Ok, according to relativity, if one twin stays on earth while the other one goes on a trip really fast, the one who went on the trip will not have aged as much as the one who stayed home. But, relative to the person taking the trip, isn't that one standing still while the one at home is moving? Why is it that time "slows down" for the one on the trip when spacetime isn't supposed to make the distinction between a fast rocket and stationary planet and a stationary rocket and fast planet?

the one that ends up younger is the one that "changes direction" to catch back up with the original. i.e. if the "stationary" twin stays where he is, and the "moving" twin turns around and races back, then it's the second one that'll be younger. If, on the other hand, the "stationary" twin decides to take off and catch up with the "moving" twin, then he'll be the younger one once they meet. It all has to do with the 3rd inertial time frame that's introduced once the action is made to get back together with each other.

Wally's got it exactly right. There are three inertial frames in the experiment as it is normally given, and it is the fact that the "rocket" twin is the one who switches inertial frames that causes the time dilation to be one-sided like it is.

BTW, it'd be possible for one twin to rocket off and then stop, and then the other twin rockets off in the same direction (and velocity) and catches up to the first. In this case, both twins would still be the same age at the end, because in this case, there are only two inertial frames.

[img]/phpBB/images/smiles/icon_smile.gif[/img]

[Editing for spelling]
_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2001-11-09 07:37 ]</font>

Okay, twins.

It's been explained before, but the idea of twins seems to be confusing people.

Why should two twins always be the same age?
Here's some stupid analogy:
Find two twins. Kill one. You'll probably be amazed to see that the one you didn't kill is still alive! Now you have something to think about in prison. [img]/phpBB/images/smiles/icon_smile.gif[/img]

Twins are two separated individuals, like you and me. Aside from have identical DNA (my biology is very very bad!) they are not linked.

Please don't use twins, they're just used to introduce an unecessary parameter to confuse. Call them "two individuals of same age" or something similar, it doesn't change anything at all. Or correct me if I'm wrong. [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

Biologically speaking, twins are almost necessarily the same age within a couple of days. I'm not sure how far apart two twins have ever been born, but I'm sure it's not too far. And we're not talking about killing anybody! [img]/phpBB/images/smiles/icon_smile.gif[/img]

Well, if it doesn't change anything, then why change it? [img]/phpBB/images/smiles/icon_smile.gif[/img]

Twins are used in the paradox because it drives home the point more. The basic idea that two people born on the same day end up years apart in age is just, well, cool!

Besides, any time you're doing an experiment with a subject and a control, it's necessary that the two be as much alike as possible to prevent any biological (or other) differences from causing differences in the outcome . . .

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

My thoughts in this thread? no that it matters
"WAS" how many Frames?
I use this string (1{2[3:4|5|:]}) Five frames?
or from from a real life event
(Earth{ocean[typhoon:ship|Gravity Wave pod
NOT THAT THIS MATTERS, probably not
but while i was in the "POD"
time did seam to slow : noticably :
and when I "LEFT" the pod
returned to the "SHIP"'s frame
the Time accumulated in the POD
was decompress "RAPPIDLY"
what can i say ? getting back was not much fun?
anyway? My other example a TV picture
of a TV picture |...| called a picture in
a picture maybe |.[]| where the time in the
small picture delayed some amount from the
primary time. the step rates the same one
second = one second but its not the same
second. to get the second second to be a
different second than the first second
I `poise that a negative Zoom maight work?
although i've not tried this yet myself
there probably is a manipulation that
would cause the small seconds to actually
be longer than the large seconds. Like I say
even I have not tried to do this "YET"
now going out five frames ({[:|what
well its to small to see ? maybe but i try it today

Don't do it HUb'!!! You'll end up seeing the back of your own head as you regress back thru time to a point where you're nothing but a fetus floating quietly thru space.

I'd hate to see that happen. . . [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

Warning! Material that is not mathematically rigorous follows!

Why does the twin moving at .7 c (209,854,720 km/sec) experience 'slow time' relative to the homebody twin? Let's imagine the traveller turning on her ship's headlights. Now the photons from the lights must travel at the same rate as viewed by both the homebody and the traveller. During one second of the homebody's time, photons from the headlight travel 299,792,458 km, and the ship travels 209,854,720 km. So from the perspective of the homebody, only 89,937,738 km seperates the photon emitted from the headlight and the ship.

So, if both twins measured time the same, the the traveller would see light moving at only 89,937,738 km/sec, or a bit less than a third of the speed of light! This will not do, cries the Universe. In order to balance the books, the Universe slows the traveller's time so that it takes a bit more than three times as long for the traveller's clock to tick as for the homebody's clock. Voila! Now, during the 'long second' of the traveller, a photon from her headlights has the time to travel a proper 299,792,458 km.

The Universe rejoices. The homebody applies wrinkle-creme and worries about grey hairs, whilst the traveller wishes she were old enough to get a driver's license.

If that is not clear as mud, then my name is not DStahl.

BIG FAT CAVEAT, added later: The above is not the way velocities are actually figured in relativistic situations. I just used simple arithmetic to make the point that the traveller's time must dilate with respect to the homebody's in order to preserve the speed of light in her reference frame. Do not use this math on your final exam. The professor will not be amused.

<font size=-1>[ This Message was edited by: DStahl on 2001-11-11 11:21 ]</font>

4:03 A.M. TO Late: I already tried it
I get two versions on my pic.pic
1. {I call the Time Tunnel} ({[]}) smaller & smaller
with a time delay at each increment
2: {I call it the Space Pace} [.].].] wher its Blue shifted ?

