Time does not run slower or faster

[JesseM]

JesseM wrote:
> ... because of his definition of simultaneity Ann was
>already much older than he or Bob ...

I wasn't aware that clock synchronization could affect aging;
it would be a strange universe if one's physical aging process
were somehow affected by the way some dude on a faraway
planet had set his clocks. (And one would have to age in
many different ways if each outside observer set his clocks
differently!)

Clock synchronization cannot affect the answer to any genuinely physical question about aging, like how much a person ages between two events on their own worldline (like the event of leaving Earth and the event of returning). But the whole point is that the question of which of two inertial observers is "aging faster" is not a physical one, it's a coordinate-dependent one, and as such it depends on your chosen coordinate system's definition of simultaneity. Since Carl and Ann were not at the same location to compare ages objectively until the moment they first meant, without picking a definition of simultaneity you have no basis for thinking the fact that she was older when she met implies she was aging more quickly.

Perhaps it will help if I rephrase the question as follows:

(Note: My rephrasing is just another view of the cited site.)

Two people of the same age pass in space while moving inertially.
We can quantify by saying that both are 5 years old when they
meet in passing.

Physically speaking (and aging is a physical process), we have
only the following (2) choices after they separate:

(1) They continue to age alike
or
(2) They don't

But which choice we make depends on our coordinate system, there is no real physical answer to the question of whether they age at the same rate or not (in a frame where both were moving at the same speed in opposite directions, they would age at identical rates, but in a frame where their speeds were different, they would age at different rates, and relativity says they're no basis for preferring one inertial frame over another).

To keep it simple, let's chose (1).

OK, then we're choosing a frame where Bob and Ann both have the same speed.

Let’s say that Ann went to the left, and Bob went to the right.

When Bob turns 10, he is meet by Copy-Bob (or Messenger Carl),
who is also 10.

Since Ann and Bob are aging alike all through the experiment
(because of (1) and the fact that they never change frames),
we know that Ann is "now" also 10.

Only if we use this frame's definition of simultaneity.

(If you insist otherwise,
then you are saying that Ann somehow actually (really) aged
differently from Bob, and we have the same problem, but sooner!)

No, I am saying that according to relativity there is no "actual" or "real" answer to whether Ann ages at the same rate as Bob or a different rate, it's totally coordinate-dependent, just like the question of whether Ann's speed is equal to Bob's or different.

When Copy-Bob goes on to catch up with Ann, he's still
only sweet 16, whereas she is a married-with-children 28!

Yes, and in the frame where Ann and Bob age at the same rate, it would be impossibe for copy-Bob to catch up with Ann unless his speed is greater than either of theirs (if his speed was the same as Ann's and he was traveling to the right as well, then the distance between them would remain constant), thus he will be aging slower than them in this frame.

This proves that these two people actually aged differently.

Not if "actually" implies a coordinate-independent statement which doesn't depend on your choice of simultaneity. It is true that in the frame where Ann and Bob age at the same rate, copy-Bob ages slower than them, but there is nothing special about this frame.

They were both 10 years old at the same time

Only using this frame's definition of simultaneity, not in other frames.

and yet later
they had actually different ages (16 versus 28).

Yes, they had different ages when they met, and all frames will agree on this, but they will disagree that the event of copy-Bob turning 10 was simultaneous with Ann turning 10.

All we have essentially are direct physical comparisons of
people as they meet in space while moving inertially.

But to support your notion that copy-Bob aged slower, you must use a definition of simultaneity which tells you that copy-Bob was aged 10 at the same moment Ann was aged 10, and this is not based on any "direct physical comparison of people as they meet in space", you can easily pick a frame which agrees about all local readings when people meet but disagrees that these two widely-separated events happened simultaneously.

There is no need to bring up clock synchronization.

Of course there is, do you think the notion that Ann and copy-Bob both turned 10 simultaneously does not depend on your clock synchronization?

But we can go your way if you wish because (as not many know)
acceleration does not affect a clock's physical rate.

In other words, the objective age difference that you agreed
to (by having a turnaround twin) _cannot_ be explained by
acceleration, so how would you explain it?

rqr

Acceleration does not affect the clock's instantaneous rate of ticking in any given frame, that depends only on its speed in that frame, but since acceleration means a change in velocity (which means the speed must be changing in at least some frames), it does affect the integral of $$\sqrt{1 - v(t)^2/c^2}$$ which each inertial frame uses to calculate the elapsed proper time on a non-inertial worldline. And if each frame does this integral using the speed as a function of time v(t) in their own coordinates, they will each get the same answer for the proper time along a worldline between two events, and mathematically you can show that this proper time is always smaller than the proper time on an inertial worldline which goes between the same two events.

It might also help if you'd read over my geometric analogy in post #10--your statement that rate of clock ticking depends only on speed rather than on acceleration is analogous to the statement that for the paths on paper in my example, the rate that their "partial path length" is increasing in a particular coordinate system is dependent only on the slope of that line in that coordinate system (analogous to velocity), not on the rate that the slope changes (analogous to acceleration), yet it's still obvious that a straight-line path with constant slope between two points will always have a shorter length than a non-straight path between those points whose slope changes.

 

[RandallB]

Bob is of course accelerating. Adding messanger Carl seems to eliminate this acceleration but in fact it doesn't. Bob cannot pass any objective information about Ann's speed or age to Carl. Carl is in a different frame of reference and will not agree with Bob about Anns age. In fact Carl will see the system exactly the same way as Bob would if he accelerated into Carls frame. In other words there is an implicit acceleration here.

Nonsense, replacing BOB’s turn around with an information transfer to CARL at the “turn around point” rather than having Bob actually turn around does demonstrate that GR accelerations do not have anything to do with the SR twins paradox.
CARL only needs the information from BOB that he has when they are both at the “turn around point”. That does not include any “current” info about ANN; only how old BOB is currently and what BOB remembers about how old both he and ANN where when he was near her at the start of the experiment. Accelerations implied or real & GR have nothing to do with working out the details of the Twins problem, just the three defined SR inertial reference frames.

Being able to establish an “objective” decision as to which one Ann, Bob, or Carl is “really” aging the fastest is a matter establishing a “preferred" reference frame. SR does not necessarily require that such a frame be defined.

 

[JesseM]

Nonsense, replacing BOB’s turn around with an information transfer to CARL at the “turn around point” rather than having Bob actually turn around does demonstrate that GR accelerations do not have anything to do with the SR twins paradox.

paw didn't say anything about "GR" accelerations, you can calculate the proper time on an accelerated worldline in flat spacetime just fine using SR alone. It's certainly true that if you have two worldlines between the same pair of events, and one is inertial while the other involves acceleration, then the one involving acceleration will always have less proper time, so in that sense acceleration is important in understanding the twin paradox. The situation with three twins doesn't actually involve two different worldlines that go between the same two events, although if you have a twin who travels away from Earth inertially and then instantaneously accelerates and returns inertially, then during the outbound leg he'll age the same amount as Bob traveling alongside him, and during the inbound leg he'll age the same amount as Carl traveling alongside him, so this shows you can compute the total proper time on his non-inertial worldline by adding segments of the worldlines of two inertial observers.

