double slits are strange and fun!

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double slits are strange and fun!

Postby monk » Fri Jul 29, 16:06 2011

So most of us have heard about Schrodingers cat (if not see paragraph after the picture). Well here is a real life version of that, its called The "Double-Slit" experiment and thankfully no cats were killed to do this experiment.


A photon emitter was set to aim through two rectangular slits in a screen, onto photographic paper; a regular interference pattern emerged.
Wanting to know how the light interacted though it, sensors were put to measure how the light went through, but this time the pattern did not emerge, and it was just a nebulous cloud, as if there was no screen at all.

When they turned off the sensors, the pattern emerged again- but the sensors were still physically there.

So, they eventually got the experiment to this: The emitter would only shoot ONE photon at a time, the sensors would measure the light, *then* be recorded by computer, *then* the computer would go through a 50/50 program to decide whether to *erase the data or not*, well after the photons (which traveled at the speed of light, and thus, at the highest limit any information in the universe can travel) were initially measured.
If the data was saved, there was a cloud. IF not, the pattern emerged.

This hints at the strangeness of what must be Quantum Mechanics:
1) A photon can *interfere with ITSELF* to cause a pattern
2) Information in the present can *change the past*

If you want the messy math of this look here

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-Schrödinger's Cat: A cat, along with a flask containing a poison and a radioactive source, is placed in a sealed box shielded against environmentally induced quantum decoherence. If an internal Geiger counter detects radiation, the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when we look in the box, we see the cat either alive or dead, not both alive and dead.
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Re: double slits are strange and fun!

Postby Aum » Fri Jul 29, 20:13 2011

I read your post 4 times and I am no closer to understanding. -_-
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Re: double slits are strange and fun!

Postby monk » Fri Jul 29, 23:28 2011

I read through it and it seems fine, but I will give it another shot.

In the picture above you see a photon emitter on the left and a standard interference pattern on the right. the thin black line represents the "screen" with two rectangular slits.

The put sensors on the screen to see how the photons interacted to create the pattern but when they turned them on the pattern disappeared like the screen wasn't even there, when they turned off the sensors the pattern reappeared, but the sensors were still in place, they were just turned off. These were passive sensors that could cause no interference to the photon beam, but just the act of observance made the photons behave differently. The math says that somehow the photons know before they get to the screen whether there is a sensor or not and choose to behave differently based on this information.
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Re: double slits are strange and fun!

Postby DruidX » Sat Jul 30, 2:26 2011

So, wait. Photons are actually sentient and therefore capricious? Also, can I use this to explain why I can, say, juggle fine on my own, but the moment someone is watching I mess up?

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Re: double slits are strange and fun!

Postby Aum » Sat Jul 30, 2:38 2011

I get it now. That's pretty amazing, but at the same time it also makes sense. The universe around us has always been alive to me, and capable of learning just as we are. It's dynamic and has an innate intelligence. Also, it's a comedian.
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Re: double slits are strange and fun!

Postby Sonic# » Sat Jul 30, 7:46 2011

I find it interesting that the way a photon behaves depends on its being observed. If it's just being watched after it goes through the slit, it behaves like a wave. If it's watched to see which slit it goes through, it behaves like a particle. And if we learn enough information about what slit the proton went through beforehand, it turns out to have behaved like a particle, though before it seemed to behave like a wave. It changes when you're looking!

What math I could understand is also quite interesting, since it's able to connect things like the rate of dispersion and the width between the two slits to wavelength, angle measures, and the distance of the source from the slits in order to describe the diffraction. You call it messy math, monk, but the relation is no less than beautiful.

Also, this:
One cannot speak of the location of any particle such as a photon between the time it is emitted and the time it is detected simply because in order to say that something is located somewhere at a certain time one has to detect it.


These basics of quantum mechanics are so fun to think about.

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Re: double slits are strange and fun!

Postby monk » Sat Jul 30, 12:33 2011

DruidX wrote:So, wait. Photons are actually sentient and therefore capricious? Also, can I use this to explain why I can, say, juggle fine on my own, but the moment someone is watching I mess up?


Totally. when you mess up juggling with an audience you just say it was photon interference and you would be glad to explain it but they need to learn quantum mechanics first.
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Re: double slits are strange and fun!

Postby SakuraSong » Sat Jul 30, 13:13 2011

monk wrote:
DruidX wrote:So, wait. Photons are actually sentient and therefore capricious? Also, can I use this to explain why I can, say, juggle fine on my own, but the moment someone is watching I mess up?


Totally. when you mess up juggling with an audience you just say it was photon interference and you would be glad to explain it but they need to learn quantum mechanics first.


Or just throw Schrodinger's Cat at them, and they'll pretend to get it.
(If anything, it works on me.)
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Re: double slits are strange and fun!

Postby God is an Englishman » Sat Jul 30, 21:14 2011

What the... hell happened to my eyes....

