# what is time?

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I love reading everything that people have to say about time. I asked this question, because felt we should have an answer to what it truly is for how can be understand space time without understanding time its self.

It depends on whether you're asking what time is or how time behaves. The latter is part of physics, the former is philosophy.

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What's the difference between observing length and distance?

None. For both you need references to objects.

There are many cases where it is not possible to isolate causes and effects and study each independently. In such cases, other approaches are used.

Also, time is not an "event", and there is no "force" slowing the clock down, so this feels somewhat irrelevant. And, as always, any objection you raise to the temporal dimension applies equally to the spatial dimensions (which also change in the example you give).

But you can't observe length. You can only observe marks on a stick, or more general, distance.

You exactly make my point. But you don't seem to realise it.

Edited by Eise
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A 3d moving point is a 4d "motionless" world line. The universe is a 4d fixed "block". We think we are moving but we are just a 4d space-time worm. We are a picture fooled into thinking we are a motion picture.

Seems Zeno's paradox: a flying arrow is motionless in every point of its trajectory. But the paradox is not in the reality, it's in the chosen mathematical model. In another model, if the arrow flies a constant speed, it has the same speed in every infinitesimal point of its trajectory.

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This discussion between you, Eise, and swansont, has been extremely interesting, if a little philosophical recently.

A little? The question 'what is time' is metaphysical through and through.

You insist time is an abstraction of change, and cannot be independently defined.

In effect, there is no time without change.

Yet how can you define change without using the concept of time ?

You cannot have change without the passage of time.

By your standards, change is also an abstraction as it doesn't exist without time.

So what are you saying, time is an abstraction of another abstraction ?

Right: we cannot explain change without reference to time. But if I ask you, what do we observe: can you observe time when nothing changes? On the other side: do we observe change? What does that tell us about what is an abstraction of what?

Indeed, he's yet to respond to this point. All he said was "time is not physical" the last time I brought this up.

Yep, and there is nothing more to say about it. Physical events/objects can be observed because they have causal influence. Time and space don't.

It depends on whether you're asking what time is or how time behaves. The latter is part of physics, the former is philosophy.

Exactly. So I wonder why you did not move the whole thread to philosophy from the beginning...

But of course, I would amend your sentence as 'how changes behave' (e.g. as function of velocity and gravity), instead of time.

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It appears that relativity says we have block time. A 3d moving point is a 4d "motionless" world line. The universe is a 4d fixed "block". We think we are moving but we are just a 4d space-time worm. We are a picture fooled into thinking we are a motion picture. The past is still there and it is next to us in the direction of time. Karl Popper once called Einstein "Parmenides" after Einstein admitted this was his view of the universe. Quantum mechanics has raised doubt's about relativity. The block universe may not be set in stone.

I have been reading some of the posts about "What is Time" but this idea that the past is till there next to us seems wrong to me. I could accept the image of the past is sort of next to us, but that it is spreading out at the speed of light in a sphere from any point.

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Exactly. So I wonder why you did not move the whole thread to philosophy from the beginning...

Because it was posted in the physics section and several people (myself included) were trying to discuss they physics aspects of it. One might just as easily ask why people simply didn't refrain from raising philosophical questions in a physics thread.

Such as

But of course, I would amend your sentence as 'how changes behave' (e.g. as function of velocity and gravity), instead of time.

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I have been reading some of the posts about "What is Time" but this idea that the past is till there next to us seems wrong to me. I could accept the image of the past is sort of next to us, but that it is spreading out at the speed of light in a sphere from any point.

The cone is the parts of the past that are observable by light. The question is if you traveled into the past would you see a younger version of yourself? If you do then the past is still there. If you don't then the past is gone. The twin paradox seems to disagree with the idea that the past is gone. Think of a 2d space-time graph. A moving point is represented by a motionless line.
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Because it was posted in the physics section and several people (myself included) were trying to discuss they physics aspects of it. One might just as easily ask why people simply didn't refrain from raising philosophical questions in a physics thread.

You were right from the beginning:

"What is time" is question of philosophy (metaphysics), not science, just as "what is length" would be. How time behaves and how it is measured are questions of physics.

But 'what is time' was exactly the original question.

Edited by Eise
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The cone is the parts of the past that are observable by light. The question is if you traveled into the past would you see a younger version of yourself? If you do then the past is still there. If you don't then the past is gone. The twin paradox seems to disagree with the idea that the past is gone. Think of a 2d space-time graph. A moving point is represented by a motionless line.

