Motives for interstellar warfare

[andrewgpaul]

Of course, this is assuming technological parity, which is in all probably not going to happen with alien cultures, but a war would be open and shut if one outclasses the other anyway.

That review of The Killing Star appears to imply that that assumption is incorrect. It appears that the aliens arrive before humanity can do anything about intercepting those missiles. (mirrors?)

[Ariphaos]

The problem with the first strike genocide is you have to be sure you got them all before they can respond. I think this is much easier said than done - I also am of the opinion that the detection and interception of a relativistic missile isn't as hard as your quoted review implies. My reasoning is if it is moving that fast, the interstellar medium is going to be impacting it rather brutally, heating it up. Your telescopes now see a rapidly closing new heat source.

The local medium is somewhat rarefied, so the 'speed limit' the ISM imposes is rather higher for us right now than most of the galaxy. However, firing relativistic rockets in such a manner is stupid in the first place. Just build a Dyson swarm and point your star at them.

Of course, the defense against starstrafing is simple if you can do that - move your planet, and build your industry inside a jovian. For assaults covering more than a few Earth radii, it becomes economical to move terrestrial planets.

If the target has spread their industry and support around the star system, this becomes little more than a way to guarantee that you -really- piss your target off.

[Patrick Degan]

The problem with the first strike genocide is you have to be sure you got them all before they can respond. I think this is much easier said than done - I also am of the opinion that the detection and interception of a relativistic missile isn't as hard as your quoted review implies. My reasoning is if it is moving that fast, the interstellar medium is going to be impacting it rather brutally, heating it up. Your telescopes now see a rapidly closing new heat source.

Put the math together, know what is coming. Point your mirrors at it and vaporize it and at the same time, point your own missiles right back at them.

And by the time you've aimed your mirror or laser at one spot, the target has already moved past it. A relativistic projectile would be just about impossible to successfully intercept. The reaction time required simply is not feasible for any human or mechanical operator.

[Ariphaos]

And by the time you've aimed your mirror or laser at one spot, the target has already moved past it. A relativistic projectile would be just about impossible to successfully intercept. The reaction time required simply is not feasible for any human or mechanical operator.

What magical force prevents people from doing relativistic math in your Universe? Even at .999c (touching on the speed limit imposed by the ISM - if it's not a long, thin rod it will get deflected) that's ~8 hours per light-year. Dyson swarm around the sun will vaporize it nicely. At .999c, the light from the sun is blueshifted by a factor of 22 already - the sun will begin boiling it away even before it reaches Earth's atmosphere.

[Adrian Laguna]

With a single blow the surface of Earth is sterilized.

Then the million independent space habitats in various orbits shoot their own missile back at them, wiping out their planet as well.

You missed the part about follow-up ships, which come and start to clean-up survivors. Incidentally, I read an excerpt of the book where it describes the attack. They didn't just hit earth, they hit every single planet and moon in the system.

In any case, the people of the Killing Star probably launch their missiles as soon as they detect intelligence life, probably hoping they'll get there before said life has the ability to retaliate. Getting a bunch of projectiles to relativistic velocities and making sure they hit the target is no mean feat.

The problem with the first strike genocide is you have to be sure you got them all before they can respond. I think this is much easier said than done - I also am of the opinion that the detection and interception of a relativistic missile isn't as hard as your quoted review implies. My reasoning is if it is moving that fast, the interstellar medium is going to be impacting it rather brutally, heating it up. Your telescopes now see a rapidly closing new heat source.

Put the math together, know what is coming. Point your mirrors at it and vaporize it and at the same time, point your own missiles right back at them.

The book states the following:
"The most humbling feature of the relativistic bomb is that even if you happen to see it coming, its exact motion and position can never be determined; and given a technology even a hundred orders of magnitude above our own, you cannot hope to intercept one of these weapons."

[Patrick Degan]

And by the time you've aimed your mirror or laser at one spot, the target has already moved past it. A relativistic projectile would be just about impossible to successfully intercept. The reaction time required simply is not feasible for any human or mechanical operator.

What magical force prevents people from doing relativistic math in your Universe?

What magickal force allows people and machines in your universe to have reaction times able to cope with objects moving at high relativistic velocities, and attempting to implement firing solutions based on data which is hours old? And what assumes the aliens who would launch an RKV strike in the first place have not taken your other objections into account in the first place?

[Ariphaos]

The book states the following:
"The most humbling feature of the relativistic bomb is that even if you happen to see it coming, its exact motion and position can never be determined; and given a technology even a hundred orders of magnitude above our own, you cannot hope to intercept one of these weapons."

What does this tell us except that the author is rather shortsighted and thinks relativity is a bit of mystic voodoo that human mathematicians have yet to puzzle out?

[Ariphaos]

What magickal force allows people and machines in your universe to have reaction times able to cope with objects moving at high relativistic velocities, and attempting to implement firing solutions based on data which is hours old?

A small bit of magical mathematics called algebra, the specific equations of which are all of 110 years old.

If you have the technology to spot the point of origin (given by the author's claim, though not out of hand considering the ever-increasing power of sensor technology), you see an object approaching you at .999c from a distance of thirty light-years away, say (anything closer would have to be exceedingly quiet). You know that, when you see it, it's ten light-days away.

