Okay, here's something uncontroversial, in case you didn;t know

[FayeKane]

The following has been known to physicists for (lemme see...) about 80 years. Einstein not only came up with this, but most importantly, he proved it mathematically. NONE of it is either thought up by me, or is my interpretation of anything.

 

But if you didn't already know it, it's really mind-blowing, and that's why I'm writing about it.

 

You can read all about it in this book. It's a textbook for physics grad students, but really, anyone can understand it. Yeah, a page full of nothing but equations looks intimidating, until you look closely and say "Hey, this is nothing but high school algebra, and it's all just the same simple equation written in different ways."

 

See, when you know what's REALLY going on, you realize that there IS no such thing as "now", other than the independent "now" which is different for every single point in the universe. Even your right hand and your left hand live in different "now"s. Every one of your blood cells has a different "right now" -- you just don't notice it.

 

But when you get the big "OMG", you see the special-relativity way of looking at things as the only reality (which it IS), and the concepts we have of everyday motion and time are really illusion, much like thinking that an elephant is a snake because all we can feel is his trunk. If all you EVER see of the elephant is his trunk then, okay, you can get away with pretending that that's what an elephant is. But it is WRONG.

 

And so is the idea of some objective "now".

 

If you sync a REALLY ACCURATE watch on your left wrist to a watch on your right wrist, then whichever arm you move first will show an earlier time than the other watch, no matter how slowly or how far you move it.

 

Neither watch is "correct", because there is no such thing as "the correct time"... or even "right now", for anything other than a single mathematical point in spacetime.

 

You don't notice it unless you're going really, really fast (like a billion miles an hour), or if you have a clock that's really, really accurate. We can't go that fast, but we have clocks that are so accurate, that if you put one in a truck (they're kind of big) and drive it across town, it will read a different time than it would have if you had left it alone.

 

And it's not just motion. Gravity slows down time too. These same clocks will run at different rates and immediately get out of sync if you put one of them on the 3rd floor and one on the fourth floor, because gravity is SLIGHTLY stronger on the 3rd floor (it's closer to the earth).

 

Also, there is no such thing as "over there, right now", EVER, ANYWHERE. There is only a set of points in 4D spacetime. The points are separated by a special kind of distance called an "interval". You can look at this separation as space, or you can look at things as being separated in time, or as a combination of the two. But none of these is exclusively "correct", because they all force you to see everything as sitting still in space and floating together down a river of "time". But that just isn't so.

 

...I mean, in case you were, like, curious about it or something.

Edited September 9, 2008 by FayeKane

[ydoaPs]

I was under the impression that acceleration breaks the symmetry and we can thus tell which wristwatch is correct.

[FayeKane]

Well, that does make sense, but my understanding of the twin paradox is that the symmetry still exists, despite acceleration. I've been meaning to read more on the twin paradox exactly for that reason.

 

I wish we had some real relativity physicists here! I sometimes read alt.sci.relativity, but it's like the wild west there, with people shooting each other, and I don't want to post there because some of the replies people get are from the certifiably crazy.

 

-flk

[insane_alien]

the twin paradox is merely a misapplication of special relativity. you need to use general relativity when there is an acceleration. apply general relativity to the problem and the paradox suddenly disappears.

[ajb]

Partly true insane_alien, you can deal with acceleration in special relativity and there is a notion of proper acceleration.

[FayeKane]

The fact remains that this is false:

 

> acceleration breaks the symmetry and we can thus tell which wristwatch is correct.

Because there is no "correct". Every object in the universe has it's own rate of passage of time, and if two of them happen to be the same for a while, it's only because neither one of them has moved since you last looked. As soon as one of them does move, even if it is nudged once by just a nanometer, then time is out of sync. They have different definitions of "now".

 

The minute hand of each clock has a different "now" than the hour hand. The arrow at the tip of the minute hand experiences time slower than the part of the minute hand near the hub. This is even true of any two molecules within a single clock. They will never be at EXACTLY the same temperature; one will be at a slightly higher temperature and therefore vibrating more.

 

The point I was making was that not only is there is no "master clock" that others may be wrong with respect to, but that the very concept of "two clocks in sync" is a platonic ideal which cannot physically exist.

 

I posted the thread item because I thought anyone who didn't know this might go "hey wow, that's cool!", not to argue about whether it's true or not.

 

But I guess I'm willing to discuss that too, if someone wants to.

Edited September 9, 2008 by FayeKane
multiple post merged

[swansont]

We can't go that fast, but we have clocks that are so accurate, that if you put one in a truck (they're kind of big) and drive it across town, it will read a different time than it would have if you had left it alone.

 

Across town? No. Across the US and back? Yes (a nanosecond or two at ~60 mph)

 

The fact remains that this is false:

 

> acceleration breaks the symmetry and we can thus tell which wristwatch is correct.

Because there is no "correct". Every object in the universe has it's own rate of passage of time, and if two of them happen to be the same for a while, it's only because neither one of them has moved since you last looked. As soon as one of them does move, even if it is nudged once by just a nanometer, then time is out of sync. They have different definitions of "now".

 

The minute hand of each clock has a different "now" than the hour hand. The arrow at the tip of the minute hand experiences time slower than the part of the minute hand near the hub. This is even true of any two molecules within a single clock. They will never be at EXACTLY the same temperature; one will be at a slightly higher temperature and therefore vibrating more.

 

 

You have to decide at what level of precision you want to have the discussion. because while what you say about the rate of the passage of time is perfectly correct, it's also true that we can't discern fractional frequency differences much better than a part in 10^16 or so, so at some point it won't cause any difficulty to say that two objects at rest are in the same inertial frame of reference and not worry about thermal motion of their center of mass.

 

The point I was making was that not only is there is no "master clock" that others may be wrong with respect to, but that the very concept of "two clocks in sync" is a platonic ideal which cannot physically exist.

 

Even if you could, it turns out that they won't stay in sync. You'll have white noise in the frequency, which when you integrate to get phase (time), turns into a random walk. So two clocks that are synchronized and running at the same frequency will random walk away from each other (and the measurement noise in current systems is way bigger than any relativistic effect)