 

[pervect]

paw didn't say anything about "GR" accelerations, you can calculate the proper time on an accelerated worldline in flat spacetime just fine using SR alone. It's certainly true that if you have two worldlines between the same pair of events, and one is inertial while the other involves acceleration, then the one involving acceleration will always have less proper time, so in that sense acceleration is important in understanding the twin paradox. The situation with three twins doesn't actually involve two different worldlines that go between the same two events, although if you have a twin who travels away from Earth inertially and then instantaneously accelerates and returns inertially, then during the outbound leg he'll age the same amount as Bob traveling alongside him, and during the inbound leg he'll age the same amount as Carl traveling alongside him, so this shows you can compute the total proper time on his non-inertial worldline by adding segments of the worldlines of two inertial observers.

If there is anyone for some strange reason who still doesn't see that Jesse is right, I would suggest reading the first chapter of "Space-time physics", a textbook on GR, available for download at:

http://www.eftaylor.com/download.html#special_relativity

(This is the first chapter of the first edition that's available for download, later editions can be found at your local library).

The twin paradox is much like the triangle inequality in standard Euclidean geometry. The difference is, that in Euclidean geometry, the shortest distance between two points is a straight line, while in a Lorentz geometry.

In a Lorentz geometry, a curved worldline between two points is shorter than the direct wordline between two events - shorter as measured by the elapsed proper time along the wordline.

 

[RandallB]

paw didn't say anything about "GR" accelerations,

I disagree, when he said "In other words there is an implicit acceleration here." it supported the often seen and incorrect view that the aging differances between twins is based on an "Acceleration" in the turn around not in the SR frames alone as your discriptions show.

 

[JesseM]

I disagree, when he said "In other words there is an implicit acceleration here." it supported the often seen and incorrect view that the aging differances between twins is based on an "Acceleration" in the turn around not in the SR frames alone as your discriptions show.

The fact that the traveling twin's worldine has less proper time is based on the fact that the twin accelerates during the turnaround, in just the same that the fact that a bent path between two points in space has a shorter distance than a straight path is based on the fact that there is a change in its slope. But this doesn't imply you're using GR, you can use an inertial frame in SR to calculate the proper time between two events along an non-inertial worldline by taking the speed as a function of time v(t) in that frame and evaluating the integral $$\int^{t_1}_{t_0} \sqrt{1 - v(t)^2/c^2} \, dt$$ where t1 and t0 are the times of the two events in this frame. If you have two worldlines between a given pair of events in flat spacetime, and one has a constant v(t) function while the other has a non-constant v(t), that fact alone is enough to absolutely guarantee that the one with the non-constant v(t) (i.e. the one that accelerated) will yield a smaller value when you evaluate this integral.

 

[vivesdn]

The twin that remains on Earth is constantly accelerating, as he is describing a cycloid orbit around the sun and daily following the Earth perimeter.
Honestly, I think that relativistic effects are due to speed. The funny thing, and hence the word relativity, is that it is not possible to know who is really moving and who's at rest, as everybody will measure light speed as being c because his time paces correspondingly. If the word 'rest' means anything at all in this context.

 

[JesseM]

The twin that remains on Earth is constantly accelerating, as he is describing a cycloid orbit around the sun and daily following the Earth perimeter.

In calculations of the twin paradox it's assumed that the so-called "Earth twin" is moving inertially--remember, this is a thought-experiment about special relativity, not an analysis of something that's actually been done in the real world! To adequately deal with the motions of the Earth you'd have to take into account the fact that the Earth is moving in curved spacetime, so you'd need general relativity to deal with this; but as an approximation for this situation, treating the Earth as moving inertially in flat spacetime would probably introduce only a slight error, the velocities and gravity are low enough that the difference in aging between a clock on Earth and a clock arbitrarily far away from it and at rest in the Schwarzschild coordinate system centered on the Sun would be very small. In any case, the point of the thought-experiment was just to show the difference between inertial and non-inertial paths in flat spacetime, you're free to imagine the inertial twin was actually moving inertially in deep space rather than sitting on the Earth.

 

[vivesdn]

JesseM,

my point was that what make time move slower is the velocity, close enough to c if an effect has to be measured, not accelerations.
I've been told that there are particles created by cosmic rays in the upper atmosphere that, given their speed, altitude and life, should not reach the surface. But they actually do, so their lifespan is higher when moving at high speed that when created on an accelerator with lower speeds.
But as I'm not an expert, I may be wrong.

 

[JesseM]

JesseM,

my point was that what make time move slower is the velocity, close enough to c if an effect has to be measured, not accelerations.

It's true that time dilation in any frame depends only on velocity, but velocity is frame-dependent, whereas it's true in any frame that if one clock travels inertially between two points in spacetime while another travels between the same two points on a path that includes some acceleration, the one that accelerated will always have elapsed less time.

I've been told that there are particles created by cosmic rays in the upper atmosphere that, given their speed, altitude and life, should not reach the surface. But they actually do, so their lifespan is higher when moving at high speed that when created on an accelerator with lower speeds.

Yes, this is true of muons, see http://www.glenbrook.k12.il.us/gbssci/phys/Class/relativity/u7l2e2muon.html for example. But the explanation for this depends on your frame--in the Earth's frame, they make it to Earth because time dilation slows their decay rate down, but in the muon's own rest frame, it's the Earth's clocks that are running slow while their own decay rate is unchanged, and the reason they make it through the atmosphere before decaying is that the distance from the upper atmosphere to the surface is shrunk by length contraction. Each frame's point of view is equally valid, there's no reason to prefer one over the other.

 

[pervect]

JesseM,

my point was that what make time move slower is the velocity, close enough to c if an effect has to be measured, not accelerations.
I've been told that there are particles created by cosmic rays in the upper atmosphere that, given their speed, altitude and life, should not reach the surface. But they actually do, so their lifespan is higher when moving at high speed that when created on an accelerator with lower speeds.
But as I'm not an expert, I may be wrong.

If you regard speed as a purely relative quantity, an angle on the space-time graph you are correct. But if you regard speed as an "absolute", you are mistaken.

An analogy can be drawn between the twin paradox and the lengthy of a hypotenuse. The twin paradox is just a triangle in a Minkowski geometry. The point is that what is important in determining the relative lengths of the triangle sides is the angle. In the twin paradox, this angle is just the relative velocity between the twins. This determines the ratio between the length of the hypotenuse (proper time elapsed for the twin that does not accelerate) vs the sum of the lengths of the two sides of a triange (proper time elapsed for a twin that changes direction).

The effect of gravity requires GR to analyze. There is an effect due to height according to GR, but it's very small.

[add]
If you consider a short enough period of time, though, it is still true in GR that the longest elapsed time between two points is a geodesic. You run into trouble if you make the time interval too long (i.e. you have to keep the time less than an orbital period).

If you have a non-orbital geodesic that is at the same height at time t and at time t + delta, what is that geodesic? It's the path of a ball that you throw up into the air at time t, and that lands on the ground at time t+delta. This is the only geodesic with those qualities for short time intervals.

This is the path that maximizes proper time. It will have a longer elapsed time than a clock that's fixed to the surface of the Earth.

 

[paw]

Nonsense, replacing BOB’s turn around with an information transfer to CARL at the “turn around point” rather than having Bob actually turn around does demonstrate that GR accelerations do not have anything to do with the SR twins paradox.
CARL only needs the information from BOB that he has when they are both at the “turn around point”. That does not include any “current” info about ANN; only how old BOB is currently and what BOB remembers about how old both he and ANN where when he was near her at the start of the experiment. Accelerations implied or real & GR have nothing to do with working out the details of the Twins problem, just the three defined SR inertial reference frames.

