4 ms·

I was under the impression that electrons moved at the speed of light, but according to [1], they "only" move at 2200 km/s, which is less than 1% of the speed o

by steeve 11y ago

I was under the impression that electrons moved at the speed of light, but according to [1], they "only" move at 2200 km/s, which is less than 1% of the speed of light (~300000 km/s). Exciting times indeed. [1] http://education.jlab.org/qa/electron_01.html http://education.jlab.org/qa/electron_01.html

DanBC 11y ago
I thought that too. My mistake was in confusing the speed of the "wave" or signal (which travels at about 50% to 99% the speed of light) and the speed of the electron, which is quite slow. I think I'm confused about waves because of early childhood text books that had person A's mouth, then some lines increasing in size (like the lines on WIFI symbols) and then person B's ear. I now think of waves as moving forward, pushing forward, rather than up and down. Is there a version of "Maths for mums and dads" that covers science? http://www.amazon.co.uk/Maths-Mums-Dads-Mike-Askew/dp/0224086359 http://www.amazon.co.uk/Maths-Mums-Dads-Mike-Askew/dp/022408...
- maggit 11y ago
  I'm unsure of what you mean to be confused about, but maybe I can attempt to clear it up anyway. There are both kinds of waves. Sound waves, propagating from mouths and into ears, and waves in a spring along the direction of the spring, are longitudinal: http://en.wikipedia.org/wiki/Longitudinal_wave http://en.wikipedia.org/wiki/Longitudinal_wave Electromagnetic waves (in an appropriate physical model) and waves on the ocean are transverse: http://en.wikipedia.org/wiki/Transverse_wave http://en.wikipedia.org/wiki/Transverse_wave
castratikron 11y ago
The drift speed of an electron in metal is actually much, much slower, somewhere on the order of millimeters per second. The electromagnetic wave in metal propagates a lot faster, and it's usually pretty close to the speed of light in a vacuum. The link you posted was talking about the speed of an electron in an atom or something.
- viggity 11y ago
  If you imagine a pipe filled with marbles and you add a marble to one end, a marble will pop out the other end instantaneously (i.e. the speed of light).
  - cmiller1 11y ago
    A fun little caveat of that fact is that if you were able to make a 100% incompressible material you would violate causality. If the glass in your marbles did not compress at all then the "information" that you inserted your marble would reach the other end before it actually happened (faster than the speed of light)
    - deleted 11y ago
      [deleted]
  - gipp 11y ago
    Actually, that particular hypothetical would happen at the speed of sound in marbles (plus some corrective factors due to the interfaces between the individual marbles). But your intuition about propagation vs drift velocity is correct.
    - lfowles 11y ago
      Could you expand more on why it's the speed of sound?
      - Geee 11y ago
        If you touch a marble at the other end, you will feel the vibration at the other end with the delay equal to the speed of sound (and also hear the vibration when it's transferred to air). The speed of sound is the propagation speed of mechanical energy in a matter.
      - mikeash 11y ago
        The speed of sound is the speed at which any disturbance in the material propagates. When you push on a marble, that's a disturbance. The other end of the marble begins to move when the disturbance reaches that end. In this scenario, the disturbance is then transmitted to the next marble, and so on in that fashion, all at the speed of sound.
cmiller1 11y ago
Optical systems don't move at the speed of light either. Typically fiber optics have a refractive index of about 1.467
jessriedel 11y ago
The drift speed of the electrons is actually not that important at all. If we communicated over the atlantic ocean by gently pushing on very long sticks, the atoms in the stick might not move faster than a few mm/s, but the signal could be transmitted at the speed of sound for the material composing the stick. So your real question is "what is the speed of sound for electrons in a conductor?", i.e., how fast do disturbances travel? Wikipedia calls this the "speed of electricity", and it's of order half the speed of light: https://en.wikipedia.org/wiki/Speed_of_electricity https://en.wikipedia.org/wiki/Speed_of_electricity So the propagation speed of signals for electrons versus photons is not a very good explanation for the potential advantages of photonic computers.
bradleyland 11y ago
This is a good example of asking the wrong question. How fast does an electron move? 2200 km/s. How fast does an electrical signal propagate from one end of a circuit to the other? This is a much more relevant, but also more difficult, question. The unit used to measure this performance is called Velocity Factor [1]. Unfortunately, I don't know what the VF would be within a modern microprocessor, but in a wire, it varies from 50% to 99% of the speed of light in a vacuum. As is typical for mass media news, this story has been summarized in a way that glosses over a lot of the detail. Photonic computers have a very, very long way to go. Even this tiny beam beam splitter is thousands of times larger than current transistors. Intel uses a 14 nanometer process for Broadwell. The University of Utah beam splitter is 2400 nanometers square. Let's not ignore the fact that a beam splitter and a transistor aren't even close cousins. A beam splitter can't perform any logic. All of this technology is still very much in the realm of basic science. A lot of the assumptions about the speed improvements are based on napkin math involving theoretical numbers. If anyone ought to know that the difference between theory and reality is often muddied by the trip through the physical world, it would be Photonics researchers. Fiber optic (probably the most widespread application of Photonics research) performance is significantly impacted by real-world factors. Far more so than electromagnetic mediums. [1]: http://en.wikipedia.org/wiki/Velocity_factor#Typical_velocity_factors http://en.wikipedia.org/wiki/Velocity_factor#Typical_velocit...
- stephengillie 11y ago
  > Photonic computers have a very, very long way to go. Even this tiny beam beam splitter is thousands of times larger than current transistors. Intel uses a 14 nanometer process for Broadwell. The POC is a major milestone. Miniaturization will come. Don't forget that at one time, we had to use flatbed trucks to move around 5MB of RAM.
  - bradleyland 11y ago
    Absolutely. A 2.4 micron beam splitter is a hell of an achievement. I don't know what the next closest is in size, but the vast majority of beam splitters I'm familiar with are measured in millimeters. I'm sure there are others doing work on miniaturization, but 2.4 micron would seem like genuine landmark progress. What we need next is a practical photonic transistor at these scales!
lanna 11y ago
Electrons have mass, therefore they can't move at the speed of light.