No matter what it's made out of, it's giving off a blueshifted spectrum. You know -exactly- how fast it's approaching. If you have the capacity to build a Dyson swarm defense/offense like I suggested, you know where it is. Sol's local ISM is only going to deflect it (or however many thousands are firing) by a few hundred kilometers at most.

So, you do the relevant basic equations (say, half an hour for triangulation data to come in), then send the command to the Swarm (eight minutes), and point your star in the attack's path.

~100 Yottawatts gets focussed on an area twice Earth's diameter, which, because of the projectile's velocity, is getting blueshifted by a factor of 22. They will quickly vaporize and slam into their own debris, propelled by sunshine and happiness, five light-days out from Earth. They also experience a not insignificant amount of momentum (in the petanewton range).

And what assumes the aliens who would launch an RKV strike in the first place have not taken your other objections into account in the first place?

...then they would skip the entire RKV idiocy in the first place, like I suggested, and point their star at us.

And hope we didn't think of it and possible defenses first (far more difficult than doing the above).

[Adrian Laguna]

Well, it should be noted that the first sign of intelligent life would probably be radio waves. It probably takes a very long time for a civilization to go from emitting radio waves into space to being able to do a relativistic bombardment. It is this long window that basically allows however gets there first to become top dog.

[Ariphaos]

Well, it should be noted that the first sign of intelligent life would probably be radio waves. It probably takes a very long time for a civilization to go from emitting radio waves into space to being able to do a relativistic bombardment. It is this long window that basically allows however gets there first to become top dog.

The thing is, the most plausible scenario where this is relevant is a schism between colonies.

[HRogge]

...then they would skip the entire RKV idiocy in the first place, like I suggested, and point their star at us.

Keeping the beam of light focused at the distance of tens of hundreds of lightyears might be "a little bit difficult".

[Patrick Degan]

What magickal force allows people and machines in your universe to have reaction times able to cope with objects moving at high relativistic velocities, and attempting to implement firing solutions based on data which is hours old?

A small bit of magical mathematics called algebra, the specific equations of which are all of 110 years old.

A cute but meaningless objection. Algebra doesn't make up for the need for updated and accurate information, nor does it erase the reaction-time problem or the lightspeed lag problem.

If you have the technology to spot the point of origin (given by the author's claim, though not out of hand considering the ever-increasing power of sensor technology), you see an object approaching you at .999c from a distance of thirty light-years away, say (anything closer would have to be exceedingly quiet). You know that, when you see it, it's ten light-days away.

Excuse me, but are you aware of how much time it takes to spot a single object in the sky in the first place, which assumes you're even looking for it to begin with?

No matter what it's made out of, it's giving off a blueshifted spectrum. You know -exactly- how fast it's approaching. If you have the capacity to build a Dyson swarm defense/offense like I suggested, you know where it is. Sol's local ISM is only going to deflect it (or however many thousands are firing) by a few hundred kilometers at most.

This is of course assuming that the attackers would conveniently wait the many decades/centuries until you've built your Dyson swarm network before launching a strike, which is not at all a given.

So, you do the relevant basic equations (say, half an hour for triangulation data to come in), then send the command to the Swarm (eight minutes), and point your star in the attack's path.

~100 Yottawatts gets focussed on an area twice Earth's diameter, which, because of the projectile's velocity, is getting blueshifted by a factor of 22. They will quickly vaporize and slam into their own debris, propelled by sunshine and happiness, five light-days out from Earth. They also experience a not insignificant amount of momentum (in the petanewton range).

Um, focus only occurs if you have a solid object to actually lock onto. Otherwise, that ~100 yottawatts is going to simply go off into nowhere. BTW, what about the problem of beam diffraction over distance?

And what assumes the aliens who would launch an RKV strike in the first place have not taken your other objections into account in the first place?

...then they would skip the entire RKV idiocy in the first place, like I suggested, and point their star at us.

A ~100 yottawatt beam maintaining coherence and peak energy level over distances of several light-years, Gracie?

[Ariphaos]

A cute but meaningless objection. Algebra doesn't make up for the need for updated and accurate information, nor does it erase the reaction-time problem or the lightspeed lag problem.

...are you even aware of what the Lorentz transformation is?

Excuse me, but are you aware of how much time it takes to spot a single object in the sky in the first place, which assumes you're even looking for it to begin with?

So, a culture targeting cold objects a nanoarcsecond wide, predicting their positions decades to centuries in the future, requiring analysis accurate to the zeptoarcsecond (you will miss with an error of one micrometer/s^2), is fine.

But heaven forbid a sensor net actually notices the YJ-scale burst that fires these things in the first place! Or the glow they give off against the background while plowing through the ISM.

This is of course assuming that the attackers would conveniently wait the many decades/centuries until you've built your Dyson swarm network before launching a strike, which is not at all a given.

I am of course arguing from technical parity, you need to build such a network to launch large masses to other stars at relativistic speeds anyway.

Um, focus only occurs if you have a solid object to actually lock onto. Otherwise, that ~100 yottawatts is going to simply go off into nowhere.

You're targeting a region you know the objects will be in. You know where they'll be, and when, when you fire.

Look, simple:

You see the mass drivers flare exactly 30 light-years away. The objects are tungsten rods which are glowing while they plow through the ISM, giving off a blueshifted spectrum of 22.46, or .999 of c. You know, right away, that they are exactly 10.95 light-days away when you make the observation, unless they are missing, in which case you don't care.

No reaction time whatsoever is involved, it's all prediction.

