Double Slit and Observer Effect - Possible to test at home?

[Albert VDS]

The double slit experiment was used, in 1801 by Thomas Young, to find out if light acts as waves or as a particle.

The results provided proof that light behaves likes waves.

Now for the observer effect, which is explained rather well by Dr. Quantum:

Which brings me to my question: Can you test this at home?

I know you can do the double slit experiment rather easily.

But how can you do the observer effect experiment?

[togfox]

Great vid. Makes something difficult to understand a little easier to get less confused.

[Sirrobert]

Nice video.

But I don't think you could do it at home. You'd need some very delicate sensors to observe single electrodes.

Ofcourse, if you had those you could probably do it at home, but I don't think that counts as home anymore

[stupid_chris]

Actually it proves that light is both particle and wave, but heh.

You could do it with a piece of paper with slits in it and a flashlight IIRC

[Sirrobert]

Actually it proves that light is both particle and wave, but heh.

You could do it with a piece of paper with slits in it and a flashlight IIRC

I thought the slits needed to be smaller than the wavelenght?

And he talked about the observation part, where you observe each individual electrode as it passes through the slits

[Albert VDS]

Yes, that's how you show that particles have a wave function shown by the interference pattern.

But not how you show the observer problem.

I'd like to know what they use to measure/observe the particle, like in the above video.

[Kryten]

The cheapest really practical way to observe single particles is scintillation counters, but 'cheapest; is very different from 'cheap'. In some experiments you can use gamma sources for your photons (which can be got or ~$50 over the internet) and geiger counters, but in this one there's no way to distinguish the photons you want to measure from background radiation.

[Yourself]

Nice video.

I actually really hate this video because its conclusions are misleading.

For example, it sets up that an electron is "like a tiny marble". Well, it's not. That's part of what this whole experiment demonstrates: that our notions of "particles" and "waves" don't really work that well at the quantum level. Electrons (and really everything) are neither particles nor are they waves, they are something which exhibits properties of both of those things. They have the discreteness of particles, but the interference properties of waves. We humans simply don't have a good intuitive model for what stuff actually is.

We definitely now have fantastic mathematical models, but the intuitive models are still stuck at "particle" or "wave", because that seems to really be what our minds understand.

But that's not even what bugs me the most about the video, I could easily forgive that. What really bothers me is that they make it seem like electrons "decide to act" in a particular way based on whether it's being observed. The electron's not "aware" of anything, least of all being watched. Most of the issue comes about because the detection apparatus has to interact with the electron to detect it. It's not like at the macro scale where you can set up a camera somewhere which passively detects light that the object. If you want to watch a single electron you're necessarily going to interfere with it. Observation is most certainly not some kind of psychic force that changes the way the world works.

Although this clip is from a movie that kind of has that premise and it was most certainly not well received by scientists.

Actually it proves that light is both particle and wave, but heh.

You could do it with a piece of paper with slits in it and a flashlight IIRC

It won't work with a flashlight because a flashlight doesn't produce coherent light. You might be able to do it with a laser pointer, but you'd have some difficulty with the setup to show the wave-particle duality. First you'd need to dim the laser to the point that it only emits one photon at a time, otherwise you're just demonstrating wave interference. You'd also have some difficulty detecting one photon at a time with paper.

Additionally, the slits in the double slit experiment should be smaller than the wavelength of the waves passing through them, otherwise you're going to get secondary diffraction effects.

Now, if you want to get into the stuff that's really weird:

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

[stupid_chris]

It won't work with a flashlight because a flashlight doesn't produce coherent light. You might be able to do it with a laser pointer, but you'd have some difficulty with the setup to show the wave-particle duality. First you'd need to dim the laser to the point that it only emits one photon at a time, otherwise you're just demonstrating wave interference. You'd also have some difficulty detecting one photon at a time with paper.

Additionally, the slits in the double slit experiment should be smaller than the wavelength of the waves passing through them, otherwise you're going to get secondary diffraction effects.

