General Relativity Question

Good reply.But not stupid. You've just digested too many dubious layman accounts and need something a little meatier. Have you tried Brian Greene's book? Try that and then maybe Hawking's Universe in a Nutshell. The best place to get a good grounding though is John Baez's site and the physics FAQ he hosts hereAbsolutely. There is no simultaneity in relativity. The "order" of events that are not causally related themselves has no meaning. If I clap my hands on the earth, and an astronaut claps his gloves on the moon within a second of each other, there is no concept of who clapped first. The order of the events purely depends upon vantage point. There is no correct answer. If the astronaut is orbiting alpha Centauri, then it is anything within 4.3 years of each other!! Well, I know what you mean. It is a fundemental property of the universe and not just a measurement problem. BUT the light time is intimately related to this property and is equally a fundemental property. Exactly... In the same way the universe guarentees that angles in triangles add up to 180 (in flat space) It is "simple" geometry within the mathematics of relativity. This catching up is simply your observation of someone else's time. This observation is very dependent upon your motion, especially whether you are moving away from them or moving towards them.Your own passage of time is a measure of "proper time" which we just integrate up over your journey which depends upon your acceleration. You would get less of an effect, becasue you would be travelling slower on average (as slow acceleration/deceleration implies spending more time proportionally at slow speeds) But there would be no qualitative difference.

Thanks!Let me try this again. In the twins paradox, you can simulate the concept of "simultaneity" by making your twins psychic. Stella is one light year away, traveling half the speed of light. Messages from Terra are one year old, and sped up. (Are they sped up twice as fast, have i got that right?) What I mean is, they have to be replayed in slow-motion in order for me to understand them, as broadcast they sound like the chipmunks.But Stella and Terra are very close, they are in fact Psychic Twins, and can communicate simultaneously with one another! Mental messages from Terra are received Now, not one year later, but they Also are sped up, and make Stella's head hurt. Terra has to learn to think Twice as slow (or whatever) in order to get ideas across. Perhaps helpful scientists can freeze her a bit!As Stella accelerates and decelerates amongst the nearby stars, the radio messages she sends speed up and slow down in relation to the relative aging of the sender, always remaining slower than the proper clock of Terra. When she orbits a planet or stops to turn around or whatever, this gets back down to the minimum so that these messages sent then are just about normal speed. But they are also delayed by the length of time it takes them to travel, which is the distance in light years.Keeping the idea of simultaneity alive for a moment longer, these mental messages also slow down and speed up relative to the senders experience of aging, but get through a period of time earlier which is about equal to that same distance in light years. Stella is now 10 light years away and traveling at 3/4 of the speed of light, her messages are 4 (???) times slower, don't get here for 10 years, her mental messages are behaving a totally different way, and we already know there's trouble.Stella is still pretty young, we are only keeping Terra alive with extreme means to keep the messages going, and now little Estrellita, who has inherited her mom and aunt's special connection, is heading back towards earth! she has one speed of light relative to one inertial frame and one relative to another! She has two speeds of light, and with her magical simultaneous connection to both reference frames she can see and change the future! (if, and only if, she can come up with a way to read some psychic messages even if they are being received in reverse)Is that it, is that what breaks simultaneity irrevocably?

Absolutely. What you are calling simultaneous is just what we conventionally call "faster than light". Yep, the telepathic messages will sometimes be in reverse, so that one twin perceives the other as travelling backwards against the flow of time. Such is the price of breaking causality with telepathy (or any other FTL mechanism). Another observer will be able to see one twin receive the message before he sees the other send it! In an extreme case, he could see the message receieved and still have time to intercept the first twin to stop her sending it! I would put it the other way round. If you want to preserve causality, you have to give up simultaneity. This applies to telepathy, hyperspace, warp drive, sub-space transmissions, etc, etc. This is why wormholes are also time machines...

