Nothing, so long as they don't care if developers ignore their schizophrenic platform.

I believed that for a long time, but actually if you get into the mathematics of Relativity it turns out that with a little cleverness virtually *any* FTL mechanism will allow you to travel back into your own past. I don't really understand the mechanism well enough to explain it, and can't find the nice almost-comprehensible explanation that finally convinced me, but it's the reason that many scientists assume that FTL is inherently impossible. In current physics the adage is: Relativity. FTL. Linear causality. Pick any two. All three can't coexist.

That does sound very weird, and way too full of plot holes for my tastes. I mean I could see dino-sapiens escaping to Mars if it was postulated that it had been life-supporting at the time, but tiny lifeless Phobos? As long as you're going someplace that will need a completely artificial environment why not just go underground on Earth? And the aliens sound downright sadistic, murdering entire planets to study the deep-frozen remains that survive an interstellar journey? Or did they include an artificial sun as well? Either way the tectonic stresses of Earth-based rockets would likely be devastating.

Yeah, what can I say? I like my SF to be at least plausible.

Hmm, hadn't thought of that, and Google isn't turning up anything useful on the equations governing the diffraction limits of a gravitational lens as wavelength, eyepiece, and target distance vary - if anyone could point me in the right direction for those it would be much appreciated. I did find the results of some other people's calculations though:

That an eyepiece at 550AU could theoretically resolve 4m details from a target 10 lightyears away
That a double-ended gravitational bridge radio-link to Alpha Centauri would need only ~100mW transmission power to virtually eliminate transmission errors.
And that a gravitational bridge to a sunlike star in the galactic bulge (~27,000ly away) would only need a transmission power of ~1kW

Negative energy as I heard it - which is stuff we've actually seen some limited evidence of.

The transmission power necessary to stand out against the broad-spectrum radio noise generated by our sun would likely be truly staggering. There is some question as to whether even our most powerful military radar signals could realistically be detected from as close as Alpha Centauri. Not to mention the small but real danger in announcing our existence to a completely unknown alien species. Why not build a gravitational telescope first and peek in their windows? We could at least see what they were like 500 years ago before deciding to try to chat. And if we do decide to chat then that gravitational lens works both ways - a relatively weak signal focused by a lens billions of miles across could be heard much more easily.

>Too much, it vaporizes (Venus)

No, as you increase the pressure the boiling point also increases. The reason water only exists as a vapor on Venus is not because of the air pressure of 90atm at the surface - that's equivalent to being only ~900m underwater, while the average ocean depth is about 4300m.

Instead it's because Venus is insanely hot thanks to the greenhouse effect of that thick CO2 atmosphere, which makes it far hotter than Mercury despite being much further from the sun.

Why? We're not remotely ready to realistically send probes to another star, and unless we plan to actually land the probe on another planet, gravitational-lens telescopes would get us all the same information far faster and cheaper, while also being retargettable to other angularly-close objects of interest. And they don't much care how far away the target is, it just changes the focal distance a bit.

True, but if there were life there then we could peek in their windows easily enough - it's only lack of sufficiently interesting targets that keeps us from building a gravitational telescopes around the sun. And if they were sufficiently technologically advanced we could potentially communicate. The lag would be horrible, but cultural and/or technological exchange could be mutually beneficial.

Nah man, chuck the probe in the opposite direction. It only needs to get 0.011 light years away from the sun to act as the eyepiece of a gravitational telescope focused on the star. Maybe not quite as exciting as sending a probe to another star, but we'll be able to count the pebbles on the planet's surface within a only a few decades.

The sun will last close enough to forever for pretty much any purpose we might have. After expanding into a red giant it is expected to collapse into a white dwarf, which will survive long past the point where the expansion of the universe will have driven all other galaxies beyond the boundary of the observable universe. If we are still around and have the technology to move a few billion people between stars in anything like a timely fashion, then it will probably be even easier to simply adjust the Earth's orbit to compensate for the changing solar output. A giant ion-drive on the moon could easily tug the Earth around without significantly affecting the planet itself. For that matter we could even put a bunch of natural-spectrum lights on the near face and take the whole planet with us to a new star. Whats a few million years transit time when you're taking your whole planet with you? While we're at it maybe we could grab Mars, Venus, and all the other small planets in the system as well, and cross the cosmos as a swarm of terraformed rogue planets.