Now, if you want to get into the stuff that's really weird:

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

Meh, optics are definitely not my forte. There's a reason I'm not planning to go in anything that involves this for my career, really my lack of interest in it gave me mediocre results in my optic classes. Glory to electricity and mechanics.

[Kryten]

If you do want to demonstrate quantum effects at home though, it is possible to demonstrate quantum entanglement with amateur-level equipment.

Edited July 14, 2013 by Kryten

[Albert VDS]

Hey thanks Kryten! Still a bit expensive, like you said.

Here's a video of a real double slit experiment, fired 1 by 1.

And here's a video of the observer effect:

[Sirrobert]

I actually really hate this video because its conclusions are misleading.

For example, it sets up that an electron is "like a tiny marble". Well, it's not. That's part of what this whole experiment demonstrates: that our notions of "particles" and "waves" don't really work that well at the quantum level. Electrons (and really everything) are neither particles nor are they waves, they are something which exhibits properties of both of those things. They have the discreteness of particles, but the interference properties of waves. We humans simply don't have a good intuitive model for what stuff actually is.

We definitely now have fantastic mathematical models, but the intuitive models are still stuck at "particle" or "wave", because that seems to really be what our minds understand.

But that's not even what bugs me the most about the video, I could easily forgive that. What really bothers me is that they make it seem like electrons "decide to act" in a particular way based on whether it's being observed. The electron's not "aware" of anything, least of all being watched. Most of the issue comes about because the detection apparatus has to interact with the electron to detect it. It's not like at the macro scale where you can set up a camera somewhere which passively detects light that the object. If you want to watch a single electron you're necessarily going to interfere with it. Observation is most certainly not some kind of psychic force that changes the way the world works.

Although this clip is from a movie that kind of has that premise and it was most certainly not well received by scientists.

And than how would you want to explain it to someone who doesn't even know what an electron is?

Cause I'm pritty sure that's the target audience of this video

And the observer effect is a very real fenomenon. Quantum physics don't work the way we are used to.

Best example I ever heard was with radioactivity. During observation of a single unstable particle, that particle will never decay. The Quantum Zeno Effect

[Yourself]

And than how would you want to explain it to someone who doesn't even know what an electron is?

Cause I'm pritty sure that's the target audience of this video

I'd probably point them to this:

http://simple.wikipedia.org/wiki/Electron

And the observer effect is a very real fenomenon. Quantum physics don't work the way we are used to.

Best example I ever heard was with radioactivity. During observation of a single unstable particle, that particle will never decay. The Quantum Zeno Effect

Yes, it's real, but it's not magic, and it's not because quantum particles have or are affected by "consciousness", which is what this particular video implies (and that implication is much stronger when taken within the context in which it's presented, in that movie I mentioned).

[codepants]

But that's not even what bugs me the most about the video, I could easily forgive that. What really bothers me is that they make it seem like electrons "decide to act" in a particular way based on whether it's being observed. The electron's not "aware" of anything, least of all being watched. Most of the issue comes about because the detection apparatus has to interact with the electron to detect it. It's not like at the macro scale where you can set up a camera somewhere which passively detects light that the object. If you want to watch a single electron you're necessarily going to interfere with it. Observation is most certainly not some kind of psychic force that changes the way the world works.

So, my question is (not to you specifically, it's just that you provide a good reference point), how do you explain, or prove to people, that the act of observing did not somehow change the experiment? This is the problem I run into when explaining the double slit experiment. As soon as I say, "The results are different depending on whether or not the electrons are observed," people go, "Ah! Well, observing them must have changed something."

To be fair, that argument does make sense. If we assume the change has a cause, and the only thing we have changed is whether or not we are observing, then does it not "make sense" that the act of observing must have caused the change? Especially if we consider that there is no case without observation where the change occurs. I am aware, of course, that correlation does not imply causation, and I am not arguing against the implications of the experiment. I am just wondering, what do you say to people who insist there must be some sort of interference? EG, the observation device has a nonzero magnetism, or something.

