Imagine you can walk at the speed of light.
It is difficult to comprehend just how large distances are in astronomy. By shrinking the scales down to something more manageable (eg Sun the size of a melon, grapefruit etc) we are still usually left trying to imagine distances that go far beyond our horizon and beyond the horizon they once more become difficult to comprehend.
So, rather than work with distances let’s work with their equivalent – speed and time.
Imagine you can walk at the speed of light or, conversely, that light travels at walking pace.
How far can you walk in the 8m 19s it takes for light to travel from the Sun to the Earth? With a walking speed of about 5km/hr (3mph) you could walk about 700m or less than half a mile. Depending on where you live this is probably about the same as walking to the local shops or to the edge of the village.
Walking to the local shops – a good manageable speed, distance and time with a sense of neighbourhood.
How much walking do we have to do to go places?
Walk for about three or four minutes and you can visit your friends a few doors down the road – Mars, Mercury and Venus. It will take about half an hour to visit Jupiter – outside the neighbourhood but doable so long as it’s not raining. Beyond that your friends are still within walking distance but you should make sure they are home before you set off (Saturn, over an hour; Uranus, hour and a half; Neptune, about four hours).
You should also note that your friends keep moving house and these times vary depending on whether they live this side of the local shops or on the other side.
And that’s it. Without making plans for an epic journey that’s as much as you can do walking at the speed of light.
OK, so let’s make an epic journey. Let’s take some time off and make plans to go on a walking holiday abroad.
By the end of the first day we can be outside the heliosphere and then…..
Lots and lots of walking and not much to see along the way. You need to make plans for walking non-stop for four years before you get to the next star (Proxima Centauri). Where we had friends in our own city within walking distance, we now have to walk for four years before we get to the next city.
If you want to do any sightseeing on our extended holiday we are going to need a car, we are going to have to drive places.
Whoa, wait a minute. If we’re already walking at the speed of light then driving a car would be faster-than-light travel. Well, we just need to break the laws of physics a little bit and imagine a technology that allows us to do that.
As we amble along the roadside a car whooshes past us at about 20 times our walking pace (this would be approaching Warp 3 in Star Trek). At that kind of speed we could get to Proxima Centauri in about two and a half months. Cooped up in a car, day after day for weeks on end, nothing to see on the journey and not a lot to see when we arrive (not that we know of anyway). Maybe not the best of holidays but at least we don’t need a gap year to do it.
What we really want is some proper sightseeing like the Horsehead Nebula, a journey of a lifetime – except that it would require a lifetime to get there (about 75 years driving our car at near Warp 3). Even that would be “local” when we think about the rest of the galaxy, let alone neighbouring galaxies and galaxy clusters.
Driving at warp speeds for a lifetime and not getting very far takes us beyond the horizon of comfortable comprehension which is what I wanted to stay within. Time to return to Earth.
Look at the night sky, find The Plough (or Big Dipper) and pick out the star that joins the handle to the bowl. That is Megrez, the nearest of the seven stars in the constellation. Imagine setting off from there on our epic holiday in our faster-than-light car. We put the car into cruise control at near Warp 3 and settle down for the ride.
It is a very long and very boring journey. Three years of nothing to see. If we are lucky then every few months we might stumble across a star system but they do little to break the monotony.
Finally we see that pale blue dot and pull the car over to a stop.
We get out of the car and walk across to take a closer look. If are walking at the speed of light then at this scale the Earth is no bigger than a golf-ball. Its surface seems iridescent with greens and blues and swirls of white. Three years of driving and here, at last, is a thing of magic, teeming with life, a precious jewel.
We pick it up and hold it gently in the palm of our hand. It is ours to keep and protect.
The idea of a solar neighbourhood is used in Adam Nieman’s project “Welcome to the Neighbourhood“.
To see the relativistic effects of walking at the speed of light (or rather lowering the speed of light to walking pace) take a look at A Slower Speed of Light from MIT Game Lab