Clock drift facts for kids

Imagine you have two clocks, and you set them at exactly the same time. After a while, you might notice one clock is a tiny bit ahead or behind the other. This small difference, where clocks slowly get out of sync, is called clock drift. All clocks, even the best ones, experience some drift over time. This means they need to be checked and reset once in a while to stay accurate. Clock drift is especially important in computers and other technology that needs very precise timing.

What is Clock Drift?

Clock drift happens when a clock doesn't run at exactly the same speed as a perfect reference clock. Think of it like two runners starting a race together. Even if they run almost identically, one might slowly pull ahead or fall behind. Over time, the gap between them grows. Clocks do the same thing; they slowly "drift apart" from each other unless they are regularly reset or synchronized.

This drifting can cause problems, especially for computers. Computers need to communicate very quickly and precisely. If their internal clocks drift too much, it can mess up how they send and receive information.

Everyday Clocks and Their Drift

Most clocks we use every day, like the ones in our wristwatches or on our walls, are not perfectly accurate. They have a small amount of drift. This means they will eventually need to be adjusted to show the correct time.

How much a clock drifts depends on several things. The quality of the clock itself is important. Also, things like the temperature around the clock and how steady its power source is can affect its accuracy. Because of these factors, the same clock might drift at different speeds on different days.

Older mechanical clocks, like pendulum clocks, often had ways to adjust their speed. For example, by slightly changing the length of a pendulum, you could make the clock run a bit faster or slower. This helped correct for drift.

Today, most everyday clocks use a quartz oscillator. These are tiny crystals that vibrate at a very steady rate. Quartz clocks are much more accurate than old mechanical clocks. They drift so little that most don't even have a way for you to adjust their speed.

Super Accurate Atomic Clocks

Atomic clocks are the most amazing clocks we have! They are incredibly precise and have almost no clock drift at all. They are so accurate that even the Earth's own spinning speed changes more than an atomic clock drifts. The Earth's rotation can speed up or slow down slightly due to things like ocean tides.

Scientists use the super-steady vibrations of a specific atom, caesium-133, to define exactly how long a second is. This incredible precision means that the best atomic clocks might only lose or gain one second in about 15 billion years! To give you an idea, that's longer than the age of the universe!

Because they are so accurate, atomic clocks are used to create the official time standards for the whole world. For example, Coordinated Universal Time (UTC), which is what most countries use, is based on a careful average of many atomic clocks around the globe.

Time Travel? Not Quite: Relativity and Clocks

Did you know that time itself can be affected by speed and gravity? This amazing idea comes from Albert Einstein's theories of relativity. He showed that there isn't one "universal" time that everyone experiences the same way. Instead, time can be relative to who is observing it.

One effect is called time dilation.

• If two clocks are moving very fast relative to each other, they will appear to tick at different rates. This is part of special relativity.
• Also, a clock in a stronger gravitational field (like closer to a planet) will appear to tick more slowly. This is called gravitational time dilation and is part of general relativity.

It's not that the clock itself is broken; it's time that is actually behaving differently! Scientists have proven both of these effects with experiments.

These relativistic effects are very important for things like the GPS satellites. GPS satellites orbit Earth, so they are moving very fast and are in a weaker gravitational field than clocks on Earth. Their clocks actually run a tiny bit faster than clocks on the ground. If GPS systems didn't account for these tiny relativistic differences, their location readings would be wrong by about 10 kilometers every single day!

Making Random Numbers with Clock Drift

Computers often need truly random numbers. These are used for many things, like making games unpredictable or creating strong security codes (cryptography). It turns out that clock drift can be a clever way to generate these random numbers.

One method uses two independent clock crystals. Imagine one crystal ticks 100 times per second, and another ticks 1 million times per second. On average, the faster crystal should tick exactly 10,000 times for every single tick of the slower one. But because no crystal is perfectly precise, the exact number of fast ticks will vary slightly.

This tiny variation can be used to create random bits (0s or 1s). For example, if the number of fast ticks is even, the computer might choose a 0. If it's odd, it chooses a 1. By doing this many times, a computer can generate a stream of somewhat random numbers. These "somewhat random" numbers are then processed to make them truly unbiased and useful.

Even software can create random numbers using clock drift. It compares the computer's main timer (like the operating system's tick) with the speed of the CPU. If these timers use different crystals, it's like the example above. Even if they use the same crystal, other things happening inside the computer (like different programs running) can cause tiny, unpredictable changes in timing. These changes can also be used to create good random numbers.

Most of these hardware methods are a bit slow. So, computers often use them to create a starting "seed" number. This seed is then fed into a faster pseudorandom number generator to quickly produce many more random numbers.

Keeping Computers Safe: Understanding Timing Attacks

Sometimes, even tiny differences in how long a computer takes to do something can be used by attackers. This is called a timing attack. It's a way to learn secret information by carefully observing the time it takes for a computer to perform certain operations.

For example, in 2006, researchers showed how a timing attack could use clock drift. An attacker could make a server's CPU work very hard, causing it to heat up. When a CPU heats up, its internal clock might drift slightly differently. By watching the timestamps (the exact times recorded) from that server, the attacker could detect these tiny clock changes. These changes could then be used to potentially identify the server or gain other information. This shows how important precise timing and understanding clock drift are for computer security.

See also

• Bit slip
• Clock skew
• Effects of relativity on GPS