posted 04-03-2001 00:59            

Would it be a usable analogy to compare time dilation effects to the effect of attempting to heat a pot of water beyond its boiling point? When you add heat to already-boiling water, the excess energy is "bled off" into the heat of vaporization, thus converting water into steam at a more rapid rate but leaving the mass of water itself at a uniform 100 Celsius - assuming ideal heat transfer, sea level STP, etc.

So would it be analogous to think of time dilation as a similar effect which maintains the constant c for any observer by simply "bleeding off" the extra... er,
well, not-velocity as a time dilation effect relative to the observer?

For that matter, can the increased in observed mass - "relativistic mass" - be considered simply to be the effect of viewing the rest mass of the object through a time dilation effect - much like the observed brightness of nebulae or galaxies in a photo being increased through the "time dilation effect" of a slow photographic exposure?

Shit, this stuff is almost starting to finally make sense on a level I can grok - I'm gonna be pissed if my analogies turn out to be a total crock of shit.

posted 04-03-2001 03:40            

First, I was a physics major, and so was Weis. But he was LAS physics, so he probably doesn't understand these difficult concepts. So, here goes. You seem to have the basics, so I'll skip them, and I'll try to avoid equations.

You're somewhat off line. You've heard that up near C (the speed of light), velocities don't add normally - clocks slow down, things get heavier, and so on. But you're looking for an analogy in normal (far below c) environments, and there isn't one. This isn't a good way to understand what is happening.

A better way is: Don't think of light as a magic thing and C as its magic speed. Think of C as a theoretical maximum speed, and light as one of several things that happen to move at this maximum speed. Another one (which we should focus on) is information.

Now, we start with the stipulation that C is a constant in all reference frames. Everything else we say comes out of that. The way to understand the repercussions of that statement is to understand that anytime we make a measurement of speed or time, we are collecting information. And information moves at C (or slower). Of course, usually this doesn't matter, but when we, or the thing we are measuring, are moving very fast (more than, say, 0.01 c), then the finite speed of light and information begins to play a role.

So when you imagine a spaceship moving at .5c, it's a bad idea to think of it "getting heavier" or "shrinking"-- you need to think in terms that its length will appear to decrease, because of the way in which we measure distance.

Similarly, as this spaceship passes, its onboard clocks will appear to you (the grounded observer) to be running slow, but this is because clocks all function on some method of measuring the speed of a process, and that speed will appear different to you because it is taking place in a reference frame moving very fast with respect to yours.

Does that make sense? The only way to grok this is to work through the low level thought experiments and see how "a constant for c in all reference frames" results in "lorentz contraction, time dilation, &c.".

Hope this helps-- just ask if not.

posted 04-03-2001 09:53            

quote:

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A better way is: Don't think of light as a magic thing and C as its magic speed. Think of C as a theoretical maximum speed, and light as one of several things that happen to move at this maximum speed. Another one (which we should focus on) is information.

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Not sure I buy that one - unless I remember incorrectly, changes in gravitational potential propagate instantaneously, regardless of distance. If this is true, you could in theory capture a quantum black hole and change its position according to a signal - say the hole ten feet to the left is a 0 and the hole ten feet to the right is a 1. Now, assuming that you can feed the hole enough matter to keep it stable, keep from killing yourself with the radiation that tunnels out of it, keep it heavily charged and contained in an electrostatic field, AND move it at will... you should be able to propagate a "signal" composed of gravitational field strength variations instantaneously over stellar, possibly even galactic or intergalactic distances - all depends on the sensitivity and filtering abilities of your gravitometer at the other end.

True?

quote:

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Similarly, as this spaceship passes, its onboard clocks will appear to you (the grounded observer) to be running slow, but this is because clocks all function on some method of measuring the speed of a process, and that speed will appear different to you because it is taking place in a reference frame moving very fast with respect to yours.

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Hmmm. So what you're saying is, although their clocks appear in transit to be operating much slower than our clocks, if a spaceship travelling at infinitesimally-slower-than-c (with instantaneous acceleration and deceleration) makes a trip from Alpha Centauri to here, the trip will subjectively have a four-year duration both for the people onboard and for observers viewing from here?

That's not the way I've always been given to understand it - I've always been given to understand that time dilation effects were "real" and not simply "observed" - like a Doppler shift is "observed", for example. What I've always been told is that, for the crew of a vessel travelling at near-c, their "time" will be wildly out of sync with universal "time", such that when they come to rest after an onboard "year" of near-c travel, they will discover that more time has passed for the rest of the universe than that has had for them - the infamous "taking a four year round trip voyage at near-light speed and arriving back several hundred years later" bit.

posted 04-03-2001 18:03           

Well, see, that's a complicated question. Everything they tend to teach you says 'gravity propogates instantaneously'. But uhh, it's one of those 'well, maybe' things. There's been some calculation and conjecture that it's something along the lines of 10^39 c. The real problem is gravity has been a tricky thing to determine exactly WHAT the mechanism of transmission is, so there's all kinds of theories running around.

But no, it's not instaneous. I seem to remember reading somewhere this made galaxy collision simulation quite a pain, but I can't provide a reference.

posted 04-03-2001 20:41            

However, it is theorized (tho maybe not proven) that information cannot be transmitted faster than light. Which means that things *can* move faster than light, as long as the have neither mass nor information. For example, if you shine a flashlight on a screen (far away) and move the light around, the point of light on the screen can move faster than C. But there never seems to be a way to use this to transmit information.

Other question is coming up....

posted 04-03-2001 21:08            

quote:

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Originally posted by Jimbo:
Hmmm. So what you're saying is, although their clocks appear in transit to be operating much slower than our clocks, if a spaceship travelling at infinitesimally-slower-than-c (with instantaneous acceleration and deceleration) makes a trip from Alpha Centauri to here, the trip will subjectively have a four-year duration both for the people onboard and for observers viewing from here?

