Up and Down in Spacetime... - Page 2

CodeSlinger · #31 (**permalink**) 21-February-2008, 06:41 PM

What's wrong is that the elevator thought experiment has nothing to do with directionality. The equivalence principle is about the equivalence between gravity and uniform acceleration; it does not say directions like up and down are equivalent to each other, as you seem to think. Want to try again?

Occams Ghost · #32 (**permalink**) 21-February-2008, 06:49 PM

But it does, from an observer's point of view... instead of boring this out - why don't you contribute something... something intelligent, something of thought... rather than obtrusion.

CodeSlinger · #33 (**permalink**) 21-February-2008, 06:52 PM

Here's the Wiki article on the equivalence principle. Educate yourself on what it really says. If you can't stand being corrected, then don't post stuff that's blatantly wrong.

Occams Ghost · #34 (**permalink**) 21-February-2008, 07:00 PM

Look... bottom line... Stop throwing that ancient relic wiki at me. I and many, have spotted mistakes in their work... in fact... Wiki is written by the people and for the people. I know the Equivalance covers many things, but you treat it as a narrow road.

--- There is no directionality in spacetime--- period... full stop/

Occams Ghost · #35 (**permalink**) 21-February-2008, 07:01 PM

And for your information, correction is NOT a problem. It's downright dogmatism hat gets me.

CodeSlinger · #36 (**permalink**) 21-February-2008, 07:28 PM

Quote:

Originally Posted by Occams Ghost

I know the Equivalance covers many things, but you treat it as a narrow road.

If you mean I prefer the correct definition, then yes.

Quote:

Originally Posted by Occams Ghost

--- There is no directionality in spacetime--- period... full stop/

Of course there is, your temper tantrum notwithstanding, unless you think heading towards the sun is exactly the same as heading away from it.

Quote:

Originally Posted by Occams Ghost

And for your information, correction is NOT a problem. It's downright dogmatism hat gets me.

This is a public science board. Bad science gets corrected. If you can't help but mistake that for dogmatism, perhaps this is not the place for you.

Disinfo Agent · #37 (**permalink**) 21-February-2008, 08:12 PM

Quote:

Originally Posted by Occams Ghost

--- There is no directionality in spacetime--- period... full stop/

Then how come white holes don't exist, in addition to black holes?

ravens_cry · #38 (**permalink**) 21-February-2008, 09:14 PM

In one sense I agree, in another I don't, Occamsghost. Yes, in space time, there is no absolute referance point, no north on the compass, no ceiling to bump our heads on and such like. but that is reletivity. On the other hand, if I go one way I will get to that star and if I go the other way, I will get to a completely differant star.In empty space time, going up is no differant then going down, except in your flight path. And yes, that down is spmeones up, but right here, right now, its down. For gravitational fields, up and down are syononiumus with in and out.

Occams Ghost · #39 (**permalink**) 22-February-2008, 12:42 AM

But Einsteins relativity states that if you travel far and fast enough, you will experience a curved path through spacetime. You are no longer accelerating in a straight line, but now in a curved path.

CodeSlinger · #40 (**permalink**) 22-February-2008, 01:15 AM

Yeah, they're called geodesics. We all travel in geodesics, regardless of how far or how fast we're traveling. And you know what? That doesn't mean there is no directionality in spacetime. Just the opposite, in fact, because without directionality, you can't distinguish between different geodesics.

The number of buzzwords you try to pull in at once is impressive; your understanding of them, rather less so.

ravens_cry · #41 (**permalink**) 22-February-2008, 01:19 AM

I am not sure about that, but I think that is if the universe is spherical. Is it? I don't know enough to say. Anyone like to support me, inform me of my errors? I only know the metephors of relitivity, like flat land, and rocket elevators, and squishy near-lightspeed trains.

Occams Ghost · #42 (**permalink**) 22-February-2008, 01:26 AM

Your wrong. A fast enough object will follow a totally different path to our slow existences. This is because very fast objects in spacetime will evade any gravitational pull by our solar system... Geodisocs are only the most effecient path for a system to take...

ravens_cry · #43 (**permalink**) 22-February-2008, 01:38 AM

up and own are relative, but they have meaning within there frame of reference. Whether that is arbitrary one in empty space (that star is 'up' and that star is 'down') or a more or a frame of reference with a more physical base, (like a gravity well) it is all relative, but still important within it's frame of reference. Being relative, doesn't mean it is irrelevant.