Being able to establish an “objective” decision as to which one Ann, Bob, or Carl is “really” aging the fastest is a matter establishing a “preferred" reference frame. SR does not necessarily require that such a frame be defined.

Evidently I wasn't clear. I'm not saying you need to include acceleration in the analysis, I'm saying the result of the analysis is the same in either treatment. Inertial Carl is equivalent to accelerated Bob. At some point the twins have to compare clocks to have an objective measure of their respective ages. Whether they physically get back together or a third inertial party carries the information back it still involves a real or implied acceleration.

JesseM gives a much more lucid explanation but I believe I'm saying the same thing. It's meaningless to say who is aging faster unless and until the twins can objectively compare ages.

 

[rqr]

to JesseM

I repeat my prior question:
In other words, the objective age difference that you agreed
to (by having a turnaround twin) _cannot_ be explained by
acceleration, so how would you explain it?

Your reply relied on coordinate systems, but there are none
in the given experiment. Also, even if there were any, then
their coordinate readings certainly could not physically
affect either one's aging or an atomic clock's rate.

Just as you age in only one way as long as you do not change
your speed, any given ideal clock will tick at only one rate
as long as its speed does not change.

Contrast this with the fact that various coordinate systems
will find _different_ "tick rates" for one and the same clock
that is moving at an unchanging speed.

There is no way that the slope of some coordinate line can
have any physical effect upon the intrinsic atomic rate of
an atomic clock or upon one's aging rate.

Math (geometry) cannot affect one's aging rate.
Math (geometry) cannot affect an atomic clock's atomic rate.

We can also look at it in this way:
You have agreed that the Two-people case contains real or
objective differential aging. Even though the Three-people
case has no accelerations, and the Two-people case does have
accelerations, acceleration per se has no physical effect
upon aging rates (or atomic clock rates). Clearly, as far
as aging is concerned, there are no physical differences
between the two cases. Therefore, if the Two-people case
has objective differential aging, then so does other case.

What is the physical cause of objective differential aging?
(It can occur even without accelerations.)
(It cannot be affected by clock synchronization.)
(It cannot be affected by coordinate values.)
(There are no coordinate systems in the given experiment.)
(There's not even a definition of clock synchronization.)
(There are only people passing/meeting in space.)

[added]
Originally Posted by rqr:
>>There is no need to bring up clock synchronization.

quoting JesseM:
>Of course there is, do you think the notion that Ann
>and copy-Bob both turned 10 simultaneously does not
>depend on your clock synchronization?

It cannot depend upon clock synchronization because there
are no clocks in the given experiment. What is does depend
on is the simple given fact that Ann & Bob aged alike.
(This was chosen as one of only two physically possible
paths, (i) Ann & Bob do not age alike, or (ii) they do.)
This simple given fact tell us that whenever Bob is 10,
so is Ann. Therefore, whenever Copy-Bob & Bob are 10, so
is Ann. However, since Ann & Copy-Bob have different ages
at the end, they must have aged differently.

We now must wonder what can be given as the physical
cause of objective differential aging when there is no
acceleration and no clock synchronization to fall back on.

If you pick choice (ii), then you have Bob and Ann aging
differently (objectively) without acceleration, so you
still have the same problem of finding a physical cause
for objective differential aging for inertially-moving
people.

rqr

 

[JesseM]

I repeat my prior question:
In other words, the objective age difference that you agreed
to (by having a turnaround twin) _cannot_ be explained by
acceleration, so how would you explain it?

Of course it can be explained by acceleration, in exactly the same way that the difference in lengths between two paths between a pair of points on a piece of paper can be explained by the fact that one is straight and the other is non-straight (which is the same as saying one has a constant slope and the other has a changing slope, in terms of any cartesian coordinate system). Did you read my geometric analogy, and if so do you see anything wrong with it?

Your reply relied on coordinate systems, but there are none
in the given experiment.

A coordinate system isn't part of the physical facts of an experiment, it's part of how you describe the experiment and calculate things about it. Again think of the analogy with lines on paper--you can calculate the length of a path using a particular cartesian coordinate system, but the actual length of the path is independent of your choice of coordinate system, it's part of the physical facts about the path itself.

Also, even if there were any, then
their coordinate readings certainly could not physically
affect either one's aging or an atomic clock's rate.

Of course they don't, but they are used to calculate the elapsed age on a worldline in just the same way a cartesian coordinate system is used to calculate the elapsed distance on a path through space.

Just as you age in only one way as long as you do not change
your speed, any given ideal clock will tick at only one rate
as long as its speed does not change.

Why do you believe this? There is no physical reason to believe there is an objective truth about the "rate of ticking" of a particular clock that's independent of your choice of coordinate system, any more than there needs to be an absolute truth about an object's "velocity". Do you believe in absolute velocity? If not, why do you think clocks must have an absolute ticking rate?

There is no way that the slope of some coordinate line can
have any physical effect upon the intrinsic atomic rate of
an atomic clock or upon one's aging rate.

According to relativity there is no such thing as "the intrinsic rate" of a clock's ticking, it's observer-dependent just like velocity. There are no physical problems with this view, since there is no need to postulate an absolute ticking rate in order to make predictions about the outcome of any physical event, like what two clocks will read when they pass each other. If you have a problem with this, it would seem that it is based on your philosophical preconceptions, not on scientific issues.

Math (geometry) cannot affect one's aging rate.
Math (geometry) cannot affect an atomic clock's atomic rate.

Again, the "rate" of a clock's ticking at a particular moment has no objective value in relativity. The total time elapsed between two points on a clock's worldline does have an objective value, and this is a function of the geometry of spacetime in just the same way that the total distance between two points on a path through space is a function of the geometry of space.

You have agreed that the Two-people case contains real or
objective differential aging.

Differential aging between two events which lie on both twins' worldlines, yes. Different objective "rate of aging" at any particular moment in time, no.

Even though the Three-people
case has no accelerations, and the Two-people case does have
accelerations, acceleration per se has no physical effect
upon aging rates (or atomic clock rates).

The three-people case does not involve a comparison of different worldlines which travel through the same pair of points in spacetime. You can only talk about an objective amount of aging if you pick two particular points on a given worldline and ask how much a clock which has that worldline will have ticked between those two points. Again, talking about a "rate of aging" at a single moment is meaningless in relativity.

Therefore, if the Two-people case
has objective differential aging, then so does other case.

Objective differential aging between what pair of events in the three-person case? In this case, none of the worldlines intersect any other worldline more than once, so you don't have any pair of events where two different worldlines go between the pair and you can compare how much time elapsed on each worldline between those same two events.

What is the physical cause of objective differential aging?
(It can occur even without accelerations.)

Not if you restrict yourself to talking about the amount a clock/observer ages between two events on its own worldline, and avoid the idea that there is an objective truth about the "rate a clock is ticking relative to absolute time" at any given instant. This is analogous to the fact that there is an objective truth about the length of a path on a piece of paper between two points along that path, but no coordinate-independent truth about the "rate of partial path length increase relative to increasing y-coordinate" (again, please read over my 2D geometric analogy if you haven't already) at a single point on the path.

 

[JesseM]

(continued)

[added]
Originally Posted by rqr:
>>There is no need to bring up clock synchronization.

quoting JesseM:
>Of course there is, do you think the notion that Ann
>and copy-Bob both turned 10 simultaneously does not
>depend on your clock synchronization?

