4 minutes ago, martin-w said:

Its relative, if their journey time is 500 years, then it remains 500 years to them. Different story for an outside observer though. An observer back on Earth would see them receding into the distance at a much slower rate, dependent on the velocity. At almost light speed, they would appear almost stationary.

Sorry, but I must correct you.

To a stationary outside observer, the ship would streak by at close to the speed of light.  It is processes for the crew and the ship that would occur at a much slower rate, making the 500 years seem much shorter *to them*.  This is why humans can indeed travel vast astronomical distances if they can accelerate to 99.9% of light speed, but they must realize that the folks back home will be long dead whereas for them only perhaps a year will have passed.  I think there is a formula for calculating the time dilation effect if you know your velocity.

Dave

2 minutes ago, dmwalker said:

These scenarios represent the extremes. Perhaps a population of 2,000 or 3,000 would be all that is required but for a journey such as this you would want a huge safety margin.

 

Some wildly different estimates. In the article below, 6300 year journey in a generation ship would require 98 people they claim. 

https://www.universetoday.com/139456/whats-the-minimum-number-of-people-you-should-send-in-a-generational-ship-to-proxima-centauri/#:~:text=Beluffi concluded that under conservative,with a potentially habitable exoplanet.&text=Any less than that%2C and,success would drop off considerably.

 

 

15 minutes ago, dave2013 said:

Sorry, but I must correct you.

To a stationary outside observer, the ship would streak by at close to the speed of light.  It is processes for the crew and the ship that would occur at a much slower rate, making the 500 years seem much shorter *to them*.  This is why humans can indeed travel vast astronomical distances if they can accelerate to 99.9% of light speed, but they must realize that the folks back home will be long dead whereas for them only perhaps a year will have passed.  I think there is a formula for calculating the time dilation effect if you know your velocity.

Dave

An example: http://physics.bu.edu/~duffy/semester2/c34_time_dilation.html#:~:text=As an example%2C let's say,10 years for the trip.

Time dilation refers to the fact that clocks moving at close to the speed of light run slow. Consider two observers, each holding an identical clock. These clocks work using pulses of light. An emitter bounces light off a mirror, and the reflected pulse is picked up by a detector next to the emitter. Every time a pulse is detected, a new pulse is sent out. So, the clock measures time by counting the number of pulses received; the interval between pulses is the time it takes for a pulse to travel to the mirror and back.

If our two observers are stationary relative to each other, they measure the same time. If they are moving at constant velocity relative to each other, however, they measure different times. As an example, let's say one observer stays on the Earth, and the other goes off in a spaceship to a planet 9.5 light years away. If the spaceship travels at a speed of 0.95 c (95% of the speed of light), the observer on Earth measures a time of 10 years for the trip.

The person on the spaceship, however, measures a much shorter time for the trip. In fact, the time they measure is known as the proper time. The time interval being measured is the time between two events; first, when the spaceship leaves Earth, and second, when the spaceship arrives at the planet. The observer on the spaceship is present at both locations, so they measure the proper time. All observers moving relative to this observer measure a longer time, given by:

time dilation: Δt=

Δto

γ

where Δto is the proper time.

In this case we can use this equation to get the proper time, the time measured for the trip by the observer on the spaceship:

Δto = Δtγ= 10 (1 - 0.952 )1/2 = 3.12 years.

So, during the trip the observer on Earth ages 10 years. Anyone on the spaceship only ages 3.12 years.

It is very easy to get confused about who's measuring the proper time. Generally, it's the observer who's present at both the start and end who measures the proper time, and in this case that's the person on the spaceship.

30 minutes ago, dave2013 said:

Sorry, but I must correct you.

To a stationary outside observer, the ship would streak by at close to the speed of light.  It is processes for the crew and the ship that would occur at a much slower rate, making the 500 years seem much shorter *to them*.  This is why humans can indeed travel vast astronomical distances if they can accelerate to 99.9% of light speed, but they must realize that the folks back home will be long dead whereas for them only perhaps a year will have passed.  I think there is a formula for calculating the time dilation effect if you know your velocity.

Dave

 

Oops, yes that's correct, I was thinking of an object falling into a black hole. 😁 I'll go and take my pills. 

This morning I played three games of chess and left my queen hanging in two of them. 🤪

 

 

Edited April 14, 20233 yr by martin-w

1 hour ago, martin-w said:

In the article below, 6300 year journey in a generation ship would require 98 people they claim. 

One of the comments on that article has a unique solution:

"I think if only females are sent along with frozen male sperms and only females are allowed to be born then crew size can be reduced dramatically."

That's a bit drastic but a traditional crew of equal numbers of men and women could carry a sperm bank with wide genetic diversity, which could be used to compensate for any genetic drift.

