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> After I found all this out, I concluded space travel isn't THAT hard, assuming you have a way to accelerate constantly at 1g for 22 years (that is the hardest part actually), you can get anywhere in the galaxy in a human lifetime, no need for generation ships, cryogenics or other crazy tech.

Unfortunately, there is also a kind of rocket equation for constant 1g acceleration, and if I remember correctly it's not really doable for any realistic amount of time, even with matter/antimatter reaction drives.

I think because of the expansion of the universe, you don't have to decelerate as much if you travel to far away galaxies.

See the novel Tau Zero by Poul Anderson. https://en.wikipedia.org/wiki/Tau_Zero

I think you're forgetting that there's a hard upper limit on velocity.

Edit: Ah, wasn't aware of length contraction. https://courses.lumenlearning.com/physics/chapter/28-3-lengt...

Not on subjective velocity. As you approach the speed of light from an outsider's perspective from the insider's perspective the universe appears to shrink.

But if you experience 22 years to make 10,000 ly's of distance, you've subjectively gone much much faster than the speed of light, haven't you? I thought that was a constant in all frames of reference.

From your own perspective you are always stationary, it's just the universe that is moving (and shrinking).

Quote from the original claim:

> After I found all this out, I concluded space travel isn't THAT hard, assuming you have a way to accelerate constantly at 1g for 22 years (that is the hardest part actually), you can get anywhere in the galaxy in a human lifetime, no need for generation ships, cryogenics or other crazy tech.

They are saying add a constant 1g acceleration and you can get anywhere relatively quickly, no warp bubble required. That's what I'm taking exception to. If we add a warp bubble, sure, but a constant 1g acceleration shouldn't ever see you having an experience of moving faster than the speed of light, and that 10,000 ly trip should still take > 10,000 ly.

No, you're misunderstanding.

From your frame of reference, the universe is much flatter in the direction of travel, and getting flatter and flatter the more time you spend. The distance between Earth and the edge of the galaxy will get smaller and smaller in your direction of travel.

Thanks - yeah, I realize now that the length actually shortens.

When you go fast, subjective experience slows down.

You'd have 22 years of thoughts, for example, even though 10,000 years of stuff happened elsewhere.

From the perspective of an observer on earth, the trip takes a long time. The people on the spaceship experience 22 years.

It's not about time dilation, I realize now it's possible due to the length actually contracting. https://courses.lumenlearning.com/physics/chapter/28-3-lengt...

If it didn't you'd still need to pass through 10,000 ly of space and if you were going at slower than the speed of light it would still appear in your frame of reference to be > 10,000 years. But contraction removes that limit and shortens the distance.

They are different sides of the same coin.

This is the realization that really made SR click for me. The fact that "in the traveller's frame of reference, the universe becomes smaller in the direction of travel" is the same thing as "in the traveller's frame of reference, time becomes dilated".

You move at a constant vector, the speed of light, through both space and time. Since you go from basically 0 magnitude in space (anything us humans do on earth) to approaching the speed of light, that robs direction from the time magnitude. Therefore, going that quickly means you experience substantially less time. Taken to the extreme, when you move at the speed of light, you experience no time; you would simply from a human perspective teleport to the place where you start decelerating (given that's possible lol).

As an example, from the perspective of a photon, the trip takes 0 seconds, the photon is emitted and absorbed at its destination with no (subjective) travel time

You never reach that limit if you are accelerating with a normal engine at 1g. You will get closer and closer to it, but never reach it.

If you somehow DO achieve lightspeed, time stops for you, completely, so from your point of view you are teleporting, but since time is stopped no machine or chemical reaction can happen, thus why we believe lightspeed machinery is impossible.

By the way, achieving near lightspeed with 1g acceleration is in a way, hard, for example the trip to alpha centauri at 1g results in you achieving "only" 94% of light speed... to do that same trip peaking at 99.0% of light speed it would take 3g of acceleration, and it would result in the trip being roughly 1/3 shorter (taking 1.7 years more or less, instead of 5)

> Crazy things happen if you decide to go on galaxy-wide trips though, to the astronauts because space contraction they perceive their own trip as happening still rather quick, a 100 000 ly trip (the size of our galaxy, for reference) takes 22 years for someone inside a spaceship with 1g acceleration, perfectly doable. But people outside still see it taking 100 000 years... (meaning they will be 100 000 years "older" from the point of view of the astronauts)

But from your frame of reference wouldn't you be moving much faster than the speed of light? If you're taking 22 years to travel 10,000 ly, that means you're moving faster than light in your perception, doesn't it? Isn't that forbidden in any frame of reference?

From your frame of reference, the universe is much smaller in the direction of travel. You'd still see light zoom past you at 1c, though.

From your perspective, the entire length of the milky way is squished into 22 light years (roughly), so not really violating anything. From the outside frame of reference you still take 10000 years (plus) to cross that distance, from your perspective, the distance gets shorter.