Are we alone in the universe?
This is a very big question, and one that is very difficult to answer! Even if the
universe was teeming with life, how hard would it be to find? The answer to this
question is – very hard indeed! The main reason for this is that the universe is
so big and so empty.
So, just how big and how empty is the universe?
Human beings have a real problem grasping any kind of scale. We evolved in a society
where until very recently most people would never have travelled more than a few
miles from their villages or towns in a whole lifetime. If you ask most people how
big the Earth is, or how far away Australia is you’ll get a lot of pretty vague
answers. Try it!
Where are we in relation to our neighbours?
We live on a small rock planet about 12000Km (8000 miles) in diameter. This sounds
pretty big. If you tried to walk a distance equal to the circumference of the Earth
it would take you about two years if you walked twelve hours a day.
- The nearest object to the Earth in space is the moon: this is about 384000Km (236000
miles). It would take about 13.5 years to walk this distance.
- The distance to our nearest star, the sun, is about 93,000,000 miles. It would take
about 5300 years to walk this distance.
- The distance to the nearest star after the sun is about 26,000,000,000,000 miles
(4 light years). It would take about 1.5 billion years to walk this distance.
- Our local cluster of stars, the Milky Way Galaxy, is about 100,000 light years across.
It would take 70,000 billion years to walk this distance.
As you can see, the universe really doesn’t do things on a human scale. The numbers
become very silly very quickly.
The Galaxy contains lots of stars: about 300 billion stars is a reasonable estimate.
If each star was an M&M you would pack 6 double decker buses to contain them
all. Yum!
The problem is with the stars is that they are all rather spread out. The distance
between each M&M, if they were packed at the same density as the stars in our
galaxy, would be on average about 386.25km (240 miles). This means that although
there is a lot of stuff out there it is spread out very, very thinly. If the Galaxy
was scaled to the size of a large football stadium it would appear like a very wispy
cloud. It would be like fine smoke, and if you walked through it you would feel
nothing.
The distances between the stars are vast. If we travelled at ten times the speed
of the space shuttle (281635.2km or 175,000 miles per hour) it would still take
us 16,000 years to reach the nearest star. So visiting any potential neighbours
is probably a long way into the future.
Any aliens in the house?
Erich Von Daniken wrote a best selling book called ‘Chariots of the gods’ claiming
that aliens had visited the Earth in the distant past. He said that the ancient
people’s had recorded their appearance in pictures and carvings, often on temple
walls. Much of his ‘evidence’ has since been disproved. It didn’t stop him selling
over 60 million books though! So how are we going to contact ET when there is so
much space to explore?
Bring on the big ear!
Just about our only chance of detecting ET at the moment is to listen out for radio
signals. This is an incredibly difficult task. Radio signals get weaker the further
they have to travel. In fact, the radio energy picked up by all the radio telescopes
in the world since the dawn of radio astronomy amounts to less energy than the single
beat of a butterfly’s wing.
The signals can come from any point in the sky and at any one of billions of frequencies.
We also have the problem of man-made radio signals. A mobile phone produces a signal
billions of times stronger than the one we’re listening for. And there are quite
a few mobile phones about.
This is a really difficult problem to crack. The things you really need are:
- a radio telescope that is really big so that it can pick up as much of this incredibly
weak signal as possible
- it must be able to point in any direction and see a large area of sky at any one
time
- it must be able to see a wide range of frequencies
- it must be able to pick out the one important signal from massive amounts of background
noise
This is what the Allen Telescope Array (ATA) is designed to do. It is funded largely
by Paul Allen, hence the name. Paul Allen (the clever one) and Bill Gates (the one
with the dodgy jumpers) co-founded Microsoft in the 1970s.
The ATA is being built in California about a hundred miles north of San Francisco.
Once it’s completed it will have about 350 antennae, each with a collecting area
of over 28 m2. The whole thing will have a collecting area of about 10,000m2. This
is the equivalent of a huge single radio telescope over 110m in diameter; this is
about the length of football pitch! The big advantage of this design and its size
is that each component is small enough to steer and can therefore cover lots of
sky very quickly. Eventually it should be able to look at around 100,000 nearby
stars. The ATA should be able to listen to over a 100 million channels simultaneously.
It does require a fair bit of computing power to do this, around the same as 100,000
PC’s!
What’s the chance of finding a signal?
This is a very good question. To answer this we first need to ask ourselves a few
questions. This is what Frank Drake did back in 1961. He produced the famous Drake
equation.
The questions go like this:
How many stars are in the Milky Way Galaxy?
(Answer – about 300,000 million)
What percentage of these stars have planetary systems?
(Answer – recent searches for extra solar planets suggest more than 90 per cent)
For each star that does have a planetary system, how many planets are capable of
sustaining life?
(Answer - tricky one this, about 1-10 maybe?)
On the planets that are capable of sustaining life, what perceentage of life does actually evolve?
(Answer – even more tricky this one. It could be anywhere between 100 per cent and
0 per cent. If we do find traces of life on Mars in the next few years, and we may, it would
put the figure close to 100 per cent. Take a guess.)
On the planets where life does evolve, what percentage evolves intelligent life?
(Answer- Very tricky one this! The Earth has had advanced life forms on it for around
500 million years. As we far as we know humans are the only example to have evolved
out of the millions of species that have existed. Say 0.0000001 per cent)
What is the fraction of intelligent life that can communicate using radio transmitters?
(Answer- Dolphins may be quite bright but they wouldn’t be much good with screw
drivers. This is another really difficult one. Shall we say about 10 per cent. What
do you think?)
For each civilization that does communicate, what fraction of the planet's civilization
survive?
(Answer- probably the most difficult question of all. The only example we have is
Earth. The Earth has been around for about 4.5 Billion years. We have only been
producing radio signals ourselves for around 100 years. Our civilization has nearly
destroyed itself once already in the 1960s during the Cuban missile crisis. How
long are we likely to survive? We could destroy ourselves tomorrow with nuclear
or biological weapons. A large meteor strike would achieve the same result; so would
unchecked population growth. Think this one out for yourselves. If our civilization
survives for a thousand years it would still only be 0.000001 per cent of the age
of the Earth).
If I put my best guesses in and multiply the numbers together to get the number
of civilizations in our Galaxy that we may be able to communicate with I get about
300. How many do you get?
If my guess is correct we are looking for one star in every billion. If we checked
a thousand stars a day it could still take 2,500 years to find the one with the
civilization!
Don’t despair though. The rate at which computers are progressing we will be able
check loads more than that soon enough. I suspect that if you are in school at the
moment we will certainly have tuned into ET before you reach middle age. How will
it change our view of ourselves to know that we are not alone? Now that really is
an interesting question! Don’t look down at your boots, look up and claim your place
in the Galaxy.
Produce a PowerPoint explaining the true scale of the universe. Include as many
real images as possible. The NASA Hubble web site is a fantastic place to start.