When Worlds Collide

[ericthered]

Originally Posted by vicky_molokh

Not always. If you choose the spaceship as the reference frame, suddenly it's the two stars that changed speed. And you can pick absolutely any reference frame, and the math is supposed to give the same results. That's the thing.

Of course, if you chose a non-inertial frame, such as 'the space ship is always at rest', you have fictitious forces (forces that don't exist but appear to because of your reference frame), and you have to account for those -- which I presume will restore the time shift to the space ship rather than the planets, because the universe is consistent. For the same reason you don't get to use newton's laws quite as written when you are in a rotating reference frame.

Originally Posted by whswhs

Doesn't that apply while the acceleration is taking place? So I accelerate Galaxy 1 toward Galaxy 2, up to .9999c, and then it crosses 50,000 light years in slightly more than 50,000 years at constant velocity. Once you have constant velocity, it's equally true that Galaxy 2 is crossing the gap at .9999c, right?

Originally Posted by Anthony

I'm trying to figure out how that's responsive to the original question. There are certain classes of virtual objects that can move faster than light, and can thus approach at faster than light, but nothing with a positive information content can do so.

There is an old situation in science fiction where a person gets on a space ship, goes to alpha centauri and back and .9c, and comes back much younger than those they knew. Its a well explored part of science fiction. A well known aspect of relativity even.

But if everything is relative, how do you know its the guy on the spaceship that gets less time than the guys who stayed on earth? from a velocity standpoint, there isn't a difference. How do you know which is which?

The answer is 'who accelerated'? The space explorer experienced much more acceleration than his friends back at earth. Thus he experiences about 4.5 years of time while everyone else experiences about 9.5.

I'll point out that from the point of view of the space explorer, light speed only applied to how much time everyone else experienced.

I'll also point out that these numbers work out even with instantaneous acceleration and even if you only consider the trip back from alpha centauri. I don't know how to run them -- this is just a hobby, but that they work is well demonstrated in theory, in experiment, and in fairly commonly shown in science fiction.

The detail about acceleration is one I had to search fairly seriously to find. Its also one that if you look at both the experiments and the more serious speculative fiction examples is obviously true. sorry I can't throw more math at you.

[vicky_molokh]

Originally Posted by ericthered

Of course, if you chose a non-inertial frame

But how do I find an inertial frame? Inertial means non-accelerating, acceleration is a difference of velocity, and velocity is always measured against some object, not against some mythical privileged immovable frame. (It's a fascinating and disturbing topic, and you seem to have a clearer idea of inertial frames, that's why I'm asking.)

[Anthony]

Originally Posted by ericthered

There is an old situation in science fiction where a person gets on a space ship, goes to alpha centauri and back and .9c, and comes back much younger than those they knew. Its a well explored part of science fiction. A well known aspect of relativity even.

Yes. However, it has no discernible connection to the original topic of FTL approach.

[jeff_wilson]

Originally Posted by vicky_molokh

Acceleration is a change in velocity. You measure velocity relative to something. Ergo you measure acceleration relative to something.

Sort of, since the something can be carried with you. Since f=ma, a=f/m; you can measure the force needed to keep something of known mass stationary relative to you. As long as it's acceleration due to contact force rather than a body force, that is.

BTW, for everyone who would like to know more or review physics, ASIMOV ON PHYSICS is cheap 2nd hand, a very easy read, and covers most real world phenomena measured before about 1965, definitely including including relativity and quantum uncertainty. This book will give you familiarity with all that and show you some basic arithmetic relationships , like frequency * wavelength = speed of light.

[ericthered]

Originally Posted by Anthony

Yes. However, it has no discernible connection to the original topic of FTL approach.

Its relevant because it describes a method for traversing millions of light years in an observed year -- without violating relativity. A great old one who can throw around galaxies with great speed CAN make it happen on a human-relevant scale if he moves the milky way at near light speed rather than (or in addition to) throwing something at the milky way. I can (theoretically) move millions of light years as fast as I want, but you can't move to me over millions of light years and expect to find me alive.

Originally Posted by vicky_molokh

But how do I find an inertial frame? Inertial means non-accelerating, acceleration is a difference of velocity, and velocity is always measured against some object, not against some mythical privileged immovable frame. (It's a fascinating and disturbing topic, and you seem to have a clearer idea of inertial frames, that's why I'm asking.)

I'm trying to come up with a definition that's not recursive. An inertial reference frame is a frame in which inertia is preserved within the frame. Meaning that if an object is moving at a velocity and it experiences no outside force it continues moving at that velocity. You can determine if an outside force exists or not by newton's law about action and reaction. The classic way to do this is to peg the frame to an object or center of gravity that has very little acceleration within the problem set you're considering. Objects like to move in ways that satisfy inertia. Look at your reference frame from a Newtonian perspective, and if you're breaking the action/reaction rule to explain behaviour, you've got an non-inertial reference frame (or magic*).

Now, you will pretty much always have a tiny bit of acceleration in your inertial reference frame. Alpha Centauri and the sun are rotating around the galaxy center, which means a frame based on the center of gravity for these two has some acceleration going on. But its tiny and you can ignore it without messing up your data too much. If you're dealing with independent galaxies you have the thrice-blasted expansion of the universe. But that is often small enough to ignore. Basic physics: ignore the really small numbers and make sure the numbers you don't want to deal with are really small. Don't overthink finding the missing acceleration. Most of the time it will be really obvious, like objects orbiting much smaller masses, separate large objects accelerating in tandem without reaction mass, or other nonsense. Trust inertia. If gravitational, electromagnetic, and kinetic inputs aren't really acting on an object or theoretical object or system of objects, you have a valid nearly inertial reference frame.

