quantum mechanics - Tagging the photons on double slit experiment

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.

Answer

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.