How Does a Clock Measure Time?

[mangaroosh]

Albert Einstein is quoted as having said "Zeit ist das, was man an der Uhr abliest" ["Time is what a clock measures"]. The question is, as per the title of the thread, how exactly does a clock measure time?

When considering this question we need to consider a few things; namely:
- what time is considered to be in the physical sciences
- how the physical processes of a clock measure a secondary physical property called time (if that is what time is asserted to be)
- how a clock demonstrates that there is a temporal dimension i.e. how it demonstrates the relationship between past and present, or present and future.Time in physics
The first point is something I'm not abundantly clear on.

Time is absolute in standard quantum theory and dynamical in general relativity.

Does time exist in Quantum Gravity?

In Einsteinian relativity, time appears to be physical, dynamical, as well as relative; motion affects its passage for different frames of reference. It appears to be different in Quantum Mechanics, where it appears to take on a more absolute form. That appears to be similar to the view under the interpretation of Lorentzian relativity, which is equally supported by relativistic experiments, as Einsteinian relativity. All three view time as a physical property of the universe - or so I believe.Measurement
A clock provides a regularly occurring, repetitive process which is used for the purpose of comparison. The repetitive process provides a standard unit in which other, different processes are expressed, and then compared to yet other processes, expressed using the same standard units.

For example, if we take the standard atomic clock, the recurring process there is the oscillations of the caesium-133 atom; what appears to be actually measured, by the atomic clock, is the number of oscillations of the atom, not some secondary physical property called time. Theses oscillations are then used to compare different processes.

For example; if we say that an object is displaced by a distance of X in 1 second, what we actually mean is that when 9,192,631,770 oscillations are counted, the distance traveled by the object will be X.

We can then compare other objects using this standard unit of comparison; if an object is displaced by a distance of X+2 when the counter of the clock reaches 9,192,631,770, then we say that the second object has moved more quickly than the first.

The question is, at what point in this process is a secondary (or tertiary), physical property called time measured - without, of course, simply assuming that it is?"Distance"
"Distance is what a ruler measures" is a comparison often used to attempt to explain how a clock measures time, but, much like time, "distance" is just a concept. If we talk about measuring the "distance" between ourselves and a remote object what we are actually saying is how many standard units of measurement - a metre stick for example - could we fit between [an arbitrarily defined point on] ourselves and [an arbitrarily defined point on] the object. If the object we are talking about is a coffee table in our living room for example, we might say how many metre sticks can we lay between ourselves and the coffee table; that number would correspond to the amount of floor between us and the coffee table. While we might say the floor exists, the coffee table exists, we exist, and the metre stick exists, "distance" is just a concept.Dimensions
"Length is what a ruler measures" is somewhat different to the notion of a ruler measuring distance, because here we are talking about the physical dimensions of an object, as opposed to the conceptual distance between objects.

Time, however, is somewhat different to the spatial dimensions of an object; the three spatial dimensions are [in general] clearly observable, however, the question is how do we discern that an object, or a process, has a temporal dimension? The time co-ordinate of an object, or a process, will always be "now"; that is, any attempt to measure a temporal dimension can only be carried out in the present moment. While we may be able to recall a previous state of an object or process, this recollection is just a mental construct, a memory; we may also be able to project a future state of the object, but this too is just a mental contsrtuct, or a concept. The same applies to any mathematical representation of the "past" and "future" states.

Given that this same reasoning applies to any clock, how can a clock demonstrate that there is a temporal dimension?
Conclusion
A conclusion that could be drawn is that time is not actually a physical property; it is, however, dynamical and relative but only insofar as it is a mental construct for each individual, and our memories and projections can be distorted. It appears as though "time" is not so much something to be measured, as it is the system of measurement, or comparison.

All that, of course, is based on the reasoning that a clock does not actually measure a physical property called time; but the question remains, how exactly does a clock measure time?

 

[lostcauses10x]

How does a clock measure time?
It counts a rate of change, or better said a length of change.

