# Just what is time dilation?

#### Northern

Hi.
It's known that for an object in a motion relative to another, time will slow down.
Now the question is why? Is it merely the mechanical effect that since the object is in motion, all its processes (which are practically motion of particles within the structure) now have to travel further due to the motion as shown http://upload.wikimedia.org/wikipedia/commons/e/eb/Time_dilation02.gif" [Broken] or is it because the very fabric of time itself slows down?

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#### Dmitry67

There is an important part missing here. Check red.

It's known that for an object in a motion relative to another, time will slow down relative to another.

#### Naty1

What Dimitry means is that if two observers are moving relative to each other at a constant velocity, each will observe the other's time as having slowed down. But neither observer sees any change in their own, local, passage of time.

An example of this is an experimental observer in a lab watching a particle decay...a faster particle appears to a stationary lab observer as lasting longer than would be expected because of the motion of that particle.

No one knows exactly why; changes in gravitational potential also affect the passage of time as measured by a distant observer.

So both motion (velocity) and gravity affect the passage of time.....somehow all three are related in a way that is not fully understood.

#### Northern

oh, so the nature itself of time dilation is not yet understood. Ok, then. It's what I needed.
Thanks.

#### Dmitry67

Well, it is well understood in the physical sense.
It is a result of the math behind the physics.

Physics does not answer questions like "but WHY 2 masses attract?" (you can replace it with anything). Physics answer questions "what is a formula which describes the force between 2 masses", etc.

The deep "why" question, depending on the context, belongs to:
1. Realm of phylosophy.
2. Advanced cosmological concepts (multiverse, antrophic principle, selection)
3. Crackpottery

http://math.ucr.edu/home/baez/crackpot.html

17.
10 points for arguing that while a current well-established theory predicts phenomena correctly, it doesn't explain "why" they occur, or fails to provide a "mechanism".

#### ExecNight

I know 2 + 2 = 4, and i know *why* 2 + 2 = 4.

If i didn't know why 2 + 2 = 4 then i would be just a database who just happens to know the data that 2 + 2 = 4.

--

My mind works in a way that i have to understand "the why" when i am talking about something. And physics exactly answers the question WHY. This new trend is a big degeneration.

Newton answered the question "WHY THE APPLE FALLS DOWN?" with gravity..
And answered what forms the gravity and how. How gravity affects and all.

But this simple answer to the question WHY led to other and harder questions. And thats exactly how science works. Right now science can not answer some why questions.

Its not because science doesn't work that way, its simply because science didn't get there yet. You will have that answer, but absolutely not by deeming it irrelevant.

Just my thought process

#### yossell

Agree why' questions are not to be dismissed as merely philosophical'; but it's also trivial to ask `why' to any answer that's given and to pretend that physics or philosophy has failed when nothing informative or non-circular can be given. Every theory must have its primitives. Why do massive bodies attract on Newton's theory? Why is space-time a 4-dimensional Minkowsian manifold? Why does the Schrodinger equation obtain?

Perhaps future theories can give exciting, informative answers. But if not, if these are just fundamental facts about the world and there is no why, well - fair enough. I see no problem.

#### Dmitry67

This is way how science WORKED BEFORE.
Why ice melts? because temperature is a chaotic movement of molecules, and when their movement is too energetic, they break the crystal. So the less fundamental thing is explained based on the more fundamental one. However, this logic does not work any longer when science hit the most (or almost-) fundamental level. There are no more fundamental things. People feel comportable getting explanations like "space is made of spacions and time is made of timions", but only until they start wondering what "timions" consist of :)

#### Northern

the "why" I was talking about is quite physical. Why does the apple fall from a tree? Because it's attracted by earth's gravity. It wasn't a philosophical kind of why but more like "what causes it"? What exactly happens when the two objects move at difference speeds that causes the time to run faster or slower for this or another one?

So, it was a pretty technical question of "what causes it?" or rather than "what purpose does it serve" or some other nonsense.

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#### Dmitry67

Why does the apple fall from a tree? Because it's attracted by earth's gravity.
It explains nothing.

Why Earth excerts gravity?
Why apple is affected by gravity?

:)

#### ExecNight

But isn't accepting current knowledge as fundamentals of the universe a bit too much?

It draws a line for the scientists that further thought in that area is unnecassary. And it kind of shows similarities with religion in its logic.

#### George Jones

Staff Emeritus
Gold Member
I don't think that "Newtonian Gravity" is an answer to a "Why?" question. "Newtonian gravity" gives a name and quantitative form (that is, formulas) to certain forces we experience, but it doesn't answer "Why?".

I see that Dmitry67 posted much the same opinion while I was typing.[/edit]

#### Northern

It explains nothing.

Why Earth excerts gravity?
Why apple is affected by gravity?

:)
Absolutely. It gives one answer which raises the other. What causes the very effect of gravity? Is it the curves in spacetime, or is it caused by gravitons? And so on and on.

As you may see, all I was asking is what causes time dilation. What exactly makes time move at a different pace for different speeds.

