# Does Time Contraction Change Substance?

• abrogard
Well no, it doesn't, not in ordinary common sense. But see above, and refer to the 'clocks around the world' experiments, weren't they all examples of time dilation being experienced? By the 'material thing' ? To such an extent that we can measure it?No, the clocks were measuring the difference in time between two inertial reference frames. Again, the traveling clock experienced time normally, it is only when it is compared to the stationary clock that the difference in time is observed.

#### abrogard

As you approach c time slows. Relative to an observer.

But it is an actual thing.

A material thing can be seen to have not aged as the observer did, right?

Well if time slows then an atom cannot vibrate at the same frequency as 'normal'.

It must be slowed.

An atom vibrating at an unnaturally slow rate would be what? A different material atom?

An atom in a different state?

Could materials retain their integrity under such conditions?

I don't understand anything you wrote after that. Starting with "actual thing". As opposed to some other kind of thing?

abrogard said:
As you approach c time slows. Relative to an observer.

But it is an actual thing.

A material thing can be seen to have not aged as the observer did, right?

Well if time slows then an atom cannot vibrate at the same frequency as 'normal'.

It must be slowed.

An atom vibrating at an unnaturally slow rate would be what? A different material atom?

An atom in a different state?

Could materials retain their integrity under such conditions?

Time dilation is a perception/calculation of a remote observer. It has no effect on the object itself.

Think about it this way. Right now as you read this you are traveling at .9999999c. You are ALSO traveling at .8c And at .1c and at 5,000mph, AND you are not moving at all. It just depends on what Frame of Reference you choose, and none of these is a preferred frame (that is, you can't say one of them is the REAL one and the others are just calculations ... they are all equally real).

BUT ... if time dilation were something you experienced, you would have to be experiencing an infinite number of time dilations all at the same time. Doesn't seem too likely, now does it?

abrogard said:
As you approach c time slows. Relative to an observer.

But it is an actual thing.
If I approach c relative to an observer, then the observer's time slows relative to me, actually to my rest frame.

abrogard said:
A material thing can be seen to have not aged as the observer did, right?
I'm a material thing and so is the observer. The effect of Time Dilation (not Contraction) is relative for inertially moving things according to the Inertial Reference Frame (IRF) of the other thing. But Time Dilation cannot be seen by any observer, it's a calculation based on the assumed speed of light according to an IRF.

abrogard said:
Well if time slows then an atom cannot vibrate at the same frequency as 'normal'.

It must be slowed.

An atom vibrating at an unnaturally slow rate would be what? A different material atom?

An atom in a different state?

Could materials retain their integrity under such conditions?
In their own rest frames, atoms vibrate at their 'normal' frequencies. In other frames where they are moving, their vibrations are slowed but this doesn't make them different material atoms or atoms in a different state.

For example, if two atoms moving away from each other at some speed give off a frequency of light that can be observed by the other atom, they will each see the same Doppler shifted frequency of light, or a slowing of the frequency. But this is not Time Dilation. If they are moving towards each other at the same speed, they will each see a speeding up of the frequency but in both cases, the same Time Dilation is in effect.

Time Dilation is not observable, it is the result of assigning an IRF to both atoms which will determine the speeds of both atoms and that establishes the Time Dilation of each atom, the faster it is moving the more its Time Dilation. You can make one atom at rest in one IRF which makes the second atom have all the speed so only the second atom is Time Dilated. Or you can have a different IRF in which the second atom is at rest and the first atom has all the speed making it the only one with Time Dilation. Or you can have an IRF in which both atoms are moving in opposite directions at some intermediate speed which will make them both have the same intermediate Time Dilation. But in all cases, they both see the same Doppler shift in the observed frequency of light.

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quote: "Time dilation is a perception/calculation of a remote observer. It has no effect on the object itself."

Hence the speed of light traveler aging less than his earthbound friends is not right? He would not return to Earth younger than his friends?

