Can anyone explain the cause of time dilation in layman terms?

[Gungalley]

All I got from searching are a bunch of calculations, or experiments that prove time dilation (2 atomic clocks, 1 on a plane 1 on ground etc...) I didn't take advanced physics in school, so my background on physics is pretty bad, anybody care to explain this phenomenon?

[Sphere]

The simplest explanation is that time is not a constant, even though we precieve it as such. In truth, the faster you travel in comparision to someone else, the slower time ticks by. This effect is really only noticiable when you approach lightspeed, though it is very important to GPS systems due to the need for exact timing, and the high velocity at which the satellites travel.

[Subuatai de Bodemloze]

http://www.oberlin.edu/physics/dstye...ein/SRBook.pdf

[Mar77]

This kinda explains it, among other things:

[Chaigidel]

time is only variable according to observers and gravitic variance-- otherwise space-time is unified in a single moment; relativity only comes into play when you incorporate observers.

[Simetrical]

Relativity has nothing to do with observers (or at least special relativity doesn't, can't say for general). Don't confuse observers with reference frames.

[Chaigidel]

either way the same space time is shared by all the universe and it is only changed ( in time dilation terms) in relation to gravitic variance-- relativity only applies because you are going over the vast distances, the same concurrent time is still carried out even if it takes a billion years to see the light, that star is carrying out current time, much as we are here.

[Simetrical]

Okay, sorry, that makes absolutely no sense.

[Chaigidel]

what makes no sense?

that alpha centauri is undergoing time under the influence of local gravitic variance; just as we are

even though the light reaches us some minutes or hours later; alpha centauri is still carrying out a normal time, producing the light which we eventually see.-- just as we produce the same for it, and regardless of the time it takes, we are all sharing the same moment of time.

time is shared in this way in the fabric of space-time; which is why it is called space-time?

[Playfishpaste]

what makes no sense?

that alpha centauri is undergoing time under the influence of local gravitic variance; just as we are

even though the light reaches us some minutes or hours later; alpha centauri is still carrying out a normal time, producing the light which we eventually see.-- just as we produce the same for it, and regardless of the time it takes, we are all sharing the same moment of time.

time is shared in this way in the fabric of space-time; which is why it is called space-time?

But that isn't time dilation.

And Sim, I always figured using the term "observer" was just a layman term for frame of reference. For instance, someone in the laboratory frame as opposed to the CG frame in a giant sphere.

[Chaigidel]

yeah observer counts as any reference, which dictates a certainty so to speak-- interactions at certain times and places are observers.

[Baron von Sky Hat]

Personally, I'd be cautious about equating observer and frame of reference. While it can make sense, certainly in this context, observer has a more specific, non-frame of reference related, meaning within Quantum Mechanics. So depending on what you're thinking of at the time, interchanging the two terms could lead to both confusion, and an awful lot of dead cats in boxes. Possibly. At least, that's my understanding of QM.

As to Chaigidel's points, it's not really meaningful to talk about a `current time' over stellar distances. Even if we do share the same space time, and clearly other places are experiencing events in a linear order as we do. If everywhere is running at different rates, there isn't easily any particular time for those places that you can call `current'. If you say that it's `current' as though you were in both places at once, then due to the different rates that current would drift away from any sort of common time.

Possibly though I've misunderstood what you mean, if you could elaborate your point more, possibly I'd see what you're trying to get at.

Nor is it the case that relativity need involve large distances, there are plenty of smaller scale examples of time dilation even on earth.

A very useful illustration of the effect is that of cosmic rays, in relation specifically to the muon. When cosmic rays hit the atmosphere of the earth, a slew of different particles can be created, one of which is the muon.

The lifetime of the muon is known to be a certain amount (in their frame of reference, which can be similar to the lab frame if they're created at non-relativistic speeds), and its speed due to creation in this manner is known (near the speed of light ish, 98%, wiki says). Based on these two elements, you'd expect to know the distance it would travel through the atmosphere (since it's not undergoing any significant acceleration effects).
However, this would give a distance that would never allow it to be detected on the ground, they should pretty much always decay before getting there. But they're detected anyway. How? Time dilation.
They are moving very fast relative to us, and so experience time `slower' from our perspective. This means that, from our perspective, their life is significantly longer, letting them reach the ground. From their perspective, ie in their frame, they live the expected amount of time.

This might appear to lead to certain contradictions, surely from their perspective, if they're living their normal lifespan, but they manage to cover a much larger distance, in their frame they'd be traveling faster? Possibly even faster than light?
However this is where the length contraction effects of relativity come in. From the muon frame, the earth is moving very fast. Very fast things appear `squashed', contracted, in the direction of movement, due to the space equivalent of time dilation.
So, they see the earth as squashed `towards' them, from their frame, the distance they travel would appear shorter. The distance to them would be the normal predicted distance if it was at non-relativistic speeds.

