Astrogation question

[Blix]

A ship jumps 4ly away, waits a week, then jumps back the same distance.
A local star base times his departure and return.
How long does that star base clock for this trip? 3 weeks? -8 years?

I don't think that question can really be answered accurately, since it all depends on the reference frames. If simultaneity and causality were somehow preserved, then one would expect that clocks on the ship and on the starbase would both say that 3 weeks had passed (assuming that the ship spends a week in jump in its own time frame); but for this to work, there would have to be an external, universal reference frame, and that does not exist in relativity. This is assumed to exist in SF settings that involve FTL travel, which is why the problem does not come up there .

If the ship travelled at about 99% lightspeed then someone on the ship would think that they travel for 0.57 years (time dilation at 99% of c is a factor of about 7), spend a week at their destination, and then travel back for another 0.57 years. So from the ship perspective, the total travel time is 1.14 years plus a week at the destination. From the starport perspective, they are gone for 8.08 years + a week.

 

[rinku]

My take on it is that the ship pretty much has to have its own separate reference frame - it has no communication with the normal universe during jump. How its reference frame is going to vary from the origin system's reference frame (or more importantly, the destination world's one) would require a bit more detail about jumpspace theory

However, since it all seems to work, I think we can safely say that the ship's reference frame is going to roughly match up with the system's. You may add or lose a few hours or days per trip, but it's much of a muchness with the days or weeks a person living in a 1.2G planet will gain compared to their neighbors living on a 0.5G moon over their lifetime. (Totally out of the air example, mind you. Do you want to do the maths on that, Blix?)

BUT (and this was my original point) very small differences in relative reference frames can mean big differences in practical terms over stellar distances. The required precision to arrive at a desired point in spacetime from one star system to another without actually travelling through normal spacetime is going to have to account for the effects of acceleration on clocks.

And no doubt quantum is going to get involved somewhere as well...

 

[Blix]

Do you want to do the maths on that, Blix?

It's probably a miniscule difference at most, since we're only talking about the difference between 12 m/s² vs 5 m/s². Compare someone (hypothetically) living on the surface of a neutron star (about 100 billion g) with someone living on Earth and then you'd get some more noticeable dilation.

It's just better not to think about relativity in an FTL situation .

 

[rust]

Compare someone (hypothetically) living on the surface of a neutron star (about 100 billion g) with someone living on Earth and then you'd get some more noticeable dilation.

Like in this novel by Robert Forward:
http://en.wikipedia.org/wiki/Dragon%27s_Egg

 

[phavoc]

The Traveller universe skates around the issue of FTL travel. It's probably more accurate to say that Trav jump drives are inter-dimensional drives. A ship departs from spacedock or a planetary body, moves to at least 100 diameters from the object and its gravitic influence, and then activates its jump drive.

When the drive is activated, the ship pumps out the hydrogen fuel in its tank to create a bubble-universe that is outside the existing one. But the bubble universe eventually dissapates, and when it does, the ship re-enters our universe, but at a different spot from whence it started its journey.

So there isn't any acceleration towards the speed of light that would cause time issues. As far as I can recall from all the various explanations about Traveller jump space, its no different than "normal" space. So if you had atomic clocks on board, and you departed from one system, jumped, refueled and then jumped back, the clocks should still be in synch with one you left behind.

At least that's my understanding of it.

 

[DFW]

So there isn't any acceleration towards the speed of light that would cause time issues. As far as I can recall from all the various explanations about Traveller jump space, its no different than "normal" space. So if you had atomic clocks on board, and you departed from one system, jumped, refueled and then jumped back, the clocks should still be in synch with one you left behind.

Yes, that was the raison d'être for Jump drive in Traveller and, indeed for Hyper Space drives and the like in Sci-fiction.

 

[Blix]

So there isn't any acceleration towards the speed of light that would cause time issues. As far as I can recall from all the various explanations about Traveller jump space, its no different than "normal" space. So if you had atomic clocks on board, and you departed from one system, jumped, refueled and then jumped back, the clocks should still be in synch with one you left behind.

Yes, that was the raison d'être for Jump drive in Traveller and, indeed for Hyper Space drives and the like in Sci-fiction.

Relativity isn't just concerned with what happens at velocities approaching or at the speed of light. It's also about reference frames, and how they work relative to eachother (hence the name "relativity").

I've demonstrated what the problems are with FTL travel in a relativistic universe. You can have two out of special relativity, causality and FTL travel/communication - you cannot have all three.

If we have the latter two, then special relativity does not apply, in which case we cannot accurately describe what happens to the clocks or reference frames involved because we don't have a theory that describes it. Maybe phavoc's description is right, but maybe it isn't; nobody has considered all the implications if it is assumed that all clocks magically remain synchronized. And why should clocks run at the same rate in jumpspace? Why should time even exist in jumpspace at all?

 

[hdan]

Why should time even exist in jumpspace at all?

That's an easy one - to give player characters time to heal, meet their fellow travellers (should they be of any interest) and to practice skills.