Or he would appear to be so without actually being so? Or he would actually be so without appearing to be so?

"Right now as you read this you are traveling at .9999999c."

Is this plausible at all?

"BUT ... if time dilation were something you experienced, you would have to be experiencing an infinite number of time dilations all at the same time. Doesn't seem too likely, now does it?"

Well no, it doesn't, not in ordinary common sense. But see above, and refer to the 'clocks around the world' experiments, weren't they all examples of time dilation being experienced? By the 'material thing' ? To such an extent that we can measure it?

abrogard said:
quote: "Time dilation is a perception/calculation of a remote observer. It has no effect on the object itself."

Hence the speed of light traveler aging less than his earthbound friends is not right? He would not return to Earth younger than his friends?

Or he would appear to be so without actually being so? Or he would actually be so without appearing to be so?

You misunderstand. Yes, they each experience time in exactly the same way(*) and yes, they are different ages when they get back together. There is no contradiction in that because they have taken different paths through spacetime.

"Right now as you read this you are traveling at .9999999c."

Is this plausible at all?
"Plausible" is a very weak description for something that is an absolute fact.

"BUT ... if time dilation were something you experienced, you would have to be experiencing an infinite number of time dilations all at the same time. Doesn't seem too likely, now does it?"

Well no, it doesn't, not in ordinary common sense. But see above, and refer to the 'clocks around the world' experiments, weren't they all examples of time dilation being experienced? By the 'material thing' ? To such an extent that we can measure it?
Yes, they were examples of both time dilation and of differing amounts of aging due to taking different paths through spacetime. They in fact support everything I have said.

(*) EDIT: just to be sure you are clear on what I am saying, consider this. They both have clocks that when put side by side tick at exactly the same rate. During the trip, the both clocks continue to tick at exactly the same rate. but when they get back together, just to use some off-the-cuff made up numbers, the traveling clock says, correctly, I have ticked 100,000,000 times and the stay at home clock says, correctly, I have ticked 500,000,000 times.

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abrogard said:
quote: "Time dilation is a perception/calculation of a remote observer. It has no effect on the object itself."

Hence the speed of light traveler aging less than his earthbound friends is not right? He would not return to Earth younger than his friends?

Or he would appear to be so without actually being so? Or he would actually be so without appearing to be so?
Did you not read and understand my post #4?

In the IRF in which the Earth remains at rest, only the traveler is in motion, therefore he is the only one experiencing Time Dilation, that is, his clock is taking longer to tick out each second than that of the Time Coordinate of the IRF. The people on Earth are at rest in this IRF so their clocks tick at the same rate as the Time Coordinate of the IRF. Therefore the traveler ages less during his trip than his earthbound friends and returns younger than his friends who were the same age when he departed. He is actually younger and he will appear younger. Isn't that easy and clear to understand?

Now we can pick another IRF, say the one in which the "traveler" is at rest while he is "traveling" away from earth, and now the earthbound friends and their clocks will be Time Dilated while the traveler and his clock are not. But think about this, it's really the Earth that is traveling away from the "traveler" while the "traveler" is at rest. Then when the "traveler" "turns around" to go back to earth, he has to actually travel at a higher speed than the Earth is traveling away from him in order to catch up to it. As a result, he will now be subject to an even higher Time Dilation than his earthbound friends are which results in him being younger overall than his friends are by the time he gets back. Isn't that easy and clear to understand?

abrogard said:
"Right now as you read this you are traveling at .9999999c."

Is this plausible at all?

"BUT ... if time dilation were something you experienced, you would have to be experiencing an infinite number of time dilations all at the same time. Doesn't seem too likely, now does it?"

