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View Full Version : A weird theory, probably worthless, still...



Professor Tanhauser
2007-Feb-21, 05:49 AM
I was about half asleep recently and thinking of the classic SFRPG "Traveller" which uses a jump drive to exit the universe as we know it, enter an alternate plane called "jumpspace" and reenter this universe a week later up to 6 parsecs away from it's origin point in this universe.

People claim it's "Impossible" bacause it violates casualty as you might appear to arrive before you left blah blah blah.

The thought occured to me that what if there's a balancing effect that makes the casualty violation moot?

Suppose someone says "Frak casualty, I'm sick of earth and am going to another star system, builds himself a starship complete with jump drive, fires it up and a week later re enters this universe 4 light years away. Now to someone in that system watching ours with an enviably powerful telescope, he might appear to arrive 4 years before he left, true.

But imagine someone from earth was watching his target system with an equally impressive telescope. He sees the traveller arrive in the system 4 years after he actually arrived.

This would seem to balance out the casualty issue. One observer sees the traveller arrive x time before he left, but an observes at the opposite end sees the traveller arrive x time after he actually did arrive. Would this equal discrepency balance things out?

Like I say, I'm probably laughably wrong, but I wanted to ask. it semed almost like a question a conceptual physicist might ask.

Jason Thompson
2007-Feb-21, 11:43 AM
I posted a reply on this and it seems to have been lost. Basically it boiled down to appearing to arrive before you leave not violating causality at all.

If a bomb goes off a few miles away, by the time you hear it the bomb has not existed for several seconds. You can see that, but you have no aural cues. In the case of your traveller, you can see that he has left Earth because he is standing next to you, you just haven't seen him depart yet. Two pieces of information about an event have travelled at different speeds. In this case the light arrives before the sound, in your case the traveller himself arrives before the light from his departure does. In the case of the bomb few people would have trouble recognising the fact that the bomb has exploded and will realise they are too far away for the sound to have reached them yet. In the latter, you can see from the arrival of the traveller that he has departed Earth, but the light from that event just hasn't reached you yet.

So no causality violation and no need for reference frames to cancel out.

korjik
2007-Feb-21, 04:15 PM
assuming that gravity propagates at c, then FTL travel violates energy conservation. for a while, you will have gravitational potential energy in two different locations at once

WaxRubiks
2007-Feb-21, 04:23 PM
if gravity propagated at c then if you traveled 4 light years in 6weeks then you would have the possibility of being gravitationaly effected by your own mass(ie from the possition of your spaceship before traveling). Would this be possible? I have my doubts.

Jason Thompson
2007-Feb-21, 04:52 PM
assuming that gravity propagates at c, then FTL travel violates energy conservation. for a while, you will have gravitational potential energy in two different locations at once

Is that a violation though? I'm not an expert in the field, so feel free to correct me if I am wrong, but since I am not actually in two places at once my two gravitational potential energy fields will be fractions of my total field, or something along those lines? Or, put another way, why is it not similar to the situation with the light from my two locations?

Lets say I start from point A and move to point B one light hour distant. At one light second from point A is a bloke with a gravity detector (I know, just work with me, OK!), and another bloke with a similar instrument is one light second from point B. At the start, bloke A can detect me at a certain strength, and bloke B can detect me at some tiny fraction of that because I am so much further away. If I transfer instantaneously from point A to point B, one second later bloke A ceases to detect my mass at the same moment the light from my departure arrives at his location, but because the light and gravity are still propagating bloke B still detects me just as he did before I left point A. At the same instant I disappear from bloke A's detector, bloke B then detects me at the same strength at point B because my gravity and light have just reached him. Now he's detcting me from two places at once, but bloke A is no longer detecting me at all because my light and gravity have propagated past him from point A, and have not yet reached him from point B, so isn't the potential energy the same overall? One hour later bloke B stops detecting my signal from point A, and bloke A starts detecting my arrival at point B when he receives a signal at the same strength bloke B was detcting me on before, and we have a neat reversal of the situation. I can't see any violation of energy conservation.

Put another way, before I leave bloke A detects me at 13 billion gravity units, and bloke B at 1 unit. Total 13 billion and 1 units.

1 second after I transfer, bloke A loses the signal and bloke B picks me up at 13 billion units, as well as still receiving me at 1 unit from my original location. Total 13 billion and 1 units.

