Let's suppose that you're driving down a highway and your speedometer reads 80 mph. Your friend is driving in the next lane, and his speedometer reads 90 mph. Here's the question - what speed is your friend moving at? And what Galileo and later Newton showed was that there's no single answer ...

To you, your friend looks as though he's moving away from you at 10 mph. That's the difference in what your speedometers read. To someone on the side of the road, your friend is moving at 90 mph away from them. Now, you might say 'hold on, Liz, he's *REALLY* moving at 90 mph because that's what his speedometer says' ... but is he? What about the earth's spin? Is he *REALLY* moving at 90 mph? What about the earth's orbit around the sun? Is he *REALLY* moving at 90 mph?

So what Galileo and Newton realised was that speed is not an 'absolute'. The speed (or more correctly, the velocity) of an object depends on who is doing the measuring. You measure your friend moving at 10 mph relative to you. The person at the side of the road measures your friend moving at 90 mph relative to them. Both measurements are entirely correct for each person.

Now, here's the next interesting question - how far will your friend travel in an hour? Well, you'll see them 10 miles in front of you. The person at the side of the road will see them 90 miles away. So, what we've established is that two different observers won't necessarily agree on how fast something is moving, and what that means is that two different observers won't necessarily agree on the distance something travels in a certain amount of time! In physics, whenever you perform a measurement of speeds and distances you have to describe what those are measured relative to. Your friend is moving at 90 mph relative to the road. Your friend is moving at 10 mph relative to you. You observe your friend traveling a distance of 10 miles in 1 hr relative to you. And so on ...

In the mid 1800s, a mathematician by the name of James Clerk Maxwell took laws of physics describing magnetic fields and electric fields and played around with them (Ampere's, Faraday's and Gauss's laws). What he found was that there was symmetry in the equations and that these laws gave rise to an electromagnetic wave. He was able to work out how fast this wave should travel - it depends on two constants (known as the permittivity and permeability of free space) - and he found he got a result close to the known speed of light at the time. Light was an electromagnetic wave, and Maxwell had made one of the most important discoveries in history! All our wireless technology is ultimately the result of Maxwell's equations ...

In the early 20th century, Albert Einstein was looking at Maxwell's equations for electromagnetic waves and he noticed that the speed of light only depends on the values of those two constants. Since those constants are in fact constant for a vacuum, irrespective of who measures them or what speed they're travelling at, that meant the speed of light was also a constant irrespective of who measured it or what speed they're travelling at.

And here's the point ... we've already said that observers will disagree over how far something travels. We've already said that observers will disagree over how fast something travels. So the only way that everyone would agree on how fast light in a vacuum travels is that they must also disagree on how much time as elapsed. If Speed = Distance / Time, and two people agree on the speed of light, but disagree on the distance traveled, then they must also disagree on the time that has elapsed! Time, like speed and distance is relative. It depends on who is doing the measurement!

So what you find is that there is a balance ... the twin on earth will measure the distance from earth to the twin's spaceship and measure the duration of the mission. The twin on the ship will measure the distance from the ship to earth and get a different result, and will measure the duration of the mission and get a different result. The two twins won't agree on how far the ship traveled and they won't agree on how much time has elapsed. And the amount of change will be exactly enough to make sure that both twins measured the same speed of light!

When you do the maths out, you find that the twin in the spaceship moving close to the speed of light will have measured less time has elapsed than the twin on earth. Therefore, the spaceship twin will come back to earth and be younger than his/her earth twin! Time travel into the future is possible - you just need to move very close to the speed of light.