 

[vitalis6969]

Why should time even exist in jumpspace at all?

That's an easy one - to give player characters time to heal, meet their fellow travellers (should they be of any interest) and to practice skills.

haha, THIS....

Simple and full of win.

-V

 

[Blix]

Why should time even exist in jumpspace at all?

That's an easy one - to give player characters time to heal, meet their fellow travellers (should they be of any interest) and to practice skills.

True, but there is also plenty of "downtime" in the days spent travelling between the jump point and the destination/departure planet through realspace in which characters can do all of that.

 

[captainjack23]

Why should time even exist in jumpspace at all?

That's an easy one - to give player characters time to heal, meet their fellow travellers (should they be of any interest) and to practice skills.

True, but there is also plenty of "downtime" in the days spent travelling between the jump point and the destination/departure planet through realspace in which characters can do all of that.

True, but, there's no need to worry about random encounters, or whatever in jumpspace; and, more dedicated time in a system with no Expereience points is better. You pretty much have to let the time pass; players, as we know seldom budget for downtime or vacations, and when they do, it does help syncronise things for the campaign.

the 1 week constant (1WC) is one of the more unique things about Traveller's pseudo-FTL, and I've found out that it helps nail home the idea that This Is Different for players.

It would be interesting to find out from MWM what design goals, if the any, the 1 week constant was intended for.

 

[Blix]

True, but, there's no need to worry about random encounters, or whatever in jumpspace

Time spent in jumpspace does not necessarily have to be uneventful either though. There are plenty of possible "self-contained' plotlines that can be done in jumpspace (the classic murder mystery, for example, or something breaking (or escaping?) on the ship).

 

[BP]

My early (CT) interpretation had jump as instantaneous for the ships occupants while in normal space, about a week had passed. Re-watching Starwars, caused me to change to the more official version (???) where the time was experienced in Jump space as well.

My take on the week game mechanic is that it was arbitrarily chosen to mimic the feel of the era of sailing ships and pre-telegraph old-west.

 

[captainjack23]

True, but, there's no need to worry about random encounters, or whatever in jumpspace

Time spent in jumpspace does not necessarily have to be uneventful either though. There are plenty of possible "self-contained' plotlines that can be done in jumpspace (the classic murder mystery, for example, or something breaking (or escaping?) on the ship).

Well, Yes, Any time can be adventure time. I get that: I once had a ship overrun by vermin while in jumpspace. Hard to deal with when you cant suit up and vent the ship very easily.

The point is it's useful, and if you want or need dead time its easy to have it there. And it helps with the age of sail feel, which is one of the things I like about traveller.

 

[zero]

Hm, yes, being in jumpspace can be a pretty good time for the ship to have a creature or more aboard :lol:

Just a quick question, we all know how the J-Drive works roughly with the hydrogen mini-universe bubble, but how would that appear to someone watching in another spaceship?

I'd say it'd be similar to the Delorian in Back to the Future, the ship hits 100 diameters, and venting the hydrogen dissapears into a mini-sun where they were that immediately dies out.

Whats your opinion on describing a jump to a witnessing ship?

 

[BP]

IMTU, visually, to non-augmented humans, the jumping ship just disappears...

With sensors, a sudden, brief, emission of radiation is detected as the very sparse matter in the near-vacuum of space 'fills' the void. Though the radiation is across the spectrum, the human eye is generally incapable of detecting anything due to the speed and scarcity of matter involved in the 'implosion'.

(P.S. - the official version has the 'hydrogen bubble' formed in a 'pocket' universe, so from the sounds of it, precludes any direct visual effect related to that aspect of Jump).

 

[rust]

Whats your opinion on describing a jump to a witnessing ship?

While a cinematic effect could be nice to describe, I see no convinving way
to get one, so my ships just disappear, too - now you see them, now you
don't ...

 

[DFW]

I decided that since the ship is moving to a different universe/dimension, it all of a sudden looks 2 dimensional and then collapses in a blink of an eye from its "edges" towards its centre. So, it looks different if you see it edge on or flat face on.

 

[TC]

never really described it, just explained to the players if they asked what happened. A hole was just ripped in the fabric of reality and the jumping vessels got sucked into it and is gone..............I left them visualize what they saw.

I've tinkered with the idea that no 2 people see it the same, being a higher dimentional event, the mind just interprets what sence it can make of the impossiblity it just experienced.

 

[zero]

So the ship jumps to a pocket universe, then vents the hydrogen? I thought venting the hydrogen was part of the jumping process and the vented hydrogen is the wall to the mini-universe (when all the hydrogen allocated for the jump is vented the ship begins to return to the universe in the parsec allocated.)

To be fair, I havent really properly read the rules for Jumps yet (I've been dealing with system ships before now) so I'll look there also.

Atm I'm going for a combo of DFW and TC's version, the ship gets sucked towards a central pinpoint that swallows it, it then appears at the parsec allocated a week later, as if the pinpoint exited there (if you get what I'm trying to describe, its like when Dr.Weir in Event Horizon describes how the Gravity Drive can go FTL)