Other civilisations in the Galaxy - are they really that likely?

I have always been fairly conservative (dismissive even) regarding the possibility of other "intelligent" life in the universe. This comes from gut feeling, aesthetics(!) and the Fermi paradox. Now it's time to get scientific with the issue, as there seems to be a good number of experts around here in the relevant fields. I'm looking at a sort of "informed" Drake Equation...By stipulating Galaxy, I'm just giving us a working number of star systems. I'm quite happy for this to branch out to the observable universe once we have a handle on the numbers.My opening comments...What's the earliest opportunity for a suitable planet to form?What conditions do we really need for abiogenesis?How long do we need to wait for abiogenesis? (~1Gyr in our case)How long for progress beyond prokaryote stage? (~2Gyr in our case)Now, where to put it? Origin of Life?

Good point. I never thought of that... Though the point remains in terms of how long we have to wait for abiogenesis to begin, assuming 1Gyr is a reasonable amount of time for bombardment to stop in a typical universe. So? It just leaves a little flexibility in the extrapolations

Hmmm, I'm often asked "don't you ever get stuck?" and I reply "apparently not!" Getting stuck while diving caves tends to be a little more concerning than getting stuck in dry caves... We (probably) need heavy elements for life, so we need to have seeded the galaxy with enough population II(III) star debris to have the building black for a) planets and b) life. In other words, life didn't start evolving elsewhere 10Gyr ago...To me, the big hurdle seems to be Prokaryote to Eukaryote. Once there, given a benign environment, I have no problem with evolution up to all our advanced lifeforms. But the "benign environment" is a question... We've had our fair share of scrapes, but we haven't had neighbourly stars going SN, and other such sterilising events.

Can I drag this back to life on Earth for a moment?I'm happy with abiogenesis. As Chirop pointed out, this pretty much happened as soon as it was possible (i.e. post heavy bombardment). And I'm happy for eukaryote to human (with a question over civilisation). But it's the 2Gyr from prokaryote to eukaryote that has always amazed me. 2Gyr is 1/7 of the age of the universe!

I believe that this is God's universe, He thought it up and made it. I believe that God is quite capable of creating a universe that brings forth life from non-life.

Always a safe bet

This is a great point. It makes the "mean-free-path" from eukaryote to civilisation much much longer. My question is then, what is the shortest this path can be? We've done it in 2Gyr... is that good?BTW, to all those saying we don't know enough to talk about any of this, one data point and all that... I think the discussion itself throws up all sorts of issues and ideas, some of which may be new and useful. We're not going to find the answer, but we may progress an extra 1% towards understanding the question.

This is related to my reply to Chirop. When could the first civilisations arise? I'm going to track down times for the first Pop I stars.

Back to the Drake eqn... for how long is any civilisation expected to survive?Ok, how long have we got if we do not colonise off-planet? At a max, it's red-giant phase, but I'm certain it is much much sooner than that.If we list all the civilisation endangering (and extinction) events with their probabilities, we should be able to get an "expected time before wipe-out".

I did thanks. I think it is a good example of why Occam's Razor is inapplicable in such situations (i.e. no communication implies no communicators). When you have very few facts, OR is not very helpful as it is difficult to define "more simple". I think this is equally true in trying to reason the non-existence (or existence in some cases) of God via OR.Back to your question of using grav waves - certainly possible except that the transmitters would have to be a little on the big side. A pair of neutron stars would serve for a small transmitter, but modulating galactic collisions would be preferable for large scale transmissions. Of course, you are still restricted to speed-of-light communication.

Intended pun?No, not really. Wormholes appear as solutions in GR. They just require effective negative energy densities to support the "throat". We see this kind of energy density in the Casimir effect and in your beloved dark energy So for a wormhole you just need GR and a bit of QM/QFT to supply the -ve energy. Of course, string theory is a good place to get both at once...

Absolutely, though I guess if we can solve that one, we won't be stopping to think of grav wave transmitters as we will have anti-grav, artificial grav, and inertial dampers to play with. We won't be transmitting... we'll be going... Spooky action is probably not the right route... it's a bit like all of the "faster than light" revelations that are always down to group velocity. It's just a naive misunderstanding.Other waves...well, we already use the best: e/m. We are just too spoilt with it that we don't appreciate it enough. It's a remarkable fact of the universe that the photon exists (an abelian gauge particle) because (and I keep saying this) without it there would be no sight... probably not much else either!

Oh, I see what you mean. Yes, something associated with this is certainly plausible. However... Don't forget that EM is not just "radio waves". It is laser, quantum interference, etc, etc. Photons are ubiquitous, long range, and they don't interfere with each other. Even with radio waves, it doesn't matter how much interference there is, the original signal photons are still there to be extracted given sufficient technology. When the photons are in the form of coherent light (lasers) then it is much easier. And "long range" quantum experiments almost all use photons.On the fermionic side, neutrinos suffer almost no interefence and so are very long range, but becasue of this they are also virtually impossible to detect. The usual recipe to catch a particular neutrino is to block its path with a planet sized lump of lead... you've then got a 50% chance of stopping it!Gravity waves are long range but there is a problem that they are non-linear so there is no superposition (in the classical sense). In other words, theoretically, two overlapping gravity waves cannot be separated into the two original signals (something we appallingly take for granted every second with our hearing, where we separate out God knows how many separate sounds from a single compression wave). Practically, gravity waves are so weak that they are approximately linear and so this should not be a problem.The other known massless and hence potentially long-range field, thestrong force, is useless because of colour charge confinement of the gluons.Now there are the theoretical scalar fields: the inflationary driver, dark energy, the string theory dilaton, etc. These are possibilities but manipulating these fields is pretty much in the same league as manipulating gravity. Good, but of course we do not need any super advanced technology to go to the stars. That is where the physics in your story goes astray (as it does in the majority of sci-fi). We can collonise the entire galaxy in our (the collonists) lifetime, simply taking advantage of time-dilation. Of course thousands of earth years will pass by, but if the whole point is to diversify mankind across the universe, then what is the problem? Far far better than sending boring robots, who will take just as long to do the job.

Of course. It's a simple case of risk management. On a sufficiently long time-scale, nowhere is safe. So diversification off-planet, extra-solar, extra-neighbourhood, extra-galactic, extra-cluster, and eventually we may just have to jump universe... or create our own