Just breaking causality doesn't bother me that much though, wouldn't it take care of itself quite naturally?Let's take your observer, call him Tiberius, let's say he does intercept little Estrellita and take advantage of her to change the past, make himself Emperor and prevent space travel from developing at all. As a result, Stella never even leaves the planet! And as a result of that, the cause of Tiberius becoming Emperor never happens, and the past is changed again!This time through though, every single little quantum event that is truly random fires differently. Every butterfly flapping his wings and making a hurricane goes a different way. Stella and Terra are never born, instead some different idiots eventually develops a different method of time-travel, and try to do something else (interfere in the Kennedy assassination I'm betting.) They find that every time they try to accomplish anything though, kidnap Oswald before he ever goes to work that day, or take a pot-shot at that guy on the grassy knoll, it all goes horribly wrong.They don't get reset though, like Tiberius, because they don't actually succeed in altering their past, they just turn out to have already been creating it before they were born.These two scenarios show breakable and unbreakable causality and how they are indistinguishable. The Kennedy guys could have succeeded in breaking causality a hundred times and not know it, because the only spacetime that remains stable and consistent is the one that eventually results in an odd string of terribly improbable events. The others won't ever have existed once they are gone.

Well, local causality is fundemental to sensible dynamic quantum field theory so we probably wouldn't exist without it! But non-local causality breaches shouldn't be too much of a problem.However, GR does not allow for multiple time-lines. There is one past, one future, and they are set in stone. Consistency is the name of the game. The observer could not stop the twin from transmitting, having seen the reception, because it did not happen. In this case his freewill (should any exist) is removed.You can postulate the ability to "change" the past, but you are moving outside of GR. And as it is GR that has actually allowed you to break causality, you are off into fairy-land...

You're welcome, if even half these models help you get some of it I don't think it will be a waste of time. I'm sure you understand I am oversimplifying everything out the yinyang in order to break it up into palatable chunks, there's no other way around it though.For example, my Crowleian distinction between mass-units and space-units is only even implicitly true down at the very lowest levels of spacetime, in the real world we always find mass and space joined together at the hip in exact proportions, and the words "fermion" and "boson" actually mean something rather different than the ideas I am using them as a place-holder for. I promise though, we will get to the business of how a helium atom could also be a boson in a bit. In the meantime, let's stick with mass and space, as it sounds a bit more scientific than "something" and "nothing".Assuming you have got the idea of how gravity could be a field, a quality of space associated with a mass, you can see that spacetime, distance, all that, is directly associated with the masses themselves. That is, space is a field that extends outward from a planet (for example) and decreases in some sort of strength or intensity the further away it is. Imagine it in terms of there being More space in the direction of the mass than away from it. As a results of this bias in direction, all the random motion represented by rest energy that would normally cancel itself out now slants in the direction of the planet.Cavediver talked through the mathematical meaning of the word "field" and I don't know that I could add a lot to that. One of the fun things to make note of with quantum mechanics though is that the reason you have to resort to averages and sheaves of probability to do proper math with them is that some of your numbers drop to 0. You know that ????? or ###### your spreadsheet gives you when one of your fields you are dividing another one by happens to be 0? And then if you get 0/0, what the heck is that ?!?That's what the quantum guys like to do to the General Relativity guys math! The way they get through it (a fellow named Cantor helped) is to go ahead and allow for "zero" and "infinity" in the equations and treat them as misbehaving variables. So for example when we take 3x and 7y and throw them through the 0/0 machine, we get back something odd of course, call them 3x' and 7y', and we dont know what x' and y' will be! But we do predict that we will get 3 of one and 7 of the other.

As Fabric was asking about fields, I thought I'd link this thread to a brief primer I made on quantum fields over in the Coffee House. If anyone wants to go further, or ask any questions, just drop in a post.My decription of classical fields is here Message 28The quantum field stuff is here: Message 90

Thanks cavediver & Iblis those explanations help me out alot, i now understand what fields are or at least i understand how Gravity is a field through out the whole of space and when
you get mass curving it the values change because space is closer together.... Right ??
obviously thats the simple way of looking at it....I have been reading about particles recently and have another question:when we talk about atoms , electrons, quarks ect are they really tiny bits of matter
or energy with different charges giving them the value they have.... What is fundamentle matter made of..??Thanks for your timeThis message has been edited by Fabric, 01-31-2006 06:20 PM

You have been reading too much Heinlein.