I stumbled upon the double slit experiment. It was very interesting to the say the least. The video stated that when being observed the electrons act as particles rather than the wave because they created pattern similar to that which would be created by a particle. This video positioned the argument that maybe conscious observation changed the way the photon behaved.

So I Googled and searched a little bit to find the argument that the act of observing in itself affected the photons causing the irregularity. This seemed more realistic than the consciousness theory, but then it leaves the experiment not account for anything as the whole experiment doesn't seem consistent.

This got me thinking, instead of observing the photons passing through the slit to see which ones are going into which slit, why not tag the photons? Then you can observe the result after it has hit the screen and determine what has happened? Has this already been done? If not, why? Isn't this technology not available?

Also, what about passive observation? What I mean is : For example, we can smell things. Of course, smelling in itself is an interaction between our nose and the molecules of the object being smelled, but we are not directly interfering with the state of object. Isn't there a technology where we can identify the photons without directly affecting the experiment? If no, can someone explain to me why that is not possible?

Sorry if I seem not very well versed with this subject. I only have a high school knowledge of physics. I'm still curious.


3 Answers 3


Photons and electrons are elementary particles and can be successfully described in the framwork of quantum mechanics. Quantum mechanics, in contrast to classical mechanics , does not predict a trajectory for a particle, only a probability distribution is exactly predicted. This means that one has to do the experiment many times with the same boundary conditions in order to see the probability distribution that the solution of the quantum mechanical equations gives.

Take this one photon at a time experiment, photons of a single frequency impinging on a double slit . The slits, width , distance apart, are part of the boundary condition "photon scattering on double slit".


What looks like random dots on the left, shows the interference pattern expected from a wave behavior on the right, which in effect is the cumulative quantum mechanical probability distribution predicted by the mathematics.

Photons are very simple particles and in any case elementary particles are not distinguishable from each other. There can not be a tag other than kinematical variables and quantum numbers . To get at the variables the photon has to interact, and all interactions change the boundary conditions, and thus are the solution of a different problem than the pure double slit scatter. Those solutions do not show the interference pattern, because different probability distributions fulfill the boundary conditions,and these do not show an interference pattern.

In this experiment they tried "tagging" electrons collectively by using a filter in one of the slits.(The mathematics is the same for electron and photon double slit).

Although the electrons (which were shot one by one) could still pass through the filtered slit, the filter caused more of the electrons to undergo inelastic scattering rather than elastic scattering. As the physicists explained, an electron undergoing inelastic scattering is localized at the covered slit, and acts like a spherical wave after passing through the slit. In contrast, an electron passing through the unfiltered slit is more likely to undergo elastic scattering, and act like a cylindrical wave after passing through that slit. The spherical wave and cylindrical wave do not have any phase correlation, and so even if an electron passed through both slits, the two different waves that come out cannot create an interference pattern on the wall behind them.

The "passing through both slits" is describing the mathematical function which describes the electron and will give the probability distribution, the final cumulative pattern on the screen.

This experiment shows that adding "tagging" changes the boundary conditions and the mathematical function the electron probability distribution has to obey.


When you talk about tagging and passive observation, you are already using terms of classical thinking. Tagging an object with something means attaching something else to it which is an interaction with it.

Observation, tagging, smelling, are all means of interaction. It's that interaction which does the job of destroying the interference pattern.

Quantum effects are only noticeable/observable on the most elementary objects. On that level you cannot avoid interaction with objects when you want to get any information about them.

Keep being curious!!


The problem is, I believe, what you say in your second-to-last paragraph, which is that observation is somehow "passive" or can at least be made so. And that's the fallacy: there is no such thing. There must always be some form of contact, or touch, even if not direct, between what is being observed and the observer. Indeed, I like to say there is actually only one sense, and it is touch. All observation involves touch, contact, with something - and you can see this by going down each and every one of your senses, one at a time, and thinking about how they work, and moreover that touch has to ultimately end up in some way at the object in question, even if it involves a bunch of intermediate stages in between, including the object touching that which transmits as well as your sense touching the proximately sensible thing:

  • Sight. This sense is of course our most informative, but also the one that feels the most passive and indeed at least in Anglo-American culture its apparent passivity is emphasized as part of the English language - as we like to say with statements like "sit around and watch" and "they just watched", and so forth. It seems very passive to us indeed because it requires no effort, and moreover, does not seem to have any obvious effects upon that which is being looked at. But let us look a bit more closely at how this sense works. Sight requires, in order for you to see an object, that light must enter your eyes and touch the receptors on the back of them, which will then start a complex series of chemical reactions resulting in the transmission of that information into your brain. You could also say that your eyes have to touch light. Going further back, however, for the light to contain any information, it also has to have touched whatever the object was that you are observing, so as to pick up its spatial configuration as a result of being modified by absorption, reflection, and transmission - all three of which involve contact, or touch, between the objects in question.

