h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

No.

Yes

Mnnn.... Maybe

To paraphrase from a great philosopher ( Calvin from the Calvin and Hobbes strip ) :

"The surest sign that there are intelligent lifeforms elsewhere in the universe, is the fact that they have not tried to contact us."


Says it all really...

Of course non-terrestrial life exists. But the chances of another comparably-advanced civilisation, recognisable as such to human beings, existing at this exact moment of time, and conveniently close enough to us to be contactable... well, they're a bit slim, aren't they?

The chances of us spotting them are fairly slim, too - NASA once, as an experiment to prepare for the Mars programme, used a satellite to scan the Earth from space for signs of life which it could recognise without human intervention. It didn't find any.

On the one hand, you're right. There is little or no evidence, from a distance, that there is intelligent life on earth.

Other than artifacts like the Great Wall of China and the tendency of night-time America to light up like a christmas tree, that is.

That said, it's possible that we've not been noticed.

On the other hand, we have been getting airborne visitors that have been identified as machines since the 1800s, possibly earlier. So perhaps there is something going on.

A *little* earlier, if you count Ezekiel and his "Wheel" in the Old Testament of the Judao-Christian Bible...

Oh, and both your examples were of artefacts, rather than life - if all life were wiped out tomorrow (weather permitting... ) there would still be a wall, and some lights would probably come on automatically.

They were looking for a means to detect life through its biological processes, not the artefacts left behind, else the "Face" and "Pyramids" on Mars would have been enough for them...

**thinks hard**
And how would you go about detecting life through it's biological processes? From a distance?

I would have thought it would be easier to send a probe to the surface and look. ("Oooo! Trees!").

That's a poser, though. Any idea what methods were employed?

You can't see the Great Wall from space. It's long enough, but nowhere near wide enough.

I'm not a biologist but presumably the composition of the atmosphere in terms of things like oxygen must be a bit of a giveaway. I believe (at least, David Attenborough said so) that the earliest life-forms on Earth completely changed the chemistry of the air. But it's whether you could detect that from a long-range survey. Anyone here know anything about atmospheric spectroscopy? That sounds like our best bet...

OK, here's how you might do it. You look for the 'wake' that a planet's atmosphere might leave in space as it gets stripped off by the charged particles emanting from its parent star. This is happeing all the time, even here on Earth, although its magnetic field protects us. One should be able to detect this wake as the planet passes in front of the star and it absorbs the light from it. Oxygen (and ozone) have pretty characteristics absorption lines, and should show up.

So then we show a planet has an atmosphere. That's only the first step.

Using this method we not only know it has an atmosphere but also roughly what that atmosphere is composed of. And it's my understanding that the presence of certain gases in large amounts in an atmosphere is a sign of life. (As would be the presence of pollutants - whether you consider that a sign of intelligent life, or quite the opposite, is up to you...)

All the oxygen in our atmosphere was generated by photosynthetic acivity. Therefore gaseous oxygen implies the presence of life. QED.

If the person searching is looking for carbon-based, oxygen dependent lifeforms. Oxygen is toxic and very dangerous in large amounts. It's sort of like that short story in the Martian Chronicles where the martians say that there can't be any life on Earth because the oxygen content is too high. An oxygen atmosphere could be as poisonous to someone else as something like a chlorine atmosphere would be to us.

I forgot what book, but i seem to recall an article on ancient biology, stateing that the first plants on earth *probably* used a much simplier, though less iffecient chemical process using other gasses(ring a bell to any one?) When the level got too low, some of the plants made a switch to using CO2, releasing O2, wich, as a poison, killed off the original plant life. I think one of the bases for this statement that jumping to Photosynthisis straight away would be difficult. I can't remember where I read that, if it was a factial book on prehistoric earth or not... sound like anything someone else has heard? I only say this to emphasize the point that life doesn't nessisarily need O2.

Any one know the accepted definition of Life?
Are viruses alive?
Is Fire alive? (it needs food, spreads it's make-up around, dies)
Would Life need to live in similar temperature zones?
Would it need to use the same light waves to see? sound waves to hear?
What are a collection of skills/physical requirements needed to build a civilization? is fire building? Or are there other ways to create heat in an enviroment lacking oxygen? is heat needed fo civilization?
Are there other bands of radio waves that could be used? other forms of waves intirely?
Can a civilization start under water? say in a watery moon surrounded by an ice sheild and with a center kept liquid by tidal forces, looking barren an the surrface?
Or how about in a gas giant? no solid ground, but plenty of heat(if needed), plenty of various gases, and chemicals.

