Double-slit Experiment at Home?

Thanks for the link, solareagle. Looks interesting. I wonder if there is a way to perform the experiment where observation changes the outcome?

I think observation defines the state of an energetic particle at that time and place....of course it can be several places at the same time because it has lightspeed energy, which is beyond the limit of human observation...at least that's my take...

By observation, I think you mean knowing which slit a photon went through. If you block the right slit, for example, you know the photons have to go through the left slit. If you do this, of course, the pattern will change and formerly dark areas could be light. This has actually been done after the photon has passed the slits, and the photon somehow knows that one slit has been blocked.

https://briankoberlein.com/2015/06/04/real-and-unreal/

https://youtu.be/H6HLjpj4Nt4

I thought that was what you were after. Short answer is no. Long answer is Yes if you have the ability to emit single particles one at a time and know their mass and the time of emission. Then you also have to have a means of detecting them on the other side of the slits, and you have to be able to see which slit the individual particles passed through. Not an easy or cheap setup to make at home!

It's not exactly "double slit", but it is the same effect. You can take a hair and hold it in the beam from a laser pointer and see the light fringes. The fringes will be parallel to the hair spaced out in a line perpendicular to the hair.

I've even noticed this effect at night from traffic lights when a hair was in front of my eyes. The fringes for the red light were more widely spaced than for the green light, due to the different wavelengths.

http://physicsed.buffalostate.edu/pubs/StudentIndepStudy/EURP09/Young/Young.html

Very interesting! I've definitely noticed the speckle characteristic of LED taillights and streetlights, but not the interference pattern. I'll certainly be watching for it now.

This shows that the concept works with (approximately) monochromatic light, not necessarily coherent. Thus no laser necessary. Single LEDs are now bright enough that it should be possible to set up a double-slit experiment with only an LED as a light source.

A laser will give you a nice clean diffraction pattern. The distance between fringes is proportional to the wavelength.

If the source is not monochromatic like a laser but just narrow band like a LED, you can consider it a superposition of many monochromatic sources with slightly different wavelengths.

So the diffraction pattern will be sharp near the center but farther out the different wavelengths will be further out of step and blur the pattern. The narrower the bandwidth, the more fringes that can be seen.

Diffraction provides a way to measure wavelength and bandwidth given a known fiber diameter, or given a known wavelength laser, the diameter of a fiber can be measured from the pattern.

Shining a laser pointer on three fine mechanical pencil refill 'leads' held parallel in a flat plane works well. You can tape the refill leads or just hold them between your fingers.

The nice thing about fine mechanical pencil lead versus fine wire is the uniform straightness. It is easy to get uniform slits.

Also there isn't as much reflection off the pencil leads.

Good idea, I like that.

What results could be observed....?
Those which are observable.
Preconceived expectations lead to inaccurate observation.
Del

In 1801 Thomas Young did not have a monochromatic, coherent laser to perform his double slit experiment. To reproduce Young's interference experiment what one needs is a bright enough point source of light. I vaguely remember many early investigators of light itself used direct sunlight shone through the hole of a camera obscura in their early experiments. I suspect this could be repeated without the requirement of modern technology.

It depends on what results you would consider as being sufficient for ''success''...

I briefly attempted Young's Experiment while still in High School, oh so many years ago...

I had a bright lamp light source a one end on the diagonal distance of our garage floor and a mirror propped up at the opposite end of the diagonal...

Near the light was a corrugated cardboard target with a 1/8'' wide vertical slot in it, and solid cardboard sheet a few feet behind it, but not good enough for my Physics teacher...)

That produced a diffraction pattern of only two fuzzy parallel vertical lines...

So, dispite relatively crude components, my (apparatus) essentially worked, but only nominally... (which was good enough for me, at that time...)

You can observe interference patterns from a single slit using non monochromatic light (daylight, say) by placing your thumbs together and looking through the gap, holding the thumbs upright and close to your eye.

As to how the photons 'bend' around the edges of the gaps, remember that the edges of the gaps are solid in the macroscopic sense, but are largely voids at the sub-atomic level and so can be influenced in their trajectory by the atoms that they pass through.