It cannot depend upon clock synchronization because there
are no clocks in the given experiment.

Fine, replace "clock synchronization" with "simultaneity". Your claims about who aged at what rate do depend on an assumption about simultaneity, namely that the event of Ann turning 10 and copy-Bob turning 10 both happened simultaneously. Just as a person might believe there is an objective truth about the "length" of an object even if there are no physical rulers in an experiment, for your claims to make sense you must believe there is an objective truth about simultaneity even if there are no synchronized clocks in the experiment.

What is does depend
on is the simple given fact that Ann & Bob aged alike.
(This was chosen as one of only two physically possible
paths, (i) Ann & Bob do not age alike, or (ii) they do.)

And what made you choose (ii)? An arbitrary whim? Why is it that you think Bob and copy-Bob, who passed each other at constant velocities just like Ann and Bob, do not age alike? And most importantly, why do you not consider the possibility that there is no objective truth about whether two separated observers are aging at the same rate or different rate? Would you say there is an objective truth about whether two observers moving apart move apart at "the same speed" or "different speeds", and if not, why should "rate of aging" be any more of an objective notion than speed?

This simple given fact tell us that whenever Bob is 10,
so is Ann.

And here you are making a claim about simultaneity.

We now must wonder what can be given as the physical
cause of objective differential aging when there is no
acceleration and no clock synchronization to fall back on.

There is no "objective differential aging" here because any claims about aging depend on your choice of simultaneity, which is totally arbitrary. I could just as easily pick a different definition of simultaneity which would tell me that copy-Bob aged more than Ann between the moment copy-Bob passed Bob and the moment he passed Ann, and you'd have no physical reason to say your definition of simultaneity is "more correct" than mine (your arbitrary whim is not a reason)

If you pick choice (ii)

A choice made by an arbitrary whim, not based on any of the observable physical facts of the experiment. Say we decide whether to go with your choice (i) or (ii) by flipping a coin--would you say that the "real physical truth" about who aged more depends on the outcome of this coinflip?

then you have Bob and Ann aging
differently (objectively) without acceleration

No, you have provided no argument whatsoever as to why we should think that there is any objective truth about who ages more, you have only shown that if we pick a totally arbitrary definition of simultaneity then there will be a truth about who ages more according to that definition.

 

[Tunji]

The little I understand...

When the twin paradox was explained to me, from what I got you (the twin on the rocket ship) are being removed from one reference frame to another passing through space-time at it's own velocity. This being so, your psychological time reference would feel unchanged because internally (assuming inertial dampners on the ship, hey it's a stretch but if the ship does have this sub-light speed tech. then...) your particles retain their initial speed. However because you are in a spacecraft moving at a speed greater than that of your previous reference point, Earth, you would expience time dilation at an effect equivalent to the given equation. Does that make any sense at all?

 

[pervect]

assuming inertial dampners on the ship, hey it's a stretch but if the ship does have this sub-light speed tech. then...)

Huh? Please do *NOT* assume fictional technology ("inertial dampers") in a serious discussion. I'm having a hard time following you at all, and your assumption of fictional technology as real is NOT helping matters.

 

[JesseM]

When the twin paradox was explained to me, from what I got you (the twin on the rocket ship) are being removed from one reference frame to another passing through space-time at it's own velocity. This being so, your psychological time reference would feel unchanged because internally (assuming inertial dampners on the ship, hey it's a stretch but if the ship does have this sub-light speed tech. then...) your particles retain their initial speed. However because you are in a spacecraft moving at a speed greater than that of your previous reference point, Earth, you would expience time dilation at an effect equivalent to the given equation. Does that make any sense at all?

This doesn't make much sense--for one thing "in a spacecraft moving at a speed greater than that of your previous reference point" would seem to assume some sort of absolute definition of "speed", which is wrong (there is no definite truth about which of two objects has a greater speed in relativity, you can pick a reference frame where either one has a greater speed or the speeds are equal). And as pervect said, the comment about "inertial dampeners" is just totally confusing, nothing in the analysis of time dilation in relativity should depend on such fictional technologies, and you'll always be aging at the normal rate in your own reference frame without the need for any special technologies to ensure this.

 

[rqr]

to JesseM once more

Quoting JesseM re the Three-people case:
>There is no "objective differential aging" here because any claims
>about aging depend on your choice of simultaneity, which is totally
>arbitrary.

No, there is no "choice of simultaneity" involved. There are only
these two mutually exclusive physical possibilities involved:

[1] Ann is not the same age as Bob and Copy-Bob whenever the
latter meet

or

[2] Ann is the same age as Bob and Copy-Bob whenever the
latter meet

Physical aging has nothing to do with your stated "arbitrary choice
of simultaneity." Aging has to do with internal bodily processes,
and each person will age in only one way in any given inertial frame.

There is no "whim" involved when I say that either [1] or [2]
must occur during the Three-people experiment.

You kept on insisting on using worldlines. That is math. There is no
math involved, so you cannot use worldlines.

Let me put it this way:
In the Three-people experiment, there are no clocks or rulers.
Therefore, there can be no usage of any math based on the use
of such instruments (such as Minkowskian math or SR math).

There can be no x, t, and v. No clocks, remember?
You cannot use any math, including any that contains
the standard variables x, t, and v.

There can be no use of any geometric analogies.

Quoting JesseM:
>Again, talking about a "rate of aging" at a single moment is
>meaningless in relativity.

I did not talk about "'rate of aging' at a single moment." I
am talking about "continuous (ongoing) rate of aging."

Now that I have laid down the ground rules, I will continue
my above-started Three-people experiment discussion.

As I said above, there are two (and only two) mutually-exclusive
physical possibilities, viz,

[1] Ann is not the same age as Bob and Copy-Bob whenever
the latter two meet

or

[2] Ann is the same age as Bob and Copy-Bob whenever the
latter two meet

To reiterate, one of these two must occur regardless of any
clock synchronization or simultaneity schemes, math, or
worldlines. Also to reiterate, this has nothing to do with
any whims on anyone's part.

Indeed, I used both to cover all physical possibilties.

When I used [1], we have the fact that Ann and Bob
aged differently even though both moved inertially.

But we got this same result when I used [2] because
then Ann and Copy-Bob aged differently even though
they both moved inertially.

As you can see, it really doesn't matter that there are
two physical possibilities because they both end up
with the same result, namely, people in different
frames aging differently.

My original question still stands:
What is there about merely being in different frames that
causes people to age differently?

rqr

 

[JesseM]

Quoting JesseM re the Three-people case:
>There is no "objective differential aging" here because any claims
>about aging depend on your choice of simultaneity, which is totally
>arbitrary.

No, there is no "choice of simultaneity" involved. There are only
these two mutually exclusive physical possibilities involved:

[1] Ann is not the same age as Bob and Copy-Bob whenever the
latter meet

or

[2] Ann is the same age as Bob and Copy-Bob whenever the
latter meet

To say that these represent "physical possibilities" is to assume that questions about relative rates of aging between observers in relative motion have some sort of "real" physical answer, and are not just dependent on your choice of coordinate system. But why do you assume this is true? Why can't "relative rate of aging" be a coordinate-dependent notion just as much as velocity or x-coordinate? What would you say to someone who said:

There are only
these two mutually exclusive physical possibilities involved:

[1] Ann's speed is greater than or equal to Bob's speed

or

[2] Ann's speed is less than Bob's speed

Would you accept without argument that these are distinct "physical possibilities", or would you point out that "speed" is an intrinsically coordinate-dependent notion and that there are no physical reasons to believe in absolute velocity? If the latter, why can't you accept that the same thing might be true of relative rate of aging?

