# Re: Another stab at the universal present moment - a gedanken..

`On Thu, Jan 2, 2014 at 11:54 AM, Edgar L. Owen <edgaro...@att.net> wrote:`
```
> Jason,
>
> Sorry, but you didn't address the argument I presented. I don't see how I
> think it through.
>
>
> And there are only 2 frames under consideration in our example.
>

Okay, let's use a concrete example from here on, because I think it will
help:

Two twins, Sam and Pam are born on the same day in the year 2000. Sam
remains on Earth, and Pam goes to Proxima Centauri (4 light years away) at
80% the speed of light and then comes back at the same speed. When the
twins are reunited in 2010, Sam is 10 years old and Pam is 6 years old.

I think you are asking me to consider the two frames of each twin. I agree
that every moment between when Pam and Sam are separated, until they are
reunited, each twin exists and is doing something, and this is necessary
the case in all possible frames from all possible external observers too,
since they eventually meet up again.

However, to me there is already one apparent contradiction in the idea of a
common present when considering this example. Sam experiences 10 years of
time, 10 years of biological ageing and 10 years of memories, yet Pam only
experiences 6. If there is a common present, how can Sam "experience more
of them" than Pam?  It seems Pam only experiences 60% of the present
moments that Sam does.  How do you account for this with P-time?

> Forget about all others. Second you are again trying to analyze present
> moment time with SR. It won't work for reasons I've repeatedly explained.
>
> 4 dimensionalism (SR and GR work great - for clock time, not for Present
> moment time which you've already agreed is a whole different kind of
> time)...
>

"Present moment time" in the twin example is equivalent to the lengths of
the paths traced by each twin through space time. Pam's journey toward
proxima centauri is the hypotenuse of a 3-4-5 triangle. She moves 4 light
years through space, and 3 through proper time (she is 3 when she gets to
the destination), but the path through space time is the hypotenuse, which
is 5 light years long. Meanwhile, 5 years have transpired on Earth and Sam
is 5 years old.

During Pam's return voyage, each twin traces out another 5 light years
through space time. So when the twins are reunited, in 2010, Sam's
coordinate time is sqrt(0^2 + 10^2) (going 0 ly through space and 10
through proper time), and Pam's coordinate time is sqrt(8^2 + 6^2), having
gone a total of 8 ly through space and 6 through proper time. Since things
only interact when their x,y,z, and (coordinate time) t are the same, the
10 year old Sam who shakes hands with Pam is shaking hands with the
6-year-old Pam, since they both have a coordinate time of 10 light years.

> In the common present moment someone is either actually dead or not dead.
> It is true that it's not alway possible to measure when this happened in
> any particular clock time frame. But that is just trying to assign a t
> value to the time of death. Nevertheless someone is always either dead or
> not dead in the actual shared present moment....
>

You can say the common universal present is all things that  have the same
coordinate time t, but only in the context of a particular inertial frame.
The moment you allow different intertial frames, there can be no agreement
on what the current coordinate time is for different things that are in
different locations.

Consider Pam's perspective during her trip from Earth to Proxima Centauri.
She might consider herself to be at rest, and Earth, Sam and Proxima
Centauri to be flying through the universe at 80% c. Since these things are
moving so fast, she measures the distance between Earth and Proxima
centauri to be length contracted to 60% of what Sam believes it to be. She
thinks it is 2.4 ly, not 4 ly.  Therefore, at Proxima Centauri's present
speed it will take 3 years to get to her (2.4 / 0.8). By the time Proxima
Centauri arrives, she believes her coordinate time is only 3 light years
(as is Sam's from her perspective), she thinks Sam is only (3 * 60%) = 1.8
years old, while Sam thinks he is 5 by the time she gets to Proxima

How can Sam believe he is 5, while Pam believes he is 1.8 (when Pam arrives
at her destination).  Note both twins agree that Pam is 3 at the time she
arrives at Proxima Centauri, and both twins agree that when they meet at
Earth in 2010 that Sam is 10 and that Pam is 6.

