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michel123456

what we know about Time

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When a dicussion about Time begins, it usually derails into philosophical blah blah after a post or 2.

I'd like to refrain from the philosophical tentation, and try to concentrate on what we really know about Time from a scientific point of vue only.

 

For example:

 

1. we know that motion requires time: nothing can move from one spatial coordinate to another in zero time, it would be a transgression of the Speed Of Light.

 

2. we know that the rate of time is related to gravity: where gravity is stronger Time flows slower.

 

3. we know that time has a "direction", commonly called the arrow the time.

 

4. we know that time is related to causality, and causality is related to c, the Speed Of Light.

 

5. we know that time is linked to space: time alone has no physical meaning, only the spacetime continuum "exists".

 

6. we know that time can transform in space, and vice-versa: what is space for an observer may be time for another.

 

What else do we know about Time? (and please correct me for any error)

Edited by michel123456

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Ah, the first point of reference for any good scientist!

 

Time is very dificult to pin down, I'm very interested to see how this thread turns out and if people can keep philosophy out of the thread...

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Ah, the first point of reference for any good scientist!

 

Time is very dificult to pin down, I'm very interested to see how this thread turns out and if people can keep philosophy out of the thread...

 

Time is what clocks measure.

 

In general relativity what clocks measure is the proper time of the world line of the clock.

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Time is what clocks measure.

 

In general relativity what clocks measure is the proper time of the world line of the clock.

 

 

That's pretty much what I expected to see and the extent of what physics has to say on the subject

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That's pretty much what I expected to see and the extent of what physics has to say on the subject

 

Indeed, I have been scratching my head on what to say here. I don't think I could say any more than DrRocket, or at least not without going into metaphysical/philosophical lines which won't help anyone!

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Indeed, I have been scratching my head on what to say here. I don't think I could say any more than DrRocket, or at least not without going into metaphysical/philosophical lines which won't help anyone!

 

 

It is annoiying that it is such a fundamental concept and yet the best we can say is "it's what clocks measure"

 

If we said the same of temperature "it's what thermometers measure" we would be laughed at

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It is annoiying that it is such a fundamental concept and yet the best we can say is "it's what clocks measure"

 

Sure, though other fundamental concepts in physics are similar. Then that is physics, as long as we can mathematically model what is going on and make sound predictions one is not usually preoccupied with the questions of what is?

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It is annoiying that it is such a fundamental concept and yet the best we can say is "it's what clocks measure"

 

If we said the same of temperature "it's what thermometers measure" we would be laughed at

 

It's inevitable, though. You define concepts with the underlying principles, but there is no deeper level to which we can refer.

 

It's true of language, too. Try defining words with other words, without referencing some abstraction or having any circularity to the definitions. Then define those words, and so on. Eventually you will fail to be able to do so.

 

The interesting thing is that nobody seems to be frustrated with the similar problem with length.

 

2. we know that the rate of time is related to gravity: where gravity is stronger Time flows slower.

 

It's actually the gravitational potential (i.e. the depth inside the gravity well), not the local value of g, that determines the change in rate.

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It's inevitable, though. You define concepts with the underlying principles, but there is no deeper level to which we can refer.

 

It's true of language, too. Try defining words with other words, without referencing some abstraction or having any circularity to the definitions. Then define those words, and so on. Eventually you will fail to be able to do so.

 

The interesting thing is that nobody seems to be frustrated with the similar problem with length.

 

Very true, but then length is not as interesting as time (in my opinion)

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The interesting thing is that nobody seems to be frustrated with the similar problem with length.

 

I agree. I think time is over-analysed...it's just a system that measures magnitude of duration relative to some arbitrary reference that has some desirable feature of inherent cyclicity or of fixed and known durational magnitude; like an hourglass.

Edited by StringJunky

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I agree. I think time is over-analysed...it's just a system that measures magnitude of duration relative to some arbitrary reference that has some desirable feature of inherent cyclicity or of fixed and known durational magnitude; like an hourglass.

