[McKay (OP)]

We cant make have a signal bout EM radiation danger that arrives to us before the radiation itself.. or can we?
Obviously, we cant have have a signal traveling faster thn c, BUT the fact that we live in atmosphere means tht radiation propagates slower than c.
So, perhaps, we can have a, basically, vacuum pipe  closed with a seal that is transculent to whatever we want to detect, facibg outward with the detector qt the closest end. The radiation will travel faster trough the pipe than trough air. I calculate that for a 10 meter pipe/ distance there would be a lag of 970 microseconds (almost a milisecond). For 100m that would be almost 10 ms.
That sure aint much, but I am wondering, could anything benifit from such a warning system? Perhaps its enough time to prepare some systems for a EMP, flicking a switch or soething.
Particle radiation would be even slower..

[McKay (OP)]

Sorry, im off by a factor - it is 970 microseconds for 100 meters

[evan_au]

Electromagnetic radiation travels at around 3x10⁸ m/s in a vacuum.
According to my calculations, this works out at 0.3us per 100m= 100m/3x10⁸ m/s.

The refractive index of air is 1.000277, which means that a burst of light will travel through air at a speed of 3x10⁸/1.000277, which will arrive later over 100m by 0.1 ns =0.0001us = 100m/3x10⁸ m/s x (1.000277-1)

0.1ns is not a lot of time to shut down equipment.
What is worse, this giant vacuum tube will allow a blast of dangerous radiation to penetrate your bunker, quickly followed by a burst of particles which have not been slowed or absorbed by the atmosphere.

During early atomic bomb testing, scientists wanted to measure the spectrum of radiation produced by the blast, so they built an airtight tube, several kilometers long and filled with Hydrogen (if I  remember correctly). This was apparently on the theory that the relatively simple atomic structure of hydrogen would create the least distortion of the spectrum. But I guess they knew when the explosion was going to happen, so they weren't so interested in early warning.

[McKay (OP)]

Ah, yes, sorry again. My calculations are WAY of. Whatwas I thinking?
Hmm, ok, so a 0.1ns - not much indeed