A cone is series of increasingly larger circles, increasing with time, but is just because you are considering a 2 dimensional situation.

In space around us light reflects off us all around, so once that light strikes a substantial object, you won't be looking for your image in that direction any more. (Like an eclipse, you can't see the Sun for a moment when the Moon is in the way.)

Time can't be made to run backwards (even though there are religious thoughts that counter/challenge that, "the old will grow young" or the "dry bones will get flesh on them again" but even then it is still some time/event in the future , not an event that happens in the past. You can be cloned but that is not turning the clock back.

I've been studying the Twin paradox and reject and/or I don't understand your idea that it shows the past is gone. Why did you say that? Both twins are still growing older.

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Right: we cannot explain change without reference to time. But if I ask you, what do we observe: can you observe time when nothing changes? On the other side: do we observe change? What does that tell us about what is an abstraction of what?

What? You're asking vague questions and pretending the answers should be obvious. You tell us the answers.

Yep, and there is nothing more to say about it. Physical events/objects can be observed because they have causal influence. Time and space don't.

Ignoring the fact that you did say more about it literally one sentence prior, how does that answer my question? You're defining time in terms of change. I want to know how you're defining change, so as to prevent your definition of time from being circular. That is, how can you define change without explicit reference to time? Saying "time is not physical" is not an answer. Even if I granted you that time isn't physical, the question still remains.

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A cone is series of increasingly larger circles, increasing with time, but is just because you are considering a 2 dimensional situation.

In space around us light reflects off us all around, so once that light strikes a substantial object, you won't be looking for your image in that direction any more. (Like an eclipse, you can't see the Sun for a moment when the Moon is in the way.)

Time can't be made to run backwards (even though there are religious thoughts that counter/challenge that, "the old will grow young" or the "dry bones will get flesh on them again" but even then it is still some time/event in the future , not an event that happens in the past. You can be cloned but that is not turning the clock back.

I've been studying the Twin paradox and reject and/or I don't understand your idea that it shows the past is gone. Why did you say that? Both twins are still growing older.

An electron doesn't grow like a circle as it gets farther in the past. If we went back in time people would not look like giant disks. The cone shows what part of the past can be observed. It takes time for light to travel from a to b.For one second in the past the sphere would have a radius of 186000 miles. The cone is a BOUNDARY. To see outside of the cone would require a signal traveling faster then light. As you go farther back the boundary grows. The particles do not grow. Traveling backwards in time would mean your worm makes a u turn. You are still moving foward in your time but your time is moving in the opposite direction of everyone else's time. Imagine a 2d graph where y is the time axis and x is the distance axis. Draw a upside down V. This would be a point that is traveling forward in time and then reverses it's direction in time. If you are observing this and you don't reverse your direction in time you could mistake this with two particles coming together and then disappearing when they meet. This is how Feynman describes matter and antimatter annihilating each other. An electron emits two photons. Instead of it recoiling the opposite direction in space it recoils the opposite direction in time. We reverse the arrow of time on the backwards traveling electron. When we do this we must also reverse its charge, spin, and matter antimatter classification. I said the twin paradox shows the past is not gone. When twins travel at different speeds there clocks get out of sync. What appears simultaneously to one would happen at different times for the other. 5 o'clock for one would be 4 o'clock for the other. If only the present exists then at 5 o'clock only one twin could exist. The other is at 4 o'clock and 4 o'clock no longer exists.
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....

I said the twin paradox shows the past is not gone. When twins travel at different speeds there clocks get out of sync. What appears simultaneously to one would happen at different times for the other. 5 o'clock for one would be 4 o'clock for the other. If only the present exists then at 5 o'clock only one twin could exist. The other is at 4 o'clock and 4 o'clock no longer exists.

If the twin was at same relative speed and again at the same location they both would be there at 4 o'clock and 5 o'clock. That would be like the situation in the first few hours after the traveling twins return. They would be having a party, and they would both be there.

Edited by Robittybob1
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If the twin was at same relative speed and again at the same location they both would be there at 4 o'clock and 5 o'clock. That would be like the situation in the first few hours after the traveling twins return. They would be having a party, and they would both be there.