Well no, it doesn't, not in ordinary common sense. But see above, and refer to the 'clocks around the world' experiments, weren't they all examples of time dilation being experienced? By the 'material thing' ? To such an extent that we can measure it?
This is basically the same as the traveler scenario that I just described in detail except that this one is two-dimensional whereas the other one was one-dimensional. In the 'clocks around the world' it is most convenient to use the IRF in which the center of the Earth is at rest. Then the clock that remains on the surface of the Earth will be traveling at a constant speed in an easterly direction, about a thousand miles an hour if it's at the equator and so it will have a certain amount of Time Dilation. The clock that flies on an airplane in an easterly direction will be traveling at a higher speed and have more Time Dilation. The clock that flies on an airplane in a westerly direction will be traveling at a slower speed and have less Time Dilation. Therefore when they all get back together again, the earthbound clock will have aged less than the westerly clock and more than the easterly clock. That's how we analyze the scenario in the "center of the earth" IRF.

But we could also analyze it in a different IRF and it will be much more complicated yet it will yield the same result at the end as far as the amount of time accumulated on each clock although the Time Dilations of the three clocks will be different than what they were in the "center of the earth" IRF.

For example, we could use the IRF in which the earthbound clock was at rest at the start. Then as the Earth turns, that clock will begin to experience Time Dilation until it reaches a maximum on the other side of the Earth from where it started. Then it will diminish until it reaches no Time Dilation when it gets back to where it started. Meanwhile the two clocks on the planes will also be experiencing Time Dilation in a similar but more complicated way. I have no desire to work out the details but I think I have given you enough explanation to see how it could happen. The point is that different IRF's yield different changing Time Dilations that are all related to the different changing speeds of each clock such that the final result is the same.

Is that clear now?

Nope. It is not clear to me. And I did not 'read and understand' post #4. I read it.

Seems to me you're (all, or together, the two of you) saying (1) we're traveling right now at as close a dammit to lightspeed.
(2) we don't feel and won't feel any effects from this.

(3) that's the answer to my atom question. it will experience no changes whatever.

I can dimly see that this is somehow perhaps the basis of the whole relativity thing: that all the laws of nature appear the same for all observers in whatever IRF. (as Prof Shankar puts it in a vid I saw).

I'm accustomed to imagine travelers in a spaceship doing 0.8c thinking everything is fine, poor deluded souls, due soon to be squashed out of existence...

But to imagine we are those deluded souls... a fraction of a knot away from infinite mass... living in the treacle of almost stopped time...

But that's the claim, is it?

And, back to where I started: I see an atom as a discrete material thing that can only exist if parts of it are rotating or vibrating - moving, is the point - at certain speeds. And if those speeds change or stop as they must given sufficient time dilation then it seemed to me the atom would lose its character and become something else.

perhaps like a phase change. I'm thinking you can't just apply these changes in time without having a deep effect.

but you say no. you can. it happens. it is happening now. to us. to the extreme of 0.99... etc of c.

you seem to be saying it is all imaginary. all objects living in their own hallucinatory world. we perceive that object to be a 'spaceship' at near lightspeed, all squashed up and frozen in time but within there, if we could get in there, we'd find everything to be perfectly normal. Apparently.

so where does the spaceship finish and the 'other' world begin? all the molecules falling off the skin of the ship, gas exhalations, rubbish, refuse, expelled, each atom of that is experiencing these relativistic effects until it separates from the ship and then it decelerates - no, it can't do that, can it? no force acting on it ... so the spaceship is a cloud of stuff... at near light speed...

Look at time dilation and length contraction as a real property - not of an object - but of the relationship between two objects.

abrogard said:
you seem to be saying it is all imaginary. all objects living in their own hallucinatory world.
No. Every object is in its own real world. It is all the observers that see a distorted view.

abrogard said:
so where does the spaceship finish and the 'other' world begin? all the molecules falling off the skin of the ship, gas exhalations, rubbish, refuse, expelled, each atom of that is experiencing these relativistic effects until it separates from the ship and then it decelerates - no, it can't do that, can it? no force acting on it ... so the spaceship is a cloud of stuff... at near light speed...
Yes. Every atom is observed from a remote observer in a disparate frame of reference to have relativistic effects.

abrogard said:
I'm accustomed to imagine travelers in a spaceship doing 0.8c thinking everything is fine, poor deluded souls, due soon to be squashed out of existence...