9 minutes ago, dmwalker said:

One of the comments on that article has a unique solution:

"I think if only females are sent along with frozen male sperms and only females are allowed to be born then crew size can be reduced dramatically."

That's a bit drastic but a traditional crew of equal numbers of men and women could carry a sperm bank with wide genetic diversity, which could be used to compensate for any genetic drift.

The alternative is a purely robotic vessel with the ability to autonomously "grow" humans and other fauna and flora from frozen sperm, seeds, or embryos at the destination...

26 minutes ago, HiFlyer said:

The alternative is a purely robotic vessel with the ability to autonomously "grow" humans and other fauna and flora from frozen sperm, seeds, or embryos at the destination...

What, no eggs? In any case, I believe frozen eggs are OK for only about 55 years and embryos even less.  I wonder if anyone has programmed a robot to change a diaper.

I dunno....

But the one thing I'm absolutely certain of...

No one is leaving this place alive.

1 hour ago, dmwalker said:

What, no eggs? In any case, I believe frozen eggs are OK for only about 55 years and embryos even less.  I wonder if anyone has programmed a robot to change a diaper.

Well, first I would assume that future Technologies could better those numbers, and second even those numbers are not that horrible once you factor in time dilation.

At an appreciable fraction of light speed, those 55 years of ship time and maybe a few hundred years of our time might put quite a few star systems in range....

55 minutes ago, HiFlyer said:

At an appreciable fraction of light speed, those 55 years of ship time and maybe a few hundred years of our time might put quite a few star systems in range....

It depends on what you mean by "appreciable". I think your example corresponds to 99% light speed. I'm sticking with travelling to alpha Centauri at 2% light speed but even at 50% light speed there wouldn't be much time dilation. If I could travel at 99% light speed I would aim for star systems which didn't require generational space ships.

Of course, nobody is going anywhere until we get conclusive evidence that an exoplanet is actually habitable. I don't know whether Breakthrough Starshot. can do that.

Edited April 14, 20233 yr by dmwalker

54 minutes ago, dmwalker said:

If I could travel at 99% light speed I would aim for star systems which didn't require generational space ships.

At 99c a 100 lightyear trip would take our space explorers about 14years (their time)

Hope they have good forcefields at that speed!

But, maybe they will! In the meantime, just like our first steps to Mars, etc. I suspect our descendants will probably (almost certainly?) end up sending unmanned probes/robots etc initially, Ala' (as you mentioned) something along the lines of the Breakthrough Starshot Initiative as our first attempts at interstellar flight.

Removing things like the need for food and life support, not to mention possibly heavy cosmic shielding might allow higher speeds for lower mass craft like light sails.

Then, even backing down to 20% lightspeed and a lower time dilation would still place Alpha Centauri "only" about 22 years away....

Stretch that out to 40years (well within the suggested 50yr expiration date of frozen eggs) and that puts about 10 stars in reach, not just of a probe, but also potentially within a modern human lifetime...

Or we could get really lucky, and... 

 

Edited April 15, 20233 yr by HiFlyer

13 minutes ago, HiFlyer said:

Hope they have good forcefields at that speed!

May the forcefield be with you.

14 minutes ago, HiFlyer said:

I suspect our descendants will probably (almost certainly?) end up sending unmanned probes/robots etc initially, Ala' something along the lines of the Breakthrough Starshot Initiative, as our first attempts at interstellar flight.

The question would be what level of detail would you need to know about an exoplanet before you would risk everything and travel there.

I wonder about manoeuverability at 50% light speed if, for example, some interstellar object came into your flight path.

6 minutes ago, dmwalker said:

The question would be what level of detail would you need to know about an exoplanet before you would risk everything and travel there.

Heck, Mr Musk already has volunteers to go to Mars on a one-way trip that he says will almost certainly end in death. Human beings are interesting Critters.

Above and beyond that, any reasonable expedition should probably expect to take along the type of supplies to allow them to exist even if no habitable planet is found.

Now of course, if there are no useful asteroids, accessible water and Etc, our colonists are probably toast, but hey, Columbus's map showed monsters at the edge of the world, and he and his Crews still went....

17 hours ago, dmwalker said:

I think if only females are sent along with frozen male sperms and only females are allowed to be born

 

Yikes, the radical feminist's would love that. 😀 And when the males are born, of course, they will be rendered mere thralls to the new feminist ruling class.  😏

13 hours ago, HiFlyer said:

Heck, Mr Musk already has volunteers to go to Mars on a one-way trip that he says will almost certainly end in death.

 

I recall, the now bankrupt, Mars One venture had 200,000 volunteers too.