[quote name='Hodgman' timestamp='1320186530' post='4879424']How on earth is this synchronisation achieved (no pun intended)?

By the way, I am having serious difficulty understanding why time is connected to speed in the first place... and don't even get me started on quantum mechanics. That is serious nonsense to me... and I've been selling quantum-based generators for a while! That nonsense actually works!
[/quote]
Just curious - what properties should a satisfying answer to "why" have for you?

IMHO unless we manage to derive all physics from pure mathematics (and possibly even then), there will always remain the question "why are the fundamental laws of Universe such and such.."

This explains why the clock on the destination planet reads 20 years from the ship-based observer, even though he knows the clock must have only been ticking for 15 years since the ship left Earth. It's because from the ship's reference frame, the two clocks were not started at the same time and the clock on the destination planet has been started 5 years before the ship's clock was started. This part can be calculated using a Lorentz transformation to compare the space-time coordinates of an event in different inertial reference frames.

Confusing stuff... But you are saying that the guy left on the planet will read 15 years on the same clock?
What happens when you land your spaceship?
Will the decelleration make the clocks "synchronize", otherwise guy A on planet and B on the ship will have a big problem deciding on a time to meet eachother...
If you never land, but have some kind of way to communicate, would guy A and B start to argue about what time the clock is showing?
Or would communication be impossible?

I imagine theory of relativity light speed blah blah like this:

The object can go at a speed of max c.

Time is movement of the stuff inside the object, wich happens at c if the object is not moving.

If the object is moving, all the stuff inside it is moving at c to the same direction, so they cannot move at all relative to each other, and thus gravity and all that will take like forever to respond. (Since the gravity particle
thingys will also be moving at c when they spawn... or something)

If the object is moving at half of c, stuff moves at half the speed and thus reacts twice slower.

Like if there is a magnet, it will pull twice slower at half c because the magnet quantum-vibrates (?) at half the speed, and thus sends the waves slower.



And i have no idea if any of that is right but at least it makes it simpler to understand.

[quote name='Hedos' timestamp='1320186908' post='4879427']
This explains why the clock on the destination planet reads 20 years from the ship-based observer, even though he knows the clock must have only been ticking for 15 years since the ship left Earth. It's because from the ship's reference frame, the two clocks were not started at the same time and the clock on the destination planet has been started 5 years before the ship's clock was started. This part can be calculated using a Lorentz transformation to compare the space-time coordinates of an event in different inertial reference frames.

Confusing stuff... But you are saying that the guy left on the planet will read 15 years on the same clock?
What happens when you land your spaceship?
Will the decelleration make the clocks "synchronize", otherwise guy A on planet and B on the ship will have a big problem deciding on a time to meet eachother...
If you never land, but have some kind of way to communicate, would guy A and B start to argue about what time the clock is showing?
Or would communication be impossible?
[/quote]

The observer on the planet will read 15 years on the ship's clock, when the ship arrives, yes. This is the same time that the ship-based observer read on the ship's clock. The ship-based observer reads 20 years on the planet's clock and the planet-based observer reads 20 years on the planet's clock. The two observers are in complete agreement.

Whether you land the spaceship or not doesn't matter! Nothing would change at all. There is no synchronization involved and there is no contradiction either.


Time has indeed been shorter for the observer travelling on a spaceship, there is asymmetry in this situation (because the spaceship undergoes changes of velocity but the planet does not).

Keep in mind that when the spaceship has just been launched from Earth, then the reference frame of the Earth and the reference frame of the spaceship disagree about when the clock on the destination planet was started relative to when the ship-based clock was started. But that's because the ship-based clock and the destination planet's clock are physically separated. When the ship arrives at its destination, then because the two clocks are at the same location, it doesn't matter at all which frame of reference you choose, they will all be in agreement about the two clocks.

Thank you for your answers. It is clearer to me now, even though I still don't understand it.

I have this one more related problem that concerns length contraction.

So, a ruler of length 100 cm moves at a relativistic speed and passes near another, stationary, 100 cm ruler.
My question is: Will the observers on both rulers see that both rulers have identical length? That is to say, will there be no length contraction?

