Proving Heisenberg’s Uncertainty Principle: the work in progress

The uncertainty principle is one of the most coveted laws in quantum mechanics. Its mysterious properties have mystified physicists for decades, and you may be surprised to find out that its counter intuitive affects can be observed somewhat easily. Shine a laser through a slit. what you see projected behind it is a spot from the laser. As you close the slit, you see the spot get thinner and thinner, and you expect this to continue until the spot is too small to see, but it doesn’t. Once the slit gets very thin (on the order of a few hundredths of an inch), you see something completely unexpected. The projected light from the laser begins to fan out, getting wider and wider until the slit closes and no more light gets through.

This experiment is a somewhat common demonstration, and if you don’t believe it, try it your self. You will get the same surprising results that so many have in the past. Heisenberg came up with a very specific law to prove explain this, but how do we prove it exactly? you could measure the amount of dispersion and the width of the slit, using the unchanged output of a laser pointer, but to me this seems like it would invite a large percent of error.

I wanted to be able to prove this myself, but in a more profound way. I got thinking. Would it be possible to shine a single photon source through a slit and prove the uncertainty principle on a level of individual particles? It seemed difficult, but I actually already had a lot of the necessary materials to do this. I wanted to perform the double slit experiment with individual photons a while ago so I bought 5 ND 3.0 neutral density filters and a 1 mW laser pointer in hopes of making a single photon source. ImageImage

I intended to test the setup by letting my eyes get fully dark adapted and then try to see the characteristic uniformly bright flashes from individual photons, but I didn’t have enough patience to do it successfully. So the setup sat for a long while, until it made its way up on my “to do list.”

I reassembled the single photon source and tried to get results with my Dad’s DSLR camera (a Rebel T1i (?)).



I wasn’t about to just shine things through slits before a control though, So I built a quick setup to hold the laser in a stationary position with an external 3.3v supply for the long exposures I made using a bulb setting on the camera with a remote shutter control.

I mounted everything on a small wood frame I made and put it in a dark exposure box.



After successive 1, 5, 10, 15, 30, and 60 minute exposures with both 5 and 4 filters, I couldn’t see any photons that were definitively from the laser pointer. The picture was being exposed by something however, as small spots getting more numerous with the longer exposures were visible, but it can be difinitively stated because of their dispersion that they were not from the laser pointer.

These initial tests while unsuccessful have shown me two things.

1. My “dark box” is imperfect. I am confident that no light from outside was getting it, but their was a point of weakness in that the laser’s power supply was included on the inside of the box, and it had an LED that was lit when it was on. I tried putting it in another box and covering it with a trash bag, but it is clear it was not enough. I will need to try to somehow keep it outside the box entirely to prevent exposure from anything other than the laser.

2. I did not have a good macro lens for the camera, which means that I could not focus very well on the laser which sat 12-18 inches away from the front of the lens. I didn’t realize it at first, but the fact that it was not focusing well could have been causing the individual photons to be too poorly defined to be identified. If the above point does not fix the problem I will have to wait until I have access to a better macro lens before I can continue this experiment.

To summarize, these past few weeks spent on this project have, while not being successful, shown me weaknesses in my apparatus and have shown me how I will have to proceed with the experiment. Keep an eye out for updates. I should have an update in a couple weeks or so, which will tell whether I will be able to get good results soon, or if I will have to wait longer for a better lens.

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