Just what is time dilation?

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To be politically correct despite the fact that scientists earn 'PH.D's, replace 'why' with 'how'.
Temper this with the fact that all definitions (theories, conjectures, etc.) are defined in terms of other definitions, which results in circular reasoning, or accepting some definitions without proof.

Isn't the linked example you gave sufficient?
I compltely agree and think you have brought up a crucial point both to this discussion as well as in a broader context.

All rational axiomatic logical strucures are inherently tautological. This applies to physics as well as formal mathematical structures.
But there is a significant difference between the two which seems to be often forgotten.
. In mathematics a valodly derived theorem consistent with the structure cannot be untrue. Axioms cannot be untrue.
Regarding the logical structure of physics , history has amply demonstrated that neither one is neccessarily valid as applied to the real world. Particularly recent history i.e. SR GR QM and the established threoms and logic they either falsified or limited.
Only a fool would fail to recognize that, not only are these systems the only way we can possibly understand and describe the world , but that the incredible power and utility of these structures renders insignificant any small glitches or required amendments.
At the same time it might be prudent to remember that history is relative and we are living it right now. That the future may reveal that a dogmatic certainty in the theorems a la mode may be as defensive and shortsighted as Ptolemists resorting to epicycles rather than examining the real questions they were confronted with. IMHO!!!!!
 
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You have to achieve certain relative speeds to experience the time dilation. So this pretty much tells us something is involved. A force, and that force changes something. And that change is percieved as time dilation.

So no, i can't accept time dilation as a cause, its an effect. So that particular question why seems valid to me. With a possible valid answer.
Just my thought process :)
Hi ExecNight

Just a note of complete agreement on the bolded statements.

On your beginning ;that is the relevant open question. You cant assert that view with certainty as it is also relative.

but it is certainly an unresoved issue and bears examination . IMHO
 
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Had a situation here.

Anyway, can please somebody explain to me what part of the question "What exactly causes time dilation" is a non physical one? That is, a question you're not supposed to direct to a physicist? What part of it is even remotely related to philosophy?
Hi Northern

I can only try to explain what many people ,"physicists" say to this question:

The affects of Lorentzian transformations do not have a physical implication in terms of cause and affect. They are purely a kinematic consequence of relative motion and it's affects on measurements between inertial frames.
I.e. they do not apply to the frame you are in. They only apply to your frame as observed from other frames.
It's always ..... "Dont blame my frame! It was the other frame did it" defence.

But I certainly agree that to ask the question and attempt to delve into the actual physics involved (if there is) is not at all a philosophical question. I think that to deny the question itself indicates a degree of metaphysical certainty regarding ultimate reality that is so far beyond determination. Certainly gravitational time dilation is viewed as having a degree of physical reality
 
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Here's one kind of answer. It's a geometrical property of spacetime. DaveC's example offers a very close analogy. The "why" of time dilation is like the question: why did the driver who went NNW not travel as far west as the driver who went due west? In Euclidean space, we can recover the total distance using Pythagoras's theorem: (total distance from point A to point B)2 = (distance travelled to the north)2 + (distance travelled to the west)2. In Minkowski space (the simplest kind of relativistic spacetime, the kind dealt with in special relativity), the square of the total "spacetime interval" between two events is s2 = -t2 + x2, where x is the spatial distance between event A and event B, and units of space and time are chosen such that the speed of light, c, is equal to 1. This formula, the Minkowski metric, gives the same answer no matter what velocity we choose to call zero, just as the Pythagorean formula gives the same answer no matter what direction we choose to call north (magnetic north, geographic north, or any other direction). The amount of time between a pair of events depends on the velocity of the coordinate system you're using, as does the amount of space, but the amount of spacetime is the same in all coordinate systems.

NOTE: Many people use the opposite sign convention: s2 = t2 - x2. It makes no physical difference; you just have to change the sign in certain equations.
Everything you have said here is undoubtedly valid but it just begs the question.
Turns the physics to spacetime. The question remains the same.
We view the geometry of flat spacetime as being different from GR spacetime in that,, in GR there is an accepted degree of physicallity, a causal relationship , a non-relative interpretation of the affects.
But not in flat spacetime. This not only creates somewhat of a dichotomy between SR and GR but raises logical and physical questions as to the separation and the meaning of spacetime in this context. I.e. accelerating through this geometric spacetime is regarded as real with real effects but the instant you turn off the motor, this spacetime is suddenly passive , abstract without any physical implications.

Would you say this situation does not raise valid questions ????
 
