I’m off this week at a workshop for an open source project that pays the bills.
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.
Ah, yes, the wise hand of government, regulating us to improve our lives.
Or perhaps just using known bad data to achieve political ends via putatively neutral regulation. There’s a persistent claim that evil private corporations killed off various public transports which is highly disputed, but we don’t have to look far to find well documented examples like this. Yet the people who complain about this as a private action don’t seem to mind if it’s done by the state for basically the same reasons.
Or government agencies willfully violating their own regulations and then claiming those same regulations prevent punishing anyone for the previous violation. That is, the regulations only protect the violators, not the innocent.
Then there’s the crack regulation enforcement teams that undertake complex and dangerous missions, to generate respect for themselves and the regulations they enforce. Because they’re the only people who can be trusted to obey regulations about dangerous items and protect endangered wildlife with uniform regulations.
I have been having some fun in various places using the slogans of the TranZis against them. They never like that, their world view is strongly asymmetric and for this reason incompatible with actual principles. For that reason I find this a brilliant twist, especially after it came up in Texas. Just use the old “if it saves just one life, isn’t it worth it?” question with regard to regulating abortion clinics and see how your local tranzis react.