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Comment by cmenge | Hacker News Reader

Meh. According to https://en.m.wikipedia.org/wiki/Intergalactic_travel, it takes a mere 28y to get to Andromeda if you have the tech to do constant 1g acceleration. You could say that is not "communication" from a civilization point of view, but if they can send us a live messenger it would sure be interesting?!

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That's 28 years on-ship time, but about 2.5 million years for an external observer in Andromeda or the Milky Way. If you want to make a return journey, then 5 million years would have passed on Earth by the time you get back.

The problem is that at something like 0.4c the radiation from hitting the very rarefied hydrogen atoms that fill the interstellar void (something like 1 atom per 1m³) becomes so hard it will kill you without a colossal radiation shield.

One way to avoid that is to project a powerful beam of electromagnetic radiation to ionize these atoms, and then swipe them away with magnetic field. But it adds significant energy requirements, and adds noticeable "friction" to your spaceship.

How much water would you need in a tank at the front of your ship to block almost all incoming radiation?

If you're going fast enough, the Bremsstrahlung in your water shield itself becomes dangerous.

The energy requirements to accelerate go up as you reach relativistic speeds, and you need to be able to decelerate as you reach the target.

Why couldn't you slingshot around the sun and planets a few times to pick up speed before leaving the solar system? In the way the slingshot racer reached super high speed on his run to the gate in The Expanse?

https://www.youtube.com/watch?v=AGmTZeiCmJY

Not for the observer on the ship.

I don’t understand how it goes up, how does the rocket “know” it’s going faster relative us?

The rocket doesn't know, but an outside observer will. If you're on the rocket, you experience a constant 1g force. The rocket doesn't "feel" that it's harder to accelerate, but an observer that remains at rest relative to the rocket's starting point will see it speed up less and less the faster it's going as it approaches the speed of light.

They continue to accelerate at the same rate for their frame of reference, but relative to earth or Andromeda their time moves slower and with their time moving slower their acceleration slows down to

It certainly going faster each time relative to the speed of light, which increases the mass. At first this is minuscule, but it starts to add up by the time you're going a decent fraction of c. It will asymptotically approach infinity, which is why nothing with mass can be accelerated to c. It also means that at some point, you won't have enough energy to continue accelerating.

Which part of Andromeda, it has a 110,000 light year radius

Might only be a few hours difference for each end.

Sure, if you just want to fly by. If you want to stick around, you'll have to decelerate.

Isn’t that galaxy also rotating? So if you wanted to hit the far right corner of that galaxy, then don’t you have to calculate where it actually is now, since it took so many light years from that part of the galaxy to reach us.

Then, you’d have to calculate where that part of the galaxy will be, by the time you reach it. Then, you aim yourself at those coordinates, and hope you don’t have any mechanical issues that will prevent you from gravitationally locking on to your destination star system.

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