[pzkpfw]

Take Alice and Bob passing each other in space, in inertial relative movement.
Alice can consider herself at rest, and Bob is passing at 100 kph.
Bob can consider himself at rest, and Alice is passing at 100 kph.
Alice has a clock that ticks at 1 second per second, but for her, Bob's clock is slow.
Bob has a clock that ticks at 1 second per second, but for him, Alice's clock is slow.
Thus, their clocks cannot be synchronised. This is basic relativity.

Well, yes, you can insert Carol who remains between Alice and Bob, for whom they are both doing 50 kph. For Carol, Alice and Bob's clocks tick at the same rate.

Your scenario seems to have the clocks meet at a common event, ...

No, I don't think it does.

... but to generalize a bit, and to remove all unnecessary observers, consider flat spacetime containing two inertial clocks at arbitrary locations, moving at arbitrary velocities, and set to arbitrary times.
In exactly one frame C will those clocks be moving in equal and opposite velocities. ...

Yes, the simple scenario implies this. Though I named two velocities to try to make it less abstract for the OP.

(Personally, I think the OP is diving off into Twins' Paradox etc, with ever increasing complexity, when they don't really grasp the significance of the basics of relativity. I think the explanations need to get simpler, not more complex.)

[hamdani yusuf (OP)]

I do acknowledge that the intended purpose of an observer is often to simply hang a name tag on a frame, so 'according to Carol' becomes shorthand for 'relative to the frame in which Carol is stationary', but 1) it matters not a hoot then where Carol is in that frame, and 2) a rock with 'Carol' painted on it serves the same purpose.

That's good to know.

[hamdani yusuf (OP)]

No, I am not forgetting the very thing I am responding to. You tend to throw out snippets that contradict your own previous snippets, or lead to consequences that don't make sense. This is one.

Take Alice and Bob passing each other in space, in inertial relative movement.
Alice can consider herself at rest, and Bob is passing at 100 kph.
Bob can consider himself at rest, and Alice is passing at 100 kph.
Alice has a clock that ticks at 1 second per second, but for her, Bob's clock is slow.
Bob has a clock that ticks at 1 second per second, but for him, Alice's clock is slow.
Thus, their clocks cannot be synchronised. This is basic relativity.

Well, yes, you can insert Carol who remains between Alice and Bob, for whom they are both doing 50 kph. For Carol, Alice and Bob's clocks tick at the same rate. (*1)

But does that mean Alice and Bob's clocks ARE in an absolute sense (or can be) synchronised?
Would Alice and Bob agree?
And, if that were true, doesn't that mean you could postulate a Carol for ANY two such clocks? (*2)
Do you think it matters to Alice and Bob if there is a Carol there or not?

Notes:
*1 For Carol, Alice and Bob's clocks will tick slower than hers of course, so you've also just moved the synchronisation issue one step deeper
*2 That's why I made my previous post

It looks like we're talking pass each other. Let's start with the basic common ground. How do you define synchronized clocks?
The same question for Alan.

Both slow down equally, thus they are still synchronized to each other.

But each appears to be running slow from the point of view of the other, and neither is in sync with the midpoint observer's clock.  So none is synchronised with any other. It just happens that, seen from the midpoint, both departing clocks are equally wrong.

[hamdani yusuf (OP)]

In exactly one frame C will those clocks be moving in equal and opposite velocities.

That's only true if we were living in a one space dimension universe.

[pzkpfw]

It looks like we're talking pass each other. Let's start with the basic common ground. How do you define synchronized clocks?

First, they must tick at the same rate as each other.

Second, some process will have been used to set them to a known start time. (Though this can be relaxed as long as their time as at an agreed event is known; their offsets can be accounted for.)

Where we differ is your claim that a special observer (i.e. one between the two clocks, for whom the clocks tick at the same rate) can claim they are synchronised. But no, to claim the clocks ARE synchronised - it's the clocks themselves (or observers at rest with them) that need to agree they tick at the same rate. That means they must be at rest with respect to each other, i.e. not like my Alice and Bob, regardless of what Carol says.

[alancalverd]

Let's start with the basic common ground. How do you define synchronized clocks?
The same question for Alan.

Two clocks are synchronised if A knows what time B is showing, simply by looking at his own clock.

If the clocks are identical, this can only be the case if there is no relative motion between them. Where there is relative motion, he must apply a relativistic correction and thus needs additional information to determine the time shown at B.

Remember, the whole point of "gentlemen, synchronise your watches" is to ensure that the army, navy and air force hit the target in the required sequence tomorrow, so there is an assumption of adequate identicality and negligible relative velocity between clocks. Nowadays we can guarantee a high degree of identicality by using atomic clocks, and measure the relativistic discrepancy at fairly small velocities. Hence no "paradox".

