# How was electron energy measured?

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## Main Question or Discussion Point

Do you know when the energy content of electrons was actually measured as .511 MeV? was the first measurement true to current value? and, most of all, how was/is it measured with precision?

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lekh2003
Gold Member
I think it was measured by a combination of physicists from the late 19th century and early 20th century. Eugen Goldstein, Wilhelm Wein, J.J Thomson, Robert Milikan. They can calculate the charge of the electrons (which were cathode rays) by measuring their charge to mass ratios. Then later on they tried narrowing down substances.

Take a look here:
https://physics.aps.org/articles/v5/9

sophiecentaur
Gold Member
Do you know when the energy content of electrons was actually measured as .511 MeV? was the first measurement true to current value? and, most of all, how was/is it measured with precision?
Wouldn't you say that knowing the Mass in kg would give a direct value to the eV equivalent value? (e=mc2)? If you read the history of the discovery of the electron, the measurement of its e/m and then the measurement of m you can decide for yourself where the point was that the "precision" was arrived at. (c was measured very accurately, way back - again, the history of that is interesting to read.)

Wouldn't you say that knowing the Mass )
I think it was measured by a combination of physicists 9
Thanks, but that was a theoretical discovery and also,at the time you mention, the equivalence mass-energy had not been ascertained.

What I clearly asked is when the theory was confirmed by a concrete experiment that revealed energy= .511 MeV, did it happen much later when pair annihilation was possible? and how is energy concretely measured?
Thanks anyway

Mister T
Gold Member
Thanks, but that was a theoretical discovery and also,at the time you mention, the equivalence mass-energy had not been ascertained.
I'm not sure what you mean by a theoretical experiment. Those researchers carried out experiments in laboratories using what was at the time, state-of-the-art equipment.

What I clearly asked is when the theory was confirmed by a concrete experiment that revealed energy= .511 MeV, did it happen much later when pair annihilation was possible? and how is energy concretely measured?
You asked when a measurement was first made, not when a theory was confirmed. What do you mean by "concretely" measured. Are there ways of measuring things that are not concrete?

Are you asking when and how it was established experimentally that the mass of an electron is equivalent to 0.511 MeV of energy?

sophiecentaur
Gold Member
What I clearly asked is when the theory was confirmed by a concrete experiment that revealed energy= .511 MeV, did it happen much later when pair annihilation was possible? and how is energy concretely measured?
I'm having a problem with the logic here. If "annihilation" was observed and appreciated for what it is, then wouldn't that imply an awareness of the mass/energy equivalence?
But measuring the wavelength of 0.5MeV EM might have been achieved with some sort of spectroscopy, perhaps; X ray diffraction? - or is the energy too high? Megavolt sources have been available for a long time.
Perhaps you have been reading some Science History and that accounts for your question. Around the turn of the 19/20 Century there was such a lot going on.

Dale
Mentor
The mass was first measured by Millikan in 1909 to within 1% of its current value through a combination of his famous oil drop experiment and previous measurements of the mass to charge ratio.

The positron was discovered in 1932, but I don’t know when the first measurements of the anhilation energy was made. Note that the energy of the photons will include the KE of the positron and electron, so they will not necessarily each be the anticipated amount. The mass is a more repeatable quantity.

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ZapperZ
Staff Emeritus
Thanks, but that was a theoretical discovery and also,at the time you mention, the equivalence mass-energy had not been ascertained.

What I clearly asked is when the theory was confirmed by a concrete experiment that revealed energy= .511 MeV, did it happen much later when pair annihilation was possible? and how is energy concretely measured?
Thanks anyway
I don't understand the issue here.

In the general physics lab, the e/m experiment using Helmholtz coils is very common. A student can find the ratio of charge to mass for an electron relatively easily. You can find the history of such experiment.

Then, using the knowledge of the electron charge, the value of the mass of an electron drops right onto your lap. The "energy equivalent" is simply a conversion from mass to energy via the infamous Einstein equation. There isn't any need to perform another experiment JUST to verify that the conversion is valid for an electron.

Zz.

I don't understand the issue here.
Then, using the knowledge of the electron charge, the value of the mass of an electron drops right onto your lap. The "energy equivalent" is simply a conversion from mass to energy via the infamous Einstein equation. There isn't any need to perform another experiment JUST to verify that the conversion is valid for an electron.Zz.
Is it really so hard to understand?
You, all, are referring to the experiment that determines just the mass of the electron, and then refer to an infamous equation that was soon disqualified by scientists that showed the circularity of the so-called derivation.
But, even conceding that the equation means something, you are deducing the content of energy in an indirect way (hence theoretically).
I am asking if and when the energy of an electron was actually, concretely, directly measured through annihilation or other, and what was the method, equipment used. If the KE is problematic, there is surely a way to overcome the obstacle, if not, then you mean that the value and the infamous equation has not yet been verified.
I hope it is clear now

Dale
Mentor
if not, then you mean that the value and the infamous equation has not yet been verified.
It is verified thousands of times a day in PET detectors across the world as a part of standard medical imaging. I just don’t know when it was first measured historically.

