What's the definition? Does matter have to exist in order for there to be time? (Think of the tree in the forest "koan")

I would sagest you look in on the search featured in the control bar obove... much has been said of this subject.

As for a definition? The present moment hardly exists. It comes out of the future at light speed and becomes history and the past in less than the time it took to notice.
The universe does not record the sequence of events or the interval between them. We do. Only we care for time is not something we have a lot of...

A word with only one definition isn't much of a word.

-- Jeff, in Minneapolis

time is like an oyster; it has few chances to become a Wall St. stock broker, but given half the chance, it will beat you at chess.

Energy has to exist, not necessarily matter. As long as events take place, there's time. And by definition, energy is required for something to happen.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

I don't have time for this.

Time means many things in many contexts. It is a sense of change for an intelligence, it is an independent variable for a dynamics equation, it is a piece of geometry for gravity, it is an operator for quantum mechanics, and it is the fundamental observable for relativity. Can you be a bit more specific? You see, the real problem here, though I may sound like a broken record to some (but it proves yet again why this is so important to say), that questions like "in what form does __(fill in a scientific concept)___ really exist?" are not meaningful questions. They have to be expressed like "how is __(fill in the scientific concept)__ used in ___(fill in a scientific theory)__ to help us understand something objective about our reality?" Put the question like that, and it can have a scientific answer.

Okay, I will ask it in a different way then.

according to mainstream, time goes slower at higher altitudes on the earth.

Clocks run slower at 14,000 ft in the mountains than they do at sea level, Correct?

No, clocks run faster at 14,000 ft than they do at sea level, due to them being further away from the Earth's centre of mass.

General Relativity tells us that clocks in a stronger gravitational field will tick at a slower rate. Special Relativity tells us that moving clocks will appear to tick slower than non-moving ones.

GR predicts that the atomic clocks at GPS orbital altitudes will tick faster by about 45,900 ns/day because they are in a weaker gravitational field than atomic clocks on Earth's surface. SR predicts that atomic clocks moving at GPS orbital speeds will tick slower by about 7,200 ns/day than stationary ground clocks. The net result is that those clocks tick faster by around 38,000 ns/day than stationary ground clocks (or at least the first one did! They adjusted the clocks of all but the first GPS satellite to compensate for GR and SR - they set their clocks slower before they are launched so that they run at the same rate when they are in orbit as stationary ground clocks).

1. My favourite definition for time is "wibbly, wobbly, timey, whimey, ball of stuff", but that doesn't really say anything :-) Time is the fourth dimension, which is relations of events. Three other dimensions are relations of distances in three different direction.

2. I can't really tell. Noclevername said that energy must exist (energy and matter being the same thing, so...) but I'm not sure about that either. What is needed, is an event or events. I our universe, energy is nesessary for something to happen, so I guess it's needed. One might say that event's like expansion of space doesn't need or have energy (exeption being the acceleration of expanding), since it's an event of space.

That seems like a rather bizarre thing to do, reminiscent of "setting the clocks ahead" instead of just doing everything an hour earlier in the summertime! It is much truer to relativity to just use a calculation to transform from our time to GPS satellite time, rather than solving the problem in an analog form because we're too lazy to make the conversion. But that does seem to be the standard approach when dealing with time.

Well, no. Matter is one form of energy, but photons aren't matter, motion isn't matter.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Time does not change. The clocks that are measuring it do.
Time can be perceived to have changed its rate if you are traveling very fast. It does not actually change its rate of progression. Only your perception of it does.
We measure the passing of time. Not always so well.

Actually that's where it get's confusing (actually too confusing to deal with in this thread). Matter (or actually mass) is the same thing as energy. Energy is always the same, no matter how it manifestates itself. But to humor you, I'm willing to take back my words, and say "matter being energy".

flies
Bandits
wounds all heels
rosemary and . . .

That probably doesn't help much, does it?

__________________
Are you my mummy?

Mass is not the same as matter.
Manifestates?
Thank you for your condescension.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

1. You're right. My bad. (But then again, that's why there is this correction)

2. Lack of better word. English isn't my native language, and I'm not exactly verbally gifted even in my native language.

But we need accurate timing signals from the GPS satellites. If the time signal from the satellite gains 38,000 nanoseconds per day, the satellite will become more and more out of synchronisation with clocks at rest on the ground as the days go by. Is it not better to predict for the amount that the GPS clock will gain, and compensate for it before takeoff? That way, you don't have to keep making extra calculations when dealing with the GPS signal - its clock stays synchronised.

It's not like the GPS clock simply runs a little ahead of ground clocks, it gains time each day. Without that adjustment, every second as measured by the clock on the satellite would be a little shorter than every second as measured by a clock on Earth.

I am a little confused by the terminology here. Initially synchronised atomic clocks do lose their synchronisation if one is put on a jet plane and flown around the world a few times and then brought back to the other clock. A twin travelling at relativistic speeds and returning home would age less than the twin that stayed at home. The decay rate of particles travelling at relativistic speeds is time-dilated, allowing us to detect muons here on Earth that should decay at the edge of the atmosphere.

If the rate at which particles decay is time-dilated by relativistic speed, surely we can say that time does change. If the twin who travelled at relativistic speeds comes home again and has indeed aged less than the twin who stayed at home, surely it is not just the perception of time that changes - they actually experienced time passing at different rates, didn't they? Am I still missing something here?

"Manifests" is the word you were looking for.
I kinda like "manifestates," though. I might have to drop that now and then.

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Are you my mummy?