#### DaveC426913

Gold Member
Hi.
It's known that for an object in a motion relative to another, time will slow down.
Now the question is why? Is it merely the mechanical effect that since the object is in motion, all its processes (which are practically motion of particles within the structure) now have to travel further due to the motion as shown http://upload.wikimedia.org/wikipedia/commons/e/eb/Time_dilation02.gif" [Broken] or is it because the very fabric of time itself slows down?
Think of this:

When I get in my car and travel due West at 60mph for one hour. I will measure 60 miles traveled. When I look at a map I will see that I have gotten from Long. 179W to Long. 180W (about 60 miles) in about one hour.

If you were to get in your car and travel at the same speed but due North West, you will measure 60 mles travelled in one hour. When you look at my map, you will see that you have gotten from Long. 179W to Long 179.707W in about one hour.

Neither of us have undergone any shrinkage in either time or space, but when we compare notes, we see that our frame of reference is not identical.

when we measured our own experiences, there was no confusion; it wasn't until we tried to use a common frame of reference (the map), that we saw our experiences didn't seem to agree. The map is biased; it marks Longitudes only East-West, not SE-NW. By your yardstick, the map is sort-changing you on miles, but only because we used my map (i.e. my frame of reference) to look at your journey.

It is the same with time. Both our viewpoints are valid, they just don't agree.

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#### Dmitry67

The reason why this discussion began because Nothern had misinterpreted the last post and wrote "oh, so the nature itself of time dilation is not yet understood"

The situation with time dilation is the same as with other 99% subjects we can discuss here: we have formulas, but we dont speak about the "true nature" etc. "not yet understood" does not mean that scientists work hard on the subject, "trying to understand it". It is a trivial part of SR.

And no, it is not drawing the line. But to cross the line, you must have more than a question WHY. You need a proof that that something is not fundamental and there are more fundamental things.

#### ExecNight

Jones i think you don't see it as an answer because you always think of the bigger picture. The question is simple here, Why does the apple fall?

The answer is simple, The earth has a mass, apple has a mass and they both attract each other and pull each other. That's why apple falls. And from now on I will call this force gravity for easier reference.

That's exactly the answer for that particular why question. It answers everything for the directed why question. It might or might not arise new questions depending on your knowledge.. But every question is another case in itself.

#### ExecNight

And no, it is not drawing the line. But to cross the line, you must have more than a question WHY. You need a proof that that something is not fundamental and there are more fundamental things.
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 :)

#### George Jones

Staff Emeritus
Gold Member
If the answer to "why does an apple fall?" is either "Because of Newtonian gravity." or "Because of the general theory of relativity.", then I think the answer to "Why does time dilation occur?" is "Because of the special theory of relativity."

#### ExecNight

If the answer to "why does an apple fall?" is either "Because of Newtonian gravity." or "Because of the general theory of relativity.", then I think the answer to "Why does time dilation occur?" is "Because of the special theory of relativity."
But i told in my answer exactly what i am referencing when i say gravity. Can you do the same with Special Relativity?

#### George Jones

Staff Emeritus
Gold Member
But i told in my answer exactly what i am referencing when i say gravity. Can you do the same with Special Relativity?
Off the top of my head, I can't give a complete list, and so the list that follows is both incomplete and redundant: inertial observers, Lorentz in variance, invariance of the speed of light, time dilation, Lorentz contraction; etc.

Listing and quantifying some physical effects and calling this list and quantification "special relativity" is no different than listing and quantifying some physical effects and calling this list and quantification "gravity."

In the case of gravity, many of the effects are everyday experiences that we have observed since we were born. Consequently, its is easy to say "Oh, that makes sense." once we see the theory of gravity. In the case of special relativity, the effects are not everyday experiences, and we have to work hard to observe these effects. Consequently, it is easy to say "Oh, this seems mysterious; why is it true?" when we see the theory of special relativity.

#### Naty1

The reason why this discussion began because Nothern had misinterpreted the last post and wrote "oh, so the nature itself of time dilation is not yet understood"
instead of nature, I beliver the poster probably meant "root, or fundamental, cause"....not being understood.

#### Rasalhague

the "why" I was talking about is quite physical. Why does the apple fall from a tree? Because it's attracted by earth's gravity. It wasn't a philosophical kind of why but more like "what causes it"? What exactly happens when the two objects move at difference speeds that causes the time to run faster or slower for this or another one?

So, it was a pretty technical question of "what causes it?" or rather than "what purpose does it serve" or some other nonsense.
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.

#### Northern

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?

#### phyti

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?
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?

#### JM

Hi.
It's known that for an object in a motion relative to another, time will slow down.
Now the question is why?
The slowing of time is the result of the light traveling at the same speed with respect to each of the observers ( or coordinate systems ) that are in relative motion. Each observer sees the light traveling a different path length, but moving at the same speed. Therefore the time taken by the light is different for each observer. See Wikepedia, Time Dilation for a more complete explanation.

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