The success of this experiment is dependent on the ability to ensure that one, and only one, photon can be aimed, fired, and tracked, without any interference from other unaccounted for stray photons. You must analyze the behavior of a single photon, not a stream of photons. And to me, that requires equipment not within the reach of the average John Q Public.

There are many possible arrangements for double-slit experiments (as illustrated by the several discussed in earlier posts here) that deal with the wave nature of light, and have nothing to do with quantum effects. They work perfectly well with continuous streams of light.

When you say "this experiment", you apparently have a specific one in mind.

Yes, that is what I had in mind when I started this thread; how to observe wave/particle duality and the quantum nature.

Demonstrating the dual nature of light in one experiment is not easy. Most light experiments rely on one or the other aspect but not both at the same time. As one of the videos above shows setting up a Young's interference experiment to measure the wavelength of light is easy to get it wrong.

The double slit experiment could be performed in a vacuum...perhaps in a high vacuum tube constructed specifically for the purpose. It coyld demonstrate wave-like behavior through a space lacking the media necessary for typical compression or shear wave.

This is where I get confused. I have a hard time differentiating a quanta-packet of massless energy with an actual particle that I subconsciously tend to equate with with mass. We all know how energy is propagated thru water-waves. But, that propagation requires the change-of-form of one type of energy to another. In other words, one type of energy encounters a multitude of water molecules, which transforms from its original state into kinetic energy of the water-molecules. You must be careful here to distinguish between the energy itself and the medium thru which it travels. Just because water can move in waves, doesn't mean that water-molecules individually have properties of waves. A single water-molecule cannot create a wave (even tho it still can transmit energy thru kinetic changes). How many water-molecules are required to produce a wave? How many photons are required to behave like a wave? How many quanta-packets are required to make a field? If you remove molecules/photons/quanta-packets one-by-one, Won't it eventually loose its wave properties?

One must always remember that quantum mechanics does not act like the classical physics that rules most of our macroscopic world. So getting confused is normal.

"Anyone who is not shocked by quantum theory has not understood it." -- Neils Bohr

https://en.wikiquote.org/wiki/Niels_Bohr

"There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there ever was such a time. There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics. So do not take the lecture too seriously, feeling that you really have to understand in terms of some model what I am going to describe, but just relax and enjoy it. I am going to tell you what nature behaves like. If you will simply admit that maybe she does behave like this, you will find her a delightful, entrancing thing. Do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will get 'down the drain', into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.

• Richard Feynman, in The Character of Physical Law (1965)"

https://en.wikiquote.org/wiki/Quantum_mechanics

That might be true. But, you could say the same thing about religion. If it doesn't make sense, how can it be called logical/science/evidence/proof?

If it doesn't make sense, how can it be called logical/science/evidence/proof

The proof of the pudding, so to speak, is in experiments. Although quantum phenomena do not make logical sense, we can predict the outcome of experiments with great precision using the mathematical formulas which have been developed.

We know how it works, we just don't know why. The problem is coming up with a mental model, e.g. it's like a water wave, or a billiard ball, etc. Apparently, whatever the nature of reality is, there seems to be no model that we can relate it to in our everyday experience.

A lot of physicists quit worrying about what it all means and just calculate with the formulas.

Well, maybe. But, consider this: Quantum Physics relies heavily on the Uncertainty Principle. And I know a lot of staticians fail to remove impossible "possibilities" from their formulae, resulting in weird "possibilities". Also, if the Higgs Field is the basic "background" in which everything exists, then maybe a single photon is producing waves in the Higgs Field instead of possessing wave properties itself. So, a proper interpretation of the data would be that the interference pattern that the Double-slit experiment exhibits, is really the reaction of the Higgs Field to the fired photon. But, that introduces another problem. How can a quanta-packet of energy (photon) give up any energy at all (including to detectors) without giving up the whole complete amount of that quanta-packet of energy, and ceasing to be that quanta-packet/ photon? It's all or nothing. A quanta-packet of energy must be treated as an irreducible unit, or it doesn't qualify as a quanta-packet, by definition. The problem with quantum physics is that if there is a minimum measurement (Planck-length, planck-time, planck-energy, etc), then that effectively prohibits knowing anything within that minimum measurement (a willful self-blindness). That can lead to all sorts of mis-information/explanations.