The time would be different for each twin if they traveled at different speeds. The present for one twin would be the past for the other twin. If only the present exists then all clocks must be the same. Only one time would exist and that would be the present. Edited by david345
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It depends on whether you're asking what time is or how time behaves. The latter is part of physics, the former is philosophy.

The physics of it. I don't care about philosophy or i would have asked there. I want to understand the nature of it like how it works when it comes to gravity. Why does time seem to change when there is more gravity. Couldn't we study the effect of time and gravity by studying the sun. For if time is effected by gravity time should be different around the sun.

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The physics of it. I don't care about philosophy or i would have asked there. I want to understand the nature of it like how it works when it comes to gravity. Why does time seem to change when there is more gravity. Couldn't we study the effect of time and gravity by studying the sun. For if time is effected by gravity time should be different around the sun.

It is different near the sun. Time runs faster as you move out of a potential well, and slower as you drop into it.

You can see the same effect if you were in a rotating reference frame; you'd have to undergo an acceleration of v2/r for a circle of radius r. Such a systems can be analyzed in terms of the acceleration or the motion at v in terms of the time dilation you get. Sitting in a gravitational field has a similar effect.

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It is different near the sun. Time runs faster as you move out of a potential well, and slower as you drop into it.

You can see the same effect if you were in a rotating reference frame; you'd have to undergo an acceleration of v2/r for a circle of radius r. Such a systems can be analyzed in terms of the acceleration or the motion at v in terms of the time dilation you get. Sitting in a gravitational field has a similar effect.

OK could we recreate these effects in a lab on a small scale. take two objects one outside the effected zone and one in the effected zone. The item at had would have to be something that we know the decay rate of. Then find out if the rate if different once each test is done. What I want to know if it does work if we could project a time field around a ship in space under the same idea so the front of the ship is moving faster and the back of the ship moving at normal rate without tearing the ship part. also if we can project gravity couldn't we build something to cause gravity to have no effect as if was not there. like two polls of the same type pushing off each other.

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OK could we recreate these effects in a lab on a small scale. take two objects one outside the effected zone and one in the effected zone. The item at had would have to be something that we know the decay rate of. Then find out if the rate if different once each test is done. What I want to know if it does work if we could project a time field around a ship in space under the same idea so the front of the ship is moving faster and the back of the ship moving at normal rate without tearing the ship part. also if we can project gravity couldn't we build something to cause gravity to have no effect as if was not there. like two polls of the same type pushing off each other.

We've measured these effects in the lab, and they are correctly described by relativity — clocks at different heights run at different rates. It's been confirmed at height differences of around a meter, with some really precise frequency standards. But you can't manipulate gravity the way you describe.

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We've measured these effects in the lab, and they are correctly described by relativity — clocks at different heights run at different rates. It's been confirmed at height differences of around a meter, with some really precise frequency standards. But you can't manipulate gravity the way you describe.

why not, is it because we lack the tech to do so or just to our understanding of gravity it can't be done. For if we could we would be able to leave earth without so much effort.

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why not, is it because we lack the tech to do so or just to our understanding of gravity it can't be done. For if we could we would be able to leave earth without so much effort.

It's not a tech issue. Nature follows certain laws, and there's not a whole lot we can manipulate. You change a planet's gravity by having more or less mass, all else being the same.

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What? You're asking vague questions and pretending the answers should be obvious. You tell us the answers.

Ignoring the fact that you did say more about it literally one sentence prior, how does that answer my question? You're defining time in terms of change. I want to know how you're defining change, so as to prevent your definition of time from being circular. That is, how can you define change without explicit reference to time? Saying "time is not physical" is not an answer. Even if I granted you that time isn't physical, the question still remains.

Don't you realise that every definition is somehow circular? That is just as true for scientific definitions. But in empirical science, some concepts are directly linked to observation. And then it is simply true: you can't observe time, you can only observe change (and objects).

Take Newtonian mechanics as example: to define force, you need mass. But the other way round, to define mass, you need the concept of a force. Newton was well aware of this problem, and defined mass halfheartedly as 'volume times density', to get out of this definitional circle.

Or another example, of Darwinian evolution: what organisms survive? Those that are the fittest. What organisms are the fittest? Those that survive. Some creationists bring this as a serious argument against evolution. But if you realise that you can attach observations to these concepts, then the problem evaporates.

Just think about a complete dictionary: if all the words of a language are in it, then all definitions of them are circular, per definition. Maybe not so directly as with mass and force, or change and time, but it will be circular.

