How can a space "radar" work?
The Question about the blockade of en entire star system makes me wonder:
How would, if possible, a space "radar" work? Or how would you scan for Ships further away than your Optical sensors can see.
The big space makes is semi impossible to look for the reflected radar rays. 0-100 Eyeball also does not work because of the hugeness of Space, same as all other optical installments.
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How about using gravity instead of radio waves? Gravity has detectable effects over great distances, and we currently have sensors that can detect and map fluctuations in gravity, so if you had a detector that was sensitive enough you could send out some kind of graviton pulse and then register where it reacted to other gravity fields and/or objects.
Even without a gravity pulse, all objects have gravity, so passive detection would be possible, especially for very large ships, or ships with artificial gravity.
A stealth ship would be one that is small, has radio/radar and light absorbing material, active cooling of the outside surface to space ambient (maybe dump the heat into an internal heat sink which means the stealth could only be used short term before the heat would need to be dumped), and no artificial gravity when in stealth mode.
So, active stealth would be cramped, hot, and weightless.
There is also a theory that the effects of gravity travel faster than the speed of light*, so you'd potentially be able to detect something faster than relativity would allow. A normal radar signal would take about 20 minutes to cross the distance between Mars and Earth.
TL;DR, it would be a lot like sonar in modern navel warfare. Big capitol ships and planet/moon/asteroid based installations actively pinging, smaller stealthy ships sticking to gravity shadows when possible and trying to remain undetected while listening for other stealthy ships...
Gravity Mapping: http://en.wikipedia.org/wiki/Gravity_of_Earth
Edit: I'd like to point out that the theory of a graviton particle, if it could be harnessed, might allow for controlled gravitational waves.
Another way to generate gravitational waves would be to generate a pair of very small black holes orbiting each other.
As the waves propagate outward detectors could look for ripples in spacetime where no object should be, or for a change in the size of the ripples caused by known objects. If an asteroid is suddenly causing a larger ripple than normal, then it would indicate that the mass had changed and a ship is probably hiding behind it.
* This is based on Newtonian physics, and doesn't hold up with modern science.
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