Using these questions as starters, which of the planets/moons/asteriods/comets/suns in our solar system could support Life?

Would other civilizations be interested in us? If they are, are they kept away from a direct meeting with us and revealing themselves due to a fear of culture shock?
Would there really be any way to contact them and learn from them? Is the light speed barrier really a barrier? How would a civilization send a ship on a voyage of many millions of years and expect to still be around when it got back?

What are the odds of Life occuring? Contacting another Life-form not of it's planet of origin?

I hope I am able to spur people into adding what knowledge about Chemistry, Biology, Physics, Math, or any other school of knowledge to a vast pool we can use open our minds to what we can comprehend.

I ask, in the name of truth, that no one adds false stats, or bad figuring to this posting, that anyone with more questions adds to the questions asked, and all help out.

'life' can be defined as having the following attributes;
movement
responsiveness
growth
feeding
reproduction
release of energy
excretion
'life' is the sum total of all of these things.

using this as a definition (which i think is quite widely accepted) it is easier to decide if certain things are alive. viruses aer, fire is not.

as to the wider question 'are we alone?'. yes, i think we are.

I can have a go at answering some of your questions. Some will disagree with some of the definitions I give, because the whole area (in biology, at least), is open to debate. Different groups of biologists think different things.

All living organisms share five characteristics:

Order - all organisms consist of one or more cells with very ordered structures - atoms make up moloecules, which make up (in some cells) cellular organelles, which are contained within a cell. In multicellular organisms, the cells are contained within a body.

Sensitivity - all life responds to stimuli.

Growth, development and reproduction - all organisms grow and reproduce. Moreover, they all possess herditary molecules which are inherited by their offspring.

Regulation - all life has regulatory mechanisms that coordinate an organisms internal functioning.

Homeostasis - all life is capable of maintaining relatively constant internal conditions eg temperature, pH.


Biologists are divided on the issue of whether viruses are alive. My personal view is that they are not - they cannot reproduce independently. They are little more than fragments of a nucleic acid (either DNA or RNA) surrounded by a protein coat (capsid).


Using the above definitions, fire is also not alive - there is very little order (as far as we know), no independent regulation, no ability for homeostasis, but most importantly, there is no element of heredity.


Even on Earth, the conditions under which life is found are quite diverse. Some archaebacteria have been detected living in deep-sea thermal vents at temperatures of up to 113C (these are known as thermophiles). Some organisms can eke out an existence in blocks of ice (known as psychrophiles). Others prefer more moderate temperatures (these are mesophiles). The trouble with high temperatures is that DNA denatures (becomes single-stranded). Whether or not this owuld be a problem is not yet (as far as I know) clear.


I can't comment on the conditions needed for civilisations to arise - I am a biology student. Perhaps the sociologists can help out there. Or perhaps I may even have some thoughts of my own (unlikely - it's a long time since I had an original thought). Will also try and develop some of the points I nmade above, but hopefully there's enough there to keep a conversation going.

BTW I would like to clarify a few points mentioned in your first paragraph.
The current theory is that the first organisms on Earth were heterotrophs (they used nutrients from the environment). They were fermentative, because there was no oxygen available. Later cells were autotrophs (photosynthetic). They fixed carbon dioxide and produced oxygen. This probably did lead to the extinction of some of the early heterotrophic cells. But it wasn't for many millions of years after this that plants evolved. Certainly, the switch from using other gases to using CO2 didn't occur rapidly. As with all events in evolution, it probably took a few tens of thousands of years.

It's also important to realise that these early cells were prokaryotes (ie no nucleus or internal compartmentation). Plants are eukaryotes. It is commonly accepted that eukaryotes came much later.

In fact, a common feature of most eukaryotes is the presence of mitochondria, which have their own genome. This genome has strong sequence and gene similarities to bacterial DNA (in particular, the rickettsias, which are intracellular parasites). It is theorised that mitochondria arose when a bacterial cell entered into a symbiotic relationship with another cell.

Sorry to go on so much

MSC

For a civilisation to arise you need intelligence to appear. But once again it's a definition question. Termites live in ordered colonies and adapt the environment to suit them (building nests) but as they're unintelligent we don't call that a civilisation. Dolphins arguably are intelligent, can communicate with each other in quite a complex way, have a rudimentary society and culture. But it's a non-instrumental culture and so we don't consider that a civilisation either. I think we face a real barrier in the shape of our own preconceptions as to what forms Life and Intelligence and Civilisation can actually take.