Star travelling without leaving home
Posted by aogWednesday, 03 July 2013 at 10:11 TrackBack Ping URL

Via Instapundit we have an article about detecting stellar engines. If you don’t want to read the article, the basic idea is that you put a large mirror on side of your star, and this results in the star moving. The article doesn’t explain well that the mirror stays in place by balancing light / stellar wind pressure with stellar gravity.

The question is, which way does the star go? The article claims that, due to light pressure on the star from the mirror, it would move away from the mirror. I think the opposite, that it would move toward the mirror.

Consider this thought experiment — suppose we could just turn off part of the stellar surface so it emitted no light. Clearly then the sphere would move toward the dark side as light pressure from the bright side would not be balanced on the dark side1. The net thrust would be from bright to dark, moving in that direction.

As far as I can tell, the stellar mirror is effectively the same thing, it “turns off” part of the star’s photosphere in terms of photon emission. Therefore the star should move in the same direction.

What of the light pressure from the reflected light? Well, consider how the mirror stays in place — gravity. To make that work the system has to be tuned so that the gravitic attraction on the mirror from the star is the same as the photonic pressure from the stellar light. But if the star is attracting the mirror, the mirror is also attracting the star and it seems to me that this would balance out for precisely this reason. There is nothing else to hold the mirror in place. On the mirror side, then, photons leave the star (imparting an impulse away from the mirror), then bounce off the mirror (generating twice the impulse the other way), and finally return to the star (generating an impulse away from the mirror) for a net of zero because there must be force on the mirror generated by the star that is identical to the pressure from the reflected photons.

This is strongly related to the “if I use a fan to blow in the sail boat’s sail, do I go forward?” issue. The turbulence free world answer is “yes” for these reasons, but in real life the turbulence of all that air motion makes it infeasible. It’s easier to see if you use lasers and mirrors in space (no turbulence, no friction). Assuming a perfect mirror, it doesn’t matter if you point the laser out the back directly or point it forward and bounce it off the mirror. In either case the system moves away from the light emitting side.


1 Yes, if you shine a flash light, it pushed back a little bit on your hand just like a rocket. Some people have done more elaborate work on the idea.

Comments — Formatting by Textile
Bret Wednesday, 03 July 2013 at 16:15

I would guess away from the mirror (like the article claims).

The stuff (radiation & particles) that accelerates away from the sun is more or less evenly distributed on average so the sun doesn’t go anywhere in particular in the base case. Sticking something in the solar system such as a mirror doesn’t change that. What it does change is that some small amount of energy (and particles?) are reflected. When the reflected stuff (which is moving away from the mirror) hits the sun (or are absorbed by the sun) it pushes the sun away from the mirror.

Cool concept - it does seem like after billions of years you could accelerate the sun to a pretty impressive speed.

Annoying Old Guy Thursday, 04 July 2013 at 07:10

But if the star is pushed away from the mirror, the mirror is pushed away from the star. Suppose the mirror was physically attached to the start, 1 inch from the surface. What would happen in the case?

In terms of traveling, it does take a while, long enough you have to worry about the star’s lifetime. This would work best with dwarf stars.

Bret Thursday, 04 July 2013 at 16:16

But if the star is pushed away from the mirror, the mirror is pushed away from the star.

Yes, if it wasn’t, the mirror would fall into the sun and that would be the end of it. The propulsion is the net force between the active star, the passive but reflective mirror, and gravity.

Suppose the mirror was physically attached to the start, 1 inch from the surface.

If the 1 inch happened to be where the sun’s gravity and solar wind pressure cancelled out, then it clearly wouldn’t matter since there would be no forces on the attachments and it would still move away from the mirror. It may be different if there’re forces on the attachments - I’d have to think about it.

Bret Friday, 05 July 2013 at 05:29

I woke up in the middle of the night realizing that I’m wrong and your right (yeah, weird and nerdly dreams:-). And according to wikipedia:

Since the radiation pressure of the star would now be asymmetrical, i.e. more radiation is being emitted in one direction as compared to another, the ‘excess’ radiation pressure acts as net thrust, accelerating the star in the direction of the hovering statite.

At minimum, I had forgotten about the acceleration of the star towards the mirror which at first made me think the system wouldn’t go anywhere at all. Then looking more closely at the mirror, it looks like it’s not designed to reflect the solar wind back to the star, but rather to reflect it around the star, causing net thrust to be away from the mirror causing the system to move towards the mirror. That seems to match the wikipedia explanation, but given that explanation is not all the clear either, and my messing up the first time, I have no confidence in what I’m writing. I probably should’ve listened to the rocket guy. :-)

Annoying Old Guy Friday, 05 July 2013 at 09:43

Two thought experiments —

First, suppose the mirror were big, enough to cover 99% of the star’s surface. If the star moved away from the mirror, you’d have a photon drive where the exhaust pointed forward, which seems a bit wrong.

Second, suppose the star only emits 6 photons at a time, in the cardinal directions. If you sum the momentum of those photons, you get zero, as expected. Now put in a mirror that blocks exactly one of the photons. Now the vector sum of the momentum of the emitted photons (those not blocked by the mirror) points away from the mirror. But the star also accelerates in that direction, creating additional momentum in that direction. Net effect is that momentum is not conserved. Expand the number of photons emitted back to the actual values and I would think the same analysis works.

Annoying Old Guy Friday, 05 July 2013 at 12:52

Bret;

Sorry, I replied before I saw your reply.

There are two very interesting things to me on this subject.

First is that this requires no really new technology, just a bigger budget. We, our civilization, is effectively capable of building something like this. It ties in to the Fermi Paradox because if we can do it, why don’t we see others doing it? OTOH would we notice?

The other is the “deep time” aspect to these. We could, for instance, get to the Andromeda galaxy using Sol, but it would be almost used up by the time we arrived. The question is, who would arrive? That’s a time period longer than the Earth has existed. Even “short” journeys to nearby stars would take longer than any of our civilizations have lasted. I read science fiction stories that play with these time scales without any understanding of the passage of that much time could really mean. Even a couple of megayears would almost certainly mean the extinction of humanity and its possible replacement by other descendant (or not) species.

This is something I liked about The Time Ships — a group of humans gets stranded 60 megayears ago on Earth (via time travel) but the protagonist, manages to escape that exile and return to the present time. The civilization he finds is utterly alien and he is mystified until his companion points out that he, the protagonist, has at best no more relationship to these descendants than he does to whales.

Bret Friday, 05 July 2013 at 17:21

Well, I do hope they don’t start taxing me for journeys to Andromeda!

This whole concept (stellar engines, moving the whole solar system, megastructures, megatimeframes, etc.) is interesting and I’ve never thought about the possibilities before.

Annoying Old Guy Friday, 05 July 2013 at 17:54

You should read up on star lifting which is about advanced civilizations managing the mass of their star and even its lifetime.

Heh. It’s also funny to read fantasy books with ruling families that span dozens (or hundreds) of generations yet have identifiable family traits. After 10 or 20 generations the family descendants are no more related to the line founders than to any other random citizen. You can’t, among humans, maintain line purity over those timescales.

Jeff Guinn Saturday, 06 July 2013 at 18:43

OT, but not quite absolutely completely.

I fixed your link and removed your followups.

erp Sunday, 07 July 2013 at 08:58

It’s almost a good enough reason to want to stay alive to see what technology will bring us both in the near and far future.

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