Dr. Quantum Explains Double Slit Experiment





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Published on May 1, 2007


Dr. Quantum explains the importance of double slit experiment in physics.

If we randomly shoot small objects, say marbles, at the screen through a slit we see a pattern on the wall where they went through the slit and hit.
Now, if we add another slit to the right of the first one, we would expect to see a second band duplicated to the right.

Now, lets look at waves. The waves hit the slit and radiate out, striking the back wall with the most intensity directly in line with slit.
When we add a second slit something different happens than with marbles. If the bottom of one wave meets the top of another, they cancel each other out - now there is interference pattern on the back of the wall: places where the two tops meet are the highest intensity and where they cancel - there is nothing!

Now, let's go quantum!

An electron is a tiny, tiny bit of matter, like a tiny marble. It behaves just like marbles. If we shoot it through a single slit, we see a single band on the screen. Now if we add another slit we should get, like marbles, two bands. But we got interference pattern!
We fired electrons - tiny bits of matter through - but we get a pattern like waves, not like little marbles.

How? It doesn't make sense!

The physicists are clever and thought that these little marbles, electrons, bounce off each other and create this pattern so they decided to shoot one electron at a time. After hours and hours of waiting they noticed the same interference pattern!

The conclusion is inescapable - the single electron leaves as a particle, becomes a wave at the slits and interferes with itself to hit the wall like a particle!

But mathematically it is even stranger - it goes through both slits and it goes through neither, and it goes through just one and it goes through just the other. All of these possibilities are in superposition with each other. But physicists were completely baffled by this! So, they decided to peek and see which slit the electron actually goes through! They put a measuring device by one slit to see which one it went through.
But the quantum world far more mysterious than they could have imagined - when they observed the electron went back to behaving like a little marble - it produced a pattern two bands NOT an interference pattern!

The very act of measuring or observing which slit it went through meant it only went through only one, not both.
The electron decided to act differently. As if it was aware it was being watched!

And it was here that physicists stepped forever into strange, never world of quantum events.

What is matter? Marbles or waves? And waves of what? And what does an observer does have to do with any of this?
The observer collapsed the wave function by simply observing!

It's Never too Late to Study:
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