If there is a truth about relative rate of aging, then do you at least agree that there would be absolutely no physical experiment that could determine the truth? That is all relativity is really saying, that there is no "preferred frame" in the sense of the laws of physics preferring one frame's point of view over another. You're free to adopt some sort of metaphysical belief that there really is an absolute truth about which frame's definition of relative aging rates or simultaneity (they are equivalent, of course) or velocity is the "correct" one, as long as you accept that only a god could know the true answer to this question, that there is no physical experiment we can perform that will determine which frame is "preferred" in this metaphysical sense. I would tend to think that occam's razor favors the view that if the laws of physics don't distinguish between different frame's answers to these questions, then that's because there is no physically correct answer and "relative rate of aging" or "simultaneity" or "velocity" are purely coordinate-dependent notions, but you're free to believe differently as long as it doesn't effect how you do physics.

Physical aging has nothing to do with your stated "arbitrary choice
of simultaneity." Aging has to do with internal bodily processes,
and each person will age in only one way in any given inertial frame.

And the laws of physics are sufficient to predict the outcome of any local physical event, like how old each person will be when they reunite in a single region of spacetime, without assuming that there is any absolute truth about the relative rate that each person was aging at any single moment during the trip.

There is no "whim" involved when I say that either [1] or [2]
must occur during the Three-people experiment.

OK, if you have the philosophical preconception that there is an absolute truth about relative rate of aging/simultaneity, then yes, either [1] or [2] must be correct. But you must admit that there is absolutely no experimental test that could distinguish between [1] and [2], therefore it's just a philosiphical view and not one that can be justified using physics. Personally, I prefer the philosophical view known as four dimensionalism (also known as eternalism) which says that the reality is the 4-dimensional spacetime manifold, there is no absolute truth about which set of events on that manifold lie in "the present" and thus no absolute truth about simultaneity or how old two people are "at the same moment" (the alternative view, which you seem to prefer, is known as presentism, it says that that there is a single objective "present moment" and thus an objective truth about whether two events happened at the same moment or different moments).

You kept on insisting on using worldlines. That is math. There is no
math involved, so you cannot use worldlines.

If you are a presentist than worldlines are just abstractions and the only reality is the arrangment of matter/energy in the absolute present. But for a four-dimensionalist, worldlines are actual 4-dimensional objects arranged in spacetime, and it's "the present" that's just an abstraction, a particular way of slicing up real 4D spacetime into a series of 3-dimensional spacelike surfaces, with the angle of the "slicing" depending on an arbitrary choice of coordinate system.

In the Three-people experiment, there are no clocks or rulers.
Therefore, there can be no usage of any math based on the use
of such instruments (such as Minkowskian math or SR math).

There can be no x, t, and v. No clocks, remember?
You cannot use any math, including any that contains
the standard variables x, t, and v.

This is a truly bizarre argument--do you believe that things cease to be true because they aren't measured? Do you think there can be no actual truth about whether the rest length of one object is greater than the rest length of another if we don't have a ruler to measure it? For that matter, how can you say that Ann or Bob is "10 years old" if you think there is no truth about statements concerning time elapsed when there are no clocks to measure it?

Quoting JesseM:
>Again, talking about a "rate of aging" at a single moment is
>meaningless in relativity.

I did not talk about "'rate of aging' at a single moment." I
am talking about "continuous (ongoing) rate of aging."

I said "at a single moment" to deal with cases where velocities change as in the standard twin paradox--there is obviously a definite truth about the total amount each one ages, my point is that there is no need to postulate a definite truth about the relative rate of aging when they have a particular relative velocity, like during the outbound phase of the trip. I suppose in your thought-experiment since no one changes velocity it doesn't matter, so in this case I'll say there is no need to postulate a truth about their ongoing rate of aging either. You can calculate things from the perspective of different frames which say different things about the rate they're aging, yet these different frames all make exactly the same predictions about local events like how old Ann and Copy-Bob are at the moment they pass. A presentist may say there is a "real" truth about which frame's definition of aging rates (or equivalently, which frame's definition of simultaneity) is the correct one, but they'd have to admit there's absolutely no physical experiment that can distinguish what this truth actually is, and a four-dimensionalist could simply dispense with the idea that this question has any single true answer, any more than the question of which person has a greater absolute speed.

Now that I have laid down the ground rules, I will continue
my above-started Three-people experiment discussion.

As I said above, there are two (and only two) mutually-exclusive
physical possibilities, viz,

[1] Ann is not the same age as Bob and Copy-Bob whenever
the latter two meet

or

[2] Ann is the same age as Bob and Copy-Bob whenever the
latter two meet

To reiterate, one of these two must occur regardless of any
clock synchronization or simultaneity schemes, math, or
worldlines. Also to reiterate, this has nothing to do with
any whims on anyone's part.

No, this is just a philosophical preconception. There is no need to believe there is any sort of physical truth about which of these is correct, since if there is no absolute present then there is no objective truth about how old two people are "at the same time", the notion of "same time" would be just as coordinate-dependent as "same x-coordinate" from the perspective of a four-dimensionalist.

Indeed, I used both to cover all physical possibilties.

No, you ignore the possiblity that there simply is no physically true answer to the question about whether events that happen far apart from one another occurred "at the same moment" or "at different moments".

 

[paw]

No, there is no "choice of simultaneity" involved.

On the contrary, there is definitely a "choice of simultaneity" involved.

Let's consider Copy-Bob for a minute. By definition Copy-Bob can't be co-moving with Ann and Bob at the moment she starts her trip otherwise you'd need to include Copy-Bobs acceleration in the treatment. Therefore, Copy-Bob must always be moving at 15/17c (0.88c).

Now Ann and Bob are co-moving in Bobs frame. Ann's ship can accelerate instantaneously to 0.6c. The very instant she presses the Go button both she and Bob flash powerful lights. Since Ann accelerates instantaneously she is doing 0.6c the moment her light flashes but there is no elapsed time yet for her to have traveled any distance. Therefore both Ann and Bob agree that the lights flashed simultaneously. Further, they are both exactly 10yo at the precise moment the lights flash.

Copy-Bob on the other hand is approaching Bobs light at 0.88c while he is approaching Anns light at 0.88-.6/(1+(.88)*(-.6)/c2)=0.59c. Copy-Bob sees Bobs light flash before Anns light. Therefore Copy-Bob doesn't agree with Bob about when Ann started the journey and by extension doesn't agree that Bob and Ann were both 10yo at the same time.

Simultanaity is definitely involved.

Physical aging has nothing to do with your stated "arbitrary choice
of simultaneity." Aging has to do with internal bodily processes,
and each person will age in only one way in any given inertial frame.

If you don't believe aging is equivalent to clocks ticking try substituting "heartbeats" for "clock ticks". It may not be as accurate as an atomic clock but your heartbeat is still a "clock" in that it can be used to time events and it responds to relative velocity the same way any other clock does.

My original question still stands:
What is there about merely being in different frames that
causes people to age differently?

