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Trying my own double-slit experiment

By: Mike Gashler


This morning I decided I needed to do some double-slit experiments with lasers. Here's the tl;dr: Achieving interference is easy. Collapsing the wave before it hits the screen is hard.

To make my double-slit, I held two razor blades together and sliced a piece of aluminum foil.

My cut could have been cleaner, but meh, it'll do.

I mounted my foil slide in some cardboard to hold it upright. I got a laser from a board game called Laser Maze.

I set a water bottle on top to push the button.

For my screen, I taped a piece of white paper to the wall.

Here's what my laser does with nothing in front of it. (It's just a dot.)

Here's it is with a single-slit slide in front of the laser. Notice that the slit is vertical.

For some reason I don't yet fully understand, the vertical slit made the light spread out horizontally. Does this mean the path of the light is affected by the edges of the slits? Apparently so, but nothing I've yet read ever mentioned that. And lots of diagrams I've seen show the particles forming clumps behind single slits that are definitely not spread horizontally like that. Hmmm.

And here it is with my double-slit slide in front of the laser. The interference pattern first discovered by Thomas Young (1773–1829) is clearly visible. I think the weird shape of my spots is due to my poor slit cuts.

Next, I measured it.

It looks like the 13 brightest spots span a width of about 37.5mm. And my screen was about 3073mm away from the foil with two slits. To estimate how far apart my slits were, I stacked six razor blades. (That's all I had available.) It looks like 6 blades is about 3.5mm wide, so my two slits must be about 0.58mm apart.

Next, I drew a bunch of circles to represent waves coming out of two slits. Let's say the lines represent troughs and the peaks are between the lines. So I filled in 5 regions where the waves reinforce each other to represent the bright spots on my paper. I counted how many wavelengths away each region was from each of the two slits. Obviously, the one in the middle is the same number of wavelengths away from both slits. And as you move away from the middle, you increment one and decrement the other. Makes sense.

Now, I think I'm ready to do some math. So I made the following diagram of my measurements:

In this diagram, w is the wavelength of my laser, and n is the number of wavelengths to the center dot. I only showed the measurements for one slit since they are symmetric. Note that

(37.5mm - 0.58mm) / 2 = 18.46mm, and

(37.5mm + 0.58mm) / 2 = 19.04mm.

Next, I set up a couple of equations using the Pythagorean theorem:

((n - 6) * w)2 = 18.462 + 30732,

((n + 6) * w)2 = 19.042 + 30732,

Then I computed the wavelength of my laser to be 0.00059mm.

Finally, I Googled it. (Really, I didn't even look this up until after I did the math and recorded my answer.) Google says the wavelengths of typical red lasers range from about 0.00063mm to 0.00067mm. So I was off by somewhere from 6.3% to 11.9%. Not too shabby! Given how poorly I estimated the distance between my slits, I think I was surprisingly accurate! I think if I worked on being more precise with my slits I could probably nail it.

So I can now add my witness to many others that I have personally validated light to behave like a wave. I can also now confirm that science really does have a basis for claiming to know the wavelengths of light. They aren't just making stuff up. I never really doubted, but it's still good practice to to validate stuff.

Trying to collapse the wave before it reaches the screen

Next, I tried to collapse the wave function. (Supposedly, interaction with a macro-scale object should cause the quantum wave function to collapse, and the light should start behaving like quantized photons instead.) But as you can see, putting large objects in the path of the wave didn't actually cause it to collapse.

I've read several different sources claiming that this behavior is extremely reproducible and well-validated. But apparently what all these sources fail to clearly describe are the precise conditions necessary to achieve it. Am I going to have to dig into the original sources to find the secret ingredients? Ug.

My first thought was that perhaps all the particles interacting with my macro-scale object did collapse, but all the ones that made it to the screen were still in a wave-like super-position. But that can't be right because supposedly even detecting the absence of a particle is still interacting with the wave function, and that is what causes the whole system to collapse into a particle elsewhere.

Some authors claim that interaction with consciousness is necessary. But I am a conscious observer, and I watched it happen in real-time. And even if you don't trust that I am conscious, you can see the pictures I took. That should have collapseed it too, right? (Well, obviously not. There is just something I don't understand. But why aren't the precise conditions necessary for this phenomenon to occur better-documented? Aarg!)

My next thought was that perhaps wave-function collapse is something that only occurs when the brightness of the laser is dimmed all the way to the point where individual photons are separately fired. But that can't be right either because even a bright beam is still made of just a whole bunch of individual photons. My finger should have been interacting with the wave functions of every single one of them!

