Ok, I haven't had time to read the General Relativity section of the book, so I'll just give a brief summary of how I understand it. In the last post, I was talking about how motion is relative, and that the way the time dilation and length contraction (slow-motion and getting squished) are symmertrical. If I'm flying past you at high speed, you appear to be in slow motion and squished, but to you,
I appear to be slow-motion and squished. This doesn't seem to make sense, somehow it has to all sync up once I start moving. General Relativity helps with that.
The first thing GR deals with is gravity. Gravity basically "warps" space-time. If you imagine space-time as a spongy mattress, sticking a heavy object on it like a bowling ball is going to create an indentation, which will cause marbles to roll towards the bowling ball and stick to it. If you roll a marble past the bowling ball, it might get caught in the dent and roll around in a circle around the bowling ball before crashing into it. This is sort of how planets and moons work in three-dimensional space. (As a side note, physicists often describe a black hole as when you put something so heavy on the mattress that it actually tears a hole in the bottom that creates a bottomless pit.)
Gravity distorts time in a similar way to motion, except with gravity, it's
not symmetric. Things in heavier gravity than you appear to be moving in slow motion, while things in less gravity appear to be moving in fast-forward. So if we live on a planet where gravity is really high on the surface (everything is really heavy) but it's very light at the top of the buildings (everything is light), if you were at the top looking down, you'd think you were moving in normal speed but everyone at the bottom was stuck in slow motion. But if you were at the bottom looking up, you'd think you were moving at normal speed and everyone above was moving in fast-forward.
Now, one of the cool things with General Relativity is that you can basically treat acceleration as if it were gravity. You know how when astronauts fly into space and they have to deal with the "g force" of their acceleration pushing on them? They call it g force because it's much like gravity, and aside from "pushing" on you and making you heavy just like gravity, it also has the same effects on time. Someone who's accelerating relative to you will appear to be in slow motion, but from their point of view, you're moving in fast-forward. This is how the "twin paradox" is solved. The twin paradox is the question of what would happen if there were two twins, and one got on a rocket and zoomed away at near light speed and then zoomed back. With just Special Relativity, both twins would have experienced their other twin as moving in slow motion, so when they meet again, which one is actually younger? General Relativity resolves the paradox. The twin who got in the rocket is the one who will be younger. The rocket twin can't just instantly start moving at near the speed of light; she needs to accelerate up to that speed. While she's accelerating up to that speed, she'll be feeling a lot of g-force, and so from her perspective, the ground twin is actually moving in fast-forward, until she hits light speed, when the ground twin starts moving in slow motion again. Whereas from the ground twin's perspective, the rocket twin is slowed down
more than normal just with special relativity. The rocket twin has to fly to the other planet, and then put on the brakes (which is just accelerating in the opposite direction), which again causes g-force, and then turn around and accelerate back to light speed, and then put the breaks on again when she gets back to earth. Each of those periods of acceleration cause the rocket twin to move forward in time and see the other twin moving in fast-forward, so that overall, there is more fast-forward from her perspective than slow-motion. In the end, when she finally gets back, she'll have moved forward in time more, and could be like four years younger than her twin depending on how long she was out there flying around.
So the moral of the story, is if you want to get to the future faster than everyone else, you should be constantly slamming on the gas whenever you drive around to get some really good acceleration, and drive as fast as you can. Sure, it would probably only add up to a few seconds after fifty years, and you'll probably get into a car accident before then, but still, the idea is solid...
artisticsolution wrote:Hi I was just having a daydream....wouldn't it be cool if information could be attached somehow to a light beam...so then it might allow for a different type of time travel. It seems the way we imagine time travel...that is by physically going back to a certain era would be impossible. But suppose we could attach some sort of information via fiber optics that could withstand 'time travel" to and fro on a light beam? So then we might be able to read the information in some sort of virtual reality type scenario, and thus be able to "walk around" in another time.
Fiber optics basically is sending information on a light beam. But, unfortunately, it seems that you can't get light, or anything else, to move faster than the speed of light. If we're talking about mass, the g-force that I mentioned comes into play. As you get closer to the speed of light you need to keep accelerating more and more, but you keep getting heavier and heavier. In order for something with mass to hit the speed of light, it would weigh an infinite amount, and require an infinite amount of energy to accelerate it. But if you did hit the speed of light, time outside your ship would literally be stopped completely, and everything in the universe will contract to a length of zero, meaning you'd be in all places along a line at the same time. If you could break the speed of light, time outside your window would start moving backwards, and space would contract so much it would turn "inside out", though I can't really picture that in my head that well.
Most scientists believe that currently it's impossible for "information" to travel faster than the speed of light, where "information" means pretty much anything that could tell one place about another place. We haven't been able to figure out a way to get anything to break that barrier. If we could, it would lead to some problems with causality. If you sent a message from here to another colony on some other planet faster than light, it would basically travel back in time as you said, and the message would get there before it was sent. That would mean if the person on the other side reacted fast enough, he could fly back to earth and stop you from sending the message. Time paradox!
Wouldn't it be totally cool to be able to send a message back in time? One problem would be getting a recipient, on the other end, to know that is what was happening and would know enough to then decipher the message. But if there was a way of just sending that info even a second or 2 back in time it would be ultra cool....because then...from that time on...science might know more about how it was done and could prepare themselves for receiving messages from future scientists. And if we got very good at receiving the messages...we could send messages to the minds of people in the past/future and be able to "walk around" in the future via virtual reality from the information we would have retrieved from signals being implanted in a host body from the past...or future.
Why should we insist on time travel being a physical thing? Why can't it be information attached to light? Is that not possible either?
So far it doesn't seem possible, but people are still trying to figure out a way around it. There might be some hope in quantum physics, with a thing called Quantum Entanglement. The quick explanation is that you can set it up so that two particles are "entangled", so that when you spin one particle this way, the other particle spins the other way. But then you can separate the two particles, even miles apart, and it would seem they still remain "entangled", with one instantly reacting to someone messing with its twin. Einstein called this "spooky action at a distance" and refused to believe it at first, but eventually came to accept it as truth. The theory is that you could entangle two particles, then bring one of them to somewhere really far away, then poke them to have instant communication, faster than light. But I believe quantum scientists still aren't sure if it could be used that way... the whole thing seems a little complicated, but here's an article:
http://www.wired.com/wiredscience/2011/ ... anglement/