# Fermi Paradox or 'Where are they?'



## skeptical (Jul 30, 2009)

One of the great puzzles in thinking about alien civilisations is the simple question of 'Where are they?" This query is called Fermi's Paradox, after the great physicist, who first posed it. A very good detailed discussion of this problem is found at :

Fermi paradox - Wikipedia, the free encyclopedia

Lots of people regard the number of stars in the galaxy (about 250 billion) as a sure indication that intelligent and technological life must live elsewhere within the Milky Way. However, some relatively simple calculations show that an aggressive and advanced species could colonise the entire galaxy within 400,000 to 10 million years, depending on which assumptions you apply to the calculation, even if we operate on the basis that it is impossible to travel between stars faster than about one fifth of the speed of light.

Since about 10% of the galaxy is made up of stars at least 2 billion years older than our own, there has been ample time for such a species to evolve and expand throughout the galaxy. At some stage in such expansion, a visit to Earth appears most probable. Remember that the one number that makes astronomical numbers look small, is the number of potential beings that result from unrestrained population growth, meaning that an aggressive species could readily expand to the point of colonising and even overpopulating every star system in the galaxy, within the time constraints above.

Here on Earth, we have fossils of the most delicate kind, including fossil jellyfish 500 million years old. Why is it that we have never seen so much as the equivalent of a fossil alien coke bottle?

The SETI project has now covered more than two thirds of the galaxy on a 'once over lightly' basis, looking for alien radio signals, with no luck. Why not?

What is your favourite explanation of the reason aliens appear so elusive?


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## Drachir (Jul 30, 2009)

My favourite explanation is the same one I use for all religious or para-religious beliefs.  *There aren't any* (aliens, gods, or whatever you want to make up).


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## TheEndIsNigh (Jul 30, 2009)

Sweeping stuff Drachir.

I prefer the soap box explanation.

We all live in a carefully regulated 'island' where what we can observe and understand is carefully regulated by the viewers.

Those stars, they aren't real, they're just pinpoint lighting points.


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## Interference (Jul 30, 2009)

Some of those dinosaurs _were_ our first alien visitors.  Their own bad luck they beamed down just as the planet got hit by an asteroid.


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## chrispenycate (Jul 30, 2009)

This discussion has already been going on for some pages. I did consider merging it (just to prove that I could, mind) but decided just to link: http://www.sffchronicles.co.uk/forum/50436-where-is-everybody-fifty-solutions-to-the-fermi.html


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## TheEndIsNigh (Jul 30, 2009)

Aye, It's unusul for Chris to emerge sword in hand from Castle Greyskull.


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## chrispenycate (Jul 30, 2009)

TheEndIsNigh said:


> Aye, It's unusul for Chris to emerge sword in hand from Castle Greyskull.



Does that mean I should be more enthusiastic in my interventions, own up to being the local galactic observer or apply for the issue of battlecat, one, spam for the elimination of?


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## mosaix (Jul 30, 2009)

Drachir said:


> My favourite explanation is the same one I use for all religious or para-religious beliefs.  *There aren't any* (aliens, gods, or whatever you want to make up).



Nice one, Drachir. I think you're right about the gods, but I hope you're wrong about the aliens.


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## Nik (Jul 30, 2009)

My humble take is the only practicable 'there 'n' back' star-drive is the Bussard ram-jet. But, as the local interstellar medium is very thin compared to average, due a ruddy great bubble blown by a supernova, we're stuck in the galactic doldrums.

We could probably get *out* using star-wisp probes, Daedalus pulsed fusion etc...


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## skeptical (Jul 30, 2009)

Interstellar travel is theoretically possible, with a velocity of 0.1c (10% of light speed) potentially achievable even using techniques within reach today.
Interstellar travel - Wikipedia, the free encyclopedia

Sadly, in spite of Larry Niven's enthusiasm, a Bussard Ramjet will not do it.
I quote from the reference above:

_"the __Bussard ramjet__, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a __proton-proton fusion__ reaction, and expel it out of the back. Though later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design, the idea is attractive because, as the fuel would be collected en route, the craft could theoretically accelerate to near the speed of light."_

In other words, it works better as a brake than a throttle!


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## Drachir (Jul 30, 2009)

chrispenycate said:


> This discussion has already been going on for some pages. I did consider merging it (just to prove that I could, mind) but decided just to link: http://www.sffchronicles.co.uk/forum/50436-where-is-everybody-fifty-solutions-to-the-fermi.html



Interesting stuff.  It is always interesting to see logic applied to what has mostly been superstition.


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## TheEndIsNigh (Jul 30, 2009)

This 0.1C business.

Given the ship/craft will have to be of considerable mass and therefore have an incredible momentum (as in oil tankers taking an age to stop) how would something travelling that fast avoid a sun or even just a large lump of dark matter that floats into it's path.

I seem to recall even the space station could get torn to shreds by relatively small objects travelling at 25000MPH so is this form of high speed travel really possible?


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## Interference (Jul 30, 2009)

Drachir said:


> My favourite explanation is the same one I use for all religious or para-religious beliefs.  *There aren't any* (aliens, gods, or whatever you want to make up).



What about "Rolos left in the pack"?

My God!  You're right!  Your proposition works in every eventuality!!


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## skeptical (Jul 30, 2009)

To End

Yes, it is possible. Just not yet.