1 hour later, bloke B, still reading 13 billion units from my new location, loses my 1 unit of signal from my original location, but bloke A picks it up from my new location. Total 13 billion and 1 units.

Or, another way, if my field is spherical it will empty from the middle (my original location) at the same rate it starts to propagate from my new location, won't it? So at any time there'll be a hollow in my original sphere which exactly matches the new sphere. Same potential energy, I've just shifted a bit of it around.

As I say, I am no expert in the field. If I'm wrong be gentle, or else I suspect it will go over my head entirely.

korjik
2007-Feb-21, 05:52 PM
Is that a violation though? I'm not an expert in the field, so feel free to correct me if I am wrong, but since I am not actually in two places at once my two gravitational potential energy fields will be fractions of my total field, or something along those lines? Or, put another way, why is it not similar to the situation with the light from my two locations?

Lets say I start from point A and move to point B one light hour distant. At one light second from point A is a bloke with a gravity detector (I know, just work with me, OK!), and another bloke with a similar instrument is one light second from point B. At the start, bloke A can detect me at a certain strength, and bloke B can detect me at some tiny fraction of that because I am so much further away. If I transfer instantaneously from point A to point B, one second later bloke A ceases to detect my mass at the same moment the light from my departure arrives at his location, but because the light and gravity are still propagating bloke B still detects me just as he did before I left point A. At the same instant I disappear from bloke A's detector, bloke B then detects me at the same strength at point B because my gravity and light have just reached him. Now he's detcting me from two places at once, but bloke A is no longer detecting me at all because my light and gravity have propagated past him from point A, and have not yet reached him from point B, so isn't the potential energy the same overall? One hour later bloke B stops detecting my signal from point A, and bloke A starts detecting my arrival at point B when he receives a signal at the same strength bloke B was detcting me on before, and we have a neat reversal of the situation. I can't see any violation of energy conservation.

Put another way, before I leave bloke A detects me at 13 billion gravity units, and bloke B at 1 unit. Total 13 billion and 1 units.

1 second after I transfer, bloke A loses the signal and bloke B picks me up at 13 billion units, as well as still receiving me at 1 unit from my original location. Total 13 billion and 1 units.

1 hour later, bloke B, still reading 13 billion units from my new location, loses my 1 unit of signal from my original location, but bloke A picks it up from my new location. Total 13 billion and 1 units.

Or, another way, if my field is spherical it will empty from the middle (my original location) at the same rate it starts to propagate from my new location, won't it? So at any time there'll be a hollow in my original sphere which exactly matches the new sphere. Same potential energy, I've just shifted a bit of it around.

As I say, I am no expert in the field. If I'm wrong be gentle, or else I suspect it will go over my head entirely.

I am going to use different numbers and units:

Take three points A,B,C. A is 5 ly and B 1 ly from C. An object travels from A to B instantly. For 1 year C sees the object at A. Then all of a sudden, it sees the object at both A and B. It would also feel the gravity of the object from both locations. Then 4 yeas later, the object dissapears at A.

For 4 years, the gravitational potental of one mass is felt twice.

Delvo
2007-Feb-21, 06:16 PM
There's no problem with that. I've seen plenty of examples, with watercraft on lakes and rivers, of a single point receiving wake all at once that was cast by the same vehicle at different distances and times.

Jason Thompson
2007-Feb-21, 06:44 PM
I am going to use different numbers and units:

Take three points A,B,C. A is 5 ly and B 1 ly from C. An object travels from A to B instantly. For 1 year C sees the object at A. Then all of a sudden, it sees the object at both A and B. It would also feel the gravity of the object from both locations. Then 4 yeas later, the object dissapears at A.

For 4 years, the gravitational potental of one mass is felt twice.

OK, but I still don't see that's a violation, just a rearrangement. For the time that C experiences the gravity twice, there is a region where there is a corresponding lack of gravity where there had been before. There isn't actually any more gravity, just a rearrangement that means some places experience more than they did before. Put an object D 1ly the other side of A from C. For one year it sees the object at A, then it loses it and stops feeling its gravity at the same time C picks it up at B. So C feels the gravity twice, but D has stopped feeling it. Where's the violation?

The object that moves doesn't exist simultaneously in two places at once, so the moment it starts generating a gravitational field at point B it stops doing so at point A, thus conserving energy.