[Anthony]

Originally Posted by ericthered

Its relevant because it describes a method for traversing millions of light years in an observed year -- without violating relativity.

No it isn't. In the reference frame where the journey time is less than a year, the journey distance is also less than a light year.

[Blood Legend]

Originally Posted by johndallman

Unfortunately that doesn't help me much. I'm totally unfamiliar with Final Fantasy, and almost as much so with anime.

I'm getting the impression from your posts that the speed of light is about as relevant to the setting as the particular flavour of cupcakes that are most popular in Poughkeepsie?

When I say Final Fantasy I'm talking about surprisingly powerful humans doing battle against giant thousand year old creatures by utilizing magic, outlandish physical techniques and other giant thousand year old creatures.

When I say "- Anime" I mean to say I'm excluding the outlandish. Without creating an argument with other people about what anime is or how I can draw a line for what's over the top in a game full of summoning monsters to fight space squids, I meant to quickly establish that I'm excluding the outlandish features commonly found in most generic anime, which tend to contain a lot of senseless posturing regarding combat prowess, intelligence, "power levels" etc, creating situations where you'll have a scrawny fellow smirking behind his glasses because he knows he can arm wrestle the incredible hulk and win because he's just that badass and knows something you don't except you do because he's making it really obvious that the outlandish 'going to arm wrestle hulk' situation isn't ruffling his feathers.

If you're throwing thugs around like you have 200lbs of muscle, you probably have 200lbs of muscle and you're likely not a 58 pound cat girl. What this means for my setting is that when you summon a meteor to smite a creature the size of an elephant, that meteor is going to behave like a meteor striking an elephant. It's still "Final Fantasy" because people are summoning meteors to kill elephants. My game won't generate odd scenarios where a T-Rex can take an anti-tank round because "Oh man, T-Rexes are scary." It's a living creature, you just put a bullet the size of a small child through its heart, it's dead Jim.

The reason I bothered to say 'Final Fantasy without the anime nonsense' is because Final Fantasy has a lot of nonsense you would readily associate with anime, but the setting, concepts, ideas, thoroughly embedded concept of life force and reincarnation, and borrowed lore you can find in a final fantasy game is pretty good for a hodge-podge setting.

The theme of the game I'm running is that if superman tried to catch an airplane, he'd tear through the front of it because of surface ratios, but he could still find other ways to save that plane like bending a wing. So while reality is fighting him every step of the way, he has to be clever to do something cool. Only instead of crashing airplanes, the problems he's facing are antediluvian nightmares from the liquid abyss, and instead of being superman he's more like Rorsarche or the Comedian.

If my elder gods make some kind of boast about crashing planets together, and I show my players an example of that power through a seeing stone or time portal as an example of how seriously the threat of an elder god is, I'd like any dramatic scene I describe to visually match reality.

I figure that there's so much empty space between stars, squashing two spinning discs full of rock and fire wouldn't actually generate a lot of collisions, but then I've vastly underestimated the size of our galaxy and the kind of damage certain speeds would actually generate. So I'm in need of a boast that involves the moving of a sizable chunk of the cosmos into another sizable chunk of the cosmos in a way that doesn't obliterate all life within the galactic neighborhood but still 1) happens in a time frame humans would care about, and 2) looks damn exciting.

[vicky_molokh]

Originally Posted by ericthered

Its relevant because it describes a method for traversing millions of light years in an observed year -- without violating relativity. A great old one who can throw around galaxies with great speed CAN make it happen on a human-relevant scale if he moves the milky way at near light speed rather than (or in addition to) throwing something at the milky way. I can (theoretically) move millions of light years as fast as I want, but you can't move to me over millions of light years and expect to find me alive.

I'm trying to come up with a definition that's not recursive. An inertial reference frame is a frame in which inertia is preserved within the frame. Meaning that if an object is moving at a velocity and it experiences no outside force it continues moving at that velocity. You can determine if an outside force exists or not by newton's law about action and reaction. The classic way to do this is to peg the frame to an object or center of gravity that has very little acceleration within the problem set you're considering. Objects like to move in ways that satisfy inertia. Look at your reference frame from a Newtonian perspective, and if you're breaking the action/reaction rule to explain behaviour, you've got an non-inertial reference frame (or magic*).

Now, you will pretty much always have a tiny bit of acceleration in your inertial reference frame. Alpha Centauri and the sun are rotating around the galaxy center, which means a frame based on the center of gravity for these two has some acceleration going on. But its tiny and you can ignore it without messing up your data too much. If you're dealing with independent galaxies you have the thrice-blasted expansion of the universe. But that is often small enough to ignore. Basic physics: ignore the really small numbers and make sure the numbers you don't want to deal with are really small. Don't overthink finding the missing acceleration. Most of the time it will be really obvious, like objects orbiting much smaller masses, separate large objects accelerating in tandem without reaction mass, or other nonsense. Trust inertia. If gravitational, electromagnetic, and kinetic inputs aren't really acting on an object or theoretical object or system of objects, you have a valid nearly inertial reference frame.

Suppose (purely hypothetically, since this is abstract math anyway) there's an infinite uniform wall of a huge mass at a moderate distance away from the objects whose movements we can measure. Everything of our analysed objects will be influenced equally by the wall, and everything will have the same acceleration relative to the wall, but not against each other. That seems to mean that 'observed inertia' is misleading.