# what is time?

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My lack of understanding of Cesium clock set aside.

My point is that time and distance are both ratios. These measurements are always described relative to something else. Time is the ratio between events we observe over a distance. The speed of light is constant in a vacuum, and the rate of events we observe change relative to the speed of light in a vacuum according to the Lorentz time dilation, an equation derived based on simple geometry, which in turn is based on the postulate that speed of light is constant and all laws of physics are the same in all inertial reference frames. So a ratio of events (time) is changing relative to a constant (speed of light) according to the Lorentz time dilation (equation based on geometry). For me this speaks of an 'underlying mechanism of nature' with cause and effect, but I understand that is a controversy in itself. However the question of the topic is 'what is time?', and I believe it is no more than a ratio of events that happen over some distance, which is also a ratio. If you are digging deeper than this, than it seems like you would ask 'what is the underlying mechanism or mathematics of the events that happen over distance'.

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My point is that time and distance are both ratios. These measurements are always described relative to something else.

Time ≠ Time measurement

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• 3 weeks later...

How is time not a measurement? Please elaborate on this statement.

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How is time not a measurement? Please elaborate on this statement.

You can measure time, but that doesn't tell you what it is, just as you can measure mass without that measurement telling you what it is. Time is going to do what it does regardless of whether you have established a standard by which you measure it.

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"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.

How does time behave?

My (simple?) opinion: time is a measure for change. Take 2 systems that are exactly the same. In one very much has changed compared to the initial situation, in another less. So in the first more time has passed.

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How does time behave?

My (simple?) opinion: time is a measure for change. Take 2 systems that are exactly the same. In one very much has changed compared to the initial situation, in another less. So in the first more time has passed.

No, that's not how it behaves. An inert material will experience the same amount of time as substances undergoing chemical reactions, or phase changes. Even though much has changed in the latter examples, the amount of time that has passed would be the same.

In your example, if the situation is exactly the same, how can you have a different outcome?

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In your example, if the situation is exactly the same, how can you have a different outcome?

Only because the times differed.

Say, I get two boxes with radio active material in it. The person who gives them to me says that into both, exactly the same boxes, he did exactly the same amount of radium. Now in one box I measure that there is less radium left than in the other. Assuming the man told the truth, I can only conclude that he did the radium into the first box first. So even that the systems were exactly the same when filled, they were filled at different times.

Now assume I fill both boxes with exactly the same amount of radium. I keep one box, the other I send on a 'relativistic twin journey'. When it comes back, I measure how much the contents of the boxes have changed. The one that was on the journey has more radium, so less change, so there passed less time for it.

So: time is the measure of change.

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Only because the times differed.

Say, I get two boxes with radio active material in it. The person who gives them to me says that into both, exactly the same boxes, he did exactly the same amount of radium. Now in one box I measure that there is less radium left than in the other. Assuming the man told the truth, I can only conclude that he did the radium into the first box first. So even that the systems were exactly the same when filled, they were filled at different times.

So the conditions were not identical.

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So the conditions were not identical.

Yes. And the difference is the time. What other difference do you see?

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if time is not real, does this possibly mean that spacetime is not either?

how do we prove that spacetime is something more than an equation based on our experience and thus relative?

please note this is just a question to test your theory about time if you feel it doesnt exist.

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Yes. And the difference is the time. What other difference do you see?

I don't see another difference for this example; I was just pointing out that there is a difference, which makes the claim that the systems are identical into an inconsistency in the example.

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Take two harmonic oscillators, Eise.

One completes one cycle and stops.

The other completes 1000 cycles and stops.

Neither has changed more than the other, they are both back at their original position, yet for the second oscillator 1000 times the amount of time has passed.

Time can pass without anything changing.

Edited by MigL
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migl,

you make a very interesting point as the thread is placed under quantum theory so why is the answer classical?

i do believe that there are still interactions going on subatomicly all the time.

since everything is considered to have a wave function, does this mean that interactions occur at the speed in which forces are carried?

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I don't see another difference for this example; I was just pointing out that there is a difference, which makes the claim that the systems are identical into an inconsistency in the example.

Yes, the only difference is the point in time. So that makes the definition of time: measure of change.

Take two harmonic oscillators, Eise.

One completes one cycle and stops.

The other completes 1000 cycles and stops.

Neither has changed more than the other, they are both back at their original position, yet for the second oscillator 1000 times the amount of time has passed.

OK. If you want to play the hard way.

We take two identical, closed systems. The temperature of the second oscillator will be slightly higher.

And, btw, this isn't fair play. I said the systems were identical. When one stops after 1 cycle, and the other after 1000, they were not identical.

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for the record, i think it would be helpful if someone could explain time as it is considered under quantum theory instead of classic approach because the question is one of quantum nature. or put the thread under the appropriate spot.

There is a time parameter in the equations of quantum mechanics. The Schrödinger equation[33] is

One solution can be

.

where is called the time evolution operator, and H is the Hamiltonian.

i will refrain from explaining this in hopes that someone more adept at math or physics can explain it much better.

to think that we are too set in our ways to test things is horrid. perhaps someone could use this to explain why it is better to use classical physics in such a case.

that way the author of the thread gets what he asked for too.

einstein, "yea, give me a couple of math guys."

edit:

i think that it may be important to note also that time is kept currently by caesium clock.

is this done by relativity or by other means?

i believe someone around here is a time keeper, am i right?

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Yes, the only difference is the point in time. So that makes the definition of time: measure of change.

Which you did not originally acknowledge as a difference. And that doesn't define time, it only makes time a necessary condition and perhaps allows us to quantify it.

