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Comment by bobcostas55 | Hacker News Reader

>because it assumes that projecting power over interstellar distances is easier than defending yourself

This seems really obviously true to me. Accelerating a rock to relativistic speeds is pretty easy, defending against a rock potentially coming from anywhere in space traveling at relativistic speeds is extremely difficult.

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1) I think you're underestimating the difficulty of accelerating any massive object to relativistic speeds, 2) doing so with enough accuracy to hit a relatively small moving target many light-years away is not exactly trivial, 3) a civilization with the capability to accelerate a massive object to relativistic speeds is probably also capable of building self-sufficient artificial habitats around its star, which is a lot of targets you'd have to take out for complete obliteration.

So in summation, I think defending against an incoming relativistic rock is at least as easy as using relativistic rocks to obliterate a distant civilization.

another thing: I believe at relativistic speeds hitting any debris in your path would cause a pretty large BOOM, and for a multi-lightyear route that seems somewhat risky.

You're definitely not thinking creatively enough.

You don't need to hit the planet directly with anything, all you need to do is destabilize the orbit of all the planets or even just the Earth itself. You could send a gravity wave for example, that would cause the Earth's near circular orbit to shift to a very eliptical orbit. The vast change in temperatures would kill all life on the planet, and then you could come and mine all the resources that you needed and leave.

If you have the capability of engineering such a gravity wave then Earth's resources aren't worth your time.

Resources not the point here, the point is removing the possibility that our civ could ever be a risk to them

IANAAstrophysicist but it seems like targeting that rock gets proportionally hard with the speed and distance. Throwing a dart and hitting an ant on the other side of the world, etc.

And the other side of that equation is: does the species who can do that have anything to fear from us? Or does the same technology make them likely able to stay ahead on defense, making it uninteresting to destroy us?

Three Body Problem introduces all this dimension folding tech that makes being the first to strike totally dominate. But the closest equivalent on Earth hasn't (yet) led to nuclear holocaust, because preventing others from getting the tech and establishing mutual detente with those who do has been better for each actor.

We can already hit tiny (Ed: planet sized) targets at interstellar distances. Planets are relatively speaking quite large targets especially if you’re tossing several rocks.

This stuff doesn’t need sci-fi style new physics, just the kind of infrastructure we could actually build.

....we have never hit anything at interstellar distances, which are several orders of magnitude farther than interplanetary distances.

I didn’t say we have, just that we can.

The closest Star is ~268,770 AU where voyager 1 is only 152.6 AU. However, being able to target a probe within 50 feet at 150AU, means hitting a planet sized target at 4.25 light years. Building a probe large enough and fast enough to do significant damage at the other side is the hard part, not targeting.

Calculating where a planet sized object would be by the time our probe got there is a different story.

Voyager is not traveling a relativistic speeds. We could definitely defend against someone sending voyager at us.

That’s moving the goalposts. I said we could hit a planet at those distances not that we can create a relativistic impactor.

No it isn’t since the whole topic here is that any civilization capable of accelerating an object to relativistic speeds and hitting a target from light years away is capable of defending against one. We can send voyager to target an object on the other side of the solar system but we are also perfectly able to defend against a potential voyager.

Just because you now claim to not be addressing the actual goal at hand doesn’t mean I moved the goalpost; if anything you’re now implying you did.

Voyager 1 isn’t a weapon so defending against it is kind of a meaningless argument. The Parker Soar probe is aiming for 0.064% the speed of light and covered in scientific instruments and shielding. So that’s arguably a solid baseline of what we could do in terms of speed. Even at those speeds we are still talking 8,000 years to the closest planet making it a poor weapon.

Still, I doubt we could detect a barrage of incoming 0.0005c weapons designed for minimal levels of stealth in time to do anything about it.

What tech are you thinking of? Remember it's not just the distance but the speed. How would we course correct a relativistic object once the target was close enough to estimate the desired change?

Well, you do that with the same kind of engines that propels them at relativistic speed.

Anyway, it's not a target and forget situation, because relativistic ships get a (relatively) lot of drag in space.

> Well, you do that with the same kind of engines that propels them at relativistic speed.

Which don't exist except as fantasy, and even most of the fantasy ones wouldn't work for targeting adjustments.

Right, if you have the tech to allow you to course correct an object traveling at relativistic speeds you have the tech to defend against on by moving it out of the way.

Yep, that's probably correct. Even more because you can just throw something into the projectile too, and destroy it... And they can throw something into your projectile, and etc.

I don't think anybody can make any claim on what side is easier. But targeting the attack does not seem to be the hard part.

The belief is that detecting the attack in time to do anything makes defense more difficult. Your warning is limited by light speed, so detecting something moving .99c at 100 AU and you get just over 7 minutes to respond.

Worse, it doesn’t need to be some solid object. A loose field of debris 1,000 miles wide could sterilize a planet with enough mass and enough velocity.

Fortunately, it is possible to imagine other scenarios. For example; it would take relatively little energy to deflect such a missile and there is likely to be a lot of time to detect it's incoming presence assuming that some sort of planet-like mass is required to host all the technology to create a relativistic speed accelerating engine.

> Accelerating a rock to relativistic speeds is pretty easy

Huh? How? I haven't seen any proposal that doesn't start with the assumption that we can inject a rock with a preposterous amount of energy let alone hit (in astronomical scales) a ridiculously small target, not to mention navigate all the gravity wells in between.

Multistage rocket using ion engines. Remember this is over interstellar distances (>4 lightyears), so you have multiple years, if not decades or centuries to accellerate.

This does get back to the "Why didn't we RKKV it millions of years before intellegent life even existed, much less had a opportunity to hide, based on biosignatures in the atmosphere?", though.

Ok, that gets us to maybe 0.3% of the speed of light on something small, which isn't very interesting.

https://space.stackexchange.com/questions/22140/what-is-the-...

> defending against a rock potentially coming from anywhere in space traveling at relativistic speeds is extremely difficult.

Also, while 'proper' stealth is difficult, you can make the rock pretty close to invisible for practical purposes by coating it in vantablack and cooling it to liquid helium temperatures.

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