by steeve 11y ago

DanBC 11y ago

I thought that too. My mistake was in confusing the speed of the "wave" or signal (which travels at about 50% to 99% the speed of light) and the speed of the electron, which is quite slow. I think I'm confused about waves because of early childhood text books that had person A's mouth, then some lines increasing in size (like the lines on WIFI symbols) and then person B's ear. I now think of waves as moving forward, pushing forward, rather than up and down. Is there a version of "Maths for mums and dads" that covers science? http://www.amazon.co.uk/Maths-Mums-Dads-Mike-Askew/dp/0224086359 http://www.amazon.co.uk/Maths-Mums-Dads-Mike-Askew/dp/022408...

maggit 11y ago
I'm unsure of what you mean to be confused about, but maybe I can attempt to clear it up anyway. There are both kinds of waves. Sound waves, propagating from mouths and into ears, and waves in a spring along the direction of the spring, are longitudinal: http://en.wikipedia.org/wiki/Longitudinal_wave http://en.wikipedia.org/wiki/Longitudinal_wave Electromagnetic waves (in an appropriate physical model) and waves on the ocean are transverse: http://en.wikipedia.org/wiki/Transverse_wave http://en.wikipedia.org/wiki/Transverse_wave

castratikron 11y ago

The drift speed of an electron in metal is actually much, much slower, somewhere on the order of millimeters per second. The electromagnetic wave in metal propagates a lot faster, and it's usually pretty close to the speed of light in a vacuum. The link you posted was talking about the speed of an electron in an atom or something.

