Wednesday, 15 September 2010

The speed of light is too slow!

The speed of light is too slow. No, seriously. This marvellous giver of life, the sustainer of every living thing, is just too darned slow.

Okay, I know you think that I'm mad. But it is true, for computer chips at least.

Almost all (*) digital computer chips rely on something called clock signals. These are the timekeepers of the chip, keeping all the operations synchronised. Want to add two numbers? Do it now. Want to fetch something? wait... wait... now! It is vital for operations to occur in the correct sequence, and the clock signal helps control this.

These clock speeds are the 25 Megahertz (MHz) or 33 MHz numbers we used to see in the early to mid-1990s. These numbers mean that the 386 or 46 chips of the day performed 25 or 33 million operations a second.

Both light and electricity travel at a smidgen under 300 million metres a second (**); that is 300 thousand kilometres every second. In the 33 millionth of a second that the fastest chips that the bearded engineers of the early 90s could design, light would travel nine metres. This is considerably larger than the roughly 16mm longest side of the chip.

However, modern chips operate much faster. A 2 Gigahertz (GHz) chip performs 2,000,000,000 operations every second (***). In this case, light can only travel 15 centimetres between each tick of the clock. That is getting very near to the physical size of the chip. Consider what this means; if the distance light can travel in a tick of the clock is less than the size of the chip, then it is impossible for the chip to use a clock signal to control all its parts. Things become much more complex. In many cases that effective distance is much less due to the convoluted path that signals have to take through the chip.

This is one of the reasons why the increase in clock speeds is slowing down. Until recently chip manufacturers proudly displayed the clock speeds of their chips; a consumer knew that a 66MHz chip would, everything else being equal, be faster than a 33MHz chip. Unfortunately clock speeds have stalled around the 2 to 3GHz mark. One of the reasons for this are the problems caused by the speed of light within the chip.

Chip designers are constantly pushing at the limits of the possible. In many cases new technologies or materials can push those limits a little further away, buying a few more years. In the case of the speed of light, however, there can be no improvement. It is a fundamental limit that cannot be broken.

(*) Some attempts have been made to make asynchronous, or unclocked chips, such as the Amulet project at Manchester University. These are rare and can be ignored for the purposes of this discussion.

(**) This is the speed of light in a vacuum.The speed in most electrical circuits is somewhat less.

(**). This is not quite true; modern chips have some parallelisation that allow multiple operations to be performed at the same time. The controlling clock still operates at this speed, however.

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