This shows you can't extrapolate from common agreements when two people are
together to common agreements when two people are apart, just because there
is agreement when they meet up again.

Jason

>
>
> On Thursday, January 2, 2014 9:56:44 AM UTC-5, Jason wrote:
>
>>
>>
>>
>> On Thu, Jan 2, 2014 at 8:50 AM, Edgar L. Owen <edga...@att.net> wrote:
>>
>>> Hi Jason,
>>>
>>> No, sadly you haven't quite gotten it yet but you are getting closer it
>>> seems.
>>>
>>> First the twins do NOT have the same (x,y,z,t) coordinate times (that
>>> would be true of an SR constant velocity example, but not the twins' GR
>>> acceleration based example). Their watches show they don't, and when they
>>> compare watches both twins agree with the readings on both watches. Not
>>> only do the twins have different ages but their clocks accurate show that
>>> age difference. Both twins agree that the traveling twin aged less because
>>> comparing their clocks both mechanical and biological confirms that.
>>>
>>> Thus they have different (x,y,z,t) coordinates yet they DO interact.
>>> Why? Only because they share the exact same present moment which is the
>>> only place interactions can occur whether clock times are the same or not.
>>> And that present moment P-time is a completely independent kind of time
>>> from clock time. There is simply no way around this.
>>>
>>>
>> You are describing coordinate time.
>>
>>
>>> Yes, you are correct the twins shaking hands and comparing watches
>>> confirms a shared present moment by direct experiment if the (x,y,z)
>>> coordinates are the same but not they they different. However the argument
>>> to deduce a common present moment when (x,y,z) coordinates are different is
>>> simple and clear. I've already posted it a couple of times but will
>>> summarize it again.
>>>
>>> The twins start and end at the same (x,y,z) coordinates. At both times
>>> we agree they share the same present moment. Their passages from point A to
>>> point B must both be represented by continuous lines, one curved, one
>>> straight. During every point during that passage both twins continuously
>>> experience their own present moment time without interruption and those
>>> present times are the same when they start and when they meet up again.
>>> Thus we must logically conclude that at every present time moment for
>>> either observer there absolutely must have been a corresponding present
>>> time moment for the other.
>>>
>>
>> You have demonstrated it for two observers at the same x,y,z, but it does
>> not logically follow for different x,y,z's.
>>
>>
>>>  This is not directly observable
>>>
>>
>> So we should maintain some doubt..
>>
>>
>>>  but is the only logical conclusion
>>>
>>
>> SR shows there is another possible conclusion.
>>
>>
>>>  based on their starting and ending at a shared present moment and both
>>> their spacetime travels being continuous with no breaks in between.
>>>
>>
>> This can also be explained by a an (approximately) continuous,
>> four-dimensional reality, in which all events are embedded.
>>
>>
>>>
>>> The easy way to understand this is that every present moment for either
>>> twin, the other twin must actually exist and be doing something too.
>>>
>>
>> In some relativistic frames, with separated twins might be considered
>> dead, and the other still alive, while in another frame, the former twin is
>> still and the other is dead.  The only sense in which the other is
>> guaranteed to exist and be doing something is that both twin's "world
>> tubes" exist and are eternally embedded in the four dimensional reality.
>>
>>
>>> There is absolutely no way around that! Thus they must share a common
>>> present moment in which they are existing and doing something even when
>>> they are separated spatially. Clearly this cannot be experimentally
>>> confirmed (measured) but it is the only tenable logical conclusion unless
>>> you think things pop in and out of existence which they don't.
>>>
>>>
>> Now again for the nth time. don't try to analyze this by relativistic
>>> clock time theory. That correctly describes how clock times change during
>>> the trip but has no relevance to present time whatsoever! Two completely
>>> different kinds of time.
>>>
>>
>> I fail to see how this is any different from coordinate time vs. proper
>> time in SR.
