This is a really interesting website on Nova. Answers some questions that were asked about time and space density.
This is a test that puts relativity in its relative perspective.
pbs.org/wgbh/nova/time/think.html
raj, the site is amazing.
know of any good sites to explain stephen hawkings?
Yeah, his website is really good 
It has an online dictionary for a lot of terms, with animations! It’s pretty cool. Of course, if you read his books written for the layman, he shouldn’t need too much explanation. I think he does a great job.
Ah, if only Aimee Sweet’s website was as extensive as that.
you can’t really travel through time, because time doesn’t flow. that’s a trick of our nervous system. but if the many worlds interpretation of quantum mechanics is true, then if we could someone go to other universes, we could use space travel to conduct what would appear to be time travel.
I didn’t want to start a whole new thread on a quick interpretation of a quote so I’ll just post here cause it’s semi related.
At the risk of making myself look like an even bigger ass … I don’t understand that.
Basically it’s that if the laws of the universe had been different there would be no-one around to say “Cor, if this law had been slightly different there WOULD be life”.
So we’re looking at the laws and saying “these are suspicious as they’re right for creating life conditions”, when these conditions had to exist in the first place for the question to ever be asked. It’s hard to think of a good example to put it simpler.
I like your explanation. Good job! 
What is time? A trick of our nervous system? Sorry, I don’t think so.
Time is, at least of what is understood to date, the atomic rate of a system. Different entitities within different systems either traveling through space at relatively different rates or systems that are effected by comparitively different gravitational forces will have different atomic rates and therefore different times. Traveling at very high speeds close to the speed of light can cause some entity to travel forward into the future of another entity. This is possible because a person traveling at more than half the speed of light relative to this planet would slow there own atomic rate, and therefore thier metabolic rate, such that upon return on some future date to this planet, the time traveler would have aged significantly less than the creatures he left behind here on earth. Approximately a 10,000 year difference for every two years traveled at half the speed of light. That means, if you could travel at half the speed of light, after two years you would return to earth that is 10,000 years older than the day you left. Weird huh?
And with respect to gravity, if you could place yourself in a system of relatively higher gravitational force than the earth, you could also slow the atomic rate of your system relative to the earthlings you were one of.
You know that at the center of a black hole (the singularity) gravity is so great that every system at that point in space will stop moving forward in time entirely, relative to a system that is at some different point in space.
Well travel into the future certainly is possible and has been proven scientificaly, given we could move at very high speeds (not unlikely one day), or if we could warp space, which is what gravity does. But what about traveling back in time?
It is said that we could never travel to a time before we invented the time machine. This is because a time machine would be a device that would allow something to be received and transmitted through time and if the machine did not exist in lets say 1992 how would a signal or any entity be received in that year? But what about traveling at greater than the speed of light? According to relativity, a time clock for one entity travaling a the speed of light relative to another entity would stop completely to the entity not moving. And, the entity traveling at the speed of light would experience time and space to be foreshortened. That means that while long distances were traveled through space the traveler would experience a very short trip both spacially and time wise. Wow! this means that you the space traveler would get from one end of the universe to the other (15 billion light years across) almost instantaneously!!! But remeber the earth people you left behind aged 15 billion years!!! HUH?
SO! if you could than travel faster than the speed of light it is determined that you could go back in time. See a “space time diagram” if you can. If you could go faster than light than you could simply travel through space and return to earth at a date prior to the date you left.
Well… I understand that. I think it’s relating it the first part that’s confusing me. I can’t explain why it doesn’t make sense but the explanation doesn’t seem to matter to the question kind of type deal.
i understand all of that- i’ve read a steven hawking book or two, as well as lots of stuff on relativity, spacetime, and black holes.
what i was saying, or at least trying to say, is that time isn’t a fixed thing that is separate and unchangable.
i understand where you’re coming from with the speed of light idea thing to use as time travel. however, from what i understand, the closer you get to the speed of light, the more energy you have to use to maintain your velocity. i think i read somewhere that for a human (that’s a lot of particles) even to reach 9/10 of the speed of light you would have to expend more energy than it’s possible for us to generate right now. plus, if you need to generate energy, you’ll need a power source on the time machine, which will in turn need even more energy due to the extra mass.
of course, my understanding of the matter may be completely wrong.
No, it’s completely right.
Travelling into the future is impossible because it violates causuality. It would require seeing the effect before its cause produced it.
why? i.e., would you care to elaborate?
Perennial objections like “if you went back in time and killed your grandmother…”.
Recent findings which I do not subscribe to account for this supposed paradox. They state that causality actually occurs in 5 space, not 4 space. This means that if you go back in time and kill your grandfather, you suddenly no longer exist. Odd, huh?
I don’t think that’s right, though… for multiple reasons. First, you have problems with thermodynamics in that it would allow for the dissolution of matter. Since anything that can happen does happen, it would be happening on a rather grand scale when you consider how entities multiply on a Einstien multidimensional paradigm.
Another problem I have is that if time is that malleble, then either A) all of time would be in constant flux such that nothing every happens, and there is no permanance (something that is completely undesireable) or B) time doesn’t actually exist and is merely a manifestation of a moment of randomness such that we actually live infinite lives for every moment we experience in this world.
B) is actually a good way of looking at the Einstein model, but the math doesn’t support it… Instead of moments running intertwined, they are supposed to run concurrently. This means that for every choice, there are an infinite other possiblities that occur at the same time. This means that all time travel would be is the shifting of perspective between other time-perspectives.
There’s several paradoxes with this, as well. For instance, the Dr. Who paradox… if you can travel back in time, and you can never go back to your time world, if you send someone back in time on a mission, will they actually execute that mission, or will they merely disappear from your reality. Well, this one is easily fixed…sorta…since there are infinite realities, surely one of them will land back in your reality, yes? This raises a problem, though. What if 100000000000 time travelers land in the same spot because they all are trying to accomplish one single mission and your world is similar to theirs? Uh oh! lol…
I prefer to say that time travel into the past is not possible. Why? Because if it were possible, we would see evidence of it, most likely, just as we do with time travel into the future. But we do not, sooo…I’m betting on Hawkin’s ideas for this one.
Yes, I agree. If people are ever going to find a way to travel backwards in time, wouldn’t we have heard from them at some point? What i mean is, wouldn’t somebody have known? I’ve never seen a history book that had a section on “The time traveller who came to watch a Jimi Hendrix concert.” The only possibility I can think of is Egypt. Maybe some visitor from the future came and showed them all of that stuff. Or maybe not.
One again i will have to ask why is not possible to travel to pass and not to the future???
If we could travel to the pass and see people from the past does peoples wuold see us as a future people, but if they return to the future with us wouldnt they being traveling to the future then while we would only be traveling to our time!
It´s like someone from our future come here today to visit us and then they let us go with them to their future! Wouldnt we be traveling to the future!? And please dont tell me that its will be impossible to us travel to the future because we would disappear or die or something like that! If there is something that is not even exacly especified is exacly if we would die traveling to our future!! it´s like everytime part of time (past/present/and future) are diferent dimensions! For us we are at the present, but for someone from the future we will be someone from their pass and for us somone in our pass will be always someone in our past but in their perpective we will be people from the future! right?
Im sry, in the first line i meant to say:
Once again i will have to ask why is possible to travel to past and not to the future???
My english still not getting better!! 
best regards