Then you're imagining incorrectly. The travelers in the spaceship are perfectly normal. They appear length contracted to you, but that's just an appearance. If very sensitive instruments were attached to everything in the spaceship (but using no information from outside the spaceship), they would detect nothing to indicate that the spaceship was moving.

abrogard said:
to imagine we are those deluded souls... a fraction of a knot away from infinite mass... living in the treacle of almost stopped time...

But that's the claim, is it?

Not if the bit about "infinite mass" and "almost stopped time" is included. Nobody is claiming that we have almost infinite mass or that our time is almost stopped. What we're saying is, again, that if you are imagining those things happening when something moves at close to light speed relative to something else, you are imagining it incorrectly.

abrogard said:
if those speeds change or stop as they must given sufficient time dilation

Again you are imagining it incorrectly. If very sensitive measurements were made of all the atoms in the spaceship, using instruments inside the spaceship (and moving with it), they would show no change whatsoever in the atoms.

abrogard said:
perhaps like a phase change.

No. A phase change is a physical change. Just moving at high speed relative to something else is not.

abrogard said:
you seem to be saying it is all imaginary

No, we are saying that it's just an appearance; something moving very fast relative to you appears to you to be length contracted and time dilated, but that's just an appearance.

And in fact it's not even a direct "appearance", in the sense that what you actually see (the light rays actually reaching you) from an object moving very fast relative to you is not length contraction and time dilation as given by the usual formulas. You see the object rotated (Google "Penrose-Terrell rotation"), and how you see its clock ticking, relative to yours, depends on which direction it's moving relative to you (if it's moving towards you, you see its clock ticking faster than yours--Google "relativistic doppler effect"). The length contraction and time dilation that are usually talked about are calculations that you make from what you directly observe, to correct for the light travel-time delay of the light rays reaching your eyes (or instruments).

abrogard said:
where does the spaceship finish and the 'other' world begin?

There is no "other world". There are just objects moving relative to each other, and light rays traveling between them and carrying information about them. They all exist in one world.

abrogard said:
Seems to me you're (all, or together, the two of you) saying (1) we're traveling right now at as close a dammit to lightspeed.
(2) we don't feel and won't feel any effects from this.
Well, it's good that it seems to you that that is what we are saying because that IS what we are saying.

I think it is common for it to be said 'as an object approaches light speed its mass increases and time slows'.

Should this not be said? Should it be said: 'As an object approaches light speed its mass appears to us to increase and its time slow.' ?

abrogard said:
I think it is common for it to be said 'as an object approaches light speed its mass increases and time slows'.

Should this not be said? Should it be said: 'As an object approaches light speed its mass appears to us to increase and its time slow.' ?
Yes, but if you want to be even more correct you would add " ... to a remote observer" to make sure it is understood that the appearance is not how it appears to someone going along with the object and if you want to be REALLY correct you would not just say "as an object approaches light speed" because your having left out any FoR makes that a meaningless statement.

A good technical statement would be "to someone in an inertial FoR, something moving at close to c in that FoR appears to them to have increased mass and to be time dilated and length contracted even though it the object's FoR it sees no change in anything".

Actually, now that I think about it, I'd leave out the part about mass because relativistic mass is a deprecated concept.

Okay, thank you.

So: 'For an observer in an inertial frame of reference objects within that frame moving at a speed close to c will appear to have time dilated and length contracted.' ?

'appear' being the operative word. Is it mere appearance?

abrogard said:
Okay, thank you.

So: 'For an observer in an inertial frame of reference objects within that frame moving at a speed close to c will appear to have time dilated and length contracted.' ?
If they are moving at a different speed than the observer then they are not in the same reference frame.

abrogard said:
'appear' being the operative word. Is it mere appearance?
No, it's real. As proven by the relativistic twin experiment.