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swansont,

how do they measure the atoms in an atomic clock?

i think this may be a good thing to shed light on for the concept of time and quantum theory.

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swansont,

how do they measure the atoms in an atomic clock?

i think this may be a good thing to shed light on for the concept of time and quantum theory.

In many of them you shine microwave radiation on them, putting them in a quantum superposition of states and let the system evolve. You then shine the microwave radiation on them again, and the final state distribution of the atom ensemble tells you how close the microwave frequency was to the standard, which is the atoms. To get the time, you effectively count the oscillations.

That tells you how you measure time. It tells you that a changing system (ideally, a cyclic system) is required to make a time standard. It doesn't tell you what time is.

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could it therefore be considered that everything has its own time?

i say this because i am considering that cooling down the atoms removes all noise in the system so that the natural frequency can be utilized.

i know this is a jump, but does that idea conform to the way time is handled with any current method?

one could imagine that if everything had its own clock based on its natural frequency then it would only be constrained by the speed of light.

in other words each has its own time and can be affected by speed of light transmissions of force carriers.

remember, this is just an idea.

edit:

under such conditions, waves themselves are constrained by the light constant and time is dependant on the object itself while interacting with its medium.

gravitational information might be passed through thier own force carier..

i am only speculating here.

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Time isn't a physical thing or interaction the way light and sound are. Time can't exert a force or impart momentum or transfer energy. It's an abstraction.

Of what? I suggest change, of course. Same as space is an abstraction of distance, or length (in 3 dimensions).

So if we ask how much time has passed, we in fact ask how much change has happened, of course in a standardised system.

And that doesn't define time, it only makes time a necessary condition and perhaps allows us to quantify it.

How can an abstraction be a necessary condition?

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Of what? I suggest change, of course. Same as space is an abstraction of distance, or length (in 3 dimensions).

So if we ask how much time has passed, we in fact ask how much change has happened, of course in a standardised system.

When you define some thing like time it must encompass everything and the problem with using 'change' in a definition is it doesn't, for example, cover the spontaneous decay of a radioactive atom. It decays in an unpredictable manner, when viewed as a single unit, but time still passes until it does.

Edited by StringJunky
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When you define some thing like time it must encompass everything and the problem with using 'change' in a definition is it doesn't, for example, cover the spontaneous decay of a radioactive atom. It decays in an unpredictable manner, when viewed as a single unit, but time still passes until it does.

Yes, you are right. But given that QM not only brings difficulties in our understanding of the concept of time, it also brings difficulties in our understanding of the concept of locality, I will not dive too deep into this problem.

Just this: you suggest that nothing has changed as long as the atom did not decay (if you suggest the opposite, then you are assuming the existence of hidden parameters). Now assume Physicist A produces artificial radioactive nuclei, say with a halftime of 1 hour. Say he produces 100 of them. After 1 hour he has 50 left. Now, (in nearly no time), he extracts the 50 left and gives them to Physicist B. Will, by measuring the decay of these 50 nuclei, B be able to determine that these nuclei already have been there for 1 hour? So, has time passed for these 50 nuclei? (Just compare with the question 'What way did the photon go in the double split experiment?'.)

Or maybe stated another way: assume a portion of absolute stable nuclei. Can we tell how much time has passed for these nuclei?

Now we have these two together: 1 gram of lead, 1 gram of the artificial nuclei. After 1 hour, I still have 1 gram of lead, but only 0.5 gram of the radioactive stuff. What is time now? Is it not just convention of us, to say that 1 hour has passed for the lead? If we do not observe change, or at least derive that change has happened (e.g. in geological layers), how can we know that time has passed?

Let's do the 'relativistic twin experiment' again. Which physicist will be more successful in checking the time dilatation: the one who sends a kilogram of a radioactive substance, the one who sends a single nucleus of the substance, or the one who sends a kilogram of lead? How far are these physicists justified in saying that they affirmed the 'twin paradox'?

Food for thought.

Even that this thread is posted in the QM forum, I think it is not possible to get grips on what time is, if you do not clarify it first in a classical context. So my suggestion: keep QM out for the moment.

Edited by Eise
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How can an abstraction be a necessary condition?

Sorry, I mistyped. It makes change a necessary condition to quantify time.

If something does not change, has time passed?

If we do not observe change, or at least derive that change has happened (e.g. in geological layers), how can we know that time has passed?

Is not knowing the same as it not happening?

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Sorry, I mistyped. It makes change a necessary condition to quantify time.

If something does not change, has time passed?

How would you know? Observing is already a process in itself, so this is an unanswerable question. Fact is that 'time' is always connected to change. So the step to state that time is change (or if you wish, an abstraction of change) seems quite logical. What can we say about something that only exists when we measure?

Is not knowing the same as it not happening?

That's too general. I don't know what is happening in Sidney right now, but of course there is a lot happening. But of something that is accessible only by something happening, as with time, I am inclined to say 'yes'. Time has no objectivity. Change has.

Edited by Eise
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How would you know? Observing is already a process in itself, so this is an unanswerable question. Fact is that 'time' is always connected to change. So the step to state that time is change (or if you wish, an abstraction of change) seems quite logical. What can we say about something that only exists when we measure?

That's too general. I don't know what is happening in Sidney right now, but of course there is a lot happening. But of something that is accessible only by something happening, as with time, I am inclined to say 'yes'. Time has no objectivity. Change has.

Time measurement is connected with change. If something does not change, I don't agree that no time has passed for it. Only that we can't use that to quantify how much time has passed.

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