# Altering Young's double slit experiment due to observation

The interference pattern on the screen is altered due to our observation, but how could this happen, as we have been watching it since we started the experiment? Is there any other meaning of 'observation' in this experiment?

• It has nothing to do with observation. The interference goes away if you block one of the slits with a detector. If you don't have photons coming from multiple slits then there is no interference. – Bill Alsept Dec 12 '18 at 16:47
• But I heard by many pof saying path of electron disturbed due to our observation. Even in YouTube – Kelvin Dec 25 '18 at 10:40
• you can’t observe something with collecting photons (stopping them from contributing at the detection screen) which changes the outcome. – Bill Alsept Dec 29 '18 at 22:25
• @BillAlsept . youtu.be/DfPeprQ7oGc at 3.49 – Kelvin Dec 30 '18 at 14:12
• the video is a little silly but at the 3:50 minute mark it makes my point. When they put a device at one slit to observe electrons, light was used. These photons diverted the electrons trajectories. Some trajectories where diverted so much the electron didn’t even make it into the slit. Even slightly diverting electrons is enough to destroy the convoluted pattern at the detection screen. So even if you or any living being were not observing, the device would still diminish the interference pattern by diverting electrons. – Bill Alsept Dec 30 '18 at 16:51

Let's compare two experiments. Experiment A is the classical double slit case. The wave function consists of two parts, $$\psi_L$$ and $$\psi_R$$, which are coherent and overlap on the detection screen. Thus an interference pattern results from the term $$2\psi_L \psi_R$$. Experiment B is altered with a detector that can unambiguously determine through which slit the particle went. The detector has two, orthogonal, states, $$d_L$$ and $$d_R$$. The total wave function now is $$\psi_L d_L + \psi_R d_R$$. The two parts are orthogonal, $$2 \psi_L d_L \psi_R d_R=0$$, due to the entanglement of the particles with the detector, so no interference occurs, even if the experimenter forgets to watch the detector. Interference can only occur if it cannot be determined through which slit the particle passes.