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Comment by swamp40 | Hacker News Reader

> jet speed is 99.99% the speed of light

Does that mean a wave of particles is coming at us right behind the light flash?

That was my first thought too. But even at that speed, we have quite a bit of time. Since the particles are 0.01% slower than the light that just reached us, the particles would arrive in: 8,500,000,000 light years away * 0.01% speed of light = 850,000 years. Humanity will be unrecognizable by that point.

Wouldn't the galatic rotation mean the earth will be somewhere far away when the jet travels that far?

On the flip side that also means that we might flow into a burst travelling through space without any warning (?)

That got me wondering how far we'd move - if my maths is correct then in 850,000 years time our solar system will have travelled 652 light years around the galactic core (230 km/sec * 850,000 years)!

How wide is the beam? (given how far it started) Light-years?

Is it still dangerous at this point? (If its energy is spread over a giant area radius billions light-years)

Yes, that and both galaxies careening through space on different trajectories.

A couple more 9's changes that drastically though. Maybe someone just truncated at 2 decimal places?

If it is 8.5 billion light years away and was traveling at the speed of light (100% instead of 99.99%) wouldn't it take 8.5 billion light years? Wonder how you got to the 850K years figure.

> wouldn't it take 8.5 billion [years]?

Yes, if you are assuming the jet was emitted right now. But the jet was emitted roughly 8.5 billion years ago. The light in front of the jet has already reached us. If the jet were also travelling at the speed of light we'd be dead right now. But luckily it's travelling slower than light so that's why we have 850k more years before the jet reaches us.

Would we actually? If so, doesn't this imply that the probability of earth just having been destroyed by one of these things was roughly a coin flip, and therefore gives us a (much higher) rough estimate of how likely such an event might be?

> If the jet were also travelling at the speed of light we'd be dead right now

Does that mean, once the jet reaches us 850K years from now, we can say that will be a mass extinction event, or even the end of life on Earth? Compared to a billion years from now when the Sun's luminosity increases.

No. The matter in these jets isn't like a spaceship where the matter is all connected together. It's largely individual plasma particles - over billions of light years they'll run into other particles, be deflected, slow down, etc. There IS friction in space.

Matter decay will have significant impact on the mass of matter ejected by the jet, as well, particularly over billions of years. As it decays into a lower energy state, mass will be turned into photons, and less and less of it will be left to impact.

Plus, we won't be in the same spot in 850k years anyway. The solar system is moving around the galaxy, and the galaxy is moving around the universe, and space in the universe is expanding.

Interesting, thank you very much.

They're saying the difference between the time the light gets here and the particles 'only' going 99.9% of C is 850K years back of the napkin.

By his logic, an object moving at 0% of the speed of light would arrive instantly.

An object moving 0% slower than the speed of light would arrive instantly after the light arrives, yes.

When doing such a calculation, would you need to take into account the fact that space is expanding while the particles are traveling? (And will have expanded a bit more in the time it takes for the particles to reach us than in the time it took for the light to reach us.)

Not necessarily. Particles will all be charged plasma and it is possible that those will be deflected by magnetic fields. But I am not astrophysicist so you would want to check on it.

Also, in contrast with the flash where all light has the same speed, the particles will have different speed so it will all be smeared in time (read -- much smaller in amplitude and hard to detect) and arriving much later than the flash.

Then from the point of view of observer on Earth surface, charged particles will not be coming exactly from the source but at a bit of an angle (due to magnetic fields present). Again, I have no knowledge about the magnitude of the effect and I also suspect that the people who know this shit have some way to account for it...

No. The light from these jets travels many many magnitudes farther than the actual material. You wouldn't want to be within a few million light years of it but these particles have mass and will be slowed by all of the matter they interact with in space. The vacuum of space still has roughly an atom per cubic centimeter, which really adds up over the distances we're talking about here.

Edit: Left out a fairly important word. few million light years*

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