[Halc]

How do you define synchronized clocks?
The same question for Alan.

I notice I do not get asked. Am I considered so correct that there's no trolling my mistakes? I do assure you that I make mistakes.

'Synchronized clocks' means that the clocks read the same value at any given time. Due to relativity of simultaneity,, the 'at the same time' part of that definition is ambiguous in the absence of a frame reference.


To be synced relative to a frame means that the two clocks read the same value at all times in that frame. This is why Einstein put out his sync convention, which is a way to get two clocks in sync relative to some frame, but that convention only works for clocks stationary in the referenced frame. There are other conventions that can be used for moving clocks, but clocks cannot be synced in any frame in which they 'tick at' different rates.

To be synced at an event means that both clocks read the same value (typically one being set to the time of the other, or both zeroed) at some event at which both clocks are present. This is an objective sync. It doesn't persist over time, and it is frame independent.

Two clocks are synchronised if A knows what time B is showing, simply by looking at his own clock.

You give an epitemological definition, one I've never seen used. The definition as worded allows clocks to run at different rates.

If the clocks are identical

What do you mean identical? What if only one is pink? If they don't both measure proper time, then are they really clocks?

this can only be the case if there is no relative motion between them.

One can only know the time the other clock says if it is relatively stationary? Have fun justifying that.

Where there is relative motion, he must apply a relativistic correction and thus needs additional information to determine the time shown at B.

So if clock B is shot from a gun from Earth at time zero (on both clocks) at 0.866c, later on, the A (the Earth observer) looks at his clock and it says 2 hours, he knows that right now in his frame, B's clock reads 1 hour. Since he knows the time on the B clock, the clocks are in sync by your definition.

I am sure that Hamdani will hang on your words, similar to his attraction to hobbyist you-tubes. Clearly he already does, and this will totally reinforce that preference.

First, they must tick at the same rate as each other.

Possibly a frame dependent requirement. They might tick at the same rate in one frame, but not another.

Second, some process will have been used to set them to a known start time.

Not the same time? If not, then what's the purpose of Einstein's sync conventions? The clocks, being relatively stationary, already by definition tick at the same rate. The convention serves a different purpose than to get the clocks synced relative to that frame in which they are mutually stationary?


If you want me to shut up, I'm all for it. If I'm wrong about any of this, say where. I don't correct Hamdani because he has no desire to learn. I correct Alan because I disagree with almost everything he says on this subject.

You and I seem to differ on definitions. I say A & B are potentially synced in the Carol frame (both are running at the same rate, even if dilated. Your definition seems to require that they're synced in either the A or B frame. This is why I say the phrase 'clocks are in sync' is meaningless without a frame reference.

In exactly one frame C will those clocks be moving in equal and opposite velocities.

That's only true if we were living in a one space dimension universe.

An assertion that is flat out wrong. If you disagree, then either come up with a 2D or 3D example of two clocks at different velocities and positions where there is no frame in which they have equal and opposite velocities, or an example where this is true in more than one inertial frame. If you can't do that, then your assertion is vacuous.

I'm giving other members the chance to use their best explanation to answer my questions. When the time is up

That time has long since come and gone. Based on your other topics going on for years with zero conclusion, the time will never be up.

[paul cotter]

I for one do not want Halc to "shut up" as I learn from his contributions. I will not however enter these long winded open ended discussions.

[alancalverd]

One can only know the time the other clock says if it is relatively stationary? Have fun justifying that.

Not what I wrote. If you can only see clock A, you cannot know the time shown on B's clock (however "identical" they may be) unless you know their relative velocity.

Something to do with relativity.  I won't bore you with the details as you set it out very neatly in your next paragraph!

[hamdani yusuf (OP)]

If the two clocks are in relative movement (the distance between them is changing), will they tick at the same rate (and according to whom)?

According to relativity principle, an observers who keep their position right between those clocks should see them synchronized, based on symmetry.

Between the clocks is one special case, yes. But does that really mean those two clocks were synchronised?

And this is not quite what you earlier said:

They can also be achieved when relative position=zero to the observer.

And ... how does this apply to GPS?

Let's put the context back to the discussion. If an observer see two clocks tick at the same rate, and both are moving at the same speed relative to him, regardless of the direction, he can say that those clocks are synchronized to each other. Assume no gravitational effect.

[hamdani yusuf (OP)]

I am sure that Hamdani will hang on your words, similar to his attraction to hobbyist you-tubes. Clearly he already does, and this will totally reinforce that preference.

How did you come up with that conclusion?

[hamdani yusuf (OP)]

I do assure you that I make mistakes.

Which one?

