This is in response to a question from Luke Robert about this article:
This would be so much better if you could ask questions. Learning isn’t about what I think that I know but what you learn. Hence, questions. I’ll try to anticipate your questions but this will still be off ifI turn left when you want to go right.
THEORY OF SPECIAL RELATIVITY
1) If a car going 30 mph hits a parked car at 30 mph, it’s a collision at 30 mph. If a car going 30 mph hits another car coming at it at 30 mph, it’s a collision at 60 mph as the speeds will add. This is common sense. So, the following makes no sense: if a photon traveling at the speed of light hits a parked rocket, it’s a collision at the speed of light. If a photon traveling at the speed of light hits a rocket coming at it at 99.9999% of the speed of light….it’s a collision at the speed of light. No change in speed. Nuts. But you have to accept that. There may be exceptions but not in general. We’ve measured this.
The photon will change frequency, shifting in the direction we call blue, or blue shift. (A photon traveling away from us has the opposite shift or red shift.) But the speed is constant. So what does change? Space. The distance that the light travels, relative to the rocket, shrinks. Relatively. To keep the speed of light constant, space shrinks or dilates. Space dilation is what special relativity explains. At high speeds, to keep the speed of light constant, relative space dilation takes place. This tells us that space is stretchy.
2) Common sense tells us about space but what about time? You travel from Maine to Florida, and you know that they are in different points in space. But when you look at the departure board at the airport, you notice that next to the point in space called Fort Lauderdale, there is also a time, when you arrive. Time is a dimension, just like space. You can’t be in both Maine and Florida as they are different places…unless you move along the time axis. Two different points in space will always be separate by distance, got it, AND they are always also separated by the time it takes to travel between them. This is what we call spacetime. And, if space is stretchy, so is spacetime.
So, if the distance the photon travels relative to the rocket coming at it gets smaller, yeah, you got it, then the time goes quicker. Why? Repeat after me: "the speed of light is constant and we don’t KNOW why.
THEORY OF GENERAL RELATIVITY
3) Believe me, here it gets easier, as you grew up in a time when black holes are accepted as part of your reality. Not so me. Anyway, so you already know if something gets close to a black hole, it gets allllll strrrrrretched ouuuuuttttttt, right? This is from the extreme gravity. Now, apply what you read above and think, what is being stretched out? Yah, spacetime. The distance close to a black holes stretches out and the time gets stretched and things are farther and farther apart and things take longer and longer to happen. At the event horizon of a black hole, as you look in, nothing moves. Time (and space) has practically frozen.
OK, remember the name of this theory is relativity for a reason. What if, while you are outside looking in at the black hole, I was inside looking out? To me, things look normal where I am, space is space and time is time. But when I look out, I see you racing around (which is pretty much the norm with you anyway.) And looking very small or dilated. From my point of view the universe has shrunk and speeded up.Relative to me. You see potayto, I see potahto.
OK, here it gets good. What happens if you decide to join me near the black hole? From your point of view, you have to travel these long long stretched out distances and it takes a long time. But you keep looking at me and I don’t change. I look out and see you moving like the Flash AND AGING as everything I see goes quicker (yes, the Flash ages faster than the rest of us. So, deal.) The forty years for you happens very very fast for me as I watch you age. When you reach me, sonuvagun, it took forty years for you and a day and a half for me and you’re 70 and I’m… not. Wow, you old fart. From your point of view, I just never changed, trapped in the stretched out time and space like a fly in amber.
4) So, there we are, trapped near a black hole ( and we’re not getting out) and you’re still asking, but what about the rockets with all those Einsteins? OK, did you notice that the gravity of the black hole accelerated your time relative to me? What if we got rid of the black hole and you just accelerated? We know from special relativity that relative speed will dilate space and time. Acceleration does that too, squared. I will still see you moving crazy fast and aging relative to me.I know that’s a bit of a jump but it’s called the Einstein Equivalency Principle: gravity acts like acceleration, acceleration acts like gravity. You ever notice how gravity increases when an elevator accelerates upward then levels off when it gets up to speed? That’s not gravity, you say, that’s acceleration. And you just proved Einstein’s Equivalency Principle.
ANSWER: Here all the nonsense above comes together. So, imagine if instead of being near a black hole, I was in a rocket accelerating away from you on Earth up to 99.99999% of the speed of light. Now it will take a long time to accelerate to that speed. In fact, you on Earth might see my whole trip take 40 years. And the light coming from me would reach you at the same speed it left me despite my acceleration (although shifted to the red). But it be traveling these stretched out distances and times between us. Over the 40 years, you would see me moving very verrrry sloooooowly. I cruise along in a circle that returns me to earth, decelerating like climbing out of the black hole. There Iam, a day and half later for me, 40 years for you, you old punk.
Quick recap:
1) the speed of light is constant over time and space. It all begins and ends here,
2) time is a dimension just like space.
3) space and time are stretchy and gravity stretches things in time and space
4) gravity accelerates things and acceleration acts just like gravity to slow things down or speed them up. Relatively.
(OK, I skipped over some of the dicey parts to get to the answer. If you google the Twin Paradox, you’ll see what I mean, but the answer there is what I said here, acceleration.)
