The Fabric of the Cosmos CH 7 (Part 2)

In the second part of chapter seven of The Fabric of the Cosmos, a development on the first experiment we studied is present.  This new experiment deals with erasing the past.  The book asks the question: Assuming you can not change the past, is there anything you can do to change something’s impact on the present?  In the experiment conducted in this section of chapter seven, an photon gun shoots photons towards a double slit screen and a detector is placed to see which slit the photon goes through.  Because the photons are being observed, as predicted two separate lines appear on a detector, but when a screen is placed in front of the detector that erases the known position of the particle, an interference pattern forms.  This experiment is similar to the earlier experiment in this chapter and I understand the results.  The thing that I do not understand about the experiment is how a screen can all of the sudden erase the position of a particle.  I do not see how that logically can work out.


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