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The item in this week's Bang Goes the Theory, about the size of the Solar System, shows a good way of getting your head around cosmic distances in our local neighbourhood, the size of the planets, and how far apart they are. However, that's just back-yard stuff when it comes to the wider reaches of space. In the programme, Dallas explains that the edge of the Solar System (where we find the Oort cloud of comets) is about one light-year away, and that the nearest star, Proxima Centauri, is about four-light-years distant.
Our galaxy, the Milky Way, is about 100,000 light-years from side to side, and the nearest other big galaxy, the Andromeda Galaxy, is about 2 million light-years away. Now, it's easy to say that light would take 2 million years to get from us to Andromeda, but can we get any idea of how far that really is?
Imagine that you could travel at a speed of 1,000 kilometres per second. That's roughly equivalent to going in a straight line from Land's End at the tip of Cornwall to John O'Groats in the north of Scotland, in the tick of a clock. This is a speed we might just be able to imagine and it sounds pretty fast, but it's still only one-third of one per cent of the speed of light.
Now imagine travelling at that constant speed of 1,000 km/s for 30 million years. That's how long it would take to cross from one side of our galaxy to the other. If you can, imagine travelling at that same speed for 600 million years. That's how long it would take to get from our galaxy to our nearest neighbour in space, the Andromeda Galaxy. Jump between Land's End and John O'Groats once every second for 600 million years, and you'd still only travel a distance as far as the nearest galaxy.
These distances might set you thinking about whether it will ever be possible for humans to travel to other stars in our galaxy, just like they do in Star Trek. What do you think? Can you see a way in which the human race might travel to other stars, or even colonise the galaxy? If we could do it, might creatures from other planets be able to do so too? 30 million years to cross the galaxy might seem a long time, but the galaxy has existed for several thousand million years, so there's been plenty of time for someone to make such a trip. Why aren't they here yet?
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George Dishman - 29 September 2010 9:01am
These distances might set you thinking about whether it will ever be possible for humans to travel to other stars in our galaxy, just like they do in Star Trek. What do you think? Can you see a way in which the human race might travel to other stars, or even colonise the galaxy?
Speeds of the order of 1000km/s are difficult but values around 300km/s may be feasible based on a "solar sail" design using say 50nm aluminium (as used in Ikaros) on a carbon nanotube lattice supporting structure (rather than the 7.5um polyimide used by Ikaros). The use of a light sail means no fuel or engines are needed and it also addresses the biggest problem, slowing down at the destination star. On arrival, the probe would go into orbit and reconfigure as a spherical communications hub. Think of a geodesic sphere with each plate being a photovoltaic panel with a fine ground wire mesh and a small antenna just above the surface. At any time, the side of the hub facing the star would produce power using the photovoltaic cells while transmission back to the launch site and to other hubs in nearby systems could be achieved using the "synthetic aperture" technique to produce high power focussed radio signals by combining the low power signals from many individual transmitters. Obviously there are many engineering challenges in such a mission but there is no new fundamental physics needed.
The most efficient way to spread across the galaxy is for such probes to utilise circum-stellar dust in the target system (e.g. small particles in the asteroid belt or material in the planetary ring systems) to create further probes thus they would be self-replicating (Freitas around 1980 IIRC). The main requirements are aluminium, carbon, silicon and oxygen (for silicon dioxide) are all common materials in asteroid material, nothing exotic. The result of such a mission would be a network of hubs spanning the galaxy but with individual communications links seldom more than 15 light years.
If we could do it, might creatures from other planets be able to do so too? 30 million years to cross the galaxy might seem a long time, but the galaxy has existed for several thousand million years, so there's been plenty of time for someone to make such a trip.
Our galaxy has had Population I stars for probably around 8 billion years while our Sun is only 4.5 billion years old. Taking the pessimistic limit in which at least one other technologically capable race exists, if we are the second then they probably reached our level 2 to 4 billion years ago. On such timescales, 30 million years is trivial so it is most reasonable to assume that, if they exist, they would have had such a network monitoring every planet in the galaxy including ours since before the dinosaurs evolved.
Why aren't they here yet?
Maybe they were. If an alien race had placed a "spy satellite" in orbit around Earth, could the Neanderthals have discovered it? Could Galileo? Since they have not made overt contact so far, it suggests they have a policy of non-interference or perhaps non-contamination towards our society (the same idea as Star Trek's "prime directive"). That suggests they would remove all traces of their existence as our technology reached the level where we could detect their hardware.
Since an inter-stellar communications link would probably use all available bandwidth, following the same policy would imply their network would have to shut down as ours expanded if theirs was to stay hidden. It is not credible that they would remove the whole network so the criterion for ending isolation in such a scenario might be when we demonstrate the ability to create such a pan-galactic network. That might be when our first hubs communicated from the nearest stars back to Earth or, more likely, on the launch of the next tranche of probes from those first stars demonstrating that we had the self-replicating capability necessary to extend the network indefinitely.
Can humans or aliens move around the galaxy using only EM communications links and locally sourced material at the receiving end ("transporter beams")? That's another question entirely, but it would be a lot safer than trying to use physical craft and of course allows "travel" at the speed of light.
best regards
George
Andrew Norton - 30 September 2010 3:15pm
Thanks George - some really excellent ideas here.
I agree that Solar sails must be humanity's 'next step' in interstellar travel. From there to self-replicating machines and spanning the Galaxy is a relatively short step!
Andrew Norton