So through the combination of the effects of relativity, everything adds up.

I don't know if I've explained it very well, and it was just an example of the effect, as opposed to actually why. But unfortunately I don't have a huge amount of time at the moment, and wasn't really sure what would be a good way to explain it even if I did... :S
So I haven't really answered the OP actual question, I'll admit...

I've always though that it's a nice illustration of the idea though, without invoking distances that we don't have day to day reference points for, while still being actually real rather than merely illustrative like some of the clock ones. I'm far from certain that I haven't made any mistakes in the time I've spent, however. My apologies if I have, if anyone spots any inconsistencies, do let me know.

edit:
http://en.wikipedia.org/wiki/Time_di..._time_dilation
Only had a brief look, but this part of the wikipedia article looks fairly good at showing why time dilation occurs, and why, like with the muon example, time dilation and length contraction are so closely linked. Basically from a different inertial frame, light can be thought to be tracing out a different path, but that from whichever inertial frame, the light will still appear to be going at the speed of light. So from the moving frame in the wiki example, the light appears to go further, and hence would take more time, say it takes 1 second. From the perspective of the mirrors, it's traveled less distance, and would appear to take, say, 0.5 seconds. So the 0.5 seconds that the mirror frame experiences, appears to take place over an entire second from the moving frame, so it's as though time for the mirrors is slower from the moving frame.

It's not the best and most full explanation, but it wasn't until I had a diagram that I could say much at all.

[Simetrical]

what makes no sense?

Your entire post. I can't figure out what you're trying to say. Science requires the use of precise, well-defined, and standard terminology, and you aren't using it, or any approximation of it.

that alpha centauri is undergoing time under the influence of local gravitic variance; just as we are

Alpha Centauri is subject to effects such as time dilation just as everything is, yes.

even though the light reaches us some minutes or hours later; alpha centauri is still carrying out a normal time, producing the light which we eventually see.-- just as we produce the same for it

Sure.

and regardless of the time it takes, we are all sharing the same moment of time.

Well, there's no such thing as objective simultaneity in special relativity. Let's say you're on Alpha Centauri, and I'm on Earth, and we both drop an object. It's possible for one person to see our dropped objects hit the ground at the same time, while another person sees them hit the ground at different times.

I don't know if this has anything to do with your notion of "sharing the same moment of time". If you're going to speak about scientific matters, and don't know the specialized terminology, I suggest you try to speak about concrete physical events and observations. What observations would you expect to result from two places "sharing the same moment in time"? What experiment could tell whether they are? If you don't have an answer, you probably aren't talking about science.

time is shared in this way in the fabric of space-time; which is why it is called space-time?

Again, this doesn't make any sense to me.

And Sim, I always figured using the term "observer" was just a layman term for frame of reference. For instance, someone in the laboratory frame as opposed to the CG frame in a giant sphere.

Yes, but it's very confusing. It brings to mind the observer of quantum mechanics, where there actually has to be something there interacting with what's being observed for it to count. The observer in special relativity is a purely formal thing, with no physical implications. Any coordinate frame in space can count as an "observer", whether or not there's anything there to do observation. So I'd prefer "reference frame", especially for popular discussion.

yeah observer counts as any reference, which dictates a certainty so to speak-- interactions at certain times and places are observers.

Observers in special relativity are just reference frames. They don't have to interact with their environment, or even exist. Do not confuse them with quantum mechanical observers (or measurement-takers or whatnot), which must exist and actively interfere with their environment to count. The significance of observation in that sense is a purely quantum-mechanical thing.

[Chaigidel]

doesnt an obeservable reaction count as a reference frame though?

[Simetrical]

A reference frame is three mutually perpendicular lines that meet at that point. That's it. It's just a way of measuring things. For instance, I might choose a reference frame centered at my current location, with one line running north-south, one running east-west, and one running up-down. Or I might choose one centered at the Sun, with the three lines pointing in some arbitrary directions: one toward the Earth's current location, say; one perpendicular to that in the Earth's plane of orbit; and the last perpendicular to both of those. "Observable reactions" have nothing to do with reference frames. You don't need matter, energy, or anything else to exist to have a reference frame.

[Chaigidel]

to have a reference frame with any relevance you do.

[Simetrical]

The laws of physics don't care about whether you think they're relevant.

[Chaigidel]

relevance I mentioned is to the laws of physics, this is a pointless arguement.

[Baron von Sky Hat]

relevance I mentioned is to the laws of physics, this is a pointless arguement.