BTW, what about the problem of beam diffraction over distance?

To hit Earth from several light-years away, you have a target window a nanoarcsecond wide, and have to account for tracking errors down to the zeptoarcsecond range. I have trouble being convinced that dealing with beam diffraction in IPM over a few light-days is the bigger of the two issues, since you need to solve said diffraction anyway if you want to fire your RKV in the first place without antimatter (assuming you want them large enough to ignore the ISM, etc. etc).

A ~100 yottawatt beam maintaining coherence and peak energy level over distances of several light-years, Gracie?

300+.

Predicting the position of an object a nanoarcsecond wide down to the same scale several years into the future while being subject to multiple gravitational influences capable of throwing your aim off, Gracie?

Then firing an object at it through several AUs of pipe, which must remain perfectly aligned throughout the process, Gracie?

...by 'point their star at us' I'm not even talking about a laser, or even something that's actually targeting the planet, though it would be a good idea to aim at their Dyson swarm for a bit. You don't want something too accurate, you want to fry their entire star system. You'll be boiling oceans anyway.

[ArcturusMengsk]

Here's a possibility:

What if an alien race which did not have FTL travel were forced for some reason to abandon their home? Resources would be scarce, and they would not have the luxury to hop from one star to another. Provided their worldships/sleeper ships/whatever were able to mantain near-C speeds for years, it is plausible they'd be forced to invade the first habitable planet they came to for resources.

[Patrick Degan]

A cute but meaningless objection. Algebra doesn't make up for the need for updated and accurate information, nor does it erase the reaction-time problem or the lightspeed lag problem.

...are you even aware of what the Lorentz transformation is?

I am quite aware of the Lorentz Transformation. How does this erase the problems you keep ignoring?

So, a culture targeting cold objects a nanoarcsecond wide, predicting their positions decades to centuries in the future, requiring analysis accurate to the zeptoarcsecond (you will miss with an error of one micrometer/s^2), is fine.

But heaven forbid a sensor net actually notices the YJ-scale burst that fires these things in the first place! Or the glow they give off against the background while plowing through the ISM.

Why would it require a single YJ-scale burst to fire an object to relativistic velocities when steady acceleration over a period of months/years will do the same job of getting the projectile(s) up to speed and not produce that big burst you insist will stick out like a sore thumb against the cosmic background?

This is of course assuming that the attackers would conveniently wait the many decades/centuries until you've built your Dyson swarm network before launching a strike, which is not at all a given.

I am of course arguing from technical parity, you need to build such a network to launch large masses to other stars at relativistic speeds anyway.

Which means your aliens must be developing at the same pace as your own civilisation and that this parity must exist... how, exactly? How is this condition a given?

Um, focus only occurs if you have a solid object to actually lock onto. Otherwise, that ~100 yottawatts is going to simply go off into nowhere.

You're targeting a region you know the objects will be in. You know where they'll be, and when, when you fire.

Look, simple:

You see the mass drivers flare exactly 30 light-years away. The objects are tungsten rods which are glowing while they plow through the ISM, giving off a blueshifted spectrum of 22.46, or .999 of c. You know, right away, that they are exactly 10.95 light-days away when you make the observation, unless they are missing, in which case you don't care.

No reaction time whatsoever is involved, it's all prediction.

You continue to assume a single energetic burst must be the means by which the rods or whichever objects are being projected instead of a carrier moving at a steady acceleration to get its payload up to the required velocity. You further assume perfect observation, perfect targeting, perfect prediction, and of course perfect operation of every system required to focus a laser on the target zone.

BTW, what about the problem of beam diffraction over distance?

To hit Earth from several light-years away, you have a target window a nanoarcsecond wide, and have to account for tracking errors down to the zeptoarcsecond range. I have trouble being convinced that dealing with beam diffraction in IPM over a few light-days is the bigger of the two issues, since you need to solve said diffraction anyway if you want to fire your RKV in the first place without antimatter (assuming you want them large enough to ignore the ISM, etc. etc).

A ~100 yottawatt beam maintaining coherence and peak energy level over distances of several light-years, Gracie?

300+.

This paper suggests otherwise. An extract:

