I What happens when we observe an electron?

[AhmedHesham]

I have learned about the principles of quantum mechanics from two sources. The first is a book called quantum mechanics concepts and applications. The second is prof. Allan Addams lectures in MIT. But they have different opinions about what happens to the interference pattern of electrons when we observe them. Allan Adams says when we send light to the electron it changes its momentum and this causes the interference pattern to change slightly if we use low frequency light and greatly if we use a high frequency light. But the book doesn't think so. It says that when we send light, the interference disappears completely regardless of the frequency as if the electrons behave like bullets .what do you guys think about that?

 

[Douglas Burn]

Hi,

I found this this link

https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/lecture-notes/

and this .pdf

http://dl4a.org/uploads/pdf/Zettili.pdf

Are these the two sources you mentioned?

 

[AhmedHesham]

Hi,

I found this this link

https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/lecture-notes/

and this .pdf

http://dl4a.org/uploads/pdf/Zettili.pdf

Are these the two sources you mentioned?

Yes.in zettili see the chapter called particles versus waves. And in Allan Adams lectures see the video of lecture 2 the part where he explains two slit experiment

 

[Douglas Burn]

Ok I'll have a look.

 

[PeterDonis]

what do you guys think about that?

Do either of these sources give references to actual experiments?

 

[AhmedHesham]

Do either of these sources give references to actual experiments?

Of course no. That's why I suspect them here and ask for help.i am a high school student. I don't know much about physics sources. Please help me. ☺

 

[Douglas Burn]

Hi Ahmed,

I attached a screenshot of the part in the lecture about which I think you were talking.

I have also attached a screenshot of two of the pages from the book which you referenced.

Initially (and I haven't looked too deeply into it) I'm thinking that the two are saying the same thing:

that when you shine light at the incident electrons there's no longer an interference pattern on the screen.

What do you think?

 

[AhmedHesham]

Hi Ahmed,

I attached a screenshot of the part in the lecture about which I think you were talking.

I have also attached a screenshot of two of the pages from the book which you referenced.

Initially (and I haven't looked too deeply into it) I'm thinking that the two are saying the same thing:

that when you shine light at the incident electrons there's no longer an interference pattern on the screen.

What do you think?

Hi Douglas, yes both of them say the interference disappears .the question is how does it happen. According to the book it happens like magic but prof Allan explained it in a way that makes sense. The question is about whether what happens is unexplainable or it's simply explained by the Compton effect. I mean prof Allan explain it using Compton effect.

 

[Douglas Burn]

The Compton effect is this stuff isn't it

 

[AhmedHesham]

The Compton effect is this stuff isn't it

View attachment 220170 View attachment 220171

Yes it is. It happens because of the quantization of light.

 

[Douglas Burn]

That's quite interesting. So The two sources you mentioned are saying that if the light source is not there then you get an interference pattern (which implies that the electrons are behaving like waves).

But if you have light hitting the electrons before they hit the screen then you get a distribution on the screen reminiscent of the distribution for the baseball / bullet example (which implies that the electrons are behaving like particles).

I reckon that this is something to do with the idea that in Quantum Mechanics if you measure a property of a quantum mechanical system then the system collapses into a definite state.

in this case the system is an electron and you are measuring it's position by shining light onto it and getting a flash when the light hits the electron which is telling you the electron's position.

As soon as you have determined the electron's position then it remains a discreet particle. But if you never measure it (no light) then it stays as a wave.

If you're familiar with the example of Schrodinger's cat then this all might sound quite familiar.