Endless loop: see loop, endless

Loop, endless: see endless loop

Edited by Eise
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Don't you realise that every definition is somehow circular? That is just as true for scientific definitions. But in empirical science, some concepts are directly linked to observation. And then it is simply true: you can't observe time, you can only observe change (and objects).

Take Newtonian mechanics as example: to define force, you need mass. But the other way round, to define mass, you need the concept of a force. Newton was well aware of this problem, and defined mass halfheartedly as 'volume times density', to get out of this definitional circle.

Or another example, of Darwinian evolution: what organisms survive? Those that are the fittest. What organisms are the fittest? Those that survive. Some creationists bring this as a serious argument against evolution. But if you realise that you can attach observations to these concepts, then the problem evaporates.

Just think about a complete dictionary: if all the words of a language are in it, then all definitions of them are circular, per definition. Maybe not so directly as with mass and force, or change and time, but it will be circular.

Endless loop: see loop, endless

Loop, endless: see endless loop

The ouroboros.

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Since you seem to understand the cone very well, a simple question in the hope to find (maybe) someone who understands me:

You wrote

What about the inside part of the cone? can we directly observe the inside part of the cone?

To see inside would require a signal travelling slower than light.

when light travels through air or some other medium it travels slower. Refractive index equals c/v c is the speed in a vacuum v is the speed through a medium. Bats see with sonar which travels at the speed of sound.
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Don't you realise that every definition is somehow circular? That is just as true for scientific definitions.

I define the length of my leg as a new unit called the elfleg. Tell me where that's circular and I'll buy you a doughnut.

Take Newtonian mechanics as example: to define force, you need mass. But the other way round, to define mass, you need the concept of a force. Newton was well aware of this problem, and defined mass halfheartedly as 'volume times density', to get out of this definitional circle.

Sorry to be the one to break it to you, but physics has advanced since Newton. Lagrangian mechanics for example, an alternative formulation of Newtonian mechanics, makes defining force extremely simple: the spatial derivative of the Lagrangian. No reference to mass whatsoever.

Or another example, of Darwinian evolution: what organisms survive? Those that are the fittest. What organisms are the fittest? Those that survive. Some creationists bring this as a serious argument against evolution. But if you realise that you can attach observations to these concepts, then the problem evaporates.

Darwinian evolution does not say that "the fittest survive." All it says is those that did survive could pass on their genes, whereas those that didn't survive couldn't.

Just think about a complete dictionary: if all the words of a language are in it, then all definitions of them are circular, per definition. Maybe not so directly as with mass and force, or change and time, but it will be circular.

I don't see how you can go from "all words are in the dictionary" to "all definitions are circular." How are you making that jump?

Regardless, the problem is not necessarily recursion. There are perfectly sensible recursive definitions. For example $\phi = \sqrt{ \phi +1}$ is a definition of the Golden Ratio. It is a useful definition because information can be extracted from it. We can figure out the numerical value of $\phi$ because despite its definition being recursive, it is still well-defined.

What you've essentially done with your "time is change" nonsense is say, "hey guys, I figured out that $x$ is actually $y$." So then I ask, "well then, what is $y$?" And you respond with "$y$ is $x$." That tells me nothing. All it says is that if $x=y$ then $y=x$: a tautology. It gives me no useful information. You've accomplished a change in variable, presumably for aesthetic purposes. That's why I earlier called it "a pedantic shift in vocabulary."

Edited by elfmotat
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It's not a tech issue. Nature follows certain laws, and there's not a whole lot we can manipulate. You change a planet's gravity by having more or less mass, all else being the same.

but black holes seem to have no mass, but large amounts of gravity. Also I do not think all laws created by man to explain how things work around us based on study as well as observation is all written into stone. It only takes something new or better to make an old law no longer valid. We need to be able to manipulate gravity.

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but black holes seem to have no mass, but large amounts of gravity.

Where do you get the idea that black holes have no mass? Mass is one of the defining characteristics of a black hole (along with charge and angular momentum).

Also I do not think all laws created by man to explain how things work around us based on study as well as observation is all written into stone. It only takes something new or better to make an old law no longer valid.

Obviously any answer can only be based on what we know now. That may change in future (but what we currently know will have to remain valid) but there is no way to give an answer based on that.

We need to be able to manipulate gravity.

It would certainly be useful. But that doesn't mean that we will ever be able to.

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