 

[Astronuc]

A clock is calibrated. The basic measures or units (seconds, minutes, hours) of time are arbitrary.

Of course, the day is based on the rotation of the earth, and the year is based on the Earth's period of revolution around the sun - and of course, the year is not some nice integer number of days.

The number of degrees in a circle, and arcminutes/arcseconds are arbitrary. Like seconds and minutes, they are based on 60.


Nevertheless, time is based on those things that affect human life.

 

[mangaroosh]

How does a clock measure time?
It counts a rate of change, or better said a length of change.

Again, if we relate this back to an atomic clock for example, what is actually counted is the number of oscillations of the caesium-133 atom; where does the physical, dynamical and relative property called time come into that equation?

 

[mangaroosh]

A clock is calibrated. The basic measures or units (seconds, minutes, hours) of time are arbitrary.

Of course, the day is based on the rotation of the earth, and the year is based on the Earth's period of revolution around the sun - and of course, the year is not some nice integer number of days.

The number of degrees in a circle, and arcminutes/arcseconds are arbitrary. Like seconds and minutes, they are based on 60.Nevertheless, time is based on those things that affect human life.

Indeed, time is the system of measurement that breaks those phenomena into units that can be used for comparison and communication; a secondary physical property called time is not measured by those phenomena.

 

[edward]

My wife is a genius, she can turn "Just a minute" into three minutes or more.

 

[lostcauses10x]

"Again, if we relate this back to an atomic clock for example, what is actually counted is the number of oscillations of the caesium-133 atom; where does the physical, dynamical and relative property called time come into that equation?"

Again a changing quantity, or a countable number of the changes. Many things can effect change including rate of decay of the atom, yet so far it is one of the more stable items. . This were the rest of it all comes into play.

If I take the rate of decay as the unit to be measured by and accelerate it from a point and leave an identical one there( at the point) the forces of nature will have will have less effect on one, than the other.

A relay crude way to explain this : A candle flame. Leave your hand in it you get burnt.
Accelerate through it and it has less effect. very crude but might get the idea across.

 

[mangaroosh]

"Again, if we relate this back to an atomic clock for example, what is actually counted is the number of oscillations of the caesium-133 atom; where does the physical, dynamical and relative property called time come into that equation?"

Again a changing quantity, or a countable number of the changes. Many things can effect change including rate of decay of the atom, yet so far it is one of the more stable items. . This were the rest of it all comes into play.

If I take the rate of decay as the unit to be measured by and accelerate it from a point and leave an identical one there( at the point) the forces of nature will have will have less effect on one, than the other.

A relay crude way to explain this : A candle flame. Leave your hand in it you get burnt.
Accelerate through it and it has less effect. very crude but might get the idea across.

Apologies, I struggle to see where "time" comes into play in any of the above.

 

[edward]

A clock actually measures motion. That motion (if the clock is built correctly) will coincide with mans concept of time.

I say mans concept of time because I have never seen an animal watching the clock.

 

[mangaroosh]

A clock actually measures motion. That motion (if the clock is built correctly) will coincide with mans concept of time.

I say mans concept of time because I have never seen an animal watching the clock.

I would say that time is merely a concept, with no physical existence; this doesn't appear to be how time is treated in physical theories, however. Einsteinian relativity appears to treat it as being physical and dynamical. I'm wondering how a clock measures time in this context, without simply assuming that it does; or how a temporal dimension can be deduced from the processes of a clock.

In relation to time being conceptual, I would say that the concept of a temporal dimension arises due to humankinds capacity for memory and projection, meaning that "past" and "future" are just mental constructs. It corresponds directly to motion, because man's concept of time is entirely based on motion and change; memory of a previous [no longer existing] state is labelled as "the past"; the current state (which is the only state that can ever really be said to exist) is the present; the projection of a state that has yet to materialise is labelled as "the future".

This obviously isn't anything original, but just worth stating for the purpose of discussion; as I have yet to hear it satisfactorily addressed.