I tried collapsing the wave function in every imaginable place. I touched the laser before the slits, after the slits, in front of just one slit, near the screen, near the laser, and so forth. And I carefully watched the interference pattern the whole time. It never stopped occurring.

Well, completely blocking one slit killed the interference pattern, of course, and blocking the whole source killed everything, but nothing I did seemed to cause the quantum wavefunction to collapse in any sort of manifestation of quantum weirdness.

Next, I thought, maybe I need to use a set-up more like the precise experiments described in the books I have been reading. So I got out the partially-silver mirror. Here's my setup:

As expected, this made two dots on my screen. For a while, I spent considerable effort aligning the dots and trying to get them to interfere with each other, but since I couldn't get the two beams close enough to being parallel, the interference pattern was always too tiny to discern.

According to the theory, the quantum wave function simultaneously propagates down both pathways. If the wavefunction along either pathway is "observed", it collapses and no interference patterns should occur along the other pathway. So next, I put my double-slit slide in front of the other pathway.

When I blocked the dot with my hand, the interference pattern in the other beam persisted. But, one might object, my hand isn't conscious! Since this is for science, I did something you're never supposed to do and briefly looked into my laser. I had my children carefully watch the interference pattern on the other beam as conscious observers while I did this. They say it never changed. I also video-recorded it, and my recording confirms what my children said. No collapse! (Also, my eyes bothered me after doing that, so I hope you appreciate it!)

Supposedly, Complementary Metal Oxide Semiconductor arrays can be made sensitive enough to detect individual photons. I doubt the one in my phone camera is anywhere near that sensitive, but I also have a hard time imagining the quantum wave function exercises sufficient intelligence to evaluate the quality of my phone. So here is video of my phone failing to collapse the quantum wave function. I personally watched the interference pattern as I did this, and it never collapsed.

So next, I put my double-slit foil in front of the partially-silvered beam splitting mirror. Now the quantum wavefunction follows four separate paths, and I get two unique interference patterns.

In this picture, the interference pattern in the lower-right is not very clear, but it was very clear in real life. Maybe I'll redo that picture sometime, but there was definitely an interference pattern in both of them. Meh, you can redo it if you want to.

So then I repeated all my experiments in every location, with children and cameras and every combination of intervention I could think of. I tried adding another mirror that projected one of the paths onto my ceiling. The idea here was to make one of the paths longer, and also make the wavefunction along one pathway go through an extra transformation, hoping to wear down the universe with more complexity while I observed the shorter pathway. But still I couldn't get that darn interference pattern to go away.

So what does this all mean?

Is quantum mechanics just a big hoax? No, of course not not! There is obviously just something I don't understand. Maybe someone who studies QM will comment below to help me identify what exactly it is. And as soon as I know what the missing detail is, I will probably feel silly.

But still, the fact that there is a mysteriously missing detail is kind of interesting. In my strong opinion, someone who lives in modern times, and who has access to the Internet, and who is reasonably well-educated, and who is willing to put in some serious effort, should be able to at least determine what science knows and what it doesn't. I fit all of those conditions. But my strong impression is that the matter is not yet fully settled. A lot of books and articles I've read talk as if quantum weirdness is very well-established and trivially reproducible, yet they contradict each other regarding what exactly counts as a measurement, and I have never ever even found anything willing to define the specific conditions necessary to make it happen.

So does the fault lie with my effort? Certainly it does to some extent. But perhaps some component of it also lies with the efforts of those who disseminate scientific knowledge. And a considerable amount of it certainly lies with those who negligently promote disinformation. Schrödinger's equation for the quantum wavefunction has been known since 1926! If we understood quantum mechanics well enough to describe it with a mathematical equation way back then, surely we should have the general big picture details sorted out a century later, right?! Sadly, that doesn't seem to be the case.

Apparently, quantum mechanics is a tough domain to study. I get that. What I don't get is what is stopping the pundits from identifying exactly what we do and do not yet know? One thing I can conclude for sure is that there are some very specific conditions necessary to trigger collapse of the wave function. I know this because I failed to meet whatever those conditions are in my pathetic attempts. I will be the first to admit that this is not my domain of expertise. And I'm not ashamed to expose the laughable quality of my equipment or the ignorance of my attempts to seek understanding. But does anyone even know exactly what is necessary to cause collapse?

And more importantly, why is the answer to this question so hard to find? Why don't people just fess up and tell it like it is? It seems everything I read about quantum mechanics is tainted by someone's agenda to push a particular interpretation, and no one seems to want to just offer how-to-test-it-yourself instructions. Grr. C'mon science, it's been a hundred years! This should be the stuff of Junior-High science fair exhibits by now!