This article does an amazing job of describing the various versions of the definition of "time" used in physics. The author limits his use of jargon, but defines them well when necessary. Highly recommended.
Time is reversible in general relativity, in the sense that if you replace t by -t everywhere the solution is still valid. I don't remember the full proof off the top of my head, and the argument I do remember is complicated.
"There was a smell of Time in the air tonight. He smiled and turned the fancy in his mind. There was a thought. What did Time smell like? Like dust and clocks and people. And if you wondered what Time sounded like it sounded like water running in a dark cave and voices crying and dirt dropping down on hollow box lids, and rain. And, going further, what did Time look like? Time looked like snow dropping silently into a black room or it looked like a silent film in an ancient theater, one hundred billion faces falling like those New Years balloons, down and down into nothing. That was how Time smelled and looked and sounded. And tonight--Tomas shoved a hand into the wind outside the truck--tonight you could almost touch Time." (Illustrated Man)
I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing.
The argument was that although it's true that your time will pass more slowly than time on Earth while you're coasting, you also clearly have to do a lot of accelerating, and acceleration itself also has an effect on the apparent passage of time in your reference frame -- and in this scenario, where you start and end in Earth's reference frame, that effect cancels out the effect of velocity (perhaps even more than cancels it out; I don't recall).
It's not true. In fact, particles "travelling into the future" is routinely measured in high energy physics experiments: a particle's decay time will be lengthened exactly as special relativity predicts. Also, the effect has been measured directly using atomic clocks. The circumferences of your hard drive platters are younger than the part near the spindle.
I'm not questioning the existence of time dilation; I'm asking about the effect of acceleration on that dilation. Perhaps this is a question for General Relativity.
Without acceleration, the system is symmetrical, so each observer thinks the other's time is passing more slowly. But that also means they can never meet up again to compare ages and see who's "right".
The observer who accelerates is the one who breaks the symmetry, and thus travels into the other's future. I haven't done the math, but Wikipedia has pictures:
The fact that Special Relativity has meant different things to different people and that even most relativist professors have made statements that imply contradictory interpretations is well known and well documented.
http://worknotes.com/Physics/SpecialRelativity/TwinParadox/a...
Okay, I read some of the material on that site, and all it says to me is that (a) not everyone who has been exposed to Special Relativity understands it, and (b) some people think that if they don't understand something, it can't be true.
If effects from causes occurred immediately, versus with a distinct delay, everything would occur at once (or loop forever, that's a bug).
Hence, because we can observe that things happen "in sequence", there is a "propagation delay". That's what time is: "Time: the process whereby effects occur after causes, in a step by step manner".
You can only edit/delete a post for an hour after it is posted.
The HN community in general is fairly against those sort of comments, preferring instead comments that add to the conversation by either bringing new knowledge or interpreting existing knowledge in a novel way. People are quick to downvote.
The only general solution is to always ask yourself "does this really bring something to the conversation?" before posting.
Also, you shouldn't worry too much about losing karma every once in a while for comments that you do feel are constructive. Karma is meaningless.