This quote representing the discussion between yourself and Simetrical as a whole, rather than a response to that specific statement:
On the contrary, firstly you specifically don't need to have a frame of reference be an actual `observer' as such. In the example I gave in my post earlier, which is a fairly standard one to be given in early/introductory university relativity courses (or even earlier than that, possibly, it is a rather nice illustration of the effects), one frame of reference is that of the muon traveling through the atmosphere. By any definition, it can't be thought of as a conventional `observer', but it can have a frame of reference in relation to it.

Equally, as Simetrical said, it needn't even correspond to a particle or energy or similar, it can merely be a coordinate system traveling at a particular velocity relative to another thing, for example. Frequently it may be helpful for the reference frame to be connected to something, but it's not necessary. In the following description I use a reference frame moving relative to two mirrors which are stationary in the `lab' reference frame, the moving reference frame isn't attached to a particle or anything, but it's still valid.

Essentially the reference frames don't exist in any physical sense, the universe doesn't `think' about reference frames as such. Everything and everywhere is moving or not relative to everything and everywhere else in the actual physical world. Reference frames in this context are kind of just a tool to analyze what happens in a situation.

I thought I'd finally attempt to fully describe the reasons for time dilation. In what relativity I did at university, much of it was explained by the maths. If you're willing to simply accept the maths behind something, and actually understand the concept through that, then that's a fine way to explain something (and the only way, pretty much, for quantum mechanics, the specifics of which are extremely mathematical). However it's less good if you don't have a decent backing in maths.

That's why I thought that the geometric argument I noticed on wikipedia was rather good. http://en.wikipedia.org/wiki/Time_di..._time_dilation
You can read the link, but I don't think it emphasizes the key points quite enough on that example alone, so I'll attempt to reiterate, and will ignore most of the maths to make it easily understandable (hopefully!).

The idea is that you have two mirrors opposite each other at a distance `L', these are stationary relative to one another, and form the lab reference frame. You fire a beam of light perpendicular out from one mirror to the other, which then bounces off and returns to the first mirror. From the perspective of this reference frame, a stationary observer in the lab, if that's easier, the light has clearly traveled a total distance of `2L' at the speed of light, `c', taking t = 2L/c seconds to travel it.
Now consider a reference frame moving parallel to the lengths of the mirrors (ie, along the edge of the mirror) at a speed `v', the mirrors appear to move relative to this frame, meaning that the light will appear to take a diagonal path (as can be seen in the second diagram on that wiki page). From this frame of reference, then, the light has traveled further. The distance it appears to travel is given by Pythagoras' theorem, hypotenuse^2 = a^2 + b^2, where a and b are the two other sides of the triangle. `a' in this case can be the distance between the mirrors, still L, and b the distance along the base, 0.5 * v * t". Where ` t" ' is the apparent time from this moving frame of reference.
So this distance `2D' the apparent distance of travel of the light, is longer than the 2L in the lab frame.

The exact maths in done in the wiki page, but the important thing to take away is that the light appears to travel a longer distance from the perspective of the moving frame. However, the beam of light is still tracing out its path at the speed of light in both frames. How can it trace out two different lengthed paths at the same speed in the same time? It doesn't, it will seem to take a different amount of time from the different reference frames. So returning to the simplistic numbers I used in my earlier post: From the lab frame, it seems as though the light takes 0.5 seconds to travel between the mirrors, say. From the moving frame, it takes longer, 1 second perhaps. So it appears as though the 0.5 seconds experienced by the lab frame takes place over a whole second from the moving frame, hence time appears dilated.

Time dilation is also symmetrical, if instead of a generic moving frame, you consider a second mirror experiment, so that each mirror experiment is traveling past the another. Then from each frames perspective, the other mirror experiment is operating in slowed time, and it's own in normal time.

I don't know whether I muddled the explanation about enough to drain any sense out of it... :S
Hopefully not, possibly it's of use to people.

[Justinian]

One of you smart guys (I'm looking at you, Sim and Baron) can correct me here, but I think in laymen's terms a simple way to put it is this:
Say you are in a magical spaceship traveling a nanometer per second slower than the speed of light, and you shoot off a beam of light towards a distant planet -- let's say Vulcan -- because you want to show them this cool picture you took. That beam of light is traveling at the speed of light towards Vulcan -- and it's traveling at the speed of light faster than you in your near-speed of light spaceship. So to the people on Vulcan, the light and your spaceship will arrive at almost exactly the same time; but to you, the light should have gotten there a light year (if the distance between you and Vulcan was a light year) before you do. So if this is a visible light and you fire it in front of your spaceship, you will not be able to see it because it is moving at the speed of light; but it will still get where you sent it only a tiny bit before you do. So time and space sort of bend in ways that don't really make sense.

Am I wrong?