## Sunday, December 16, 2007

### The 'Now' Calculator

One of the strangest results from special relativity is that a distant spacecraft moving away from or towards the earth at the present moment will consider that what is happening 'right now' on earth (according to the spacecraft crew) are events which to us are either in our past, or perhaps more surprisingly, in our future. For example:

1. A spacecraft in the Andromeda Galaxy moving away at 300 km/sec, at a time 'right now' according to us, considers the birth of Jesus to be happening 'right now'.

2. A missile at the distance of the moon (~400,000 km) and travelling towards us at 225 km/sec considers 'now' on earth to be one millisecond ahead of when we do.

3. An alien in a very distant galaxy, ten billion light years away, can move its 'now' for the earth plus or minus a human lifetime (70 years) simply by walking away from or towards the earth at 2 metres per second (4 mph). And we can do the same for it.

I have put together a small (25 kB) Excel online calculator here.

Input the event in the future (or past) you want to think about (the next election?) and the distance of the assumed spacecraft - typically some number of light-years. The calculator will then tell you how fast the spacecraft would be moving so that its crew - right now! - think that your event is happening 'right now', from their point of view.

Sobering for any of us who still think the future 'doesn't exist' because it 'hasn't happened yet'.