because it is. No less a person than Einstein was troubled by it, and the resolution (by Bell, which is why it's called Bell's theorem) is that quantum entanglement is what we call a non-local phenomenon. To be fair, the person with the particle a light-year away won't notice anything weird about their particle once you measure yours; it's only once you bring your particle together with theirs (or the information from it, both of which are limited by the speed of light) can you observe the states of both particles. Contrary to what you may believe based on everything you just read, it has nothing to do with you, the observer. All this talk about measuring and observing has hidden the real truth here: in order to make these observations, we've needed to make a quantum particle interact with the particle we're trying to observe. And if we want to make these particular measurements, we need that interaction to take place above a certain energy threshold!
It has nothing to do with you or the "act of observing:' and instead everything to do with whether you interact with sufficient energy to make an observation;' or in non-anthropomorphic terms to constrain the particle into one particular quantum state or another. For an electron passing through a slit, that means forcing an interaction with a photon that can constrain its position well-enough to be definitively through one slit. For a photon of either spin +1 or -1, that means making a measurement sensitive to its polarization, which means having an interaction that is sensitive to the type of electromagnetic field the photon creates. An observation is a quantum interaction that is sufficient to determine the quantum state of a system.
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