I'll try lay-mans this for you. Time doesn't really slow down.

You have a reference frame, your reference frame simply describes something along the lines of "where you are". "Where you are" could be how close you are to something massive (a star, a planet) or how fast you are moving.

If you have a watch on your own wrist it will always tick at 1 second from your own perspective (ignoring extreme cases like black holes, where your hand could be in a very different reference frame from your face). It will always be correct and time will never slow down or speed up. It will be exactly the same 1 second that everyone in the universe experiences from their own perspective. However, if someone looked at your watch with a telescope from another reference frame it would be ticking faster or slower than their own watch.

This is where relativity gets its name from "something in RELATION to another." Without checking your relation to something else you can't determine that time flows at different rates.

Think of time as a smooth river. The further along the river you are the more time you have experienced (our classical notion of time does not exist, it has been replaced by distance). If you move into the area of the river that has a strong current you will move through the river faster but from your own perspective the water and people directly around you will still be the same. You would only be able to tell that you are in a strong current if you could see someone who is in the weaker current.

Therefore our clocks do exactly what they should be doing, measuring 1 second by ways of our own frame of reference.

Yep, this was the most difficult thing to grasp when I started learning this stuff. I thought that somehow the body knows to slow things down when it is in motion. But when you think through it, it is easy. Its all in the reference frames. Imagine yourself going from Earth to Alpha Centauri in a constant velocity. In SR terms, this means you are in motion with respect to earth. Now imagine there is nothing else in the universe except you. Take away Earth, Alpha Centauri, all stars, galaxies, etc. Nothing but infinite empty space. Imagine yourself doing the same movement (I know it is a stretch and I am butchering physics here, since I asked to take away your source and destination, but you can form a mental picture). Now, in SR terms, I cannot say anything about your motion. You are neither moving nor stationary and it is pointless to say anything. But based on old Newtonian model, you can be considered "moving" in this big empty room of Universe (which later became theorized into Ether). When you grasp this and understand that motion is relative, also understand that time is relative. There is no giant universal clock which dictates what time it is for this giant room (which is again what Newton imagined). Einstein simply said there is no such thing (aka special reference frame). Once this is grasped, other concepts like time dilation will flow from it.

The issue is, first, if there is a 'dumb' clock (and an accompanying 'correct' clock), then there would be a special reference frame in physics. What you can do is make a convention: "I'm measuring my time in reference to the instantaneous inertial frame A of my location on Earth", and other nearby users from other inertial frames can adjust their clock rate using Lorentz transformation (I need not even mention value synchronization), such any user from A will see the clocks going at a fixed rate. Second, after adjustment, all users with different velocities will experience their clock going slower.

This is the special relativity view and it creates the twin paradox: if your time is going slower and no reference frame is special, couldn't we say his time is going slower instead? Who will age faster? This isn't really a paradox in special relativity, since in SR (i.e. assuming all observers are in fixed inertial frames) the observers can't "age" to meet at a later time: their clocks will diverge, but they will get away from each other so they can never compare how they are aging -- indeed this is a symmetric position. Once you allow them to change frames so they're able to compare their own aging, some frame will have to experience acceleration, and hence break symmetry. That is, the paradox resolves in general relativity.

All this is well explained on wikipedia (much better than I can do).

http://en.wikipedia.org/wiki/Time_dilation

http://en.wikipedia.org/wiki/Twin_paradox

Time really slows down. GPS satelites wouldn't have worked if it wasn't taken into account.

http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_P...

That is again just the clock going haywire.

It's the clocks on all 24 GPS satellites, all 24 Glonass satellites, and the 8 (so far) Galileo satellites, all going "haywire" by precisely the amount predicted by a theory developed before the first satellite went into space.

And it's not a paltry amount - if we didn't understand and plan for this amount, the GPS system would have an error drift of 10km (~6 miles) per day

No, it's an attribute of time itself, not mechanical clock issues. I _highly_ recommend the book "About Time"

http://www.amazon.com/About-Time-Einsteins-Unfinished-Revolu...

It's difficult because it is completely unintuitive, and I'm not sure we can ever experience it - or even conceive of experiencing it.

It helped me to wrap my head around it by comparing it to gravity. If you pick up 1kg of material and place it on a scale, it'll weigh 1kg. If you take that same scale to the moon, and place 1kg of material on it, it'll weigh 1kg. And we can objectively experience that - if we pick up that 1kg on the moon, it'll feel like a bag of sugar.

And this should be the same for time. The clock taken to relativistic speeds will still measure 1 minute as 1 minute. The difference is that in the case of gravity, we can measure that same mass from a distance - we can bring it home and experience it again relative to a different gravity. It's much less intuitive to measure time from a distance.

A kg is a measure of mass, not weight. Weight requires 2 general objects, and is dependent on gravity (between them). Mass does not.

1kg of material in outer-space will weight zero. And on the moon, about 1/6 of what it weights on earth.

While its mass will remain the same.

Your scale would have to be re-calibrated for the change in gravity to produce the same reading between the measure on earth and the moon - for both weight and mass of an object (a scale can only measure weight, which then has to be converted to mass using the already predetermined gravity force at the location)... Unless it was a really fancy one with some mechanism that could also measure gravity.

I'm using an uncalibrated scale intentionally, as that's exactly what we're doing when we talk about taking two clocks and moving one of them at relativistic speeds. That two identical instruments in different gravitational fields will exhibit different measurements.

With mass this makes sense to us, because we can compare with a constant - we can take the same item with a constant mass and measure it in two different gravities - and because it happens on scales we can perceive. Men bouncing around the moon is a perceivable change. An astronaut coming back from a year on the ISS, being biologically 12.5 seconds younger than if he hadn't gone, is far too glacial for humans to perceive.

Of course, we could just argue that relativity is a myth and the clocks just need to be calibrated.

Please define "dumb" in the context of a system solely guided by one of the most basic, non-emergent process conceivable (caesium oscillator) ?

by dumb, I meant, just because our clock says the time is so and so doesn't mean the time itself has slowed down. Maybe just the clock is having a bad day(acceleration).

If I travel fast for years will I be younger when I return home to my twin? or just my clock will say so?

Sorry, going really off topic.

You will be younger. What "really" happened is a question for philosophers, but every measurable periodic physical or biological process, including your own ageing, will have behaved as though time really was running slower. (And how could it not? We can use any such mechanism as a clock - Galileo, possibly first physicist, used his own heartbeat to time falling objects).

> including your own ageing, will have behaved as though time really was running slower.

This. This is what I wanted to confirm, thanks. Is there any experiment or some paper which proves this?

The only direct measurements of relativistic effects have been in the nanosecond range, e.g. http://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experime... - not a scale on which you could measure human ageing.

But it's really just utterly implausible that it would happen any differently, because biology is just (complex combinations of) ordinary physical processes that follow the ordinary laws of physics like any other. Compare e.g. http://lesswrong.com/lw/hq/universal_fire/

There is most probably no experiment on how relativistic time dilation effects aging because the time dilation is very small and aging occurs so slow (in context).

However, there are loads of experiments and papers that show or rely on the fact that everything in one place viewed from another place seems to slow down or speed up due to relativity.

That aging also slows down or speeds up is concluded by induction.