The propulsion system consists of a large, stationary laser of
extremely high power. In order to achieve the low divergence required,
the laser is focussed by a lens. Forward assumes a lens of 1000
kilometer diameter. This large diameter lens is constructed as a Fresnel
zone-plate, or "O'Meara para-lens"; that is, as a series of rings of
ultra-thin transparent plastic sheet, alternating with vacuum. The
transparent sheets are chosen to have a thickness exactly chosen such
that the delay of the wavefront at each element is one half of the light
wavelength. Since the lens structure is extremely flimsy, the structure,
although a third the diameter of the moon, has a mass of only 560,000
metric tons.
The large lens is required because of the fundamental divergence of
a light beam emitted from a finite aperture due to diffraction. The
minimum spot size which can be achieved, Dspot, at a distance d from the
lens, is:
Dspot = 2.44 d (lambda)/Dlens (1)
where (lambda) is the wavelength of the light used. This equation
defines the diameter of the first zero of the Airy diffraction pattern;
84% of the light is contained within this circle. Any units can be used
for d, (lambda), and Dlens as long as the same units are used
consistantly; it is convenient here to use meters.
The laser power is focussed onto a spacecraft consisting of a thin
reflective "sail" plus a small payload. By Einstein's relation, the
amount of momentum in a light beam of energy E is:
p = E/c (2)
By reflecting the incident light, a momentum is transferred to the
sail equal to twice the momentum of the incident photons (neglecting, in
this approximation, the relativistic correction due to redshift, small
the the speed considered). In conventional terms, this produces a force
6.7 newtons per gigawatt. While this is an extremely low force by the
standards of the rocket-based systems used for conventional spaceflight
systems, no reaction mass at all is expended. This makes the lightsail
system extremely attractive for interstellar missions, which would
otherwise require prohibitive mass-ratios to accomplish.
The reflective sail is assumed to consist of an ultra-thin foil of
aluminum, plus structural elements required to keep the sail roughly
flat. Solar sails, which operate by reflection of sunlight, have been
extensively considered in the literature. The sail analyzed by Forward
differs from most proposed solar sail designs in that most solar sail
designs typically assume that the reflective (aluminum) layer is a thin
coating on a plastic (e.g., Kapton) sheet. Forward assumes that the
plastic sheet can be elimiated, and that the aluminum alone is sufficient
to serve as the structure. Forward also assumes that the aluminum layer
is extremely thin, considerably thinner than that used by most solar sail
proposals, in order to minimize the mass required.
The probe analyzed by Forward reaches a top speed of 0.11 c,
somewhat lower than the speed analyzed by Mileikowsky. Parameters for
the baseline mission are shown in Table 1. While the reference also
considers other mission concepts which decelerate to a stop at the
destination, the simple fly-by probe analyzed does not. Travelling at
11% of the speed of light, the Forward probe requires 40 years before
arrival at Alpha Centauri. Including three years for acceleration and
four years for the information sent by the probe to return, results from
the Forward flyby probe can be expected 47 years after the start of the
mission.

Table 1
Reference laser-pushed lightsail flyby mission
Mission
Velocity 0.11 c
acceleration 0.36 m/sec^2 (thermally limited)
distance at laser cutoff 0.17 LY (1.6 10^12 km)
Laser
Laser wavelength 1000 nm
Lens diameter 1000 km
Laser power 65 GW
Laser pulse duration 3 years
Total laser power 1.7 10^12 kW-hr

Sail
diameter 3.6 km
area 10.2 km^2
material Al
thickness 16 nm
thermal emissivity e 0.06
temperature 600 K (2/3 of Tm)
material density (D) 2.7 gr/cm3
reflectivity 82%
Mass
Total mass 1000 kg
Areal density (total) 0.1 g/m^2
Sail 0.043 g/m^2
Structure 0.03 g/m^2
payload 0.027 g/m^2

And that's simply the numbers involved in the problem of getting a flimsy sailcraft up to .11c with a thousand kilometre focussing lens.

Predicting the position of an object a nanoarcsecond wide down to the same scale several years into the future while being subject to multiple gravitational influences capable of throwing your aim off, Gracie?

Then firing an object at it through several AUs of pipe, which must remain perfectly aligned throughout the process, Gracie?

Riiiight. Because tracking a planet orbiting a star following fixed trajectories through galactic space at sub-relativistic velocities will be the same problem as spotting a far smaller object moving at far greater velocity and which will not be slowing down until it hits something.

...by 'point their star at us' I'm not even talking about a laser, or even something that's actually targeting the planet, though it would be a good idea to aim at their Dyson swarm for a bit. You don't want something too accurate, you want to fry their entire star system. You'll be boiling oceans anyway.

Uh huh. Of course, you assume that you're having the entire luminosity of the sun at your disposal for the laser shot instead of the fractional amount you'll be able to actually harness through the orbiting lenses (which depends upon the actual number of said lenses deployed), which will never be at a number sufficient to collect that much energy in one go for one use, and the numbers of which would be delimited by how many you could feasibly place in orbit before the nested clusters would be of sufficient density in orbit to start interfering with one another, reducing collection efficiency.

[GrandMasterTerwynn]

Here's a possibility:

What if an alien race which did not have FTL travel were forced for some reason to abandon their home? Resources would be scarce, and they would not have the luxury to hop from one star to another. Provided their worldships/sleeper ships/whatever were able to mantain near-C speeds for years, it is plausible they'd be forced to invade the first habitable planet they came to for resources.

Still makes no sense. You'd have to rather severely fuck over the native ecosystems to give yours a fighting chance at survival. Not to mention that whatever resources you need, you can extract from comets or asteroids without having to pay the energy cost of boosting it into orbit from a large planetary body. Really, the first star with asteroids and comets would suit the needs of our hypothetical race of survivors . . . since if they've licked the life support issues that arise in the years and decades it would take for such interstellar travel, then they could easily orbital habitats, or convert their starships into orbital habitats, while slowly converting the rest of the new starsystem's asteroids and cometary bodies into a Dyson swarm.

[Ariphaos]

I am quite aware of the Lorentz Transformation. How does this erase the problems you keep ignoring?

You know the point of origin, you know its speed, you assume the target is Earth and other critical installations. Where it is becomes an issue of simple math - you're not reacting to anything, you have plenty of sunlight at your disposal.

Why would it require a single YJ-scale burst to fire an object to relativistic velocities when steady acceleration over a period of months/years will do the same job of getting the projectile(s) up to speed and not produce that big burst you insist will stick out like a sore thumb against the cosmic background?

