Astrogation question
- Thread starter bsg1970
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alex_greene
Guest
It's a pretty good reason for ensuring at least one sophont versed in Astrogation skill. Same reasoning behind Niven's hyperdrive, the TARDIS in Doctor Who and Slipstream drives in Gene Roddenberry's Andromeda.
iainjcoleman
Banded Mongoose
DFW said:
Yes, it does. FTL travel between any two bodies will result in the same problem. Travelling between two bodies at slower-than-light speeds (or even at lightspeed) does not cause this issue.
Faster than light travel is the problem here. It does not matter the destination is a planet in the same system, or a star a few lightyears away, or a star in another galaxy, or if a ship jumps instantly from one place to another or takes a week to travel a distance of 1-6 parsecs, or if it travels by warp drive or hyperspace; if it gets there faster than light can travel between its departure and arrival points, there then causality problems ensue.
It's handwaved away in most Sci-fi settings, but it is a serious problem when it comes to investigating FTL travel in reality, according to our current understanding of how the universe works.
alex_greene
Guest
Well, consider the following idea. What if the ship made the Jump to the star as usual, but the computer had forgotten the simple fact that planets move around their star, and once in a while go behind the star relative to the point of origin?
The ship could get dejumped one hundred diameters from the destination star, and the planet on the far side of it.
This really needs to be thought through. Where exactly does the ship emerge from Jump? The aim is to emerge from Jump as close to the main world as possible, but not necessarily within the 100 planetary diameters.
That limitation only covers entering Jump space - not to emerging from it. Theoretically, assuming really accurate precision, you could have a fleet of ships emerge from Jump within a planet's atmosphere: something to think about if you're planning on some kind of fleet stunt a la Babylon 5's Endgame, and similar to how the Enterprise decanted into Titan's atmosphere in the 2009 Star Trek movie.
Most times, I imagine the ship having to plot a course that gets it as close as possible to the main world, but if that means emerging at some weird point in the system several days out because the main world's on the other side of the main star from the origin, that's how it's got to be.
The ship could get dejumped one hundred diameters from the destination star, and the planet on the far side of it.
This really needs to be thought through. Where exactly does the ship emerge from Jump? The aim is to emerge from Jump as close to the main world as possible, but not necessarily within the 100 planetary diameters.
That limitation only covers entering Jump space - not to emerging from it. Theoretically, assuming really accurate precision, you could have a fleet of ships emerge from Jump within a planet's atmosphere: something to think about if you're planning on some kind of fleet stunt a la Babylon 5's Endgame, and similar to how the Enterprise decanted into Titan's atmosphere in the 2009 Star Trek movie.
Most times, I imagine the ship having to plot a course that gets it as close as possible to the main world, but if that means emerging at some weird point in the system several days out because the main world's on the other side of the main star from the origin, that's how it's got to be.
There are even more possibilities for unnerving incidents, for examplealex_greene said:
when one of the outer planets is in the flight path towards the target pla-
net. Just imagine a ship thrown out of jump space by the mass of Pluto,
stranded 30+ AU away from Earth, without enough fuel remaining for a
microjump there, and the gas giants unavailable for fuel skimming be-
cause they are currently on the other side of the sun ... :shock:
alex_greene
Guest
You could still mine the frozen atmosphere of the plutoid that dumped the ship, but they'd have to do it quick. It could get mighty cold out there ...rust said:
alex_greene said:
That's not possible in MGT "Gravity can cause a Jump bubble to collapse
prematurely, bringing a ship back into normal space early (so if a
ship tried to Jump from Earth to Mars when the Sun was between
the two, the vessel would fall out of Jump space as soon as it came
within one hundred diameters of the Sun."
DFW said:Yep but, irrelevant as we aren't dealing with FTL travel. Which was my original point that you both missed.
I think this exchange is what started this tangential discussion:
rinku said:
DFW said:
"Relativistic effects" is a somewhat fuzzy term as relativity includes discussions about time dilation, reference frames and causality.
Our current understanding of the universe tells us that causality is violated whether the ship remains in this universe and gets to its destination faster than light would get there, or whether it jumps out of this universe and reappears elsewhere in it in less time than it would take light to travel there in the normal universe.
In both cases, the reference frames of the ship, departure point and arrival point are not synchronized and cannot be synchronized if FTL travel is involved. Causality is only preserved if the ship arrives at its destination if it travels there at STL speeds or at lightspeed.
My intent was to clarify this as you seemed to be implying that there would be no relativistic issues and that the reference frames were synchronized, when in fact that is not the case.
DFW said:
See - http://www.theculture.org/rich/sharpblue/archives/000089.html - and then please explain why my statement is incorrect (also, see http://sheol.org/throopw/tachyon-pistols.html ).
Simultaneity and Causality break down if FTL travel is possible, because there is no universal reference frame.
DFW said:
Of course it's superluminal travel, why wouldn't it be? If you can travel to Alpha Centauri from the Sun in a week, then you are travelling faster than light (because light takes 4.3 years to get there).
You may not be getting there instantaneously, but you still become an "event" outside of the lightcone of an observer stationed at Sol because you arrive at AC faster than light would be able to get there. The stars are moving relative to eachother so their reference frames are distorted relative to eachother too. If two stars are moving in exactly the same direction at exactly the same velocity then perhaps there would be no causality violation, but in all other cases there should be and the diagrams in the reference will apply.
Why don't you explain why you think you are correct?
DFW said:
It doesn't matter in what space you travel. What matters is whether you end up within an event's lightcone or outside it. The lightcone is defined by the speed of light, and getting to a destination in space in a time faster than it would take light to travel there will put you outside that lightcone. It doesn't matter how you do it, you will reappear outside it.
Put another way, if you look at this graph from the link I posted earlier:
If (using the same Alpha Centauri example) you imagine that each grid increment is 4.3 lightyears of distance in the space direction (the X axis) and 4.3 years of time in the time direction (the Y axis) then the yellow line that defines the lightcone represents something travelling at the speed of light (it travels 4.3 lightyears in 4.3 years).
A starship travelling that same distance in only a week of time (by any means) will move one grid increment to the left or right on the x-axis but at this scale will move very slightly upwards on the y-axis, which puts it outside of event P's lightcone. This makes it equivalent to event Q (which equally represents any point outside event P's lightcone) in the examples on that website, and therefore everything that applies to Q applies to our starship too.
Since event P can be thought of as the ship's departure point in spacetime and event Q (and its equvalents) can be thought of as the ship's arrival point in space time, the starship travels to its destination faster than the light from its departure point would reach it. You should be able to see that it makes no difference if it travels within this universe or not; the only difference it makes is in whether or not you can plot a worldline between P and Q, but that does not affect the conclusion, which is that Q is outside P's lightcone and to get from P to Q requires that the ship has travelled faster than light.
Perhaps you can point out to me which "201 material" I am supposed to be referencing on a site like this website (certainly none of my university physics courses covered instantaneous travel, I'm not sure about yours
). Or better still, just explain yourself properly here.A question about this jump v. relativity.
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 understand that after the jump, if the ship had a sufficiently powerful sensor, it would be seeing the departure world's "past" from 4 years ago, and the departure world would not detect the ship's return jump for another 4 years; that's all pretty straight forward wave propagation.
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 understand that after the jump, if the ship had a sufficiently powerful sensor, it would be seeing the departure world's "past" from 4 years ago, and the departure world would not detect the ship's return jump for another 4 years; that's all pretty straight forward wave propagation.