viggity 11y ago
If you imagine a pipe filled with marbles and you add a marble to one end, a marble will pop out the other end instantaneously (i.e. the speed of light).
- cmiller1 11y ago
  A fun little caveat of that fact is that if you were able to make a 100% incompressible material you would violate causality. If the glass in your marbles did not compress at all then the "information" that you inserted your marble would reach the other end before it actually happened (faster than the speed of light)
  - deleted 11y ago
    [deleted]
- gipp 11y ago
  Actually, that particular hypothetical would happen at the speed of sound in marbles (plus some corrective factors due to the interfaces between the individual marbles). But your intuition about propagation vs drift velocity is correct.
  - lfowles 11y ago
    Could you expand more on why it's the speed of sound?
    - Geee 11y ago
      If you touch a marble at the other end, you will feel the vibration at the other end with the delay equal to the speed of sound (and also hear the vibration when it's transferred to air). The speed of sound is the propagation speed of mechanical energy in a matter.
    - mikeash 11y ago
      The speed of sound is the speed at which any disturbance in the material propagates. When you push on a marble, that's a disturbance. The other end of the marble begins to move when the disturbance reaches that end. In this scenario, the disturbance is then transmitted to the next marble, and so on in that fashion, all at the speed of sound.

cmiller1 11y ago

Optical systems don't move at the speed of light either. Typically fiber optics have a refractive index of about 1.467

jessriedel 11y ago

The drift speed of the electrons is actually not that important at all. If we communicated over the atlantic ocean by gently pushing on very long sticks, the atoms in the stick might not move faster than a few mm/s, but the signal could be transmitted at the speed of sound for the material composing the stick. So your real question is "what is the speed of sound for electrons in a conductor?", i.e., how fast do disturbances travel? Wikipedia calls this the "speed of electricity", and it's of order half the speed of light: https://en.wikipedia.org/wiki/Speed_of_electricity https://en.wikipedia.org/wiki/Speed_of_electricity So the propagation speed of signals for electrons versus photons is not a very good explanation for the potential advantages of photonic computers.

bradleyland 11y ago

This is a good example of asking the wrong question. How fast does an electron move? 2200 km/s. How fast does an electrical signal propagate from one end of a circuit to the other? This is a much more relevant, but also more difficult, question. The unit used to measure this performance is called Velocity Factor [1]. Unfortunately, I don't know what the VF would be within a modern microprocessor, but in a wire, it varies from 50% to 99% of the speed of light in a vacuum. As is typical for mass media news, this story has been summarized in a way that glosses over a lot of the detail. Photonic computers have a very, very long way to go. Even this tiny beam beam splitter is thousands of times larger than current transistors. Intel uses a 14 nanometer process for Broadwell. The University of Utah beam splitter is 2400 nanometers square. Let's not ignore the fact that a beam splitter and a transistor aren't even close cousins. A beam splitter can't perform any logic. All of this technology is still very much in the realm of basic science. A lot of the assumptions about the speed improvements are based on napkin math involving theoretical numbers. If anyone ought to know that the difference between theory and reality is often muddied by the trip through the physical world, it would be Photonics researchers. Fiber optic (probably the most widespread application of Photonics research) performance is significantly impacted by real-world factors. Far more so than electromagnetic mediums. [1]: http://en.wikipedia.org/wiki/Velocity_factor#Typical_velocity_factors http://en.wikipedia.org/wiki/Velocity_factor#Typical_velocit...

stephengillie 11y ago
> Photonic computers have a very, very long way to go. Even this tiny beam beam splitter is thousands of times larger than current transistors. Intel uses a 14 nanometer process for Broadwell. The POC is a major milestone. Miniaturization will come. Don't forget that at one time, we had to use flatbed trucks to move around 5MB of RAM.
- bradleyland 11y ago
  Absolutely. A 2.4 micron beam splitter is a hell of an achievement. I don't know what the next closest is in size, but the vast majority of beam splitters I'm familiar with are measured in millimeters. I'm sure there are others doing work on miniaturization, but 2.4 micron would seem like genuine landmark progress. What we need next is a practical photonic transistor at these scales!

lanna 11y ago

Electrons have mass, therefore they can't move at the speed of light.