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> Swarms for this application seem inefficient to me because they duplicate so much of the mass-- mass that could be used not as single basket but as adequate redundancy in a single craft.

They're using swarms because we can't accelerate an object that weighs more than a couple grams to relativistic speed with realistic technology. Therefore we have to use a small craft. And since that small craft can't do everything, we send a bunch.

> How does any object as small as what they're proposing at the extreme head or tail of the string whose only energy source is a laser several light years away lower (or increase) its velocity enough to reposition itself several hundred astronomical units to form a lens 100,000 km across and then increase (or decrease) its velocity in order maintain formation?

That's not at all what they're proposing. Each craft would have its own energy source. There is also no string. The "string" and "mesh" here refer to geometry, not to actual real objects.

edit: as to how they come together, that's the previous sentence from your quote:

> Initial boost is modulated so the tail of the string catches up with the head (“time on target”). Exploiting drag imparted by the interstellar medium (“velocity on target”) over the 20-year cruise keeps the group together once assembled.

answering more:

> Is the lens pointed at the target, for imaging, or pointed at earth for communications?

The coms lens has nothing to do with the lens shape of the probe mesh or with the instruments used to collect data. You use two different things for taking images and sending them.

> If the former how does it achieve the gain needed to send a signal to earth?

From the article:

> .. periodically building up a single short but extremely bright contemporaneous laser pulse from all of them. Operational coherence means each probe sends the same data but adjusts its emission time according to its relative position, such that all pulses arrive simultaneously at the receiving arrays on Earth.

> What is the energy density of a 100GW laser beam that has diverged to 100,000km at a ±50k km radial distance because I assume that the swarm components at the edges of the lens will need power the same as those at the center?

Irrelevant since that's not the power source.

>> Initial boost is modulated so the tail of the string catches up with the head (“time on target”). Exploiting drag imparted by the interstellar medium (“velocity on target”) over the 20-year cruise keeps the group together once assembled.

That is the actual, literal, impossible part. If the head is launched at speed x and the tail at speed y when they catch up they cannot stay assembled, unless "drag" is code for "magic".

>Irrelevant since that's not the power source.

What is the power source for the device, which weighs "GRAMS"? 1 gram is several dozen grains of rice. So what is the power source, expected to last decades, power data acquisition, processing, and transmission, and inter-swarm communications and station-keeping that weighs several dozen grains of rice?

More quantum-quantum-quantum antimatter nonsense?

I was trying to be generous by assuming energy harvesting, and not delving into fantasy.

The lens direction is VERY RELEVANT because the idea of getting signals back to earth is a broad array of devices all signaling simultaneously so the lens would ideally be perpendicular to earth. For example if the lens was pointed at the target it would present a smaller profile to observers on earth (maybe even a thin line) which would be more difficult to detect than a 100k km wide circle. The same rules apply to observations: any synthetic apertures created would be useless unless grossly pointed in the general direction of the target.

> That is the actual, literal, impossible part. If the head is launched at speed x and the tail at speed y when they catch up they cannot stay assembled, unless "drag" is code for "magic".

Drag is just drag. The craft are all launched at roughly the same speed and will start slowing down due to drag. By changing orientation they can control the speed/direction in which they slow down. This is a small effect, because the drag imparted by gases in interstellar space is minimal, but over 20 years at .2c it seems like it should work.

If there's a problem with this plan it's more likely to be that the encounters with the interstellar medium is more energetic than we expect and the craft either slow down too much or are destroyed by gases constantly impacting at .2c for years. But assuming the craft have enough shielding, the idea of using the drag to maneuver should work fine.

> By changing orientation they can control the speed/direction in which they slow down.

I apologize for not communicating clearly but that is the impossible part.

It will only work if the interstellar wind is, to borrow nautical terms, "in irons" or "running" (in line with either from ahead or behind the direction of travel) and that is impossible to either know, predict, or assume. From all other directions there are lateral forces that are impossible to overcome.

For example, if you are in a sailboat following another sailboat in calm waters with consistent wind and the lead sailboat slows down or the trailing puts out more sail to speed up to narrow a gap, one of the two will fall out of the line of travel due to lateral forces and will be forced to apply rudder to compensate. These things have no rudders.

The same thing happens to airplanes. If they increase or decrease drag either altitude or speed (or both) changes and control inputs are needed maintain position.

There is no ocean of water or air in space in which to steer.

I suppose if we launch and preposition several hundred billion space weather stations along the route in advance, we will understand the forces involved and be able to set the swarm components off on the trajectory needed so that the drag plan will work.

I think the analogy would be more like craft falling through the atmosphere. I found one NASA article that says interstellar wind speed is 26 km/s, which is four orders of magnitude smaller than .2c. Practically speaking I think you can model it as a constant .2c headwind.

> will be forced to apply rudder to compensate. These things have no rudders.

These things are pretty hypothetical, but I think the concept pretty clearly requires them to have something resembling a rudder since that's their only realistic means of attitude control.

Or, essentially they would be like people in wingsuits falling.

I don't think you need to worry about winds when travelling at 0.2c...

and there's plenty of stuff in space - nebulas are just when stuff is dense enough to see visually, but hydrogen lines reveal molecules everywhere

I'd like to see a stellaser. Asimov wrote about them, but we'd just need to get one close enough to the Sun and have unlimited power to the darker parts of space

Could be a cover project for something classified, such as a ground based laser paired with an orbital mirror on lets say an X-37 or something.