But us still being around is a pretty serious question-mark. Consider, estimates are that the sun won't begin its red-giant phase for another 5 billion years, that's about 25,000x longer than our species has existed in anything resembling it's current state, and 10x longer than it's been since the Cambrian explosion of multicellular life.

Well, if life-bearing worlds are rare enough to be interesting then if our planet was noticed there's a fair chance that aliens would have at least put a gravitational-lens telescope around their star focused on us, if only as a study in xenobiology. In which case they could quite possibly have been counting our ancestor's lice hundreds of thousands of years ago.

>The sooner we launch one, the sooner our descendants get to hear back from it.

Not necessarily. Or more precisely by the time they hear back from it the information will likely be completely redundant.

At present all our mature propulsion technology is very much focused on planetary usage. Rocketry is the only one at all suitable to operating in space, and it's *horribly* inefficient in terms of specific impulse, which will be *the* deciding factor for interstellar travel. Ion drives show immense promise, already completely trouncing chemical rocketry in terms of specific impulse, but it's a technology very much in its infancy and the absolute thrust current engines can produce is miniscule, useful for little more than station-keeping and lining up gravitational slingshot maneuvers. If we launched an interstellar probe with today's technology then it's quite likely that a second probe launched 50-100 years from now would be able to make several round trips before today's probe ever got anywhere close to the target. For a mission whose expected payoff is centuries away that sort of thing is well worth considering. Much like Voyager making its pokey way out of the solar system, the value of an interstellar probe built on current-gen technology would be primarily in learning about the beginning of the path, not the destination. And unless there's some completely unexpected navigation hazard in the gulf between stars there's unlikely to be much to learn worth the cost of the probe.

Now what might be an interesting mission with current or near-term technology is a gravitational-lens telescope - rather than sending a probe towards Kepler-186f we send a telescope "eyepiece" in the opposite direction, and when it reaches a distance of only about 700AU (0.011 light years, ~10x Voyager 1's current distance) away from the sun we could start to use the sun's gravitational field as an immense lens in a telescope so powerful we could count the pebbles on 186f's hypothetical beaches. Maybe even individual grains of sand. Not to mention everything else we might see in that general direction. The downside to such a telescope is that it's extremely difficult to substantially change the target. With a telescope 700AU long even a few degrees of change requires moving your eyepiece across a distance rivaling Pluto's orbit. Still, with a clever flight plan we could get immensely detail information about dozens or hundreds of other star systems as our eyepiece slowly swept out a few degrees of motion. The only real question is, is 186f really interesting enough to be the first target? I would imagine looking toward the galactic core would offer far more interesting things to see.

Not at all. Einstein says nothing about FTL, accept that it's impossible to accelerate across the lightspeed barrier in normal space. There are however numerous ways in which we could conceivably "cheat" even without postulating any fundamentally new physics - from wormholes to Alcubierre warp drives. Of course if Einstein's theories are correct then any such cheating mechanism would inherently double as a time machine with rather serious implications to our concept of causality, but by this point we should all have accepted than "intuitive understanding by humans" is *not* a consideration for the laws of physics.

The maximum limit on an an arbitrary heat-conversion system is that doesn't break accepted theory is the Carnot-cycle heat engine, where eff 1 - T_cold / T_hot (as measured from absolute 0). But it's a rare real-world engine that gets anywhere near the Carnot efficiency limit - a car engine might run at 1100K for an ideal efficiency of around 73%, but the reality in most cars is closer to 25%. Being solid-state a thermoelectric device could potentially operate at very near the ideal (no mechanical losses), roughly tripling the efficiency. Assuming 90% efficient electric wheel motors the total system efficiency could be nearly as high.