Length contraction is real too but it returns to normal as the frames of reference come together.

abrogard said:
Okay, thank you.

So: 'For an observer in an inertial frame of reference objects within that frame moving at a speed close to c will appear to have time dilated and length contracted.' ?

'appear' being the operative word. Is it mere appearance?
Of course it is mere appearance. We've beat that horse to death already. It is real in the sense that it is a real observation, but it does not reflect something that is being experienced by the object.

Nono, it's quite real. A twin on a relativistic journey really does return younger than his twin who remains behind.

I don't want the OP to go away thinking this is "just" an observation thing.

It might help the OP to understand that time and space are not an absolute, universal thing. The twin who stays here on Earth does not enjoy preferred or special frame of reference. He is sjust as length and time contracted to a passing observer as the passing observer is to him.

phinds said:
A good technical statement would be "to someone in an inertial FoR, something moving at close to c in that FoR appears to them to have increased mass and to be time dilated and length contracted even though it the object's FoR it sees no change in anything".

Aren't you saying there that the observer and the moving object are in the same frame?

No?

So it should be: 'For an observers in an inertial frame of reference objects moving within an observed second frame of reference at a speed close to c will appear to have time dilated and length contracted.' ?

What's this post of Phinds? Serious, sarcasm, mockery?

abrogard said:
Aren't you saying there that the observer and the moving object are in the same frame?

No?

So it should be: 'For an observers in an inertial frame of reference objects moving within an observed second frame of reference at a speed close to c will appear to have time dilated and length contracted.' ?

What's this post of Phinds? Serious, sarcasm, mockery?
No, you are misunderstanding again. You can't change FoR's and be consistent. The moving object has to be moving in the FoR of the observer for the observer to see any time dilation.

To address Dave's point about it being real, he is talking about a non-inertial FoR where two object take different paths through spacetime and then meet up again. Meeting up again requires a breaking of the symmetry that says that to the moving observer what the stay-at-home observer thinks is happening to the moving observer is actually happening to the stay-at-home observer while he, the moving observer, experiences nothing out of the ordinary.

I am trying to help you understand the simple IRF situation before you get into ones that break the symmetry.

Yep. One thing at a time. I'm simple almost to the point of imbecility.

Can I get this reference frame thing sorted first?

I am in my own Inertial Reference Frame (IRF).

I am observing something moving at near light speed.

Is that thing within my IRF ?

phinds said:
a non-inertial FoR where two object take different paths through spacetime and then meet up again. Meeting up again requires a breaking of the symmetry that says that to the moving observer what the stay-at-home observer thinks is happening to the moving observer is actually happening to the stay-at-home observer while he, the moving observer, experiences nothing out of the ordinary.

Isn't this the situation where the 'twin' returns home and is found to be the one with the time dilation? i.e. it actually happened to 'the moving observer' not to the 'stay at home', as you say?

The 'breaking of the symmetry' is the changing of reference frames experienced by the traveler as it stops, turns and comes back - is all what I read recently during my stumbling around in this stuff..

DaveC426913 said:
The twin who stays here on Earth does not enjoy preferred or special frame of reference. He is sjust as length and time contracted to a passing observer as the passing observer is to him.

But I've been understanding it is quite the contrary. Observations are identical so each party thinks the other party is doing the traveling at whatever speed. It is relative. However in truth only one is doing it and that's the one that will experience the relativity effects.

That one used to be said to be the one that experienced the acceleration. It was shown that acceleration could be written out of the calculation (so to speak). My understanding is that it was finally shown that the one that changed reference frames is shown to be the one that has been moving and that is the one that experiences the relativity effects.