Building something several light-months long? Oooookay. Keep in mind energy is also going to keeping its position stable (you are targeting down to the nanoarcsecond after all). If you want it -completely- hidden against the CMB, it's not an RKV anymore.

Something moving at a speed of .86 of c through our local ISM (extremely sparse - .1 atoms per cc, though we'll be moving into a much denser region in about 50kyears) plows through 258,000,000*2 (contraction factor) * 1,000,000 = 0.000000000000086 kg/sec, or 7,428 watts per square meter, or (very roughly) 800 Kelvin or so. Not bad, but you're also having this civilization do some pretty insane tracking...

For example, take Ixion at its aphelion. Its acceleration on Earth (and most of the central Solar System) is about .67 picometers per second per second. If the attacker is 30 light-years distant, Ixion alone will drag Earth two and a half kilometers off target. At 100, it's 29. If for some reason you've missed Neptune, you're going to miss. Even Mercury is significantly perturbing Earth's orbit at these sorts of time and accuracy scales, and I'm even ignoring time dilation here.

Which means your aliens must be developing at the same pace as your own civilisation and that this parity must exist... how, exactly? How is this condition a given?

See my post above - the most probably situation for this sort of event is for the civilizations in question to be rival colonies. Far, far more plausible than two sapient civilizations arising at anywhere near the same time within a few hundred light-years of eachother (beyond which firing RKVs quickly becomes a joke as you are firing from outside the local bubble no matter what in addition to individual moons throwing your aim off).

You continue to assume a single energetic burst must be the means by which the rods or whichever objects are being projected instead of a carrier moving at a steady acceleration to get its payload up to the required velocity. You further assume perfect observation, perfect targeting, perfect prediction, and of course perfect operation of every system required to focus a laser on the target zone.

Yeah well, so are you.

This paper suggests otherwise. An extract:

Dspot = 2.44 d (lambda)/Dlens (1)
where (lambda) is the wavelength of the light used. This equation
defines the diameter of the first zero of the Airy diffraction pattern;
84% of the light is contained within this circle. Any units can be used
for d, (lambda), and Dlens as long as the same units are used
consistantly; it is convenient here to use meters.

And that's simply the numbers involved in the problem of getting a flimsy sailcraft up to .11c with a thousand kilometre focussing lens.

DSpot = 2.44 *1E18 * 1E-6 / 1E6

Or 2,440 kilometers for a distance of 100 light-years, at a wavelength of a micrometer (rather large) and the same megameter lens. That's actually -way- too focused for what I was describing, and is why I mentioned simply using mirrors. You want the diffraction to cover much of the opposing star system.

Even if you do want to target like that, sunshine and happiness is still superior to your RKV.

Riiiight. Because tracking a planet orbiting a star following fixed trajectories through galactic space at sub-relativistic velocities will be the same problem as spotting a far smaller object moving at far greater velocity and which will not be slowing down until it hits something.

It is hitting something.

At these speeds, the ISM qualifies as -something-. At .86 c, it's 7,500 watts per square meter worth, in our local, extremely rarefied ISM. In fifty thousand years, we move into a denser cloud (expanding supernova shockwave), and that drags it past the temperature any known substance can survive at.

For most of the galaxy, it would be ~75,000 watts per square meter or so.

At the same time, even if you can't see it, preparing with a simple gravity tug defeats it anyway.

Uh huh. Of course, you assume that you're having the entire luminosity of the sun at your disposal for the laser shot instead of the fractional amount you'll be able to actually harness through the orbiting lenses (which depends upon the actual number of said lenses deployed), which will never be at a number sufficient to collect that much energy in one go for one use, and the numbers of which would be delimited by how many you could feasibly place in orbit before the nested clusters would be of sufficient density in orbit to start interfering with one another, reducing collection efficiency.

I said 300+, so 80% or so, not the entire luminosity. I wasn't even talking about lasers (I mentioned that specifically in my post, why did you ignore it?) because you -don't- want something that focused. Any enemy that suspects a focused attack can defeat it with a simple gravity tug or mass driver (dedicating a few dozen petawatts or so to the defense of Earth and Mars in such a manner).

The only defense against starstrafing is to build a particle shell.

[Ender]

A real life nebula is still not very dense - little more than vacuum still. They are lightyears across, so actually mining one isn't very plausible.

Depends on what you mine it for - Nebula are usually associated with young, high energy stars. In theory, you could try to mine antimatter from them, though it would likely be better to try it from gas giants with close orbits.

[Ariphaos]

You continue to assume a single energetic burst must be the means by which the rods or whichever objects are being projected instead of a carrier moving at a steady acceleration to get its payload up to the required velocity. You further assume perfect observation, perfect targeting, perfect prediction, and of course perfect operation of every system required to focus a laser on the target zone.

Yeah well, so are you.

I guess I'll qualify this bit of flippancy.

The author gave that the initial launch was known at the soonest possible moment. At that point, if you intend to protect your most valuable targets, you know its exact speed and probable target (or targets). If it's actually off course, no worries. If you protect too many targets, no worries. The result is simply an elementary school train wreck problem. If the train is off the rails, it's missing anyway. There is a very narrow window through which it has to pass in order to strike your planet.

Likewise, a number of much colder (compared to their distance from our Sun), and non-blueshifted objects need to be tracked, and have their mass measured, in order to properly track Earth's position decades into the future. Not just gravitational attraction, but at these scales time dilation will play a role as well (which falls of linearly with distance).