That's not the way I've always been given to understand it - I've always been given to understand that time dilation effects were "real" and not simply "observed" - like a Doppler shift is "observed", for example. What I've always been told is that, for the crew of a vessel travelling at near-c, their "time" will be wildly out of sync with universal "time", such that when they come to rest after an onboard "year" of near-c travel, they will discover that more time has passed for the rest of the universe than that has had for them - the infamous "taking a four year round trip voyage at near-light speed and arriving back several hundred years later" bit.

So what's the real deal?

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We're getting tricky now. I didn't mean to imply that there is a global "time", and local parts of it can appear out of sync because of relative speed. One of relativity's biggest things is that there is no global time. As you watch the spaceship going by, you see their clocks slow (and they see yours slow as well) and they really are... they both are! This is really hard to do without diagrams.

But as to the twin paradox- The twin paradox is always a boggler. It's really better to get into grokking SR/GR for abstract stuff before tackling it... The answer is, yes, there is a time discrepancy. Meaning when spaceship twin gets home, he will have aged less than his twin. But did time "slow down" for him? Only from his twin's reference. The important thing is, though, that we are into a different territory here, because we have introduced acceleration (i.e. the traveling twin must accelerate to return to earth), which affects the mathematics.

Once again, this is really tough without diagrams. Here is a pretty decent site I found- check it out: http://math.ucr.edu/home/baez/physics/twin_paradox.html

But once again, I have to say that the twin paradox is really tricky, one of the trickiest, and it will be better to read some pages on more mundane SR first. Questions like, "what happens when a .99c speed train enters a tunnel that is longer than the train normally, but shorter than the train's lorentz-contracted length?" What happens if we shut the doors at both ends when the train is "inside" the tunnel?

Stuff like this should make better stepping stones.. But once again, I really can't help much without a bar napkin to draw on....

Hope this helps, some, and ask anytime...

posted 04-03-2001 21:18            

That's as far as I can go in this topic though. Physics makes my head hurt and somehow makes me want beer. The theory of relativity and optics both constitute cruel and unusual punishment in my book. Give me organic chemistry any day.

posted 04-06-2001 20:19            

"If GR allows me to choose any reference frame I choose, then how come I can't choose the reference frame of the travelling twin? In the space-twin's reference frame, the planet-twin travels away and back again at great speed, while the space-twin remains motionless. So doesn't the planet-twin age slower?"

And the short answer to that complaint is, No, because the space-twin accelerated and the planet twin didn't. But that's shortening the explanation to the point of inaccuracy, go read that site Fen linked if you want more info.

As for the possibility of time travel, I believe Fen is right in that GR does not disallow travelling backwards in time. Theoretically, traversing a closed time-like loop should make one travel backwards in time, but I'm not sure if that's actually possible. I read a sci-fi book that posed an interesting solution: travelling around a rotating cosmic string (circling opposite to the string's rotation). A rotating cosmic string would be massive enough to produce relatavistic angular momentum, which would mean there would be a cheese-wedge shaped angular deficit in its conical spacetime that would trail along behind the string as it travelled. Travelling across that cheese-wedge would be a spacelike path, so running circles around the string would create closed timelike loops. So all we need to do is find a cosmic string, not get killed by it, and fly around it repeatedly.
Of course, cosmic strings tend to move near the speed of light; hence, travelling around the string as it travels would require us to move even closer to the speed of light, producing time-dilation. So even if this would technically make one travel backwards in time, I don't think you could use it to meet your ancestors.

posted 04-06-2001 21:42            

fen said earlier that things moving faster than light can not have mass or information. i'm not clear on the mass issue, but i believe that they can indeed have information. the common belief is that any particles that move faster than C also view C as a boundry, but they can not travel less than C, and no information can be communicated across the boundry. additionally, (and this is something i first thought of many years ago before reading it in books, yay me!) particles moving faster than C are moving backwards in time, except they dont know it of course. the speed of light is supposed to be viewed as some sort of demented axis of symmetry over which nothing can cross.

if we could move faster than light, however, we would also be moving backwards in time.

posted 04-07-2001 06:47            

tsunamitsu - Many popular hallucinogens (e.g. adrenaline) cause subjective distortion of time, but it's just that, subjective. Relatavity doesn't really pertain to that, since relatavistic effects are measurable. That being said, I don't know of any reason that we have to experience the flow of time at that rate that we do, but that's more of an intangible for philosophers to wrestle with. For instance, would it be fair to say that someone with unnaturally fast reflexes experiences time at a different rate than someone slow and ponderous? Possibly, but it's not something we'll be able to measure anytime soon. As for the physicist's definition of time, that's kind of a big kettle of fish, but the short answer is it's a dimension - just like the spatial dimensions, except that we travel through it at an constant (?) velocity.

posted 04-07-2001 08:47            

1) How long is an hour when you're sitting in an economics lecture given by a 70 year old guy with a lisp and a monotone?

2) How long is an hour when you're with your sweetie doing something fun?

I'd say one lasts about 5 hours and the other about 5 minutes until you look at a clock.

posted 04-08-2001 20:15            

As for traveling backwards in time but not knowing it, I don't know what that means. Time is, so far as we know, a one-D quantity, meaning two directions, but the direction in which it "flows" does not necessarily come from "first principles"-- imagine viewing a simulation of planets and orbital motion. Now view the simulation backwards. How can you tell which was which? Maybe the first one was backwards?
Do a google search sometime on "arrows of time" -- Lots of interesting stuff there.