So the other one, the one that stays home, experiences no length and time contraction, in fact. Though he may look like it during the time of observation. That's the time of the 'twins paradox'. During the time of observation.

abrogard said:
Isn't this the situation where the 'twin' returns home and is found to be the one with the time dilation? i.e. it actually happened to 'the moving observer' not to the 'stay at home', as you say?
This is NOT time dilation. It is aging. It is a result of their taking different paths through spacetime. Maybe this is where some of your confusion is coming from. You are equating relative aging, which is a REAL occurance due to different paths through spacetime, with time dilation, which is just something that one observes happening to someone who is moving fast in your own IRF.

The 'breaking of the symmetry' is the changing of reference frames experienced by the traveler as it stops, turns and comes back - is all what I read recently during my stumbling around in this stuff..
Yes

To add to the above post, the stay-at-home observer A calculates that the person B traveling fast in his IRF is time dilated and slowed down, but he KNOWS that this is not something that is being experienced by B it is just what he observes/calculates. It is not until the symmetry is broken that things change. Prior to that, B observes/calculates A to be time dilated and slowed down while he, B, is not time dilated. Both are right.

After the turn around, the very same situation exists. Both continue to calculate that the other is slowed down and both know that that is not what's actually happening to the other. All of this assume the unrealistic idealized case of an instantaneous turn-around but however long the turn around takes, it is during this time that the symmetry is not maintained because B is accelerating.

Constant velocity in an IRF is relative. You have to say motion relative to what. Acceleration, on the other hand, is not relative. With no acceleration, A can say that he is standing still and B can say that HE is standing still. When B undergoes acceleration, A can still say that he is standing still but B knows that he is accelerating.

And by the way, why are we off on the twin paradox? The topic of this thread is
Does Time Contraction Change Substance?

Do you get that the answer to that is "no" ?

If you are interested in the twin paradox, there are hundreds of threads on this forum discussing it.

abrogard said:
I am in my own Inertial Reference Frame (IRF).

I am observing something moving at near light speed.

Is that thing within my IRF ?

The term "reference frame" can have a number of different meanings. It can mean a family of objects (like the measuring rods and clocks that Einstein talks about in a number of his relativity papers and articles) that are all at rest with respect to each other, or with respect to some chosen reference object, so that two things that are moving relative to each other cannot be in the same reference frame. DaveC426913 was more or less using the term this way in post #15.

But "reference frame" can also mean what is more correctly termed a "coordinate chart", which is just a way of assigning 4-tuples of real numbers to events in spacetime, subject to certain conditions (the mapping has to be one-to-one, for instance). The same coordinate chart can be used to describe objects that are in arbitrary states of motion relative to each other, so in this sense of the term, you and the thing you are observing are both in the same reference frame, yes.

I personally think the latter usage of the term is more useful, but that's a matter of terminology, not physics.

we were off on the twin thing because:

phinds said:
Time dilation is a perception/calculation of a remote observer. It has no effect on the object itself.

and now we've concluded that there IS an effect on the object.

and now it has been stated that it is not a time dilation effect but an aging effect.

To me time slows down as you near c. That's time dilation to me. That effects your aging. You age more slowly. Or you age not as much. All the same thing to me. Where's the difference? And here I am talking about the final effect upon whoever/whatever experiences it - I'm not talking about some perception shared by the two observers in their different reference frames and one of them in error as we shall subsequently discover when the 'paradox' is resolved. I'm talking about after the resolution.

'You' means every atom in your body. Hence the atoms have all been effected by the time dilation ( or whatever you want to call it).

I wonder at the nature of that effect and the residual effects.

This is an edit:

I just read the post about reference frames. Seems to me that if that is the case it is perhaps necessary to define which kind of reference frame one is using or referring to? Would that be useful?

Regarding the answer 'No' : yes, I read it but I wouldn't say I 'get' it at this point. I'd like to know more.

I am aware that in many forums the experts have their patience sorely tried by obtuse newbie posters who seem thicker than two short planks and incapable of comprehending a simple answer.

