The thing I always found a bit unnerving is that if they've left and are traveling at a speed close to the speed of light we'd never know until they arrive. If something is traveling at the speed of light towards us (whether a ship or a death star laser) we wouldn't know until it arrived.

Now we've been sending radio signals for several decades you have to begin to question if there's a civilisation out there maybe 10-100 light years away that may now be aware of us and have sent something our way which may be about to arrive.

There is an explanation for Fermi paradox that suggests maybe alien civilisations choose to be quiet or immediately wipe out other transmitting civilisations with speed of light weaponry.

Why? Well, if there's no way to have prior warning of an attack traveling at the speed of light your only defence is some combination of remaining hidden and extreme hostility towards other civilisations.

Welcome to The Dark Forest.

Most likely explanation? You can't travel at close (even 1%) of the speed of light without dealing with interstellar gas compression, object collision, passing through any moderately strong magnetic field without deviating, etc

I suspect future people will solve these problems.

If you have enough acceleration , you can travel 1000 light years in a human lifetime.

It’s just that when you return, at least 2000 years will have passed on earth.

so you're implying speed atleast 10x the speed of light? i think you'll have better luck increasing the human lifespan instead.

Not that's not what he is implying. He is talking about time dilation.

How does one travel 1000 light years in a span of human lifetime (~80 yrs) if you’re not going faster than the speed of light?

Time dilation! Time is relative. For an outside observer, over 1000 years pass, but aboard the ship, much less time passes. There are calculators for this, e.g. at [0]. If you constantly accelerate at 1g, flip around at the halfway point and accelerate in the opposite direction for the rest of the journey, roughly 13 years would pass from the traveler's perspective.

This site also has a "Newton's universe" mode, which pretends that relativity doesn't exist. Interestingly, in that case you would take over 44 years!

[0]: https://www.omnicalculator.com/physics/space-travel?c=EUR&v=...

I am pretty rusty in special relativity, but IIRC, in the ship it basically seems like the distance is contacting as you accelerate. And from earth, it seems like time is going slower inside the moving spaceship.

With relativistic time dilation. 1000 years will pass on Earth, but on tge ship, it will be only a couple of decades, assuming they're close enough to the speed of light.

""" Time dilation is the difference in elapsed time as measured by two clocks, either due to a relative velocity between them (special relativity) or due to a difference in gravitational potential between their locations (general relativity). When unspecified, "time dilation" usually refers to the effect due to velocity. """

https://en.wikipedia.org/wiki/Time_dilation

wow thanks! this was super fun, do you happen to have some souce about the calculations that go into determining the time dilation based on different velocity? i'm aware about black holes being capable of well, doing time dilation thingy due to their massive gravitational force but nothing else.

edit: souce as in something a regular average joe can make sense of haha

Well, depending on your preferred format there's a great book called A Brief History of Time - https://en.wikipedia.org/wiki/A_Brief_History_of_Time , or lots of videos about it at https://www.youtube.com/results?search_query=time+dilation

More broadly, the series Cosmos with Carl Sagan was excellent, and is worth a view. The bit about time dilation is at https://www.youtube.com/watch?v=kWgEzxi_A0w . The more recent one with Neil Degrasse Tyson is also good (but while I enjoy both, I think I prefer the older one)

The time dilation equation from special relativity only needs basic maths: https://www.omnicalculator.com/physics/time-dilation

1g constant acceleration is enough. But we don't have the technology yet.

Even simpler: you send out self-replicating AI drones in advance—to every star in the entire galaxy. That guarantees permanent security. The drones assess if alien intelligences have any potential to become a threat, and if so annihilates their planet, by autonomous decision.

Takes a few million years to set up... but that's just a blink of an eye on cosmic time scales; a brief era in the galactic-scale lifespan of an advanced civilization. (The civilizations that don't do something like this don't live that long).

[delayed]

Our sun isn't massive enough to supernova, thankfully.

But those evolve later and come back to you, biting you in the ass.

That's something we are very good at

Huh, sounds like those Von Neumann probes from Star Control II

We are the self-replicating drones.

It's taking a few million years to get from violent confused simians to violent confused simians with autonomous AI. But we're getting there.

Slow, long-duration mission have their own issues. Even a jellyfish or fungus has to eat. Carrying enough food for a multi-thousand year mission is a problem. And an AI needs power, which might be hard to come by between the stars.

> Unless 'it' left a long time ago and is already here.

Might be posting on HN right now. Never know.

All your hypotheticals are not entirely unreasonable, but they are also so far beyond our current ability to test, that it becomes pretty futile to speculate, except as a source of entertainment.

Space is reaaaaaaaaaaaaaally big, much bigger than people really have a conception of. Bigger than I have a conception of, and I try really hard to understand how bad my conception of it is.

A space fungus from 120 light years away would presumably only reach us by almost 100% chance, which is something I am comfortable saying is almost 100% unlikely. This is true of most hypothetical threats.

Fantasies about aliens or AI coming here (or sending big rocks) to exterminate are as weird to me, as I image "space is big haha" is to you.

There is no reason for anyone even to try to cross interstellar distances, especially not for war. The expected gain will never be worth the cost, outside of some fantasy FTL-for-free engine, which is just that: fantasy.

But, it is fun to think about it all.