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A Nicoll-Dyson beam is the business end of a Dyson swarm used to power a phased laser array, and is capable of vaporizing a planet within hours at a range of hundreds of light-years. (Link here. Scroll to the cowboys, start at "Griefers".)

Assuming you were on the leeward side when it hit (with Earth between you and the beam), what, if anything, would you notice before your fiery demise?

The beam is collimated, not focused, so the portion of the emitted energy hitting Earth depends on the swarm's radius. Obviously, the brightness of the star determines the total energy emitted. The power of an N-D beam could vary by a few orders of magnitude, so I'm just looking for a rough idea.

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A Nicoll-Dyson laser that is collimated, not focused (i.e. its diameter is that of the Dyson sphere used to generate it) is only a few times as bright as the incoming sunlight at the sphere. So it will be merely an inconvenience to the Earth (i.e. killing everything on or near the surface, but maybe not even boiling the oceans) rather than vaporizing it.

So lets look at a focused beam that can vaporize the planet within hours.

Planetary vaporization on this timescale corresponds to removing about kilometer per second across the exposed hemisphere. This material will be blown off the planet and sideways out of the beam. Let's say at a few times the 11 km/s escape velocity.

This gives a big kick to the planet (equal and opposite reaction) that will move as a shock wave through the planet, and would be lethal when it hit you about 10 minutes later. But you don't have to worry about that.

Before the shock, the thing you on the far side notice is that the debris cloud becomes visible over the horizon, all around you. The worrisome thing is not that you have rock heated to incandescence all around you: that would be no worse than dropping into a blast furnace.

The real problem is that the laser is likely not going to be perfectly focused to precisely the diameter of Earth. Instead it will extend a bit beyond the Earth and hit the debris cloud where it is visible to you. The scattered laser light will be much brighter than the mere incandescence of the debris cloud, and that is the first thing to kill you.

Or if the Moon is above the horizon and in the beam, you'll get the effect of its reflection and you won't have to wait for the debris cloud from the opposite side of the Earth to appear.

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