CodeSlinger · #44 (**permalink**) 22-February-2008, 01:56 AM

Quote:

Originally Posted by Occams Ghost

A fast enough object will follow a totally different path to our slow existences.

Yes, objects at high speeds follow their own geodesic; in fact, there's a different geodesic for each velocity. But again, everything travels along geodesics, not just "fast enough objects".

Quote:

Originally Posted by Occams Ghost

This is because very fast objects in spacetime will evade any gravitational pull by our solar system...

Um, no. Even photons, the fastest traveling objects, are not exempt from gravity.

CodeSlinger · #45 (**permalink**) 22-February-2008, 01:56 AM

Quote:

Originally Posted by ravens_cry

up and own are relative, but they have meaning within there frame of reference. Whether that is arbitrary one in empty space (that star is 'up' and that star is 'down') or a more or a frame of reference with a more physical base, (like a gravity well) it is all relative, but still important within it's frame of reference. Being relative, doesn't mean it is irrelevant.

Bingo, I couldn't have said it better

Occams Ghost · #46 (**permalink**) 22-February-2008, 02:18 AM

I'm tired of this now, so i am going... but when i said the gravitational pull, the object needed to be ''just'' outside any critical pull, and to top it off, and make it really safe, moving at really high speeds.

CodeSlinger · #47 (**permalink**) 22-February-2008, 04:26 AM

If you're trying to say that objects can achieve high enough velocity to escape our solar system's gravity, sure. I believe some space probes have done just that, and are sending us incredible information about our solar system even as they depart it. But even they are traveling along geodesics.

publius · #48 (**permalink**) 22-February-2008, 05:21 AM

Geodesics, in space-time, are simply the "force free" path an object will take in a given space-time. In flat space-time, these geodesics are simply straight lines (with slope less than 'c' for objects with rest mass). In curved space-time (or curved coordinates, if you happen to be accelerating yourself in flat space-time), these geodesics are not straight lines, but curves. Light follows a special limiting class known as "null geodesics", and these are generally curved in curved space-time.

The (weak) Equivalence Principle is simply that these geodesics do not depend on the mass or composition of an object. The feather falls at the same rate as the brick. Further, it says the force free path an object (or light beam) will follow is its geodesic.

Now, the Equivalance Principle is sometimes seen as saying two different things, being stationary in a locally uniform gravitational field vs accelerating in flat space-time are merely indistinguishable. That is, they are different, you just can't tell the difference.

Well, the proper view informed from the elegant picture of General Relatvity is that they are in fact the same thing. There is no difference at all (save for coordinate choices, which are arbitrary and one is as good as any other) between the two as far as GR is concerned. It *is* the Equivalence Principle, living and breathing.

-Richard

ravens_cry · #49 (**permalink**) 22-February-2008, 06:11 AM

One thing I don't understand about the equivlance princible, is that gravity doesnt seem to need energy to stick me to the ground but accelerating to stick me to the floor does. Am I missing something?

tusenfem · #50 (**permalink**) 22-February-2008, 08:22 AM

Quote:

Originally Posted by Occams Ghost

Your wrong. A fast enough object will follow a totally different path to our slow existences. This is because very fast objects in spacetime will evade any gravitational pull by our solar system... Geodisocs are only the most effecient path for a system to take...

You seem to be of the opinion that a timeline in a spactime diagram is the same a a path through physical space, it is not.

In physical space I can go from a to b by:
1) moving at constant velocity through space in a straight line from a to b
2) moving at ever increasing velocity (acceleration) through space in a straight line from a to b

Now, in physical space I will have traveled a straight line from a to b in both cases. However, in a space-time diagram the two timelines will look different. For 1) it looks like a straight line at an angle ~v/c with the time axis; for 2) it looks like a parabola. I assume you mean this parabola as the "totally different path".

The fact that you state here that "This is because very fast objects in spacetime will evade any gravitational pull by our solar system" is naturally preposterous, and shows that you lack a basic knowledge of physics. Very fast objects are too, gravitationally attracted by the sun. Heck, even light cannot "evade" the gravitational potential well of the sun, like one of the experiments proving relativity showed during a solar eclipse. The only thing is that the faster an object moves, the shorter the interaction time is.