Electron - massless or not

Cosmo16

It was my understanding (however my understandings are wrong 99.9% of the time) that a electron was massless, and that it took the form of a wave that collapsed into a specfic point when it was "located"

however, I was told that im my friends AP physics class, there are told to use the mass of an electron.

So, is an electron massless or not?

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ZapperZ

Staff Emeritus
2018 Award
Cosmo16 said:
It was my understanding (however my understandings are wrong 99.9% of the time) that a electron was massless, and that it took the form of a wave that collapsed into a specfic point when it was "located"
however, I was told that im my friends AP physics class, there are told to use the mass of an electron.
So, is an electron massless or not?
Where did you get the idea that electron is massless? Ernest Lawrence won the Nobel Prize in Physics for finding the e/m ratio of the electron. This ratio is not infinite.

Zz.

Tom Mattson

Staff Emeritus
Gold Member
Step 2: Type in "electron mass".
Step 3: Hit "Search".

At the tippy-top of the page it will say:

electron mass = 9.10938188 × 10-31 kilograms

DaveC426913

Gold Member
An electron masses about 1/2000th of a proton.

Tom Mattson

Staff Emeritus
Gold Member
Yeah, but what if he also thinks the proton is massless!

Cosmo16

maybe I was thinking of somthing else.

right now my brain is on overload. For me overload usually equals insaness whcih usually equals stupid questions

Pengwuino

Gold Member
I bet he is talking about an electron. I know I didn't even know what a neutrino was before college and he hints at being in HS

George Jones

Staff Emeritus
Gold Member
ZapperZ said:
Where did you get the idea that electron is massless? Ernest Lawrence won the Nobel Prize in Physics for finding the e/m ratio of the electron. This ratio is not infinite.
Zz.
I'm being overly pedantic, but Ernest Lawrence won the Nobel Prize for the cyclotron. Through the work of J.J. Thomson, Millikan, and others, the e/m ratio of the electron was already known (probably to at least 3 significant figures) when Lawrence built his first cyclotron, so if Lawrence added anything to this issue, it was just more digits.

At the risk of adding to the confusion, the Cosmo16 may have had the following misconceptions in mind. Light is wave a and light doesn't have mass, therefore when the electron is a wave it doesn't have mass. All particles have mass, so when the electron is a particle it has mass. Thus, an electron only has mass when it switches its character from that of a wave to that of a (massive) particle. How does the massless wave become a massive particle?

Finally, an interesting bit of trivia with respect to the "wave-particle duality" of the electron. In grossly over-simplified (enough to be quite misleading) terms, J.J. Thomson won the Nobel prize for showing that the electron is a particle, and his son, George Paget Thomson, won the Nobel prize (along with Davisson) for showing that the electron is a wave!

Regards,
George

ZapperZ

Staff Emeritus
2018 Award
George Jones said:
I'm being overly pedantic, but Ernest Lawrence won the Nobel Prize for the cyclotron. Through the work of J.J. Thomson, Millikan, and others, the e/m ratio of the electron was already known (probably to at least 3 significant figures) when Lawrence built his first cyclotron, so if Lawrence added anything to this issue, it was just more digits.
You're right. I should have been more careful. Millikan had more to do with measuring e/m.

At the risk of adding to the confusion, the Cosmo16 may have had the following misconceptions in mind. Light is wave a and light doesn't have mass, therefore when the electron is a wave it doesn't have mass. All particles have mass, so when the electron is a particle it has mass. Thus, an electron only has mass when it switches its character from that of a wave to that of a (massive) particle. How does the massless wave become a massive particle?
Finally, an interesting bit of trivia with respect to the "wave-particle duality" of the electron. In grossly over-simplified (enough to be quite misleading) terms, J.J. Thomson won the Nobel prize for showing that the electron is a particle, and his son, George Paget Thomson, won the Nobel prize (along with Davisson) for showing that the electron is a wave!
Regards,
George
Unfortunately, now its my turn to be "pedantic".

When you solve the Schrodinger equation for electrons, the MASS of the electrons is in there. The kinetic energy term explicitly contains the mass. Thus, even when you get a "wave" solution for the electrons, the mass of the electron is explicitly in the solution. So it is not correct to say that when it can be described via the wavefunction, it is massless.

Also, the "wave" of light is NOT the same "wave" as for electrons. We must not confuse the two simply because the name "wave" is being used for both. The wavefunction is not a physical wave like water waves. The QM wavefunction exists in a "configuration space" that can be complex. Physical wave, such as water waves and the solution to Maxwell equation, exists in real space. So those two are not the same beast and shouldn't be used as an analogy of one another.

Zz.

George Jones

Staff Emeritus
Gold Member
ZapperZ said:
Unfortunately, now its my turn to be "pedantic".
Why is this unfortunate?

I said that these points were all misconceptions that Cosmo16 might have had in mind.

I was that hoping that my wording would provoke you into pendantry mode, so that you would explain to Cosmo16 why these points are misconceptions.

Regards,
George

ZapperZ

Staff Emeritus
2018 Award
Oh, sorry. Now I see your point.

I blame it on lack of sleep. That's my story and I'm sticking to it!

:)

Zz.

Gokul43201

Staff Emeritus
Gold Member
For the sake of pedantry (or historical correctness, if you will)...

The e/m ratio was first measured by JJT at about 1.3*1011C/kg (correct value about 1.76...) using a modified Crookes' tube (the precursor to the modern electron gun). This was in 1897. The charge itself (from which the mass would be deduced) was also first measured by JJT and colleagues (Wilson & Townsend) using ionized water droplets which were collected in an acid and changed its pH. They got an approximate value of 1*10-19C by 1901. Millikan used ionized oil drops instead of water droplets and was able to make a much more accurate measurement of e (getting about 1.6 ...) using a tunable E-field to suspend the droplet against gravity. His work culminated in 1913 (he started in 1906, and spent roughly 7 years on just this one experiment !).

Last edited:

armandowww

Electron mass has a series of very important consequences: starting with cyclotronic frequency that is inverselly proportional to m... (Lawrence example retains its mean) ...so it got a certain inertia... passing through another question like antimatter interaction (if e- meets e+ you got a photonlike energy of twice electron mass in c squared units as it occurs on LEP facility at CERN).
Nevertheless... back to atomic level... it is basical reminding that fundamental magnetic behaviour is provided, yes, by spin of the electron, but it just becomes a powerless subject in massless hypothesis: Bohr magneton formula keeps mass, too (i.e. no mass no magnetic momentum).
It appears also in Bohr radius: realizing a mu-mesic atom you can think of electrons and muons as equals but their masses are different.
In beta decay, relativistical approch says electron mass is meant to grow up... but this is not a trouble (on this way you find starting point for Compton effect).
In crystals effective mass is a theorethical concept that is furnished with a dynamical meaning. But this is another story, because it depends on system complexity.
More generally in matter wave theory overall, hamiltonian operator, that straightforward leads to energy eigenvalues, has a mass-including kinetic term... so: no mass, no meaning.
If matter is a wave, that wave is to be a material one. We can't get away without that mass!

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