Time travel is cool

That, is an attention-grabbing title.

I'm often mildly disappointed with our education system, because it fails to teach a lot of the scientific literacy that is needed in order to understand some of the concepts that science is discovering today, and to appreciate just how cool some of this stuff is. Today, we're talking about light, the speed at which it travels, and the closest thing to a time machine we'll ever actually see.

Let's start with the speed of light. Light is a funny thing. Einstein realized, and then proved scientifically that no matter how fast you're travelling, no matter which direction you are moving in, you will always measure light as moving away from you at 300,000 km/s. This speed is referred to with the constant c. This is counter-intuitive to the way we humans think. Let's say I'm standing still, and my friend Davey runs by me. Let's say I measure that he is moving away from me at 10 km/h. Now, in scenario two, I'm running at 5 km/h in the same direction as Davey. When he passes me and starts to move away from me, I will measure him as moving away from me at 5 km/h, rather than the previous 10. This is because I am moving foward at 5 km/h myself, rather than standing still.

Here comes the counter-intuitive bit - in both of those situations, if a beam of light travels past me and starts to move away, I will always measure it as travelling away from me at c (300,000 km/s). Interestingly, the scientific experiements and evidence collected also shows that this is the theoretical speed limit of the universe. As far as we can tell, nothing can travel faster than this speed.

Okay, we're past the first bit of math.

Let's get into the time travel I alluded to in the title. Because of the rate of speed at which light travels, most of us think of it as travelling instantaneously. That is to say, you turn on your desk lamp, and the light doesn't travel to light up your desk, it's just there. This is a convenient way to think about light on a day-to-day basis, our daily lives do not involve interacting over distances greater than 300,000 km. However, once we start to talk about the stars in the sky, things get a little bit different.

The thing about stars in the sky is that they are very, very far away from us. Sirius, one of our closest neighbours in the galaxy, is about 9 light years away from us. Light years can be a bit of a confusing term, because they contain two concepts: distance and time. A light year means this: The distance that light can travel in one year. So, even though the name includes a notion of time, a light-year is always really a distance measurement, and one light-year will always take one year for light to move that distance.

What this means is that if Sirius the star were to suddenly dissapear from the universe and stop producing light (this is highly implausible, but will work as an example), it would take us nine years before we had any idea that this had occurred. The coolest thing about this is that when we look up and see the star Sirius in our night sky, we are actually seeing it as it looked nine years ago. Remember, it takes nine years for the star's light to reach us.

So, we've got the speed of light down, and we've talked about Sirius. Let's take things up to the next level and talk about stars that are much further away from us. Astronomers these days have some incredible tools at their disposable. The Hubble space telescope is able to look at stars that are billions of light years away. This is incredibly far, but not only that, it means that we are looking back at light that was created and sent on its heavenly journey billions of years ago. This is the most incredible part, and the most difficult for many people to wrap their head around. The further away from us that astronomers are looking, the further back in time they are seeing the state of the universe.

Perhaps some ascii art to help clarify?

1. (Sirius) * ---light travels 9 light-years---> * (Earth)

2. (Starry McOldey) * ---light travels 10 billion light-years---> * (Earth)

The most important thing to remember here is that once light has been emitted from a star, it doesn't matter what happens to the source of the light - the light itself will continue to travel on its path, eventually reaching us.

I've got two helpful analogies too! Think of a really powerful sprinkler: the sprinkler shoots out a blast of water, and then changes its position and shoots a second blast of water. The first blast of water is still travelling in the original direction, regardless of the fact that the sprinkler has now changed its orientation. Analogy number two - imagine two cities that are separated by 1000 km. The first city sends out a messenger to the second city, and it takes him two weeks to get there. By the time the messenger arrives in the second city, the information he's bringing with him is two weeks older. For the sake of this analogy, you can think of light as acting like a messenger from a star (though a messenger that has done an insane amount of steroids).

So, I, and possibly you, think this is pretty cool. By looking into deep, deep space, scientists are able to see the universe as it was when it was very young (most astronomers, through scientific evidence, place the age of the universe to be roughly 13.7 billion years old). They aren't really traveling back in time, but they are able to see a universe from an earlier time.

Interesting stuff right? No? Whatever. If you like reading about this kind of thing, you should check out the Bad Astronomy Blog. Phil Plait, a widely-known astronomer in skeptic circles, runs the blog and mixes things up with the latest cool astronomy news, reviews of bad movies using bad science (his review of The Core is pretty good), and skepticism regarding things like the Moon Landing Hoax. Worth a browse!