But I must point out that for us, the 'newbies', it is a 'one to many' relationship and we have no way to estimate the relative worth of the components of the 'many'.

Which wouldn't matter were the experts always in agreement with each other.

Sadly, that's not often the case.

Whereas for the experts it is a many to one relationship and it is easy for them to adopt a herd like mentality and 'gang up' on the hapless newbie, exchanging amongst themselves in their posts perhaps unkind estimations of the nature of the newbie.

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nitsuj
phinds said:
To address Dave's point about it being real, he is talking about a non-inertial FoR where two object take different paths through spacetime and then meet up again.

This is probably going to cause confusion the way you are stating it. If we are in flat spacetime (which we are assuming here), then for two objects to take different paths through spacetime and then meet up again, at least one must be moving non-inertially for some portion of his trip (normally we label that one as the "traveling" twin). However, there is no need to use a non-inertial frame (in the sense of "coordinate chart") to describe their motion. Any valid coordinate chart, inertial or not, will do.

abrogard said:
and now we've concluded that there IS an effect on the object.

and now it has been stated that it is not a time dilation effect but an aging effect.

Yes, because we're really talking about two different things.

The "aging" effect is a direct observable. You and I start out together, with both of our clocks set to zero. You remain floating in space in your spaceship, in free fall, while I fire my rocket engine and travel out to Alpha Centauri at near the speed of light and then come back again. When we meet up again, you will have aged more than me; we can verify that directly by comparing the readings on our clocks--your clock will read more elapsed time than mine.

The "time dilation" effect is a convention, based on using inertial reference frames (in the sense of "coordinate charts"). In the inertial frame in which you always remain at rest in the scenario I just described, my clock will appear to run slower than yours. However, in the inertial frame in which I am at rest during my outbound leg to Alpha Centauri (we will suppose that, after firing my rockets for a short period to get up to near light speed, I shut them off and "coast" for most of the trip), your clock will appear to run slower than mine. And in the (different!) inertial frame in which I am at rest during my return leg from Alpha Centauri, your clock will also appear to run slower than mine.

Personally, I think that the "time dilation" effect is best ignored, because it causes more confusion than it solves. The aging effect can be understood without even thinking about time dilation: you just have to realize that, as Dave and phinds said, observers that take different paths through spacetime can age differently. Trying to understand it, as in the scenario I described above, using time dilation just raises more questions, such as: how can I have aged less than you when it seems like your clock appears to run slower than mine in my inertial frames? To see how that works you have to bring in other things, mainly relativity of simultaneity; but that just means that "time dilation" by itself won't allow you to correctly analyze these scenarios. You will always have to at least include relativity of simultaneity in your analysis (and often you will have to include length contraction too). It's easier, IMO, to skip all that and just look at the paths through spacetime that each observer follows, and use spacetime geometry to calculate the different lengths of each path, since those lengths correspond to the elapsed times along each path. It's simple, and it avoids a lot of needless confusion.

The Usenet Physics FAQ article on the twin paradox, which I recommend reading, gives a more detailed discussion of all this:

abrogard said:
.

To me time slows down as you near c.
. But is doesn't. First of all "as you near c" is a meaningless statement. Nears c RELATIVE TO WHAT?

That's time dilation to me. That effects your aging. You age more slowly. Or you age not as much
No, you age at exactly the same rate that you always age. One second per second. You are getting confused because the AMOUNT of aging is dependent on your path through spacetime, as I have attempted to explain several times now.

abrogard said:
time slows down as you near c. That's time dilation to me. That effects your aging. You age more slowly. Or you age not as much. All the same thing to me. Where's the difference?

The difference is that, if you and I are spatially separated, there is no invariant way to compare how much we have aged, because there is no invariant way to know which events on our respective worldlines happen "at the same time". That's because simultaneity in relativity is a convention; it's not a physical thing.

