# Double Slit Interference Pattern

It is said that even if we were to send single electrons through a double slit or a single slit we would see an interference pattern on the screen (as long as we dont setup a detector to tell us which path the electron took).
How do you send SINGLE electrons into a slit? There are electrons everywhere all around us. How do we prevent all the rest from going through the slit and causing an interference pattern and only get that one electron through?

thanks

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DrChinese
Gold Member
It is said that even if we were to send single electrons through a double slit or a single slit we would see an interference pattern on the screen (as long as we dont setup a detector to tell us which path the electron took).
How do you send SINGLE electrons into a slit? There are electrons everywhere all around us. How do we prevent all the rest from going through the slit and causing an interference pattern and only get that one electron through?

thanks
Yes, there are electrons all around us. But most of them are inside an atom (attracted to a nucleus), occupying a position in one of the orbitals. If an electron is excited (by imparting energy to it, which happens when it absorbs a photon) then the electron goes into a "higher" orbital (one further from the nucleus). If you impart sufficient energy to an electron, it will leave the atom altogether and act as a free electron. It is one of these free electrons that can be sent towards the slits.

What is the speed (not velocity) of this free electron?

daniel_i_l
Gold Member
The electron can be accelerated or deaccelerated with an electric or magnetic field.

The electron can be accelerated or deaccelerated with an electric or magnetic field.
So you are saying that an electric or magnetic field changes the speed not just the velocity of the free electron?

daniel_i_l
Gold Member
Yes. (I need 10 chars)

So my understanding that the theory of relativistic quantum mechanics indicates that electrons always travel at the speed of light is false?

daniel_i_l
Gold Member
How can an electron which has a non zero mass ever travel at the speed of light?

Doc Al
Mentor
So my understanding that the theory of relativistic quantum mechanics indicates that electrons always travel at the speed of light is false?
I certainly hope so, otherwise I'm going to have to go back and revisit all those electron scattering experiments I did a few decades ago!

I certainly hope so, otherwise I'm going to have to go back and revisit all those electron scattering experiments I did a few decades ago!
Are you sure you did not measure the velocity instead of the speed of the electrons?

Doc Al
Mentor
Are you sure you did not measure the velocity instead of the speed of the electrons?
Quite sure. Where did you ever get this idea of electrons traveling at the speed of light?

So you are saying that the instantaneous speed of an electron is most definitly not c?
If so, what according to you makes the instantaneous speed of an electorn vary?

Perhaps this is just a confusion between velocity and speed.
Like a bumble bee's speed versus his effective speed over a fixed distance.

Doc Al
Mentor
So you are saying that the instantaneous speed of an electron is most definitly not c?
That's what I'm saying. As daniel_i_l points out, how could a massive particle move at speed c, considering relativity?
If so, what according to you makes the instantaneous speed of an electorn vary?
An electric field, for one.

Perhaps this is just a confusion between velocity and speed.
I don't see how. Velocity is just a speed in a direction; instantaneous speed is just the magnitude of the instantaneous velocity.
Like a bumble bee's speed versus his effective speed over a fixed distance.
Not sure how that relates to electron speed.

How can an electron which has a non zero mass ever travel at the speed of light?
The theory of relativity is a classical theory it does not describe quantum level phenomena.

Speed and Velocity

Speed is a scalar quantity and has only magnitude. Velocity is a vector quantity and has magnitude and direction. I would try Heisenberg: A consequence of the Quantum Mechanical nature of the Universe is that particles can appear in places where they have no right to be (from ordinary, common sense [classical] points of view)!

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What is the speed (not velocity) of this free electron?
Do you mean "free" as within the bands of a conductor or in as in a vacuum?

The former "net drift" travels at almost c, dependent on the material. The latter can be controlled as desired (e.g. 40% of c), but not 100%.

how could a massive particle move at speed c, considering relativity?
What MeJennifer may be teasing you with, is the fact that for electrons described by the Dirac equation, the eigenvalues of the velocity projections are always plus or minus c (see, e.g., P.A.M. Dirac, The Principles of Quantum Mechanics) - this is the well-known zitterbewegung

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Electrons Travel at Speed of Light

If you calculate the instantaneous speed of electron using the theoretical models, it comes out to be the velocity of light. However, we cannot measure instantaneous speeds, but only speed averaged over some time scale by measuring position at two instants of time. When we do this, the speed is always less than the velocity of light. Theoretical models also predict average velocity less than speed of light. This only applies to speed of light in vacuum. Electrons can , and do, travel at speeds faster than speed of light in some media. Check out Argonne National Labs.

Thanks for the good replies people.
It was stated earlier by DrChinese:
"If you impart sufficient energy to an electron, it will leave the atom altogether and act as a free electron. It is one of these free electrons that can be sent towards the slits."

How do physicists ionize an atom and send the electron on its merry way towards the slit? How do you aim it so it goes through the slit? What atoms are generally used? Does this involve lasers?
If one was interested in only sending one electron in at a time, how would i ensure that only this one electron that ive ionized from an atom is going through and not others?

the reason i ask all this is because in Dirac's textbook on QM he says
"Each photon then interferes only with itself. Interference between two different photons never occurs."
i would just like to know how experimentally we can isolate the system
so well as to be 100% sure we are only sending in one particle at at time and still seeing an interference pattern.

thanks!

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Hello blumfeld0,

you may find these websites interesting:

1) Hitachi double-slit experiment with electrons:
http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm [Broken]

Also have a look to the paper here (paper number 8):
http://www.hqrd.hitachi.co.jp/em/tonomura_pub.cfm [Broken]

2) Physicsweb article on the double-slit experiment:
http://physicsweb.org/articles/world/15/9/1

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Thanks!. that's pretty much exactly what i was looking for.

jaketodd
Gold Member
The electron can be accelerated or deaccelerated with an electric or magnetic field.
Anyone please: So the double slit experiment will still work even if you accelerate the particle during its' transit to the detector wall behind the slit wall?

DrChinese