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Re: double slits are strange and fun!

Postby rowan » Mon Aug 1, 15:18 2011

If you want to mess around with parameters yourself you can play with the PhET simulations:

http://phet.colorado.edu/en/simulation/ ... terference

it's a java app that you can download and run that I use in my physics classes. On the top are tabs, and you can select "light" as one of them. You can also have one or two sources, as well as adding walls, slits, etc. (there are lots of other fun sims too; I like electric field hockey)
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Re: double slits are strange and fun!

Postby callingcolleen » Mon Aug 22, 0:25 2011

you mentioned that events in the present could potentially effect events in the past. i think this is really cool, especially after i found out that stars in distant galaxies could be giving off this wave/particle light bent around a similar manner. so not only is it possible, but it's probably happening and somewhere in outerspace, there's a big natural screening of the past.

gotta love physics.

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Re: double slits are strange and fun!

Postby Aum » Sat Sep 3, 4:06 2011

I need to revive this thread with more queries!

monk, I have thought about this experiment and one thing doesn't add up for me. I get the theoretical experiment with the cat in the box, but how would it be possible to create a box that shields from environmental decoherence? Isn't that just a scientific way of saying a box that is totally cut off from the outside universe?

Well... how do you make a box that is sealed off from the universe? The box itself would have to be separate from the universe, which is impossible. The box IS the universe, just like anything else is. It would also mean that the cat has ceased to exist once in there - which, yes, would put it back into the realm of potential probabilities - but humans can't do that with any known means.

The double-slit experiment demonstrates that observation changes photonic probability, but it was not done in the theoretical quantum vacuum that exists within the Schrodinger's cat experiment. The result is still very cool though.
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Re: double slits are strange and fun!

Postby monk » Sat Sep 3, 14:03 2011

Xinzang wrote:Well... how do you make a box that is sealed off from the universe?


Right at this moment, we can't.


What you have to realize Xinzang is that all these discussions with words about cats and boxes are just metaphors that represent Mathematical theorems. These theorems have been studied and reviewed by a bunch of math geeks who mostly agree that the math involved is correct. Now some mathematical theorems are easy to demonstrate in real time, 1 + 1 =2, but as I'm sure you're aware math gets more and more complicated the deeper you get. I linked to the math for this experiment in my OP, and that's what makes it so cool because it's a physical experiment that reinforces the theorem and help assure us the math involved is actually correct.

Someday maybe we'll be able to make a box that is sealed off from the Universe. Unfortunately my brain isn't big enough to even guess how to do this(or maybe it is but I'm too lazy to put in the work), but I bet there is some physicist/engineer/math geek out there brain sweating the problem as I write this.
and hopefully they aren't using actual cats :)
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Re: double slits are strange and fun!

Postby Aum » Sat Sep 3, 14:09 2011

I don't think it's possible to make a box that is separate from the universe. Any material we use is necessarily part of said universe. Maybe though, we could create another dimension/universe that is separate from this one somehow, and do the experiment in there. But that seems like a lot of work just to demonstrate probabilities.

Also... once you get down to that level, there is no "inside" or "outside".
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Re: double slits are strange and fun!

Postby monk » Sat Sep 3, 18:41 2011

Xinzang wrote:I don't think it's possible to make a box that is separate from the universe.
C'mon Xinzang, give us a century or two of really trying. 110 years ago these statements were also said:

"I don't think it's possible for people to fly through the air"
"I don't think it's possible to walk on the moon"
"I don't think it's possible to speak to someone in China with a box smaller than my hand attached to nothing."

"Things can't just float on nothing"

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Re: double slits are strange and fun!

Postby Rainbow Dolphins » Sun Sep 4, 11:23 2011

^One of those things would make the most AWESOME knife block in the world. (And dangerous. But awesome!)
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Re: double slits are strange and fun!

Postby Snuppy » Sat Mar 24, 20:31 2012

IT is extraordinary. THIS is my take, and probably many others, but the thinking came to this head, DON'T know why, or how, but its how I feel. Of course, I may be totally confused and do not have the physics nor the maths to take this much further. It is almost as if the world is just a bunch of energy photon soup, and our action of observation via our knowledge, perhaps learnt, perhaps inherited, as the knowledge to build the body is all inherited, life span, everything, so why not the visible, and perceived world, including ourselves, LEARNT, via the genes)made from photons/intelligence etc) themselves. OUR observation is turning the soup into a known and believable world. THat is what I feel this is what this demonstrates. Perhaps that is far fetched, but its a hunch. Btw, I dont care if this has been said a zillion times, If I am repeating something i have read, I would say that.

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Re: double slits are strange and fun!

Postby rowan » Sat Mar 24, 20:56 2012

Certainly some philosophers have said similar things. There are some philosophy of quantum mechanics books out there that you might be interested in.

This is a pretty dead thread, though, so unless anyone has any especially enlightening thoughts, let's leave it dead.
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