Some pretty impressive feats of measurement are given in both instances.

[Patrick Degan]

Xeriar, it really should not be necessary to point out just why your yottawatt laser idea is a non-starter to begin with. But then, it also should not be necessary to point out another flaw in your argument: your faith in targeting-by-prediction.

Sure, it's more than possible to do a Lorentz calculation of a relativistic object. That's not the problem, however. That comes from one of the conditions you yourself mentioned earlier in this thread concerning what happens when the .9c projectile is ploughing its way through the heliopause. Ablation will no doubt occur as well as braking and quite possibly deflection as well. Which means that both velocity and trajectory are now non-constant values, which junks the prediction set and therefore the targeting solution will have to be recalculated.

The next problem is that thousand kilometre wide focussing lenses massing a half-million metric tons can't exactly be turned on a dime. This is where the reaction-time problem comes in, because it will not be possible to do precision adjustments on the lens angles in the time you have left before the terminal strike occurs, and the closer in the projectile gets toward Earth, the more the interception window shrinks.

Sure, the sun pumps out 4E26 watts of power. Unfortunately, you're not going to have 100% of that output available for this task. Half your collectors will be out of orbital position on the opposite side of the sun at any given time, and you'll have at most the luminosity of the side you have available lenses to focus on the incoming object to work with. Then, there's that pesky Inverse Square Law to consider: intensity halves with the square of distance, which is further reducing the energy available for your lenses to collect and focus into a beam. Additionally, the total amount of power you'll be able to harvest from your collectors and lenses will be delimited by their surface area. This is absolute.

If you had five billion of the aforemetioned 1000km-wide focussing lenses available, you'd still be harvesting only .00008% of the sun's total output as usable power. Assuming that those are in the same orbital distance as the one mentioned in the extract from the laser sailcraft paper quoted earlier.

So much for the yottawatt laser. You'd have a better chance of defending against the incoming R-bomb by putting asteroids or clouds of debris in its path or actually moving the Earth itself out of position —a difficult bit of orbital engineering but possible in principle; though it depends upon how much time there is before hammer-fall.

[Ariphaos]

Sure, it's more than possible to do a Lorentz calculation of a relativistic object. That's not the problem, however. That comes from one of the conditions you yourself mentioned earlier in this thread concerning what happens when the .9c projectile is ploughing its way through the heliopause. Ablation will no doubt occur as well as braking and quite possibly deflection as well. Which means that both velocity and trajectory are now non-constant values, which junks the prediction set and therefore the targeting solution will have to be recalculated.

Same benefit. If parts get deflected, they aren't heading to Earth anymore, now are they? You are covering a focal region the size of the planet to begin with, with a very predictable path length.

Oh, sure, some unlucky satellite might get hit, and in many cases Earth's atmosphere is going to light up, but five light-days is well beyond the heliopause to begin with.

As for the actual deflection caused by the IPM, it's actually fairly negligible. I was hard pressed to come up with a figure of more than a hundred kilometers - unless your star system passes into a nebula, you make one, move your planet, or do something like point your star in their path, if they've got good prediction on your planet's future position, they won't miss.

The next problem is that thousand kilometre wide focussing lenses

Well, five light-days, so, 2.6E10 kilometers.

25,000 km (DSpot) = 2.44 * 2.6E10 * 1E-9 (micrometer, still large) / Dlens

Dlens = 2.44 * 2.6E10 * 1E-9 / 25,000

Dlens = 2.5 meters.

Defense rests. Literally. Especially considering the tiny angular adjustments you'd need to make if you wanted to make them -anyway-.

massing a half-million metric tons can't exactly be turned on a dime. This is where the reaction-time problem comes in, because it will not be possible to do precision adjustments on the lens angles in the time you have left before the terminal strike occurs, and the closer in the projectile gets toward Earth, the more the interception window shrinks.

Er hrm.

Are you under the impression that light pressure + terrestrial IPM will deflect the rods being fired, and if the attackers account for this, they'll hit Earth anyway? Is that why you're bringing this up?

If so, it's pretty baseless, like I said. I needed to cheat to come up with the 100 km IPM deflection.

Sure, the sun pumps out 4E26 watts of power. Unfortunately, you're not going to have 100% of that output available for this task. Half your collectors will be out of orbital position on the opposite side of the sun at any given time, and you'll have at most the luminosity of the side you have available lenses to focus on the incoming object to work with. Then, there's that pesky Inverse Square Law to consider: intensity halves with the square of distance, which is further reducing the energy available for your lenses to collect and focus into a beam. Additionally, the total amount of power you'll be able to harvest from your collectors and lenses will be delimited by their surface area. This is absolute.

Vaporization of any known substance would require something on the order of 15 MW per square meter. For a radius of 12,500 km, that's ~1E20 watts. Since this is occurring at an extreme angle, pumping that up by a few orders of magnitude is desired, though going beyond 1E23 does seem rather silly.

If you had five billion of the aforemetioned 1000km-wide focussing lenses available, you'd still be harvesting only .00008% of the sun's total output as usable power. Assuming that those are in the same orbital distance as the one mentioned in the extract from the laser sailcraft paper quoted earlier.

Well, for the Solar System, you want to put a Dyson Swarm in the Vulcan orbits. I calculated the numbers once and they were truly obscene, but ended up being "Scrape the top three kilometers off of Mercury" when you broke it down into raw materials needed (I'll need to run the numbers again, it's been awhile).