 

 

I cant help feeling that there is more to it than that...

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If we said the same of temperature "it's what thermometers measure" we would be laughed at

 

You can say the temperature is the average kinetic energy of the molecules of the substance in question. At that point, to become more fundamenta,l you are stuck with explaining what energy is and that is just as mysterious as what time is.

 

But wait, you say, KE=1/2 mv^2. But when you think about it, to measure v you need to know length and lenght is "what rulers measure". Not to mention that pesky "mass" thing, which becomes even more sticky.

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7. we know that information needs time to travel, it's an extension of point 1 of the OP. As a direct consequence we know that any observation is observation of the past.

 

8. we know that distance is related to time: to more an object is far away, the more he is observed in the past (another consequence of point 1)

 

9. so we know that if you take a close object, then propulse it far away, you are throwing the object into the past (and not in the future as comonly believed). And that is coherent with point 8. because the object is continuously observable along its path. As the distance increases the object falls into the past.

 

10. We know (we suppose) that time is one dimensional, and space is 3-dimensional. The one-dimensional part of space is called distance. So we know that when time transforms into space (following point 6 of the OP) in fact time transforms into distance.

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7. we know that information needs time to travel, it's an extension of point 1 of the OP. As a direct consequence we know that any observation is observation of the past.

 

8. we know that distance is related to time: to more an object is far away, the more he is observed in the past (another consequence of point 1)

 

9. so we know that if you take a close object, then propulse it far away, you are throwing the object into the past (and not in the future as comonly believed). And that is coherent with point 8. because the object is continuously observable along its path. As the distance increases the object falls into the past.

 

10. We know (we suppose) that time is one dimensional, and space is 3-dimensional. The one-dimensional part of space is called distance. So we know that when time transforms into space (following point 6 of the OP) in fact time transforms into distance.

 

You are really confused.

 

More reading and less writing might be in order.

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12. we know from the laws of motion that no external force is required for an object to remain under constant motion. So once we gave the impulse to the object, it will continue to travel further away forever.

 

13. So we know from point 12 that no external force is required to travel into the past. Not exactly: the impulse was required at first in order to put the object in state of relative motion. But after relative motion was established, no other force is required to maintain the travel into the past. I know that sounds gibberish, but it is a direct consequence of what we know, or correct me.

 

14. we know that time is relative: there is no absolute past. The past as described above is the observer's past, that is the specific past relatively to the guy throwing the ball away.

 

You are really confused.

 

More reading and less writing might be in order.

 

Oops, where did I go wrong?

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Oops, where did I go wrong?

9. so we know that if you take a close object, then propulse it far away, you are throwing the object into the past (and not in the future as comonly believed). And that is coherent with point 8. because the object is continuously observable along its path. As the distance increases the object falls into the past.

Post #14.

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I can't speak to everything you mention, but for the most part it seems you are not describing what we know about time, but what things have time as a component.

 

1. we know that motion requires time: nothing can move from one spatial coordinate to another in zero time, it would be a transgression of the Speed Of Light.

This is something we know about motion.

 

2. we know that the rate of time is related to gravity: where gravity is stronger Time flows slower.

This is something we know about the model of Relativity.

 

5. we know that time is linked to space: time alone has no physical meaning, only the spacetime continuum "exists".

This is something we know about the model of Relativity.

 

7. we know that information needs time to travel, it's an extension of point 1 of the OP. As a direct consequence we know that any observation is observation of the past.

This tells us about information.

 

8. we know that distance is related to time: to more an object is far away, the more he is observed in the past (another consequence of point 1)

This is something we know about the the speed of light.

 

9. so we know that if you take a close object, then propulse it far away, you are throwing the object into the past (and not in the future as comonly believed). And that is coherent with point 8. because the object is continuously observable along its path. As the distance increases the object falls into the past.

I am not sue what this is.