Nothing. As has been said many times, there is no "objective measure" of aging between different inertial frames.

 

[JesseM]

No, there is no "choice of simultaneity" involved.

On the contrary, there is definitely a "choice of simultaneity" involved.

As I interpret him/her, rqr is not saying that simultaneity is not an issue, but rather he/she believes there is an absolute truth about whether two events are "really" simultaneous, so that there is no "choice" in the matter. The question of whether or not there is such a thing as absolute time is a philosophical one, not a physical one--this is what I was trying to make clear to rqr in my last post. rqr is free to believe in presentism since there's no logical reason you couldn't have a "metaphysically preferred" definition of simultaneity even in the absence of a physically preferred frame, but rqr needs to realize that this is a matter of untestable philosophical assumptions rather than testable physical ones, and that the philosophy of four-dimensionalism or eternalism is equally compatible with all the physical evidence (and I'd say it's more of a 'natural fit' for relativity and should be preferred if we accept occam's razor, but this is itself a philosophical judgment). If rqr agrees to this much, then we can "agree to disagree" on the question of whether presentism or four-dimensionalism makes more sense as a philosophy, or perhaps debate it in the philosophy forum rather than here.

 

[richard.coppack]

I am new to the forum, so I hope my question is relevant to the discussion. By the equations of the Lorentz transformation ; from a photon's frame of reference, time = 0. By the same equations x,y and z coordinates are also = 0. Does that mean a photon does not have any coordinates in spacetime.

 

[JesseM]

I am new to the forum, so I hope my question is relevant to the discussion. By the equations of the Lorentz transformation ; from a photon's frame of reference, time = 0. By the same equations x,y and z coordinates are also = 0. Does that mean a photon does not have any coordinates in spacetime.

Which equations are you using for the Lorentz transformation? If we use these:

$$x' = \gamma (x - vt)$$
$$y' = y$$
$$z' = z$$
$$t' = \gamma (t - vx/c^2)$$
with $$\gamma = 1/\sqrt{1 - v^2/c^2}$$

Then if you plug in v=c, you find that the x' and t' coordinates of an event would be infinite, not zero, and you also find that the y' and z' coordinates would be unchanged from the original coordinate system. Anyway, the answer is that it's understood by physicists that you can only meaningfully talk about the inertial rest frame of a sublight object, the photon doesn't have its own rest frame (if it did, it would violate one of the basic postulates of special relativity, which says that the laws of physics should work the same way in all inertial frames--there are no sublight frames where a photon can be at rest!) Look at this thread for further discussion.

 

[Tunji]

...riiiight

Thanks for all the encouraging input. Let's break this down. So we now have technology that is capable of carrying out this 'hypothetical' experiment of moving large matter at these immense velocities... I guess my inertial dampners went too far. Learn something new everyday. Now for this aging question where everyone ages at their 'own' rate and there is no faster 'speed' ( I'm generalizing the main arguments and rebuttals I've seen here). In the Lorentz transformation a ship, ummm sorry, Bob or Ann approaching light velocity would experience length contraction of their path. They would pass through a shorter path of space-time than the twin or whomever remained on Earth that their age was being judged against. Thus would be younger. I guess if it seems so simple it must be wrong so I'll wait for all my rebuttals. There seem to be a lot of 'experts' here so maybe I can learn something.

 

[Tunji]

Maybe I missed something. Can people travel at a speed approaching light without being torn apart?

 

[JesseM]

In the Lorentz transformation a ship, ummm sorry, Bob or Ann approaching light velocity would experience length contraction of their path. They would pass through a shorter path of space-time than the twin or whomever remained on Earth that their age was being judged against. Thus would be younger.

You're confusing different issues here, length contraction is just for the spatial distance between two objects (or the ends of a single object) as seen in a frame where the objects are moving, it doesn't tell you the time measured along a path through spacetime (a 'worldline'). In the Earth's frame, the traveling twin is moving at high velocity during both the trip away from Earth and the trip returning to Earth, so her clock is ticking slower and thus she ages less because of time dilation (not length contraction). You can analyze it from the perspective of a different frame, but it has to be the frame of an inertial observer who doesn't change velocities, so even if the traveling twin is moving slower than the Earth during the trip away from Earth in the frame of this observer (meaning the Earth's clock is ticking slower during this period, from this observer's perspective), she'd then have to be moving faster than the Earth during the trip back to Earth in the frame of the same observer (so her clock would be ticking slower during this period), and the result would be that the average rate her clock was ticking on both parts of the trip would still be slower than the Earth, so this observer would also predict she'd be younger when she returns to Earth.

 

[JesseM]

Maybe I missed something. Can people travel at a speed approaching light without being torn apart?

What would tear them apart? One of the main points of relativity is that there's no absolute notion of speed (the Earth is moving at a 'speed approaching light' right at this moment in some frame, and this frame is just as valid as the frame where the Earth is at rest), and the laws of physics will work exactly the same in the frames of different observers moving at constant velocity (unchanging speed and direction). If you have two observers in sealed rooms which are both moving at different constant velocities in space, they will both feel weightless, and there is no experiment they can perform inside the room that will come out differently depending on their velocity relative to the Earth (or any other landmark), all their experiments will give the same results even if they are moving at 0.99c relative to one another. When you feel G-forces in a moving vehicle, that has nothing to do with your velocity, the G-forces are only produced by acceleration (changing velocity). Even the science-fictional notion of "inertial dampeners" is only meant to prevent starships from getting torn apart by large accelerations, not large velocities (and again, whether a velocity is large or small depends on an arbitrary choice of reference frame, there is no such thing as absolute velocity in physics).

 

[robphy]

Relativistically speaking,
a different way to say this, which corrects an often heard saying "Speed Kills", is
"speed doesn't kill... acceleration does".

 

[pervect]

Relativistically speaking,
a different way to say this, which corrects an often heard saying "Speed Kills", is
"speed doesn't kill... acceleration does".

This reminds me of the saying - it's not the fall that kills you, it's the sudden stop at the end...

 

[Ich]

Hmm... acceleration gradients kill? Killing Tides?

 

[rqr]

Originally Posted by rqr
>>There can be no x, t, and v. No clocks, remember?
>>You cannot use any math, including any that contains
>>the standard variables x, t, and v.

Originally Posted by JesseM:
>This is a truly bizarre argument--

Not for a test of special relativity theory.

Yes, the experiment that I presented was a simple test of SR.
In order to test SR, one must necessarily exclude all that is
SR, including its definition of simultaneity and its slowed
clocks and contracting rulers.

This is why the given experiment contains no clocks and no
rulers. This is why the given experiment contains no definition
of "simultaneity." Indeed, this is why it neither contains nor
needs any coordinate system of any kind, including Einstein's.

Here's an equation to help you see what this means:

SR coordinate system = SR's results guaranteed.

Therefore, it is "illegal" to insert any SR "stuff" into
the given experiment.

This includes SR's relative simultaneity.

Looking back at the given experiment, we see that it contains
only people and their aging.

Originally Posted by JesseM:
>...how can you say that Ann or Bob is "10 years old" if
>you think there is no truth about statements concerning
>time elapsed when there are no clocks to measure it?

Because "10 years old" is a physical state having nothing
whatsoever to do with any coordinate time.

In fact, it doesn't matter if they are 10 or 120; all that
matters is that their ages at their meeting point are the
same.