So much for the yottawatt laser. You'd have a better chance of defending against the incoming R-bomb by putting asteroids or clouds of debris in its path or actually moving the Earth itself out of position —a difficult bit of orbital engineering but possible in principle; though it depends upon how much time there is before hammer-fall.

Moving Earth is the most feasible defense for such targeted attacks, as I already mentioned. Building particle screens is really only feasible for systems that you place near insane value on - IE you're actually trucking matter back from Tau Ceti to build it.

Besides, you need this same sort of power generation capability to fire your RKVs in the first place.

Beyond and because of all that, sunshine and happiness > your RKVs.

[Patrick Degan]

Sure, it's more than possible to do a Lorentz calculation of a relativistic object. That's not the problem, however. That comes from one of the conditions you yourself mentioned earlier in this thread concerning what happens when the .9c projectile is ploughing its way through the heliopause. Ablation will no doubt occur as well as braking and quite possibly deflection as well. Which means that both velocity and trajectory are now non-constant values, which junks the prediction set and therefore the targeting solution will have to be recalculated.

Same benefit. If parts get deflected, they aren't heading to Earth anymore, now are they? You are covering a focal region the size of the planet to begin with, with a very predictable path length.

Oh, sure, some unlucky satellite might get hit, and in many cases Earth's atmosphere is going to light up, but five light-days is well beyond the heliopause to begin with.

As for the actual deflection caused by the IPM, it's actually fairly negligible. I was hard pressed to come up with a figure of more than a hundred kilometers - unless your star system passes into a nebula, you make one, move your planet, or do something like point your star in their path, if they've got good prediction on your planet's future position, they won't miss.

Uh huh. Nevermind that the beam won't have a dwell on the target for long enough to actually effect it. Nor can the beam be shifted to track the target with such a large focussing lens being utilised for it.

The next problem is that thousand kilometre wide focussing lenses

Well, five light-days, so, 2.6E10 kilometers.

25,000 km (DSpot) = 2.44 * 2.6E10 * 1E-9 (micrometer, still large) / Dlens

Dlens = 2.44 * 2.6E10 * 1E-9 / 25,000

Dlens = 2.5 meters.

Defense rests. Literally. Especially considering the tiny angular adjustments you'd need to make if you wanted to make them -anyway-.

Still don't get the essential problem, do you? At a velocity of .99c, the target would cross that dwell spot (which we'll use for purposes of argument here) in about eight-tenths of a second. It won't contact the target long enough to do anything to it and you can't adjust the lens afterward given how long it would take to shift such a massive object by even a degree.

massing a half-million metric tons can't exactly be turned on a dime. This is where the reaction-time problem comes in, because it will not be possible to do precision adjustments on the lens angles in the time you have left before the terminal strike occurs, and the closer in the projectile gets toward Earth, the more the interception window shrinks.

Er hrm.

Are you under the impression that light pressure + terrestrial IPM will deflect the rods being fired, and if the attackers account for this, they'll hit Earth anyway? Is that why you're bringing this up?

If so, it's pretty baseless, like I said.

I'm not responsible for your fantasies.

I needed to cheat to come up with the 100 km IPM deflection.

Then your evaluation is fairly well useless given its basis on wholly arbitrary numbers you've plucked out of thin air.

Sure, the sun pumps out 4E26 watts of power. Unfortunately, you're not going to have 100% of that output available for this task. Half your collectors will be out of orbital position on the opposite side of the sun at any given time, and you'll have at most the luminosity of the side you have available lenses to focus on the incoming object to work with. Then, there's that pesky Inverse Square Law to consider: intensity halves with the square of distance, which is further reducing the energy available for your lenses to collect and focus into a beam. Additionally, the total amount of power you'll be able to harvest from your collectors and lenses will be delimited by their surface area. This is absolute.

Vaporization of any known substance would require something on the order of 15 MW per square meter. For a radius of 12,500 km, that's ~1E20 watts. Since this is occurring at an extreme angle, pumping that up by a few orders of magnitude is desired, though going beyond 1E23 does seem rather silly.

It also requires sufficient contact time for that process to actually occur. But then this is among the many aspects of this problem you are deliberately choosing to ignore. Just as you are pointedly ignoring the figures provided by the aforementioned laser sailcraft paper describing the 1000 km focussing lens which do not support your conclusions.

If you had five billion of the aforemetioned 1000km-wide focussing lenses available, you'd still be harvesting only .00008% of the sun's total output as usable power. Assuming that those are in the same orbital distance as the one mentioned in the extract from the laser sailcraft paper quoted earlier.

Well, for the Solar System, you want to put a Dyson Swarm in the Vulcan orbits. I calculated the numbers once and they were truly obscene, but ended up being "Scrape the top three kilometers off of Mercury" when you broke it down into raw materials needed (I'll need to run the numbers again, it's been awhile).

Are you seriously under the impression that a Dyson Swarm is nothing but focussing lenses? Please say that's not what you're really suggesting here.

And, um, if you mean an inter-Mercurial orbit, you have to realise that this would not be an ideal place for anything you actually want to stay in long term solar orbit.

So much for the yottawatt laser. You'd have a better chance of defending against the incoming R-bomb by putting asteroids or clouds of debris in its path or actually moving the Earth itself out of position —a difficult bit of orbital engineering but possible in principle; though it depends upon how much time there is before hammer-fall.