I must admit, DStahl, that you have the most consistently accessible and informative explanations. I can't comment on the technical accuracy of your stuff, but if I was taking astronomy at school, I feel I could learn from you. G'job.

Ok, if twins aren't good enough, then let's use clones instead. You can have as many of them as you like, as long as they were all produced at the same time. [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

Yep, light in a vacuum has got to travel at exactly c. The Universe will bend and break laws as it sees fit, as long as that prime directive is upheld.
So if my rocket is travelling at c/2 and i measure the speed of photons coming to me from my destination I will not measure 3c/2 like Newtonian dynamics says I would. The Universe holds it's arms up in disgust and sayeth "Thou shalt see'th slower time!" and so since my seconds become longer, then I measure the photons coming towards me at exactly c.
So if I travelled at light speed, then time for me would have to stop so that I could measure photons hitting me at light speed. Interesting.

__________________
"We want a few mad people now. See where the sane ones have landed us!" - George Bernard Shaw

<<...The Universe will bend and break laws as it sees fit, as long as that prime directive is upheld...>>

And that characterization, advocating any other dissimilar (even a personally prefered) point of view on how the Universe operates, itself isn't an attempted prime directive? And one with considerably less empirical evidence to support it.

It's not to say you're certainly wrong, just certainly lacking in proffered supporting evidence for your particular hypothesis.

I call your bet. Show your cards.

We also have another thread in another forum discussing the clone paradox. I mean, twin paradox.

It's not your name. DStahl is your nickname, remember? [img]/phpBB/images/smiles/icon_razz.gif[/img]

I hate nitpicking! [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

<font size=-1>[ This Message was edited by: Mr. X on 2001-11-11 14:48 ]</font>

Mr. X:

Oh do not misrepresent us any more misrepresentations! You do so like nitpicking. And...maybe my name is really DStahl D. DStahl, though my parents call me DeeDee. Well, it's not, but maybe in some parallel universe? *grin*

Garrette:

Thanks for the kind words. If the stuff I spout is too inaccurate or just plain wrong I hope fergawdsake that someone will speak up! There are several places on the Internet where one can find the proper formulae for adding velocities and figuring time dilation in relativistic situations. It's not obscenely difficult to do, either.

<font size=-1>[ This Message was edited by: DStahl on 2001-11-12 13:44 ]</font>

DStahl (if that is your real name),

You're description is slightly misleading . . .

What about when the traveller turns on her taillights? All observers need to see that light moving at the same velocity as well, no?

A stationary observer says that after one second, the headlight light has moved 299,792,458 km; the taillight light has moved 299,792,458 km; and the ship has moved 209,854,720 km. Therefore the headlight light is only 89,937,738 km away from the ship, as you stated, but the taillight light is 509,647,178 km away!

So, how does the traveller explain the fact that one photon has traveled more than five times as far in the same amount of time?

(BTW, I do know the answer to this, but it's something for you and others to think about . . .) [img]/phpBB/images/smiles/icon_wink.gif[/img]

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

I objectionationate...nation...ate...
I objectionationatenationate!
I demand you provide proof to dismiss my wild suppositionitions and accusationations! [img]/phpBB/images/smiles/icon_biggrin.gif[/img]
At which point I will proceed and do some more!

SeanF: //So, how does the traveller explain the fact that one photon has traveled more than five times as far in the same amount of time?//

Tail lights! Why did I never bother to think of that? Well, you're simple question forces me to think hard--which reveals my fundamental understanding of relativity to be shallow to begin with, my thinking to be sloppy and weak...or both!

Thanks for pointing out that my explanation was incomplete. I, er, really appreciate it. *sighs, scratches head*

Geez, DStahl! You have lots of problems! I'm just glad I am not you right now! [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

:-| In a 1922 book, Einstein defined
a stationary frame as one where the laws of physics are simplest. An inertial frame is one where all the observers are moving at a constant velocity (speed and direction) in the stationary frame. Not every frame is an inertial frame. An accelerating frame is one where the laws of physics are more complicated than in an inertial frame.
The law of reciprocity states that the physical laws are the same in all inertial frames. The law of reciprocity is the most important postulate of special relativity. However, it does not apply to accelerated frames.



No! Einstein did not say that "the one taking the trip is moving." He said that an observer in one inertial frame sees the system in another inertial frame slow down. This applies as long as the "one taking the trip" is in an inertial frame. For example, Newton's third law applies to everything on the ship, and that is as simple as one could get. For every reaction force on one body, there is a reaction force on another body.
Pretend that the one taking the trip thinks that he isn't moving. The one taking the trip has to turn around. During the interval when he turns around, the laws of physics grow more complicated. For example, the observer feels an action force ((weight pushing him down) but no reaction force (a large mass exerting gravitational pull). Of course, someone actually standing still would see the third law still applied, that the "reaction force" is the thrust of the rockets. The rule that an observer in one inertial frame sees the system in another inertial frame "slow down time" does not apply. In fact, he would see his earth bound twin speed up in time.

Space time does have a distinction between an a rocket under thrust and a stationary planet. The thrust is a force. The law of reciprocity, and all derived from it, only applies to observers on whom no force is applied.

Although there are variations where forces are not involved (e.g., the ships passing problem), all of them can be resolved using the definition of inertial frame.


The law of reciprocity states that all inertial frames have the same physical laws, which are simpler than frames where the observer accelerates. The time dilation formula was derived using the law of reciprocity. Therefore, the time dilation formula does not apply to observers that are accelerating, where accelerating here means being under the influence of a force.

<font size=-1>[ This Message was edited by: Rosen1 on 2001-11-16 16:10 ]</font>

<font size=-1>[ This Message was edited by: Rosen1 on 2001-11-16 16:12 ]</font>