  • Hearing. This is another one that may be considered passive, but sometimes we don't - e.g. we talk of "listening with intent", and things of that nature. In this case though we're not really talking about the process of listening itself but rather of cognizing, for the process of hearing is always going on whether we are processing the information in the conscious parts of our brain or not. Regardless, the act of the sensation itself still feels passive. Nonetheless, it's also not - and works rather similarly to how sight does, except here the interaction is with a medium that does not appear and disappear as photons do (when they are absorbed and emitted), namely air. To make audible sound, an object must be in contact with - touching - the air and then must press against it in small blows (vibration) at a suitably high speed, at least 20 times per second and at most between 10 000 and 20 000 times per second depending on the age of the listener (assuming of course sie has the sense of hearing at all :) ). These blows cause the air molecules to push against other air molecules and transmit them to another set of air molecules, then those push against another set and transmit to another set, and so on and so forth, until they start pushing against our ear and our ear starts feeling them. Then the information goes on, as with sight, to our brain.

  • Smell. This is the last one we talk about, and you mention it. In order to smell something, the receptors in our nose must be able to feel - touch - molecules of that something that have been released into the air, and moreover, in order to get there, those molecules had to come off of it, and that means that it had to alter in some way (e.g. the molecule had to volatilize or evaporate from the surface).

The other two senses more obviously involve touch so I won't mention them. But I believe the point is clear. There is no such thing as a passive observation that does not in some way cause a physical effect upon the thing being observed. When that is understood, then the idea that observation changes that which is observed should not be hard for you to accept. What's different is that in quantum mechanics we can actually see that happen, but we can't so easily see it with our other senses, because the effects are so minute.

Why is that difference there? Well the answer ist hat quantum mechanics operates on precisely that very minute scale! So if we're probing around there, we will naturally expect large observer effects (that's the term, and it's actually more general than this context - if you ever play around with enough computer programming, you will come across them too, because of how computers work, "debugging" a piece of computer program, i.e. trying to find an error you made somewhere in the program, with an active debugging tool, requires changing how the computer runs that program and in some cases that can cause the bug (error) to appear to go away, or to change its behavior, than when you are not doing so. This is another example of how that making an at least more detailed observation changes the outcome. Observer effects are not bizarre, what is bizarre is how the popularization of quantum theory has been done and how it's talked about in some ways that seem to convey the idea there is some magic here.).

So now what about what you say, about "tagging" the photons? Well, it turns out there is actually a very good way to do that, and it's by setting up the direction of their spin - loosely, you can think of the photon as a little ball that is whirling around an axis, like when a basketball player holds up a ball balanced on his finger, whirling. I say "loosely" because a) photons are not balls and b) quantum mechanics also changes the conception of "rotation" of very small objects considerably from the sense you are used to, in that all the "weird" strictures you are beginning to encounter dipping your toes into it are tuned up to 10 in this setting. The point is just to really imagine the spin as having a direction and what the gist is of its correspondence, not to take the analogy too seriously or literally. In any case, ALL photons are "spinning" in this quantum sense, and the rate of spin is constant and independent of which photon it is: it's literally a universal constant. But the direction need not be - some might be spinning clockwise, others counterclockwise, for example (with of course all the little quantumie caveats in place that I'm not putting down here - when you get a little deeper you'll see that more generally each photon can be in a "superposition" of these where in a sense it doesn't quite "know which way it's turning" and this a big part of the quantum "weirdness".). You could, say, have that when they go through the left slit, the photons spin clockwise, but through the right slit, they spin counterclockwise (just to be simple).

However... the way you do that is by either having two light sources that emit photons with separate spin directions and then plastering them against the slits so only photons of that spin go through, or you emplace something on the slits that changes, or better, selects photons with only the right spin. Such a thing is called a polarizing filter, and you can get them at an optics shop. The trouble is that in both cases you've modified the setup, and so it should be no surprise either that you get a different result. In particular note that the polarizer requires interaction, too - it has to absorb some photons and let others through.

With this in place we can now accurately frame what the real weirdness of quantum theory is and it's actually not any of this stuff, but rather the ways in which these effects behave. In particular, the laws of quantum mechanics predict a sort of "conspiracy" among the forces of the Universe which will be clear as to its reasons why if you go in a little deeper and that's that in all setups you will either only have that there is information present at the slits about which slit the particles are going through and no interference pattern on the back screen, or you will have no information at the slits, and the presence of an interference pattern. (OK, there may be some setups that kind of "interpolate" between these two extremes, but I'm not sure about that or what they'd be.) That's the real magic!

And why is it? There's a number of interpretations, but the simplest one I like is that quantum mechanics basically is telling you the Universe contains, in a sense, "less information" than you might expect it to given your experience with particles and objects in the everyday world. In particular, there is not enough information in the system to have some pairs of sets of physical properties or parameters fully well-defined at the same time. That's what Heisenberg's "uncertainty" principle is, if you come across that, or will be, when you do, really telling you: that there is a limit in the total information that can exist between a particle's position and its momentum, so that if one tries to make there be a lot of information in the position, there will be little in the momentum, and conversely.

Moreover, in mathematics, we describe such situations of reduced information through the use of probability distributions. If you want to know about that, just ask another Q, though maybe on Maths :) Thanks.


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