Moving Earth is the most feasible defense for such targeted attacks, as I already mentioned. Building particle screens is really only feasible for systems that you place near insane value on - IE you're actually trucking matter back from Tau Ceti to build it.

Besides, you need this same sort of power generation capability to fire your RKVs in the first place.

Or you use antimatter rockets on steady acceleration. Doesn't matter if it takes a while to get up to speed.

Beyond and because of all that, sunshine and happiness > your RKVs.

As you wish...

[Darth Smiley]

Topic drift much?

The point is WHY two spacefaring civilizations might fight each other. We've already agreed that 'kill them before they kill us' is a plausible motive. And barring excessive amounts of handwavium, it is easy to kill a planet from a freaking long way away. Killing dipersed habitats is another issue, but...I digress. The exact mechanics aren't neccessary.

For plausible motives, what about political gain on the originating planet? I could definitely see a situation where a political faction trumps up 'the alien threat' to stay in power (or raise taxes).

[Ariphaos]

Uh huh. Nevermind that the beam won't have a dwell on the target for long enough to actually effect it. Nor can the beam be shifted to track the target with such a large focussing lens being utilised for it.

2.5 meters is large? Especially as slowly as it has to track in the beginning.

Incident angle is 180 gm x 129,600 gm so it's about 36 million kilometers long, while covering the affected area.

Still don't get the essential problem, do you? At a velocity of .99c, the target would cross that dwell spot (which we'll use for purposes of argument here) in about eight-tenths of a second. It won't contact the target long enough to do anything to it and you can't adjust the lens afterward given how long it would take to shift such a massive object by even a degree.

How is something 2.5 meters wide massive? How is anything going to cross 36 gigameters in .8 seconds?

Even beyond that, at that range (5 light days), how would turning even a megameter lens be a problem? -Earth- turns orders of magnitude faster than a lens four hundred thousand times beyond the needed size would have to.

I'm not responsible for your fantasies.

But apparently, I'm responsible for doing your math, and instead of showing any of your own (beyond utterly pointless and obvious stuff like 'it would take a lot of orbiters to make a Swarm!), or even acknowledging mine, you spew lines like this.

Then your evaluation is fairly well useless given its basis on wholly arbitrary numbers you've plucked out of thin air.

Are you offering to show the relevant math, or you going to continue this hypocrisy, thus forcing me to do it for you again.

It also requires sufficient contact time for that process to actually occur. But then this is among the many aspects of this problem you are deliberately choosing to ignore. Just as you are pointedly ignoring the figures provided by the aforementioned laser sailcraft paper describing the 1000 km focussing lens which do not support your conclusions.

1: I disproved the need for a 1,000 km focusing lens several posts ago, using the same equation you provided me. Which you have repeatedly ignored, yet somehow are getting on a horse and complaining about me ignoring something.
2: Pre-trigonometric math is enough to prove you wrong on the length of the targeting window. I thought it was too obvious to need pointing out.

I have endeavored to answer every last point of yours. If I have missed something, point it out to me, and I will apologize. You, however, have missed a very important refutation, and have done so for multiple posts now, even after I've pointed it out multiple times.

Are you seriously under the impression that a Dyson Swarm is nothing but focussing lenses? Please say that's not what you're really suggesting here.

Nope, a silicate collection array, most likely, but I'm seriously beginning to worry about your grasp on math.

And, um, if you mean an inter-Mercurial orbit, you have to realise that this would not be an ideal place for anything you actually want to stay in long term solar orbit.

Vulcan orbits are between .08 AU and .21 AU, roughly, and stable enough for a Dyson Swarm's purposes (losing some is inevitable, of course, but compared to the number you need to make in stable outer shells, why build it anywhere else?

Or you use antimatter rockets on steady acceleration. Doesn't matter if it takes a while to get up to speed.

...you believe that building an antimatter rocket, and hiding its gamma ray output is more feasible than harnessing the power of your parent star?

...and keep the entire apparatus trained on a target a nanoarcsecond wide while doing so, given the amount of AM you are using.

As you wish...

Apparently, you didn't quite get the math last time.

Was there a problem with me showing that a 1,000 km lens could reasonably focus on a planet out to a hundred light-years, using infrared wavelengths?

Was there a problem with me showing the defensive lens only needs to be 2.5 meters, using the same infrared wavelengths?

Was there a problem with my claim that light pressure and the Interplanetary medium would cause a negligible amount of course deflection? Do I have to write this out for you too, since you seem so incapable?

Is there a problem with me pointing out the angle of the defensive laser, which means that the focal beam is going to remain on target for ten minutes (on average), assuming it -doesn't move-? If the firing array is in front of Earth instead, it won't be off target until some time after the projectiles are.

[Ender]

Topic drift much?

The point is WHY two spacefaring civilizations might fight each other. We've already agreed that 'kill them before they kill us' is a plausible motive. And barring excessive amounts of handwavium, it is easy to kill a planet from a freaking long way away. Killing dipersed habitats is another issue, but...I digress. The exact mechanics aren't neccessary.

For plausible motives, what about political gain on the originating planet? I could definitely see a situation where a political faction trumps up 'the alien threat' to stay in power (or raise taxes).

Problem is time - political pressures and gains tend to be - relatively - short term. Whereas travel times to another star system are definitely not.