Originally Posted by JesseM:
>But you must admit that there is absolutely no experimental
>test that could distinguish between [1] and [2], therefore
>it's just a philosiphical view and not one that can be
>justified using physics.

I do not need to "distinguish between [1] and [2]"; all I
need to do is to declare their physical existence. Indeed,
as I said, I included _both_ in the results, so there was
never any need to even consider distinguishing.


Originally Posted by rqr
>>To reiterate, one of these two must occur regardless of any
>>clock synchronization or simultaneity schemes, math, or
>>worldlines. Also to reiterate, this has nothing to do with
>>any whims on anyone's part.

Originally Posted by JesseM:
>No, this is just a philosophical preconception. There is no
>need to believe there is any sort of physical truth about
>which of these is correct, since if there is no absolute
>present then there is no objective truth about how old
>two people are "at the same time", the notion of "same time"
>would be just as coordinate-dependent as "same x-coordinate"
>from the perspective of a four-dimensionalist.

Again, I am not trying to decide which of the two is correct;
both are used in the given results.

Originally Posted by JesseM:
>Would you accept without argument that [[1] and [2]] are
>distinct "physical possibilities", or would you point
>out that "speed" is an intrinsically coordinate-dependent
>notion and that there are no physical reasons to believe
>in absolute velocity? If the latter, why can't you accept
>that the same thing might be true of relative rate of aging?

Speed is not analogous to aging. Aging is a definite physical
process that is absolute. Speed, on the other hand, can vary
depending on the coordinate system used. There is no way that
the use of any coordinate system can control my aging process.

There are no relative speeds in the given experiment because
there are no coordinate systems.

And there was no mention of absolute speeds.

As I said, this is a simple test of special relativity theory;
it is a test of SR's claims that aging (for inertially-moving
people) has something to do with coordinate measurements,
worldlines, odometer analogies, accelerations, frame jumping,
different "space-time paths," or whatever.

The test shows that objective differential aging exists, and
that SR has no physical explanation for it.

Two people who have the same age when they meet in passing
do not have the same age when they meet again.

As I said, this is guaranteed to happen whether we have [1]
or [2]. (Since Copy-Bob is a direct copy of Bob (age-wise),
it is clear that "when they meet again" can apply to either
Bob or to Copy-Bob.)

There is no philosophy here.

rqr

 

[JesseM]

Originally Posted by rqr
>>There can be no x, t, and v. No clocks, remember?
>>You cannot use any math, including any that contains
>>the standard variables x, t, and v.

Originally Posted by JesseM:
>This is a truly bizarre argument--

Not for a test of special relativity theory.

Yes, the experiment that I presented was a simple test of SR.
In order to test SR, one must necessarily exclude all that is
SR, including its definition of simultaneity and its slowed
clocks and contracting rulers.

This is why the given experiment contains no clocks and no
rulers. This is why the given experiment contains no definition
of "simultaneity." Indeed, this is why it neither contains nor
needs any coordinate system of any kind, including Einstein's.

Here's an equation to help you see what this means:

SR coordinate system = SR's results guaranteed.

Therefore, it is "illegal" to insert any SR "stuff" into
the given experiment.

No, you're confused, it would be quite possible to use SR coordinate systems and find that the physical predictions of SR were wrong. For example, if actual physical clocks did not show time dilation, they would not all tick at the same rate as the time coordinate of their rest frame in the SR coordinate systems. More generally, if different observers in relative motion determined what equations would be used to express the laws of physics in their SR rest frame, if the laws of physics were such that the equations they found would differ from one SR frame to another, this would show SR was wrong. If Newtonian physics was correct, for example, the equations to describe it would have to be different in different SR frames.

Originally Posted by JesseM:
>...how can you say that Ann or Bob is "10 years old" if
>you think there is no truth about statements concerning
>time elapsed when there are no clocks to measure it?

Because "10 years old" is a physical state having nothing
whatsoever to do with any coordinate time.

I agree, but you're missing the point--the point is that saying that two people are "10 years old at the same time" is not necessarily a physical state that can be true or not true independent of your coordinate system; there would be an objective truth about whether two events happened at the "same time" if you accept the philosophy of presentism, but if you accept the philosophy of four-dimensionalism, the reality is just a static four-dimensional "block universe" filled with different worldlines similar to a block of ice cube with strings embedded in it, and there is no more of an absolute truth about whether two points in this 4-dimensional structure happen at the "same time" than there is about whether two marks drawn on different strings in the ice cube have the "same height" (which depends on which spatial axis you define as height).

Originally Posted by JesseM:
>But you must admit that there is absolutely no experimental
>test that could distinguish between [1] and [2], therefore
>it's just a philosiphical view and not one that can be
>justified using physics.

I do not need to "distinguish between [1] and [2]"; all I
need to do is to declare their physical existence.

But if there is no physical truth about whether two events happen "at the same time", there is no physical distinction between [1] and [2]. Do you think [1] and [2] would represent distinct physical possibilities if four-dimensionalism were correct and there was no absolute present, just a static four-dimensional spacetime?

Originally Posted by JesseM:
>No, this is just a philosophical preconception. There is no
>need to believe there is any sort of physical truth about
>which of these is correct, since if there is no absolute
>present then there is no objective truth about how old
>two people are "at the same time", the notion of "same time"
>would be just as coordinate-dependent as "same x-coordinate"
>from the perspective of a four-dimensionalist.

Again, I am not trying to decide which of the two is correct;
both are used in the given results.

But you are assuming that one or the other must be correct. Four-dimensionalism would say this is a false assumption--don't you understand that? You're free to disagree with four-dimensionalism, but this is a philosophical belief, and you admit there's no physical way to test between [1] and [2].

Originally Posted by JesseM:
>Would you accept without argument that [[1] and [2]] are
>distinct "physical possibilities", or would you point
>out that "speed" is an intrinsically coordinate-dependent
>notion and that there are no physical reasons to believe
>in absolute velocity? If the latter, why can't you accept
>that the same thing might be true of relative rate of aging?

Speed is not analogous to aging. Aging is a definite physical
process that is absolute.

We're not talking about the aging of one person, we're talking about whether two people at different locations are the same age or different ages. If there is no absolute truth about whether two events happen at the "same time", how can their be an absolute truth about whether two people are the "same age" or not? If the ultimate reality is just a static 4-dimensional spacetime with various worldlines embedded in it like strings in a block of ice, there's certainly a definite truth about how old anyone is at any given point on their worldline, but no truth about whether a point on my worldline and a point on your worldline happen "at the same time" or "different times", so if those two points are me turning 30 and you turning 30, there's no absolute truth about whether we reached these ages "at the same time" or different times".

Speed, on the other hand, can vary
depending on the coordinate system used. There is no way that
the use of any coordinate system can control my aging process.

There are no relative speeds in the given experiment because
there are no coordinate systems.

And there was no mention of absolute speeds.

I wasn't claiming there was, I was using speed as an analogy to point out that it's wrong to simply assume there must be a definite physical truth about whether property X of Ann is different than or equal to the same property X of Bob; you admit that if X=speed there would be no definite physical truth, so I'm trying to show you that the same may be the case if X=age.

As I said, this is a simple test of special relativity theory;
it is a test of SR's claims that aging (for inertially-moving
people) has something to do with coordinate measurements,
worldlines, odometer analogies, accelerations, frame jumping,
different "space-time paths," or whatever.