I mean, is the length contraction just an illusion caused by light lag?

Thank you, and thank you in advance.

Thank you for your answers. It is clearer to me now, even though I still don't understand it.

I have this one more related problem that concerns length contraction.

So, a ruler of length 100 cm moves at a relativistic speed and passes near another, stationary, 100 cm ruler.
My question is: Will the observers on both rulers see that both rulers have identical length? That is to say, will there be no length contraction?

In the frame of reference of ruler A, the ruler B will be shorter. In the frame of reference of ruler B, then ruler A will be shorter.

You might be interested in the ladder paradox for a conceptual application of this phenomenon.

I mean, is the length contraction just an illusion caused by light lag?

That's a very good question indeed. I am really not sure, but I would take the point of view that it is "real" (whatever that means), rather than just an illusion, because length contraction is analogous to time dilation which is clearly very "real". Moreover, the ladder paradox above suggests that length contraction IS real and not a mere illusion.

I imagine theory of relativity light speed blah blah like this:

The object can go at a speed of max c.

Time is movement of the stuff inside the object, wich happens at c if the object is not moving.

If the object is moving, all the stuff inside it is moving at c to the same direction, so they cannot move at all relative to each other, and thus gravity and all that will take like forever to respond. (Since the gravity particle
thingys will also be moving at c when they spawn... or something)

If the object is moving at half of c, stuff moves at half the speed and thus reacts twice slower.

Like if there is a magnet, it will pull twice slower at half c because the magnet quantum-vibrates (?) at half the speed, and thus sends the waves slower.



And i have no idea if any of that is right but at least it makes it simpler to understand.

That is entirely right, and the way these things should be explained imo, rather than with some axioms and algebra. Even most credentialed physicist manage to walk away from that without any actual 'understanding'.

To make things even more explicitly clear; that 'magnet quantum vibration' you are referring to, is usually just called a 'photon' . Indeed, the busier photons (and all force-carrying particles) are moving in bulk, the less they get around to bouncing back and forth between matter particles. That bouncing back and forth between matter particles is what determines chemistry and spacing between atoms in a crystal lattice and everything else. Hence, time dialation and length contraction.

Just curious - what properties should a satisfying answer to "why" have for you?

Yeah, I suppose I should just give up. I'm still surprised people discuss this so easily - this thing makes my head hurt.

[quote name='Waterlimon' timestamp='1320253570' post='4879756']
I imagine theory of relativity light speed blah blah like this:

The object can go at a speed of max c.

Time is movement of the stuff inside the object, wich happens at c if the object is not moving.

If the object is moving, all the stuff inside it is moving at c to the same direction, so they cannot move at all relative to each other, and thus gravity and all that will take like forever to respond. (Since the gravity particle
thingys will also be moving at c when they spawn... or something)

If the object is moving at half of c, stuff moves at half the speed and thus reacts twice slower.

Like if there is a magnet, it will pull twice slower at half c because the magnet quantum-vibrates (?) at half the speed, and thus sends the waves slower.



And i have no idea if any of that is right but at least it makes it simpler to understand.

That is entirely right, and the way these things should be explained imo, rather than with some axioms and algebra. Even most credentialed physicist manage to walk away from that without any actual 'understanding'.
[/quote]
This helps to visualize/memorize/make sense certain aspect of STR, but IMHO is not sufficient in philosophical sense.

Basically you say "If the speed of light is limit, then time in moving reference frame should lag..." - so you are taking "the speed of light/maximum possible speed is constant in all reference frames" as axiom and try to make sense of the geometry of space-time. You can, of course derive other properties of STR from it...

The algebraic way is to say something along the lines: "The space-time coordinates of moving reference frames are projected to each other according to Lorenz transformations". This feels more general statement and is easier to accept, as the fundamental ontological property of Universe. The speed of light being constant then simply follows from the geometry of space-time transformations and is nothing special by itself.

Other than that - the most counter-intuitive (IMHO) thing is STR are the relativity of simultaneity and the fact that moving FTL is equivalent to moving backwards in time (which follows from the former). Especially the latter is hard to accept unless one takes the position, that the geometry of space-time just happens to be such-and-such. At least I have not yet seen any convincing "explanations" of these - although it is easy to show, that they "hvae to" happen, if time and space are dilated the way they are.

