• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Double Slit Question

rjh01 said:
3. Repeat infinitely step 2 for every straight line

The result is a simulated coastline.
Click to expand...

Why infinitely? Surely the recursion must stop before the characteristic length reaches the atomic scale.
 
I could make a pirate joke about walking the Planck here, but I won't.
 
Madalch said:
I forget how the math works, but I know that if you coat a microscope slide with tar, and put two razor blades together to make a double slit in the tar, they are the right distance apart to diffract light.
Click to expand...

Visible light.

And yes, this works. It's a low-tech trick I've used before in the lab when money was tight.
 
Possibly stupid question time again.

What's hitting the detector, a photon? the same photon that was emitted? how far did it travel to get there or how long did it take as the distances for each path are different?
 
alexi_drago said:
What's hitting the detector, a photon?
Click to expand...

Yes.

the same photon that was emitted?
Click to expand...

Yes.

how far did it travel to get there or how long did it take as the distances for each path are different?
Click to expand...

The photon is not a point particle. As such, it is neither emitted nor detected at an instant of time, but over a period of time. This makes any path length difference irrelevant: the difference will be less than the "length" of the photon. If that is not the case, you won't get interference.
 
Indeed, any measurement that could reveal which path the particle took will wash away the interference. This is most definitely one of my favourite experiments in physics.
 
Hijacking this thread to ask another double slit question :)

When they use 'a detector' to tell which slit a photon went through I can see several problems and I'd be interested to know how they are resolved: If the detector interacts with photons to the extent of detecting them then presumably photons that get detected never make it to the screen? Is it possible to detect a photon and still have it travel to the screen? Similarly, If the detector is close to a slit, wouldn't it be effectively blocking that slit? But if it is far away from the slits how can you tell which slit the photon came through at all?
 
Ziggurat said:
<snip>
The photon is not a point particle. As such, it is neither emitted nor detected at an instant of time, but over a period of time. This makes any path length difference irrelevant: the difference will be less than the "length" of the photon. If that is not the case, you won't get interference.
Click to expand...

This is not quite right. The photon goes through both slits (how good is that a single photon goes though two different paths. It can do this because it is a wave not a particle). It then travels to the photon detector. Now then what happens next depends on the distance the wave took on both paths. If they were the same distance or the difference is exactly a multiple of the wavelength then the photon causes a reaction with the detector. It can do this because it is a particle not a wave. If the distance is a odd multiple of half a wavelength then nothing happens. If the distance is between these two extremes then a smaller reaction happens. Hence you get an interference pattern of a number of lines.

Please note that I said both that the photon is a particle and a wave. Both statements are correct. I also said that the photon goes through both slits at the same time. This is only a theory not a fact. It is the only way we can explain this interference pattern. If you put detectors on the slits to find out which slit the photon went through the interference pattern will go away.

To understand Quantum theory one thing you must reject is common sense.
 
rjh01 said:
This is not quite right. The photon goes through both slits (how good is that a single photon goes though two different paths. It can do this because it is a wave not a particle). It then travels to the photon detector.
Click to expand...

None of that contradicts anything I said. But if the photon you send at both slits is a VERY short pulse, short enough that the two parts do not overlap in space and time at the detector due to path length differences, then it will not interfere with itself. The photon must be longer than any path length difference in order for any interference to occur. And if it's longer than the path length difference, then you cannot use arrival time to determine which path it took.
 
Last edited:
Ziggurat said:
Yes. The basic effect (wave interference) behind the 2-slit experiment is also the principle behind diffraction, which can be done with x-rays, neutrons, electrons, and even helium atoms.
Click to expand...

And is it possible to know the velocity and time taken from emmision to detection for any of those? or is that where the fact that making measurements affects the conditions messes everything up?
 
Ziggurat said:
None of that contradicts anything I said. But if the photon you send at both slits is a VERY short pulse, short enough that the two parts do not overlap in space and time at the detector due to path length differences, then it will not interfere with itself. The photon must be longer than any path length difference in order for any interference to occur. And if it's longer than the path length difference, then you cannot use arrival time to determine which path it took.
Click to expand...

Wikipedia says it better than I can.
The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present. (The experiment works with electrons, atoms, and even some molecules too.)
Click to expand...

Source http://en.wikipedia.org/wiki/Double-slit_experiment

Bolding on both quotes is mine. I cannot see how the two bolded statements can both be right.
 
rjh01 said:
Bolding on both quotes is mine. I cannot see how the two bolded statements can both be right.
Click to expand...

The two situations are not the same. In the Wikipedia case the photon has a well-defined wavelength. In the other case the photon has a sharply-defined position (when created) which implies a spread of wavelengths, due to Heisenberg, causing the interference pattern to become smeared. Have a look for Heisenberg's uncertainty principle on wiki - I'd post a link but I'm not allowed yet :-(

Ugh, it's 2.30 in the morning. Time to get some rest. My apologies if this post is utter rubbish.
 
Last edited:
rjh01 said:
To understand Quantum theory one thing you must reject is common sense.
Click to expand...

I'd say more that to understand quantum theory you have to reject the idea that the fundamental particles behave like very very small billiard balls. We all know how billiard balls work, and somehow when we extend those rules to particles we end up completely boggled, as the very very small has its own set of rules.

A
 
ctamblyn said:
The two situations are not the same. In the Wikipedia case the photon has a well-defined wavelength. In the other case the photon has a sharply-defined position (when created) which implies a spread of wavelengths, due to Heisenberg, causing the interference pattern to become smeared. Have a look for Heisenberg's uncertainty principle on wiki - I'd post a link but I'm not allowed yet :-(

Ugh, it's 2.30 in the morning. Time to get some rest. My apologies if this post is utter rubbish.
Click to expand...

No, a photon of light can only have one wavelength. This can be known.

The Uncertainty principle says
In quantum mechanics, the Heisenberg uncertainty principle states that certain pairs of physical properties, like position and momentum, cannot both be known to arbitrary precision.
Click to expand...
 
You must log in or register to reply here.

Back
Top Bottom