Or you can simply accept that the universe is alive. Then Quantum, and chaos coexisting in the perfect harmony of a day care. In fact, I think a crowded daycare is a good model to study in trying to understand the universe.

My point is, in life, every time you do it, it comes out a little different.

Interestingly enough, they have shot one photon at a time through a double-slit setup. Each one makes a dot somewhere on the screen where it hits. When enough have accumulated, they form an interference pattern.

How many photons are required to behave like a wave?

So the answer is one. It doesn't make any logical sense, but there it is. Go figure...

That was interesting. I do have a theory though. If you consider a photon to be an irreducible "thing", then it must behave as an irreducible unit. If it does not behave as an irreducible unit, then it must still be reducible. This is the point I've been trying to make about an infinity existing within everything. It seems to me that quantum physicists try to deny that, and come up with all kinds of "explanations" for it (that doesn't make sense). What I'm trying to say, is that even though a photon is a fixed quanta-packet, it still consists of an infinitely reducible amount of energy that might not be considered a photon. It's that energy that's making the "waves", and not the photon itself, per se.

I have thought of doing this experiment many times but found that by holding two fingers together and looking through the thin gap between them at a reasonably bright light I could see the typical dark and light bands. This set me thinking so I started crawling around in the dark hallways of my mind that haven't been explored for 53 years and found a dim memory of actually using the DSE to measure the wave length of the sodium light we were using. Careful measurements were needed. Taking this to the extreme, why not measure the output of a photonic device placed where the screen would normal go, to see the particle side of light. It is a vague memory and I may be way off but if any one has any ideas or even wants to try this I would be interested in knowing the result.

The double slit experiment does not demand expensive equipment. Please see this reference which does depend on the equipment sold for educational purposes and is very versatile, which does drive up the price. However, as an example, instead of commercially mounted lenses you can buy some simple lenses from a company at low cost. If you are in the USA Surplus Shed sells over the Internet and would offer the lenses at less than $5 each plus shipping. In other countries, you can find similar sources (I have seen them in Germany, India and China, but I am sure there are quite a few other places I don't know because I have never tried. ebay might even help (it would be good enough in Germany, for example)).

A LASER is shown but these are cheaper than a cup of coffee these days, and a LASER pointer would be fine. In fact, a properly used light bulb or LED would work with a filter (desired because different wavelengths are diffracted different amount) or better, a red, green blue or other color LED should be good if bright enough. A narrow range of wavelengths is the goal and colored LEDs actually would be better than a white one with a filter. Offered also at ebay! I suggest red because of its longer wavelength.

The final problem is slits. If you are careful you might be able to arrange strips on a sheet of glass (Al foil or thin, black paper?). I am not sure this would work but with a good printer you could try printing on an overhead projector foil. With the right foils this could work with either LASER or ink jet printers. You could shrink scanned images or use a drawing program to make the images to the right size. Do use a high-resolution printer, better than 300 dpi.

The nice way would be to photograph onto old-fashioned film. Negative film would be best but positive, color film ought to also work well enough. Make an enlarged object and photograph from far enough to get the desired small size. A single picture could carry several double slit pairs. For black and white negatives the slits would be black on a white background for the layout to be photographed.

Good luck, but I would bet it can be done°!

Setting up a double slit experiment is actually fairly easy.

You take a light source and put a color filter between it and a baffle with a pinhole in it, about pushpin diameter. The color filter puts the light at close to a single wavelength. The pinhole effectively columnates the light into a slightly spreading beam.

The more difficult part is making the two slits. When I did it, we rubberbanded two doubledge razor blades together face to face so that the edges were the thickness of a razor blade apart. The slits were made by taking a glass slide, melting and flowing a bit of candle wax onto the surface and then smoking the waxed surface with the candle until it was opaque. The you place a straight edge across the smoked surface and scored the smoked wax with the rubberbanded razor blades. The slide is placed so the slits are in the pinhole beam and on the far side of the slits you get an interference pattern projected on white paper in a darkened room.

The key is a good score across the waxed and smoked slide where the slits are close together.