[quote name='Eelco' timestamp='1320277769' post='4879939']
[quote name='Waterlimon' timestamp='1320253570' post='4879756']
I imagine theory of relativity light speed blah blah like this:

The object can go at a speed of max c.

Time is movement of the stuff inside the object, wich happens at c if the object is not moving.

If the object is moving, all the stuff inside it is moving at c to the same direction, so they cannot move at all relative to each other, and thus gravity and all that will take like forever to respond. (Since the gravity particle
thingys will also be moving at c when they spawn... or something)

If the object is moving at half of c, stuff moves at half the speed and thus reacts twice slower.

Like if there is a magnet, it will pull twice slower at half c because the magnet quantum-vibrates (?) at half the speed, and thus sends the waves slower.



And i have no idea if any of that is right but at least it makes it simpler to understand.

That is entirely right, and the way these things should be explained imo, rather than with some axioms and algebra. Even most credentialed physicist manage to walk away from that without any actual 'understanding'.
[/quote]
This helps to visualize/memorize/make sense certain aspect of STR, but IMHO is not sufficient in philosophical sense.

Basically you say "If the speed of light is limit, then time in moving reference frame should lag..." - so you are taking "the speed of light/maximum possible speed is constant in all reference frames" as axiom and try to make sense of the geometry of space-time. You can, of course derive other properties of STR from it...

The algebraic way is to say something along the lines: "The space-time coordinates of moving reference frames are projected to each other according to Lorenz transformations". This feels more general statement and is easier to accept, as the fundamental ontological property of Universe. The speed of light being constant then simply follows from the geometry of space-time transformations and is nothing special by itself.

Other than that - the most counter-intuitive (IMHO) thing is STR are the relativity of simultaneity and the fact that moving FTL is equivalent to moving backwards in time (which follows from the former). Especially the latter is hard to accept unless one takes the position, that the geometry of space-time just happens to be such-and-such. At least I have not yet seen any convincing "explanations" of these - although it is easy to show, that they "hvae to" happen, if time and space are dilated the way they are.
[/quote]

Let see what good old albert had to say on the matter:

Taken from:
http://philsci-archi.../1/bubfest2.pdf

Einstein would have preferred a constructive account of these relativistic e?ects, presumably based on the nature of the non-gravitational forces that hold the constituent parts of rods and clocks together. But as we have seen, for Einstein the elements of such an account were not to be had in 1905. The price to be paid for the resulting strategic retreat to a principle theory approach was not just loss of insight...

One is struck [by the fact] that the theory [of special relativity] . . . introduces two kinds of physical things, i.e., (1) measuring rods and clocks, (2) all other things, e.g., the electromagnetic ?eld, the material point, etc. This, in a certain sense, is inconsistent; strictly speaking measuring rods and clocks would have to be represented as solutions of the basic equations (objects consisting of moving atomic con?gurations), not, as it were, as theoretically self-su?cient entities. However, the procedure justi?es itself because it was clear from the very beginning that the postulates of the theory are not strong enough to deduce from them su?ciently complete equations . . . in order to base upon such a foundation a theory of measuring rods and clocks. . . . But one must not legalize the mentioned sin so far as to imagine that intervals are physical entities of a special type, intrinsically di?erent from other variables
[/quote]
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The principle theory albert proposed was nothing but a patch waiting to be replaced by a theory of what holds matter together to actually form rules and clocks. Unlike Einstein, we have that nowadays (QED, interference patterns of virtual photons going back and forth, and so on). Properly understood, what the MM-experiment illustrates, is that the two-way speed of light of the photons inside the apparatus equals the two-way speed of light of the photons making up the apparatus. And like any good tautology, it holds in every reference frame.

Axiomatic relativity should have gone the way of the dodo half a century ago (as eloquenty argued by Bell too, cant find his original paper, but this is a decent drop in replacement: http://en.wikipedia.org/wiki/Bell's_spaceship_paradox), but people are too busy kissing Einsteins ass to even read what the man said himself.