The test shows that objective differential aging exists, and
that SR has no physical explanation for it.

There is no test of your claim that either [1] or [2] must be physically correct--that is just a philosophical assumption you make. It might be correct if presentism is the right philosophy of time, but if four-dimensionalism is the right philosophy of time it isn't.

There is no philosophy here.

Clearly you don't understand the philosophy then. Can you explain in your own words how even if the four-dimensionalist philosophy of time is true and there is no absolute present, there still must be a physical truth about whether [1] or [2] is correct?

 

[rqr]

2 jessem

Quote:
[JesseM wrote:]
>But you are assuming that one or the other must
>be correct. Four-dimensionalism would say this
>is a false assumption--don't you understand that?
>You're free to disagree with four-dimensionalism,
>but this is a philosophical belief, and you admit
>there's no physical way to test between [1] and [2].

I repeat, there is no need to "test between [1] and [2]."
(More on this below.)

Quote:
[JesseM wrote:]
>We're not talking about the aging of one person,
>we're talking about whether two people at different
>locations are the same age or different ages. If
>there is no absolute truth about whether two events
>happen at the "same time", how can their be an
>absolute truth about whether two people are the
>"same age" or not?

I repeat, there is no need to possesses absolute time
or the "absolute truth" re time as far as the given
experiment is concerned; in fact, that was one of the
prime reasons for the particular design of the given
experiment.

Perhaps a quick example will help:
Suppose there are two inertially-moving rods in
space very far apart in different frames; despite
their physical separation, we know that there are
only three physical possibilities re their intrinsic
lengths, viz., (i) they are equal, (ii) rod A is
longer, or (iii) rod B is longer. Note carefully
that these three physical possibilities have nothing
to do with coordinate measurements; indeed, such
measurements can come up with many different
"lengths" for one and the same rod that is moving
at a constant velocity. Also note the fact that
this has nothing to do with absolute time or with
relative time. And there is no philosophy here.

Similarly, in the given experiment, there are only
two physical possibilities, namely, [1] Ann's and
Bob/Copy-Bob's ages differ when the latter meet in
passing, or [2] they are the same.

But, as I said, I don't care which it is. There is
no need to find out which is the case. There is no
need to test for which is true because I used BOTH
in my solution. (It's not either/or; it's both!)

To reiterate, here is how I used [1]:
If Ann's and Bob/Copy-Bob's ages differ when the latter
meet in passing, then this proves that objective age
differences occur sans acceleration because neither
Ann nor Bob accelerated.

To reiterate, here is how I used [2]:
If Ann's and Bob/Copy-Bob's ages are the same when the
latter meet in passing, then this also proves that objective
age differences occur sans acceleration because neither
Ann nor Copy-Bob accelerated, and yet their ages differed
when they met in passing at the end.

This proves the deficiency of special relativity theory
by showing that it has no physical explanation whatsoever
for this objective differential aging; indeed, if we could
ever get beyond this simple conclusion, we could then see
clearly that the only possible physical explanation must
be that which SR has denied all meaning to.

There are many other ways to show how SR is deficient;
for example, and contrary to all that we have been told,
it cannot correctly measure light's one-way speed. In
order to do this, one must use absolutely synchronous
clocks, but SR has no such clocks.

Quoting Einstein's 1905 paper (from Section 2):
"Observers moving with the moving rod would thus
find that the two clocks were not synchronous,
while observers in the stationary system would
declare the clocks to be synchronous."

This makes it clear that, to Einstein, the phrase
"relative simultaneity" is equivalent to the phrase
"relative synchronization," which cannot be absolute
synchronization. Therefore, SR has no absolutely
synchronous clocks, so it cannot correctly measure
any one-way, two-clock speed, including light's.

rqr

 

[JesseM]

I repeat, there is no need to possesses absolute time
or the "absolute truth" re time as far as the given
experiment is concerned; in fact, that was one of the
prime reasons for the particular design of the given
experiment.

My point is that even if you do not know the "absolute truth" about simultaneity or absolute time, you are still assuming that such an absolute truth exists. From the four-dimensionalist point of view, there simply is no absolute truth about whether two events happen at the "same time" or not, not even an unknowable absolute truth.

Perhaps a quick example will help:
Suppose there are two inertially-moving rods in
space very far apart in different frames; despite
their physical separation, we know that there are
only three physical possibilities re their intrinsic
lengths, viz., (i) they are equal, (ii) rod A is
longer, or (iii) rod B is longer.

No, I don't know that! You keep ignoring the possibility that there simply is no absolute truth about certain questions. Is this idea really so hard to understand? Didn't you already admit that there may be no absolute truth about whose speed is greater?

Note carefully
that these three physical possibilities have nothing
to do with coordinate measurements; indeed, such
measurements can come up with many different
"lengths" for one and the same rod that is moving
at a constant velocity. Also note the fact that
this has nothing to do with absolute time or with
relative time.

Sure there is. The "length" of a moving object is just the distance between the back end and the front end at a single moment. If I say that the event of the back end of the moving rod passing the Earth happened "at the same time" as the event of the front end passing the moon, then I will say the length of the rod is equal to the distance between the Earth and the moon; if you disagree that these events happened simultaneously, and you say instead that the event of the back end passing Earth happened "at the same time" as the event of the front end passing Mars, then you will say the length of the rod is equal to the distance between Earth and Mars. If there is no absolute truth about whether two events happened at the "same time", then there is no absolute truth about an object's "length".

Similarly, in the given experiment, there are only
two physical possibilities, namely, [1] Ann's and
Bob/Copy-Bob's ages differ when the latter meet in
passing, or [2] they are the same.

You keep ignoring the argument that a four-dimensionalist who doesn't believe in absolute time would deny that these are two distinct physical possibilities! Do you not see that by saying there must be an absolute truth about whether [1] or [2] is true, you are saying there must be an absolute truth about whether the event of Ann turning 10 and the event of Bob turning 10 happened "at the same time" or not--in other words, you are assuming there is such a thing as absolute time?

But, as I said, I don't care which it is. There is
no need to find out which is the case.

Do you not understand that the four-dimensionalist would deny that either one is "the case" in any objective sense, just like you would deny that either [1] Bob's speed is greater than or equal to Ann's or [2] Bob's speed is less than Ann's must objectively be true while the other is objectively false, since you don't believe in absolute velocity? Why do you keep assuming that one or the other must objectively be true in your example?

This discussion will never go anywhere if you keep ignoring the basic point of my argument, that your assumption that either [1] or [2] must be true is based on the idea that there's a definite truth about whether two events happen "at the same time", and that since a four-dimensionalist would deny this, he'd also deny that there's any objective truth about whether [1] or [2] is the case. For a four-dimensionalist, time is just another dimension in four-dimensional spacetime, and asking whether two events happen at the "same time" is just as arbitrary as asking whether two dots on a sheet of paper have the "same height along the y-axis"--obviously there is no objective answer to this question, it depends on an arbitrary choice of how you decide to orient your y-axis! If you wish to continue this discussion, you can't just keep on ignoring my argument and asserting that either [1] or [2] must be true without even addressing the reasons a four-dimensionalist would deny that assertion. So if you choose to post again, please give a detailed answer to the final question from my last post, namely: "Can you explain in your own words how even if the four-dimensionalist philosophy of time is true and there is no absolute present, there still must be a physical truth about whether [1] or [2] is correct?"