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.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"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?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

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
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"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?

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
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"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.

I wouldn't really think so-- it seems like it would be easier and more flexible to just do a calculation to interpret the timing data. They pretty much have to do that anyway. But I don't really know any of the details, there might be perfectly good reasons to do it that way. It just does violence to the conceptual framework of relativity to change the clock instead of the interpretation.You can take out a fixed rate of desynchronization, but will still have to correct for everything that is variable. You avoid the accumulating shift, but that doesn't seem like that big of a deal. What is the technology for using a slowed clock anyway?

The terminology is very tricky here. Both experience their own time normally, so in that sense the time has not changed, but they conceptualize the other's time differently. Does that count as a change? The way I like to picture it is as if each person was making a sound recording of their own experiences in real time, but since they are not in the same place, it's not so clear what part of one tape corresponds to what "simultaneous" part of the other tape. Depending how you do that matching up, listening to (or conceptualizing listening to) the other person's tape might make it sound very slow or very fast in certain places. The tapes themselves are recorded normally-- but one tape is shorter than the other (in the twin situation) via a trick in the "matching up" of corresponding moments on the two tapes. I don't see the "matching up" as having the power to "change" time, but it certainly changes the comparison between two proper times.

In relativity, time is relative to a distance in space. Therego, time is effected by how fast a thing is accelerating through space. If it isn't moving, there is still, paradoxically movement. This way nothing can be steady. Everything is moving.

According to the equation G_ab = T_ab, we learn that matter is the distortion of space and time, and time warps, including space bends are but a fluctuation ... or curl in space and time caused by the presence of something accelerating or disturbing the fabric of spacetime... causing curvature.

Because of this, physicists say they are in fact on of all the same thing... and again, for this reason, matter, energy, space and time are all the same thing. If you removed one of these things from the arena, everything else would follow!!!

This is the truth.

Three questions
1/ What the hell are you talking about?
2/ Why do you try to baffle people with poorly understood technospeak?
3/ Since you've used it on more than a few occasions, when did "therego" become a word? Ergo or therefore, take your pick. Unless you want to be misunderestimated.

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This is not an idea to be tossed aside lightly - it should be thrown with great force

Speedfreak, Nearly and not quite. When I said time does not change. I meant time does not change.
To better explain this try... If you were to travel very fast and return here yes the time balance is 'broken' As you traveled fast your time slowed some. Not the time itself just your experience of it.
The progression of time forward does not change. It can not change. Only your experience of it can. When you return you will be younger than me by as much as the time difference shows. Your high velocity will have shrunk the passing time for you. The time passage for me would not have altered its rate as I stayed still.
If the above is unclear or wrong... Would Ken G please have some input to clear this conception error mark.

I remember someone on this forum saying that time is only a method of how humans percieve reality and it doesn't really exist, therefore it is impossible to time travel in question as there is no such thing as past or future. Can anyone comment on this?

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I Wanna Go Home CLUB

I do hope that was not me... cos now I'm going to say that has got to be rubbish.
The past is the moment gone...
The future is its still comming...
Yes time travel is not possible...

Many have said that- more a perception than a force.
Similar to Einsteins gravity.

It's not a rigorous argument. One could also say that "here" and "there" are human methods of perception to organize degrees of nearness, so it is impossible to walk from here to there because they don't actually exist. I think what precludes time travel is not that past and future don't exist, but simply that time, whatever it is, just doesn't work that way.

Nope. Why did you switch to the Pound/Rebka experiment?

Time was developed based on the earths rotation of 1 day/24 hours and then broken into minutes and seconds.

That 24 hours is based on the radius of the earth at sea level...I assume. or some given average radius from r=0 to the earths surface.

So at 14,000 ft a clock/time is running slower than it is at sea level, and this is a well know concept, and KenG or someone else should have stepped up and explained that this is 'another' time concept as well.

My questions are these...

1. How can Time be both running slower in one case at altitude and faster in another case at altitude?
2. How fast would time be if I were at the center of the earth. at r=0 based on the convention of the rotating planet to derive time.
3. If Earth was as big as the solar system in radius, how slow would time be at the surface.? If it were as large as the radius of the visable universe/13.7 BLY?...if it were infinitly large?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!