On a slightly different note, I read an interesting interview a few years back which had a section on this experiment. It was essentially an extrapolation of the observer effect. Namely, the interviewee suggested that if there are aliens in a distance universe, then they could be causing changes in our universe simply by observing electrons that make it over here, and we could be causing changes in their universe QED. I really wish I could find that article again, it was really interesting. Of course, the more you extrapolate, the more people resist (sensibly: The more you extrapolate, the less evidence you have. And the more we learn about physics, the more it seems that extrapolating leads to misleading assumptions).

Edited July 15, 2013 by codepants

[Sirrobert]

The mistake everyone makes with the observer effect is that it's mostly quantum physics, and people try to understand it using what they know of NORMAL physics.

That just doesn't work

[K^2]

And the observer effect is a very real fenomenon. Quantum physics don't work the way we are used to.

Best example I ever heard was with radioactivity. During observation of a single unstable particle, that particle will never decay. The Quantum Zeno Effect

Yourself is absolutely correct about this one. The interaction between observed and observer alters the dynamics. There is nothing "magical" about Quantum Physics. It's very straight forward field mechanics. Same deal with Quantum Zeno.

They have the discreteness of particles, but the interference properties of waves.

They are not, strictly speaking, discrete, either. You can store "half" of an electron in a box. You simply can't measure "half" of an electron. Any measurement is going to result in a discrete quantity, and so the overall dynamics is such that we can talk about superposition of discrete states instead.

So, my question is (not to you specifically, it's just that you provide a good reference point), how do you explain, or prove to people, that the act of observing did not somehow change the experiment?

You do change the experiment. That's not the point, however. The statement is far more fundamental. You cannot make an observation that does not alter the outcome in such a way. In other words, any interaction sufficient for you to make a determination is sufficient to offset the balance in the system.

The proof of quantum mechanics, however, is not in such qualitative experiments so much as it is in quantitative measurements. Certain predictions of Quantum Mechanics are among the best tested predictions of physics.

Edited July 15, 2013 by K^2

[jebbe]

A simple way to do something similar at home is to use a CD and a laser pointer. Point the laser to the data side of the CD and watch the reflection pattern. Careful - only look from the front side of the CD, such that the laser points roughly in your direction (not into your eyes!), with the CD in front of you. Project the reflection pattern e.g. to a white wall.

A CD is basically a fine refelctive grid rather than a single/double slit, so all the reflection patterns from all the "slits" overlay and give you very distinct maxima, and wide minima.

A CD works better than a DVD (let alone a bluray), because the coarser the grid, the closer the maxima. And modern technology produces such fine grids, that the maxima can be really far apart.

[K^2]

Diffraction experiments are easy to set up. But diffraction isn't the point of double-slit experiment. The point is that diffraction effect goes away when you observe which slit the particle passed through.

[icemasterpt]

the universe is created by consciousness XD

[PakledHostage]

the universe is created by consciousness XD

Care to elaborate? In my experience with this sub-forum, people tend to take you more seriously if you provide a reference when making extraordinary or controversial claims...

[Sirrobert]

Care to elaborate? In my experience with this sub-forum, people tend to take you more seriously if you provide a reference when making extraordinary or controversial claims...

I think he's extrapolating the 'observing changes things' into joke teritory

[tiwibiwi]

I think a good "homemade" double split experience.

http://bit.ly/148SiI4

[Sirrobert]

I think a good "homemade" double split experience.

http://bit.ly/148SiI4

It's amazing how many people don't read

This topic is about the OBSERVER EFFECT, not about the double slit

[tiwibiwi]

It's amazing how many people don't read

This topic is about the OBSERVER EFFECT, not about the double slit

Don't get mad, just a little vid of homemade double split experiment. I currently trying to find a good article of observer effect experiment

[Cheebsta]

Throw a rock in the mud. The hole allows you to observe that the rock has passed through the mud, but the medium used to make that observation has also altered the behavior of the rock. You have to get absurd about it to make massive examples, lol. Normally its things like the teeny bit of energy absorbed or release by a thermometer changing the temperature by 0.000000000000000000001 degrees or something like that.