Edited by zapatos

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It has been my experience that discussions of "past" really muck up the works, physics-wise. What SR quantifies are things like time intervals and rates.

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Ah, the problem is here:

"if you take a close object, then propulse it far away, you are throwing the object into the past."

 

What is wrong with that?

We know that an object far away is observed now as it was in the past.

When you propulse an object far away, the same happens.

Why is that incorrect?

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Ah, the problem is here:

"if you take a close object, then propulse it far away, you are throwing the object into the past."

 

What is wrong with that?

We know that an object far away is observed now as it was in the past.

When you propulse an object far away, the same happens.

Why is that incorrect?

First, it makes the past sound like a place you can go.

Second, it did not go to a 'time' you are not in. It simply moved in space.

Third, it is not going to a different time (the past), it is simply moved in space to a location where photons take longer to reach you than they did before you threw it.

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First, it makes the past sound like a place you can go.

Second, it did not go to a 'time' you are not in. It simply moved in space.

Third, it is not going to a different time (the past), it is simply moved in space to a location where photons take longer to reach you than they did before you threw it.

That is (edit-almost) correct. I never intended to say the throwned object made a time-travel.

I say that when you move in space, you move in time.

When you say "it is simply moved in space", well it is simply moved in time too.

 

What you say that "photons take longer to reach you" works for all the galaxies around us. If you ever send a spaceship to a planet of a far away galaxy, you will see the spaceship landing as it was landing in the past, since it will be at "a location where photons take longer to reach you".And not only photons, but any other physical interaction, like gravity for example.

 

Third, it is not going to a different time (the past),

 

I disagree. it is going to a different time. Only the objects that are at rest with you can be considered as being "in the same time".

Edited by michel123456

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That is (edit-almost) correct. I never intended to say the throwned object made a time-travel.

I say that when you move in space, you move in time.

When you say "it is simply moved in space", well it is simply moved in time too.

 

What you say that "photons take longer to reach you" works for all the galaxies around us. If you ever send a spaceship to a planet of a far away galaxy, you will see the spaceship landing as it was landing in the past, since it will be at "a location where photons take longer to reach you".And not only photons, but any other physical interaction, like gravity for example.

I don't understand why you want to equate time and space. You can describe everything by only discussing space. Overlaying space with time only confuses things.

 

When you say "it is simply moved in space", well it is simply moved in time too.

 

This is only true if space and time are the same. I guess I need to understand why you belive that. It just seems to add complexity with no benefit, and requires time to have properties that cannot be shown to exist. For example, if I throw a ball to you, it seems as if you are saying that it moves into the past (from my perspective), into the future (from your perspective) and not all through time (from the ball's perspective), all at the same time. Very confusing to me why it is true, and why you want to connect them in this way.

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Ah, the problem is here:

"if you take a close object, then propulse it far away, you are throwing the object into the past."

 

What is wrong with that?

We know that an object far away is observed now as it was in the past.

When you propulse an object far away, the same happens.

Why is that incorrect?

If I throw a ball at you will it hit you before I throw it or after?

 

What you say that "photons take longer to reach you" works for all the galaxies around us. If you ever send a spaceship to a planet of a far away galaxy, you will see the spaceship landing as it was landing in the past, since it will be at "a location where photons take longer to reach you".And not only photons, but any other physical interaction, like gravity for example.

But the spaceship will not land before it left Earth.

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Ah, the problem is here:

"if you take a close object, then propulse it far away, you are throwing the object into the past."

 

What is wrong with that?

Pretty much everything.

 

It would be rather nice to be able to throw an object into the past. I would use that ability to throw a message to the 2004 version of myself: "Buy Google stock as soon as it goes IPO. Oh, and buy some Apple stock too. Buy as much as you possibly can."

 

We know that an object far away is observed now as it was in the past.

When you propulse an object far away, the same happens.

Why is that incorrect?

Because you are still seeing the object after you threw it. You are not throwing into the past.

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