We have to bear in mind that the first interstellar journey by humans will not take place for 500 to 1000 years, perhaps. We can anticipate vastly improved technology by then. The big improvement, in the context of your question, is detecting obstacles. 

You are correct in saying that a vessel travelling at 0.1c will have enormous momentum, and will not turn or decelerate rapidly. In fact, my guess is that, to decelerate from 0.1 c to rest relative to its destination will take about 10 years. To turn sufficiently to avoid an obstacle in interstellar space will also take time. 

However, interstellar space is remarkably empty, and such obstacles are very rare - so much so that it is probable that, over a 5 light year journey, there is insignificant risk of striking anything as big as a sand grain. Even so, our hypothetical vessel will have highly advanced technology for detecting such sand grains. And no, I do not know what it will be, except that 500 plus years of technological development will bring magnificent new techniques.

Incidentally, assuming 10 years acceleration plus 10 years deceleration, plus cruising at 0.1 c, it will take 55 years to get to Alpha Centauri, the nearest new solar system. I would suspect that, by the time humanity does this journey, human lifespan will be massively extended, and 55 years will not seem such a long time.


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## TheEndIsNigh (Jul 30, 2009)

skeptical:

I tend to disagree on the emptiness of space. At those speeds even the occasional hydrogen molecule every ten miles or so will create incredible friction. Plus if the theories about dark matter (rubbish in my opinion but I'm prepared to be wrong) then space is full of dirty great chunks of invisible stuff. Let alone the remnants of planets, commets, super nova and Splogwandian space turds.

I suspect interstellar travel will be more waiting for a gap in the traffic than actual progress forward.


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## chrispenycate (Jul 31, 2009)

Stars are easy; you know where they are, you know where they are going to be for the next ten thousand years or so. Plan your course in advance not to fly through any of them. Or close to; stars have a tendency to have planetary systems, and, even though most of the space is full of vacuum (i.e. empty of worthwhile blobs of matter) it's a higher risk region than most of interstellar space. We know interstellar space contains a statistically insignificant percentage of matter because it isn't diffusing the light from stars the other side of it. But this doesn't mean zero, just less likelihood of hitting a free comet than being struck by lightning in Hendon, and Murphy's arm is long.

We can improve our odds. Once we are up to speed there is no real argument about building a kilometre wide telescope, capable of detecting anything big enough to do damage at several light weeks distance – plenty of space to decide whether to steer round it or laser zap it. Dark matter is indetectable but since at last hearing it can interpenetrate normal matter with no interaction we can ignore it totally. Which leaves us with dust clouds. 

Now, we know there isn't much in the way of interstellar dust in the region (same way as we know about comet density) but we really don't need much friction to heat everything up and slow everything down. We'll start with unmanned drones, of course, which will partially map out the region, but we might turn out to need the Bussard as a vacuum cleaner, in which case we'll definitely collect the reaction mass to throw away.

And really, I'm not expecting to hit more than a few kilograms of matter on the entire trip.


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## TheEndIsNigh (Jul 31, 2009)

You'll be taking plenty of mice with you then Chris. 

(Care of Douglas Adams)


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## skeptical (Jul 31, 2009)

End

You may be right, but on the basis of what science currently knows, it appears unlikely that, after leaving the solar system, you will encounter much.  Hydrogen molecules, even at 0.1 c are not an issue.   Grains of sand are.

On dark matter, chris is correct.   Take note.  Large objects such as comets will be so rare as to be essentially not worthy of worrying about.   However, running into a grain of sand, or even dust, at 0.1 c would be a disaster.

The biggest risk will be while still inside the solar system.  However, the velocities involved will also be way less, and in 500 years, we should be able to plot a clear course through that part of space.


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## Xelebes (Jul 31, 2009)

skeptical said:


> Interstellar travel is theoretically possible, with a velocity of 0.1c (10% of light speed) potentially achievable even using techniques within reach today.
> Interstellar travel - Wikipedia, the free encyclopedia
> 
> Sadly, in spite of Larry Niven's enthusiasm, a Bussard Ramjet will not do it.
> ...



Bussard's ramjet uses a parabolic receptacle, but makes no mention of a magnetic receptacle which could serve two purposes: 1) deflect/intake harmful radiation and particles for conversion and 2) reduce drag by having things not rub against each other.


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## skeptical (Jul 31, 2009)

xelebes

I kind of wondered if a Bussard ramjet field could be used to delelerate an interstellar craft.    If you could accelerate to 0.1c using something like a super-iondrive engine, and use a bussard ramscoop to do the decelerating, then you could cut the amount of reaction mass you had to carry quite dramatically??


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## TheEndIsNigh (Jul 31, 2009)

skeptcal:

You're not by any chance trying to have it both ways are you 

Not many hydrogen molecules for me to run into yet enough to slow the ship with the scoop


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## Urien (Jul 31, 2009)

We had a super-ion drive on our old Datsun when I was a kid. My dad dug it up from the potato patch. We found it next to the wax sealed Roman cylinders. 

Simultaneously we proved there was no Fermi paradox and (from the contents of the cylinders) that the Romans reached America.

The potatoes also made good chips.


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## skeptical (Jul 31, 2009)

To the End
Sure - lots of Hydrogen atoms to run into.  However, even at 0.1 c, their mass is so tiny as to make the impact negligible.   However, with a wide spanning ramscoop field, there may be useful braking???


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