[hamdani yusuf (OP)]

I don't correct Hamdani because he has no desire to learn. I correct Alan because I disagree with almost everything he says on this subject.

How can you read my mind?
I analyzed different explanations for the twin paradox from various sources. So far, Mahesh' explanation is the most general, effective and efficient one, compared to the alternatives. It consistently includes the frame changes at the beginning and the end of the journey, unlike Minutephysics'.
If you think you have a better explanation, please describe it. What's wrong with Mahesh' explanation?

[hamdani yusuf (OP)]

That time has long since come and gone. Based on your other topics going on for years with zero conclusion, the time will never be up.

I'm giving you the chance to give your best shot. Take your time, no need to hurry. Good things come to those who wait.
Some of my other threads already have best answer. Which means the number of conclusions are more than zero. I assume you know how to count.

[hamdani yusuf (OP)]

With enough data, patterns appear.

Which data took your attention the most?

[hamdani yusuf (OP)]

I'm glad you found one you like. No video is tagged with that name, but I'm guessing the one in post 184.

This one, I added his name in the post.

Why twin's paradox is NOT about acceleration?

Chapters:
00:00 What is the twin's paradox?
00:48 Why acceleration doesn't solve twin's paradox
2:24 Twin's paradox without acceleration (Earth's frame)
4:42 The traveling frame
7:13 My new website - floatheadphysics (ad)
8:48 Earth's frame again - with the flag
11:38 Travelling frame again - with the flag
13:30 The resolution!
14:45 Relativity of simultaneity
17:02 Isn't the root cause the acceleration?
18:20 What do they 'see'?

In this video, we'll intuitively resolve the twin's paradox. This version of the twin's paradox involves no acceleration. And no, you don't need equivalence principle, and you don't need general relativity to solve it. Twin's paradox can be completely solved using special theory of relativity and the correct usage of relativity of simultaneity.

Let's see if anyone has objection to the explanation given in this video, which is an improvement of previous video by the same author, Mahesh Shenoy from Floatheadphysics.

It should show my intention to learn, but somehow you can't see it.
On the other hand, if you are already convinced that your current explanation is the correct one, and the others are wrong, you won't be able to learn.

[alancalverd]

and both are moving at the same speed relative to him

There's the catch. He needs to know something more than just what the clocks are telling him.

[Eternal Student]

Hi.

I'm giving you the chance to give your best shot.

    I think a lot of people have already made a lot of attempts to explain and more generally just to discuss the twins paradox.

    The main issue isn't really about being able to predict something or explain something in one way that is somehow better than another way.   The twins paradox is something all physicists will explore several times over in their career and they will see something slightly differently each time if they're lucky.

     Perhaps it's just about being human and reconciling how Nature seems to be,  or ought to be, compared to how it actually might be.    I don't know.   I suspect it's the investigation that is worth doing rather than coming up with a definitive "best answer" for some scenario.    Whatever best video or explanation you ( @hamdani yusuf ) decide upon now, that will probably change in a few years, if you're lucky.     

     It also probably won't be the best explanation for someone else, we're all different.   In the kindest possible way, I hope they do disgaree with your opinions and are lucky enough to have their own journey trying to understand the Twins paradox and deciding whose frame of reference is worthy of more attention than some other frame.   Ater a while of worrying if everything works in everyones frame,  you often come round to recognising that it was never all that important to assume that time is some important variable.   Time isn't something you need to understand or deserves any special attention.   In SR there is only a space-time interval that is important and will be invariant.    Trying to worry about where different observers have their disagreements over the passage of time for Alice or Bob may be putting the emphasis in all the wrong places.

Best Wishes.

[alancalverd]

Perhaps it's just about being human and reconciling how Nature seems to be,  or ought to be, compared to how it actually might be.

One of the enduring problems of science!

From a human perspective, a flat earth, geocentric universe, static species and synchronous travelling clocks are "good enough" and Newtonian (but not Aristotelian) mechanics  put a man on the moon, so it is obvious that Hipparchus, Copernicus, Darwin and Einstein were Sinning against God (a "good enough" explanation of the blindingly obvious) by suggesting that things might be otherwise.

Kruger-Dunning Syndrome affects us all to some extent, but once you have appreciated the Pound-Rebka and Haefle-Keating experiments (why do critical advances in psychology and physics have hyphens?) or used a GPS navigator, you have to set aside such childish things  and realise that twins or clocks only remain synchronised if they don't move relative to one another, and "good enough" may not be universally true.   

The paradox is why anyone thinks it is a paradox. And the answer is inertia and vanity.

[hamdani yusuf (OP)]

and both are moving at the same speed relative to him

There's the catch. He needs to know something more than just what the clocks are telling him.

Yes, he needs to know the speeds of both clocks, which can be obtained from several methods.