In PET imaging the anhillation photons are absorbed by a scintillation crystal, which produces a large number of low energy photons for each 511 keV photon. The number of scintillation photons is proportional to the energy of the high energy photon. The number of scintillation is used as a measure of energy in order to reject photons that have undergone scattering and keep only photons that have traveled uninterrupted. This reduces the background counts, and thereby improves image quality and sensitivity. So the energy measurement of 511 keV photons from positron anhilation is an important part of routine technology in daily use globally.

an infamous equation that was soon disqualified by scientists that showed the circularity of the so-called derivation.
But, even conceding that the equation means something,
I am quite uncomfortable with this comment. Please review the forum rules. Personal speculation is not tolerated. The energy mass equivalence is well established both theoretically and experimentally.

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I

I am quite uncomfortable with this comment. Please review the forum rules. Personal speculation is not tolerated. The energy mass equivalence is well established both theoretically and experimentally.
The fact that it was confirmed ,( it would be nice if you told us where to find more details), doesn't mean that Einstein proved it, it was a lucky guess based on experiments by Hasenhorl on heat and others on 'electromagnetic mass'. In itself it has no scientific value, even theoretically, and everybody knows that, which is not my personal speculation . If you think to the contrary, I challenge you to explain the logical/scientific structure of the derivation, if you can

sophiecentaur
Gold Member
The fact that it was confirmed ,( it would be nice if you told us where to find more details), doesn't mean that Einstein proved it, it was a lucky guess based on experiments by Hasenhorl on heat and others on 'electromagnetic mass'. In itself it has no scientific value, even theoretically, and everybody knows that, which is not my personal speculation . If you think to the contrary, I challenge you to explain the logical/scientific structure of the derivation, if you can
I'm not sure what you are driving at here. You initially asked a question based on Scientific History but now you seem to be doubting the validity of a very well established concept. I think that it is really up to you to produce a good reason to justify your doubt; that's the way it works. If it happens that the 'right answer' came from a bit of a fudge at the start then that is not a rare thing to happen in the advance of Knowledge.
If you look at the work on fundamental particles that has been going on for a long time now, you must acknowledge that the theories hang to gather rather well and they are based on the Energy involved in 'Events'. If the electron Energy were wrong, then wouldn't you expect a few other serious inconsistencies? You'd have to disagree, not just with the one point but with most of the conclusions about the system.

Lord Jestocost
Gold Member
Mister T
Gold Member
The fact that it was confirmed ,( it would be nice if you told us where to find more details), doesn't mean that Einstein proved it, it was a lucky guess based on experiments by Hasenhorl on heat and others on 'electromagnetic mass'.
Einstein never claimed he had proven it. In fact, quite the opposite. He was always curious about experimental results that might reveal flaws in his theories. That's the nature of the way humans advance knowledge.

In itself it has no scientific value, even theoretically, and everybody knows that, which is not my personal speculation . If you think to the contrary, I challenge you to explain the logical/scientific structure of the derivation, if you can
Which derivation? There are hundreds of them. Thousands of experimental confirmations.

You seem to be worried about the validity of the claim that the electron has a rest energy of 0.511 MeV. Every question you've asked could be asked about every thing that's ever been measured. For example, the mass of the Earth is about $6 \times 10^{24}\ \mathrm{kg}$. Has it ever been concretely measured? Has this theory ever been proven?

Einstein never claimed he had proven it. In fact, quite the opposite.
Thanks for the support, Mister T.quite so. Nobody,except some members of this forum consider that paper as a proof, as anyone can check on the web. But I look forward to Dale's arguments!

Maybe, this presentation provides some hints:
Electron-positron Annihilation - SCIPP - UC Santa Cruz
Thanks, that's is a rather appropriate and adequate answer to my query

Mister T
Gold Member
Thanks for the support,
As far as I know Einstein was never involved in the first measurement of the electron's rest energy, or any of the other measurements, most of which were made after he died.

You seem to have somehow conflated that first measurement with the confirmation of some theory of Einstein's?

Einstein did indeed discover the equivalence of rest energy and mass. And that equivalence has been confirmed by an amount of evidence that's so overwhelming it leaves no room for any reasonable doubt about its validity. Einstein didn't put forth a proof, he put forth a theory.

Dale
Mentor
Staff Emeritus
2019 Award
Given the OP's, um, questioning attitude towards relativity, it's not 100% clear what will satisfy him. I think the crux of the question is "do we know E/m for an electron at rest is c^2?" This doesn't come from positrons, and it's not clear to me where it actually does come from. I suspect there is not a single experiment that does this, and it comes from the ensemble of relativistic kinematics experiments done in the 1st half of the XXth century.

Given that Einstein wrote down relativity in 1905 and the positron wasn't discovered until 1933, it's not surprising that the latter was not cast as a "test of relativity": For what it's worth, the Anderson paper identifies the positron as a positively charged particle with less than 2 units of charge, most likely one unit, and weighing less than 20x the electron mass. He determined that this was a new particle because of its ionization properties; he doesn't cite Bethe, but this was being worked out in the relativistic case about this time.

Interestingly, Anderson observed pair production (Figure 3 of the discovery paper) but he did not correctly recognize its significance.

lekh2003
Gold Member
I do not seem to understand how this thread has derailed from a simple question as to how different properties of electrons were confirmed, to a full blown discussion of questioning the mass-energy principle.

Dale
Mentor
I do not seem to understand how this thread has derailed from a simple question as to how different properties of electrons were confirmed, to a full blown discussion of questioning the mass-energy principle.
Looking at the OP’s past history of posting, that is an agenda that he/she was deliberately trying to promote.