Alone in a Crowded Milky Way

Re:Alone in a Crowded Milky Way by hnuser123456 on Thursday January 02, 2020 @03:37AM (#21933814)

>In fact, already if you send a generational spaceship to a star 100 ly away to reach its destination in a thousand years, by the time you receive message from their arrival, both their as our language will have evolved by 1100 years.

It is not necessarily true that more time is spent aboard a ship than light would spend taking the same path, as measured by an external observer.

If you had a ship capable of accelerating at 1g indefinitely, then as measured by the crew, they could cross the galaxy in only 24 years.

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

I like everything else you wrote.

[ Parent | on HN ]

Re:Alone in a Crowded Milky Way by hnews_account_1 on Thursday January 02, 2020 @11:50AM (#21936970)

I don't understand how this is possible. To accelerate the spaceship at 1g becomes more and more difficult as you approach the speed of light. That's the whole point of relativity. And yet the article says "the spaceship will approach the speed of light in the middle of its journey, as measured from the planet's frame of reference".

[ Parent | on HN ]

Re:Alone in a Crowded Milky Way by aricblunk on Thursday January 02, 2020 @03:07PM (#21939241)

It only becomes "more difficult" as measured by people not onboard the spaceship. For people onboard, time effectively speeds up for the rest of the universe, so that there is no brick wall to your acceleration. Therefore, it's also possible to time travel into the future, but not into the past.

[ Parent | on HN ]

Re:Alone in a Crowded Milky Way by hnews_account_1 on Friday January 03, 2020 @11:18AM (#21947225)

That is not how it works from what little I know. Neither the spaceship, nor the outside observers will EVER see it hit the speed of light. That is the only constant thing they agree on. All of relativity hinges on this point. The Lorentz transformation is based on the fact that the speed of light is constant for all observers whether they're "stationary" or moving at 0.999999999c . So even at very high fractions of light speed, you know for a fact that you haven't hit light speed because when you turn on your spaceship's lights, the photons will look like they're travelling away from you at the speed of light (due to the own time dilation and length contraction you experience).

This is why when you're calculating the actual value of time dilation, it considers what fraction of speed of light you're traveling. Because if you were the traveler, time dilates and length contracts EXACTLY enough to make light appear to travel at 1c (regardless of your own speed).

[ Parent | on HN ]

Re:Alone in a Crowded Milky Way by Retric on Friday January 03, 2020 @03:20PM (#21949899)

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

Time dilation is only 1/2 the story. The other half is distance measurements get distorted.

People in the spaceship see the same maximum velocity of objects they pass aka c. However, if a huge measuring tape with say light year tick marks passing them they are going to measure it’s speed at say .99 c and the time between tick marks is X. Alternatively, at .999c they would get a new time period Y. However the difference between X and Y is not simply .999/.99 but rather a more complex relativistic adjustment.

However, this is only important for people on the spaceship. For people staying home the it takes 3.5x the energy to hit .999c vs .99c yet it’s only shaving 1% off the observed travel time. Which is unlikely to be considered worth it by just about any civilization baring significant changes to the laws of physics.

[ Parent | on HN ]

Re:Alone in a Crowded Milky Way by est31 on Thursday January 02, 2020 @11:04PM (#21943041)

Not really, even if you have optimal rocket fuel, you still have huge fuel demands: http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/r...

[ Parent | on HN ]

Re:Alone in a Crowded Milky Way by Retric on Thursday January 02, 2020 @08:11AM (#21935057)

That’s irrelevant for people on the planet sending the spaceship.

Particle physics get matter to 99.9999% of c which causes crazy time dilation to the particles and nothing to the scientists. Further, constant 1g acceleration of a starship to high fractions of c is basically science fiction. Not even matter anti matter reactions can keep up those energy demands for very long.

[ Parent | on HN ]