>>
>>
>>>
>>> Thus the only possible conclusion is that there is a common universal
>>> shared present moment time which is completely different from clock time.
>>>
>>>
>>>
>>
>> Why doesn't four dimensionalism work?
>>
>> Jason
>>
>>
>>>
>>>
>>>
>>> On Wednesday, January 1, 2014 3:15:27 PM UTC-5, Jason wrote:
>>>
>>>> Edgar,
>>>>
>>>> I believe I may understand your point about a universal present, but it
>>>> is something relativity handles, as far as I can see, without having to
>>>> postulate anything new.  Anything having the same (x, y, z, t) coordinates
>>>> can interact, where t is coordinate time. It seems like you believe that
>>>> because the twins are different ages (in different proper times), that they
>>>> cannot interact. But they can, because each has traced exactly 10 light
>>>> years through space-time (their coordinate times are the same).
>>>>
>>>> So you might say everything with the same coordinate time, at the same
>>>> place (x, y, z) the same, shares a present moment. But you cannot use this
>>>> fact to extrapolate to spatially separated things sharing a present. For
>>>> this, the definition of a present (what things exist having the same
>>>> coordinate times) differs in different reference frames.
>>>>
>>>> Jason
>>>>
>>>>
>>>> On Wed, Jan 1, 2014 at 3:01 PM, Jason Resch <jason...@gmail.com> wrote:
>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Wed, Jan 1, 2014 at 8:41 AM, Russell Standish <
>>>>> li...@hpcoders.com.au> wrote:
>>>>>
>>>>>> On Mon, Dec 30, 2013 at 01:20:35AM -0800, Edgar L. Owen wrote:
>>>>>> > Jason,
>>>>>> >
>>>>>> > That's a totally off the wall answer. When the two shake hands it's
>>>>>> not
>>>>>> > just photons that are interacting, it's the electrons, protons and
>>>>>> neutrons
>>>>>> > of the matter of their hands which don't travel at the speed of
>>>>>> light.
>>>>>> >
>>>>>> > Goodness gracious!
>>>>>> >
>>>>>> > Edgar
>>>>>> >
>>>>>>
>>>>>> Jason is correct - electron-electron and electron-proton interactions
>>>>>> are mediated by photons. Only nucleon-nucleon interactions are
>>>>>> mediated by different stuff (gluons in that case), but for all
>>>>>> practical purposes, the strong force is irrelevant to the phenomenon
>>>>>> of handshaking.
>>>>>>
>>>>>
>>>>> And if it were, say in some particle accelerator, the gluons also
>>>>> travel at the speed of light and their present is spread across all proper
>>>>> times.
>>>>>
>>>>>
>>>>>>
>>>>>> Which gets us to the more important point. You idealise a handshake as
>>>>>> instantaneous as a demonstration of your "present moment", but in fact
>>>>>> those interactions Jason was alluding to are smeared out over a
>>>>>> temporal duration of the order of a few picoseconds (a duration well
>>>>>> measurable by current day technology - my laptop's CPU clock cycles on
>>>>>> a sub-picosecond timescale, for example).
>>>>>>
>>>>>
>>>>> You must have a VERY fast laptop! :-)
>>>>>
>>>>> Jason
>>>>>
>>>>>
>>>>>> This doesn't matter much for human affairs, but becomes quite
>>>>>> significant when extrapolating over cosmological scales.
>>>>>>
>>>>>> Cheers
>>>>>> --
>>>>>>
>>>>>> ------------------------------------------------------------
>>>>>> ----------------
>>>>>> Prof Russell Standish                  Phone 0425 253119 (mobile)
>>>>>> Principal, High Performance Coders
>>>>>> Visiting Professor of Mathematics      hpc...@hpcoders.com.au
>>>>>>
>>>>>> University of New South Wales          http://www.hpcoders.com.au
>>>>>> ------------------------------------------------------------
>>>>>> ----------------
>>>>>>
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>>>>>
>>>>>
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