6 ms·

Fun fact: electrons in an metallic wire under a potential difference move only in the order of mm/s. See e.g.[1] As electrical signals still travel at about ha

Fun fact: electrons in an metallic wire under a potential difference move only in the order of mm/s. See e.g.[1] As electrical signals still travel at about half the speed of light, that seems besides the point. However, electrical signals do not need to span the entire width of a chip in order to have an effect. That is true for your example of accessing memory, but not for the actual calculations happening inside the CPU. On each clock cycle, all transistors are 'fed' by the transistors before it. The state of a transistor only depends on the states of the immediately connecting transistors on the clock cycle before it. That means the signal only has to travel the length of a single transistor on each clock cycle. We could have CPU's working at 3THz and a much larger focus on the difference between CPU-bound and IO-bound tasks. [1] http://amasci.com/miscon/speed.html http://amasci.com/miscon/speed.html

jacquesm 15y ago
The easiest to understand analogy to illustrate this that I've found to date is to imagine a tube full of marbles, push one more marble in on one end and another one will pop out instantly on the other end. As long as the marbles are all the same colour it is as though the marble you pushed in has miraculously teleported to the far end of the tube. It appeared to have moved at the speed of light even though in reality all the marbles have shifted only an amount equal to the diameter of a single marble.
- Confusion 15y ago
  There is a problem with that analogy: if you only apply pressure to a single marble, the marble at the other end will not be pushed out instantly. In fact, the 'signal' will travel with the speed of sound in the material of the marbles and you can measure the time difference it takes for the last marble to move after the first one has moved. The marbles will be slightly compressed and decompressed while moving, accounting for the extra length needed to accommodate the 'not moving instantly'. [1] A better analogy would be one where all marbles are pushed simultaneously. That is more like what happens in logic circuits. [1] People doubting relativity often try this thought experiment: I have an incompressible metal bar of a lightyear long. I press on one side. The other side must move instantaneously and not after 1 second (or longer). In fact, this proves the reverse: relativity is incompatible with the existence of incompressible metal bars. As we know, all metal bars are in fact compressible, so that is not a problem. And with compressible metal bars, the thought experiment fails, because the push will travel with the speed of sound.
  - jacquesm 15y ago
    That's true, but it is only an analogy, and like every other analogy it breaks down at some level (after all, it isn't the 'real thing'). The fact that there is a pressure wave set up in the materials is possible because marbles are made of some material (glass, stone, metal, whatever). If you wanted a 'perfect' picture you'd have to explain about electron migration in detail and then we're looking at a completely different picture. You'd not have a pressure wave in an electron to begin with, and they're not 'pushing' against adjacent electrons either. But it serves well to show how a slow move can have an apparent instantaneous effect at a distance.
    - ScottBurson 15y ago
      [Electrons are] not 'pushing' against adjacent electrons Sure they are. Like charges repel. Charge density in this case plays the same role as mass density does in the case of a sound wave. It's a very good analogy.
  - bluedanieru 15y ago
    Edit: I had a thing written here, but I figure I'm not understanding what you mean by 'all marbles are pushed simultaneously'. Can you elaborate? Other than the conflation of the speeds of light and sound (which is not such a big deal, really), it's a good analogy.
    - Confusion 15y ago
      As for the 'all marbles are pushed simultaneously': a key characteristic of useful transistors is that they operate as amplifiers[1]. This is possible because effectively, every transistor has its own power source. As such, on each clock cycle, then can all get a simultaneous push from their power sources. [1] http://en.wikipedia.org/wiki/Transistor#Simplified_operation http://en.wikipedia.org/wiki/Transistor#Simplified_operation BTW, I'm not conflating the speeds of light and sound. If you bang on one side of an iron bar, a wave travels through the material, quickly compressing and decompressing the bar where the wave passes. Such waves travel at the speed of sound in that material, no at the speed of light. That makes sense, because sound is nothing more than the physical modulation of the density of a medium, most commonly air.
powertower 15y ago
> Fun fact: electrons in an metallic wire under a potential difference move only in the order of mm/s. Your link says otherwise, when you read down into the comments... Basically, only a few atoms are able to "conduct" an electron in a wire... Mostly the ones near the surface. And in turn, the true speed of the electrons traveling throught a wire in DC is in the m/s range, not mm/s.

jacquesm 15y ago

The easiest to understand analogy to illustrate this that I've found to date is to imagine a tube full of marbles, push one more marble in on one end and another one will pop out instantly on the other end. As long as the marbles are all the same colour it is as though the marble you pushed in has miraculously teleported to the far end of the tube. It appeared to have moved at the speed of light even though in reality all the marbles have shifted only an amount equal to the diameter of a single marble.

Confusion 15y ago
There is a problem with that analogy: if you only apply pressure to a single marble, the marble at the other end will not be pushed out instantly. In fact, the 'signal' will travel with the speed of sound in the material of the marbles and you can measure the time difference it takes for the last marble to move after the first one has moved. The marbles will be slightly compressed and decompressed while moving, accounting for the extra length needed to accommodate the 'not moving instantly'. [1] A better analogy would be one where all marbles are pushed simultaneously. That is more like what happens in logic circuits. [1] People doubting relativity often try this thought experiment: I have an incompressible metal bar of a lightyear long. I press on one side. The other side must move instantaneously and not after 1 second (or longer). In fact, this proves the reverse: relativity is incompatible with the existence of incompressible metal bars. As we know, all metal bars are in fact compressible, so that is not a problem. And with compressible metal bars, the thought experiment fails, because the push will travel with the speed of sound.
- jacquesm 15y ago
  That's true, but it is only an analogy, and like every other analogy it breaks down at some level (after all, it isn't the 'real thing'). The fact that there is a pressure wave set up in the materials is possible because marbles are made of some material (glass, stone, metal, whatever). If you wanted a 'perfect' picture you'd have to explain about electron migration in detail and then we're looking at a completely different picture. You'd not have a pressure wave in an electron to begin with, and they're not 'pushing' against adjacent electrons either. But it serves well to show how a slow move can have an apparent instantaneous effect at a distance.
  - ScottBurson 15y ago
    [Electrons are] not 'pushing' against adjacent electrons Sure they are. Like charges repel. Charge density in this case plays the same role as mass density does in the case of a sound wave. It's a very good analogy.
- bluedanieru 15y ago
  Edit: I had a thing written here, but I figure I'm not understanding what you mean by 'all marbles are pushed simultaneously'. Can you elaborate? Other than the conflation of the speeds of light and sound (which is not such a big deal, really), it's a good analogy.
  - Confusion 15y ago
    As for the 'all marbles are pushed simultaneously': a key characteristic of useful transistors is that they operate as amplifiers[1]. This is possible because effectively, every transistor has its own power source. As such, on each clock cycle, then can all get a simultaneous push from their power sources. [1] http://en.wikipedia.org/wiki/Transistor#Simplified_operation http://en.wikipedia.org/wiki/Transistor#Simplified_operation BTW, I'm not conflating the speeds of light and sound. If you bang on one side of an iron bar, a wave travels through the material, quickly compressing and decompressing the bar where the wave passes. Such waves travel at the speed of sound in that material, no at the speed of light. That makes sense, because sound is nothing more than the physical modulation of the density of a medium, most commonly air.

powertower 15y ago

> Fun fact: electrons in an metallic wire under a potential difference move only in the order of mm/s. Your link says otherwise, when you read down into the comments... Basically, only a few atoms are able to "conduct" an electron in a wire... Mostly the ones near the surface. And in turn, the true speed of the electrons traveling throught a wire in DC is in the m/s range, not mm/s.