Alan's Quantum Physics Page
Page updated Mar14


After giving up on T.V. as it just got so bad as to be a waste of time even turning the thing on, I cancelled my T.V. licence and was forced to resort to reading books instead.....
I have been reading lots of popular science books and have found the world of physics really is a fascinating one.  After reading all about quantum physics and of course all about the classic double slit experiment I decided I needed to see it for myself.

- for more info see THIS video or THIS wikipedia page.


Double slit experiment

BTW - If you want to learn more about this subject, I highly recommend having a look at THIS book by Brian Greene - it is by far he best beginners type book I know of and contains a staggering amount of information.

I had already tried a couple of times before to do this experiment but failed to see any interference pattern, my problem it turned out was that I had not realised you need to project it several meters rather than the 30cm or so I had been doing - Doh! -  Once I figured this out it is very easy and I think it something everyone should give a try as the implications of this simple experiment are mind blowing......

What to do:
Firstly you need a laser, any laser pointer should do (you can get them for a couple of pounds on eBay or even some times from a local pound shop), put this on some kind of mount to hold it (a big blob of Blue tack is good enough), you need to keep it switched on, some tape over the button will do this. 
In front of this you need to place the slits.  To make the slits place a small piece of tin foil onto a paper pad and with a ruler and craft knife cut a couple of parallel small slots approx 1mm apart (they only need to be a few mm long).  BTW - Instead of using slots you can just make a couple of pin pricks if you prefer.
You now need to place this slit in the laser's beam, again some blue tack or a clothes peg will do the job fine.
Now you need to project the image across the room (if you try to view it any closer you will not see anything of interest, as I discovered).

Here are a couple of examples of the sort of image you should see:

The different images are from varying the gap between the 2 slots

Quantum Eraser Experiment

The next thing I wanted to try for my self was a Quantum Eraser Experiment (more info here). This gets even weirder - you can make a change to the light after it has left the slits and it still changes the lights mind as to if it will be a particle or a wave today.

The simple home experiment I tried involves putting a polarizing filter in front of the two slots so that light from one slot is horizontally polarised and the other vertically. Because the photons can now be identified as to which slot they went through (depending on how they are polarised) it no longer produces an interference pattern. but, if you now put a polarizing filter between the slots and projection screen which is at 45 degrees this has the effect of twisting the polarization of the light from the slots putting them both to the same, now that you can no longer identify which photon came from which slot it returns to producing an interference pattern.

It is a bit fiddly getting the polarizing filters in front of the two slits. I cut a small square of filter, cut this into triangles so they could be rotated relative to each other and blue tacked onto the tin foil behind the slits.

You then just see a solid bar instead of an interference pattern (because the information exists to identify which slit each photon went through) but if you put a polarizing filter anywhere along the beam it changes to an interference pattern (because you have destroyed the which slit information in the photons of light)

Just think about this though!!! Each photon of light must have left a slot heading for a certain point on the screen, so this event/decision is now in the past but by removing the which slit information this has changed ???

THIS video explains more about this experiment (although I have not managed to get it to work myself using a pin)

There are lots of experiments which have been done on this and it gets even weirder - take a look at THIS.

If you found this interesting, you may like to have a look at my cloud chamber -  HERE

Have a look at these physics lectures, they are a real must see - PhysicsForFuturePresedents

Brian Greene's Fabric of the Cosmos videos

You can contact me on - alanesq@disroot.org

Back to my homepage