Had a situation here.

Anyway, can please somebody explain to me what part of the question "What exactly causes time dilation" is a non physical one? That is, a question you're not supposed to direct to a physicist? What part of it is even remotely related to philosophy?
It is a physical question, in my opinion. One observes time dilation because light signals (or any information signals) from a fast-moving source arrive at your detector in a slower sequence than the sender perceives them to be sent.

Likewise, if you communicate back to the sender, your light signals arrive in his detector in a slower sequence than you perceive them to have been sent.

The basic cause of this effect is the law that every observer must see that light propagates with the same speed. No explanation 'why' is offered for this law, it is a first principle which (so far) agrees with experiments.
 

George Jones

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It is a physical question, in my opinion. One observes time dilation because light signals (or any information signals) from a fast-moving source arrive at your detector in a slower sequence than the sender perceives them to be sent.

Likewise, if you communicate back to the sender, your light signals arrive in his detector in a slower sequence than you perceive them to have been sent.
I think that some care is needed; it seems that you're describing the Doppler effect, although I could be wrong. In reality, the phrase "a moving clock runs slow" does not necessarily mean "a moving clock is seen visually to run slow." A clock moving directly away from an observer appears visually to run slow, but a clock moving directly towards an observer appears visually to run fast. In both cases, what is seen visually is given by the Doppler expression, which is always different than the time dilation expression. In both cases, the time dilation expression, used appropriately, does apply.

Consider the following example.

Assume that Alice is moving with constant speed directly towards Ted. When Ted uses his telescope to watch Alice's wristwatch, he sees her watch running at a faster rate than his watch. Ted sees Alice's moving watch running fast, not slow! Ted sees this because of the Doppler shift. Because Alice moves towards Ted, the light that Ted sees from her watch is Doppler-shifted to a higher frequency. But the rate at which a clock or watch runs is like frequency, i.e., a second-hand revolves at a certain frequency, and all frequencies are Doppler-Shifted., so ted see Alice's wristwatch running fast.

To explain what "A moving clock runs slow." means, I first have to explain how Ted (with help from Bob) establishes his frame of reference.

Starting from Ted, a series of metre sticks, all at rest with respect to Ted, are laid end-to-end by Bob along the straight line joining Alice and Ted. At each joint between two consecutive metre sticks, Bob places a small clock. The metre sticks and clocks all are at rest with respect to Ted. Initially, none of the clocks are running; before turning them on, the clocks have to be synchronized. To do this, Ted directs a laser pointer along the line joining Ted and Alice, and then sends a flash of light. Each clock is turned on when the flash of light reaches it. The speed of light is not infinite, so the time taken for the light to travel from Ted to each clock has to be taken into account. To do this, the clocks' hands are set initially as follows. The clock one metre away from Ted is set to the time taken for light to travel one metre; the clock two metres away from the tower is set to the time taken for light to travel two metres; ... .

This whole setup of metre sticks and clocks establishes Ted's reference frame.

Now, As Alice moves toward Ted, Ted uses his telescope to watch Alice's wristwatch, and to watch his clocks. First, he watches one of the distant clocks in his reference frame. The time he sees on the clock is the time at which the light he sees set out from the clock, so Ted sees an earlier time on the distant clock than he sees on his wristwatch. Because the clock is stationary in his frame, Ted does, however, see the distant clock running at the same rate as his watch. Similarly, Ted's sees all the clocks in his frame running at the same rate as his watch.

As Alice approaches Ted, she whizzes by clock after clock of Ted's reference frame. Using his telescope, Ted sees that Alice is beside a particular clock, and he notes the time on her watch and the time on the clock adjacent to her. Some time later, Ted sees Alice beside a different clock, and he again notes the time on her watch and the time on the clock adjacent to her.

Ted checks his notes, and he finds that the time that elapsed on Alice's watch as she moved between these two clocks of his frame is less than the difference of the readings of the two clocks. This what is meant by "A moving clock runs slow."

Unfortunately, "time dilation" in general relativity and "time dilation" in special relativity often have different operational meanings. Suppose observer A hovers at a large distance from a Schwarzschild black hole, and that observer B hovers near the event horizon. If observer A uses a telescope to observe B's watch, A will see B's watch running more slowly than his own watch. In this context, gravitational time dilation is something that is seen visually.
The basic cause of this effect is the law that every observer must see that light propagates with the same speed. No explanation 'why' is offered for this law, it is a first principle which (so far) agrees with experiments.
 

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