If you and I are at the same place in space, then we can directly compare our clocks to see who has aged more and who has aged less. But that only works if we are at the same place. So a difference in aging is only a direct observable when two people start out together, separate, and then meet up again.

abrogard said:
Can I get this reference frame thing sorted first?

I am in my own Inertial Reference Frame (IRF).

I am observing something moving at near light speed.

Is that thing within my IRF ?

The phrase "in my reference frame" should be read as "using a reference frame in which I am not moving". If some object is moving at a constant speed relative to me, then there is a reference frame in which I am rest and the object is moving ("my" frame), a reference frame in which it is at rest and I am moving ("its frame"), and an infinite number of others in which we are both moving at various speeds (but in which some observer moving relative to both of us is at rest).

An analogy that might be useful: a red car and a blue car approach each other on a straight road, each doing 30mph. Two people stand, one on each side of the road.

One of the pedestrians will say: there's a person in front of me, a red car approaching from the left at 30mph and a blue car approaching from the leftright at 30mph.

The other pedestrian will say: there's a person in front of me, a blue car approaching from the left at 30mph and a red car approaching from the leftright at 30mph.

The driver of the red car will say there are two people in front of me, approaching at 30mph, and a blue car approaching at 60mph.

The driver of the blue car will say there are two people in front of me, approaching at 30mph, and a red car approaching at 60mph.

You can see that on the face of it, all of these descriptions are contradictory. Which car is traveling left to right? Is one of the cars stationary, the other, or neither? This is all because these descriptions use different frames of reference. One pedestrian is looking one way, one the other, so they don't agree on what "left" and "right" mean. Nevertheless, you can see that it all fits together when you account for the difference in perspective. It is a coherent description of what happened.

Relativity extends this somewhat. It tells us that terms like "now", "length" and "clock tick rate" are like "left" and "right". They are not universal descriptions. They depend on who is using them. That's what time dilation is; it's why the twins don't agree on whose clock is running slow. They have different perspectives.

Now, back to our cars. After they pass each other, they circle round the block and come back. You'll get the same circumstance as before - except that one of the cars will have been on the "outside" and one on the "inside" of the loop around the block. The "outside" car's odometer reading will be higher, and it'll get back a moment later.

All four observers agree on that, whatever their point of view. Differential aging, as in the twin paradox, is like this. Everyone will be able to agree who has aged more.

In the example of the cars, you can see how all the bits fit together because it's a situation you are familiar with. You've got the experience. With relativity, you haven't. Trying to expain it with language that was never developed for such circumstances is a chancy process, as you are discovering. Your best bet is to learn to manipulate the maths yourself. If not, you will have to accept our word that we are describing a consistent scenario, but trying to do it with language that doesn't fit the concepts well.

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Nugatory
Edit: This post is now irrelevant, since Nugatory kindly fixed the typo in my previous post. I've left it here as a grim warning to myself. :D

Ibix said:
One of the pedestrians will say: there's a person in front of me, a red car approaching from the left at 30mph and a blue car approaching from the left at 30mph.

The other pedestrian will say: there's a person in front of me, a blue car approaching from the left at 30mph and a red car approaching from the left at 30mph.
*Sigh* Try again, Ibix.

One of the pedestrians will say: there's a person in front of me, a red car approaching from the left at 30mph and a blue car approaching from the right at 30mph.

The other pedestrian will say: there's a person in front of me, a blue car approaching from the left at 30mph and a red car approaching from the right at 30mph.

Ibix said:
You can see that on the face of it, all of these descriptions are contradictory.
...and are, in fact, completely contradictory because I made a stupid typo. D'oh.

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My internal autocorrector, the thing that makes me such an ineffective proofreader :) slid right over that one until you pointed it out... But I've taken the liberty of correcting it for you above.

Thanks! I know what you mean about autocorrect - you work out what it must say and don't notice that it doesn't say that. I don't know how I missed this one this morning, though. It hit me the second I looked at it just now.