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Comment by unsupp0rted | Hacker News Reader

Could an object like 3I theoretically tell when eyes are being pointed at it?

Probably not ground-based eyes, but possibly Hubble.

i.e. "the inhabitants are scanning us"

If it had several kilometer wide imaging array, then yes. Hubble and other telescopes are totally passive optics. You need to see the distant dot and have enough resolution to identify it as a telescope and see that it is pointing at you. Seeing hubble from more than 1 au would require optics about 8km in size. This doesn't need to be one giant mirror, could be two cameras 8km apart, but synchronized optically (distance known to a small part of wavelength of light). Seeing Hubble as more than 1 pixel to tell that it's a telescope pointing at you needs something much larger.

I don't believe so, unless the object had such a powerful telescope it could image Hubble itself in high definition and literally see where it was pointing, which would require an absurdly large lens. A telescope like Hubble absorbing light is purely passive, it doesn't emit anything "back" that would tell the observed object it is being observed.

Extremely unlikely, given that there is no "active" scanning happening (no "give me a ping, Vasily"), we're just passively looking. I'd think the most likely way to find out would be listening in on our radio broadcasts (I assume someone's been talking about this object on the radio) rather than being able to directly detect the Hubble being pointed at it.

We have a LOT of energy outbound in the form of space radars and interplanetary comms. We’re lit up like a christmas tree.

Sure, but none of that was pointed at this comet specifically.

Ah, fair point - I see what you mean.

Theoretically, any sentient being with the necessary technology to get a craft to our solar system and spot the aiming of a telescope at that distance would have even better technologies for detecting what we are doing.

It's a logistics problem: if you can build one of something, you can definitely build two.

If you can build two, then build three. In fact, always do things in threes.

Why build one, when you can build two at twice the price?

Observing passive observation systems over those distances is pretty unlikely even with an optimistic view of future technology. But perhaps not impossible.

We have all kinds of space radars though that are active.

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