Originally posted by: CTho9305
The misinformation in this thread is horrible.
The speed of light is related to computation in that signals have to travel some distance across a chip for the various pieces to communicate. It's often convenient for the signals to take at most a full clock cycle worth of time to do this, which means a chip whose longest length is 1cm could be clocked no higher than
30 GHz. In reality, you would allow paths that took more than one clock cycle if you wanted to run faster. Of course, you'd need to allocate some of the cycle for the sender to create the light pulse and the receiver to receive it, and we don't even have optical interconnect on chips yet anyway. If you want to do any logic, that comes out of your time budget too. The wavelength of light doesn't really come into play here (I'm sure it matters... multi-meter radio wavelengths would probably be a bad idea... but it's not the stuff BitByBit was talking about).
Now, as I said, we don't currently use light for sending signals. We don't even use the fastest way of signaling electrically. Wires are effectively chains of
resistors and
capacitors; you send a 1 by charging the capacitors up, and a 0 by discharging them. An easy way to think about this is a trough or wide pipe, which you fill with water or empty of water (the capacitance is basically how much water it takes to raise/lower the water level, and the resistance is whatever slows the flow of the water into / through the trough). With this method, you can currently send a signal all the way across a modern CPU in a few cycles at clock frequencies of a few GHz. The signals propagate incredibly slowly relative to the speed of light.
A faster way to send signals (which I don't understand well) is transmission line signaling - imagine the same trough of water filled up half way; to send a signal you just create a wave on the water's surface. Signaling this way gets you much closer to the speed of light, but unfortunately the sending and receiving circuits are physically gigantic relative to the normal stuff, and it's more complex (for example, you have to worry about waves being reflected from the receiver, just like waves in a trough of water). If you read and
understand this paper, you'll know more about this than I do (so explain it to me

). Look at Figure 3 in the PDF to understand just how large this stuff is compared to normal systems.
Getting back to the wavelength of light, it comes in to play when you're manufacturing chips, rather than when you're using them. Basically, you can draw nice and sharp lines and shapes with light as long as the wavelength of the light is smaller than the shapes you're trying to draw. When you want to draw something smaller than the wavelength of the light, you run into problems - it gets hard to control widths, and you have to resort to a lot of tricks to get a useful result. Right now, the industry draws 65/45nm features on chips using 193nm light. The nice rectangles we draw when doing layout
show up as blobs. Intel has spent years and huge amounts of money trying to get the lithography process working with 13nm light, but I guess the results haven't been great.
Keep in mind that the
speed of light is a measure of how far light goes in a given amount of time, and it's distinct from the
frequency of the clock in a CPU. It's handy to call the "clock frequency" of a chip the "clock speed", but the word "speed" is being used differently in that sense than it is when we're talking about the "speed of light". "Clock speed" is a count of clock ticks in a second; "light speed" is measuring the distance light travels in a second.
Coming to the frequency of light, it's not really related to the clock frequency either.
Radio waves have frequencies in the range of 1MHz (for AM radio), 100MHz (for FM radio), to 2.4GHz (wireless networks, microwaves) and higher. Visible light is in the hundred-terahertz+ range. UV light, x-rays, and gamma rays go from the petahertz to the exahertz ranges. We can generate all of these waves easily, but they're really unrelated to the frequency of the clock in a CPU.
The fastest clock frequency (assuming we want to send the clock as an electrical signal rather than light) we could theoretically generate right now is probably around 500GHz (based on IBMs most recent BJTs), but that's not useful for a CPU, because you need to do a certain amount of work in a clock cycle, and a single gate takes many picoseconds to produce its output. You'd also have a heck of a hard time distributing the 500GHz clock around the chip for reasons discussed above.