But just to try and stick to the strict topic of the thread; if we consider the processes of a clock, where is the physical entity called "time" actually measured? The counter of an atomic clock counts the number of events (or oscillations) in the clock i.e. it measure the number of events; where does the measurement of the temporal (and physical) element of spacetime occur?

 

[chiro]

We measure it relative to some other process: this process could be mechanical in the form of a clock like using gravity and a pendulum to utilize a measure of periodicity that eventually translates into 'one second' or one 'time unit', or alternatively use a process like that found in the cesium atom way of measuring one second.

From these things we use some kind of change to effectively measure time. The changes can be something as simple as the periodicity of a pendulum in a huge grandfather clock to something a little more complicated like the entropy increase law in thermodynamics as we currently understand it.

Also its important to remember that all of the things that are usually utilized to measure time have a huge dynamic component. In other words if you have some kind of process that does not change, then its really hard to use that process to measure time so naturally we want to get some kind of process that is dynamic with properties that are well understood enough to extract the appropriate information about what kind of unit of time we are looking for.

One final thing: in order to get any universal measurement it is a good idea to use any universal constant(s) that we can use. If we choose things that are not constants we run into trouble getting different answers for the same thing.

Since in our current understanding the speed of light has so far looked to be a constant, this gives us a good candidate for measuring something in a more standardized way since so far it has passed the 'universality' test.

As long as we have some kind of standardization (the pendulum utilizes gravity which is for the most part well understood in the context that it is in in terms of its mechanics), then we can be sure that to whatever appropriate level of accuracy, that there will be the right standardization so that it can be used in many reference frames and therefore have everyone agree on it.

 

[Gokul43201]

Again, if we relate this back to an atomic clock for example, what is actually counted is the number of oscillations of the caesium-133 atom...

No, it isn't.

But why is this in General Discussion?

 

[Jimmy Snyder]

No, it isn't.

Sometimes it is.

A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time.

http://tycho.usno.navy.mil/cesium.html

 

[arildno]

My wife is a genius, she can turn "Just a minute" into three minutes or more.

And if she is to spend "just a minute" in a shop??

 

[jim hardy]

Time? Like light, do we even know if it's discrete or continuous?

 

[russ_watters]

But just to try and stick to the strict topic of the thread; if we consider the processes of a clock, where is the physical entity called "time" actually measured? The counter of an atomic clock counts the number of events (or oscillations) in the clock i.e. it measure the number of events; where does the measurement of the temporal (and physical) element of spacetime occur?

As I've said many times, there is nothing fundamentally wrong with our concept of time for this question to be useful. You wouldn't ask where (or when!) on a ruler the measurement of length occurs, would you? It's meaningless and you've got to stop seeing a problem with time in such questions: the problem isn't with time, it is with your meaningless questions.

 

[zoobyshoe]

In relation to time being conceptual, I would say that the concept of a temporal dimension arises due to humankinds capacity for memory and projection, meaning that "past" and "future" are just mental constructs. It corresponds directly to motion, because man's concept of time is entirely based on motion and change; memory of a previous [no longer existing] state is labelled as "the past"; the current state (which is the only state that can ever really be said to exist) is the present; the projection of a state that has yet to materialise is labelled as "the future".

No memory = no time. The present is the only reality. How long is the present? For humans it's as long as the psychological smear of past/present we can hold in consciousness, and the vast bulk of that is, in fact, memory. In fact, the present is more like a dimensionless euclidian point: a 'location' with no actual dimensions. (A hypothetical being with consciousness but no memory would perceive the world as a static phenomenon.) We don't know, and I don't think there is a way to know, the authentic rate that point "travels", because we would need some other kind of time to compare it to, and there isn't any.

 

[wuliheron]

The best established linguistic theories assert that words only have demonstrable meaning in specific contexts and the newest theories in quantum mechanics suggest this principle of contextualism applies to physical observations as well. Whether the cat is perceived to be dead, alive, or in superposition could be merely a question of the specific context in which we take the measurement. Relativity makes a similar assertion that whether we perceive something as time or space merely depends on the context of our relative motion. Thus the simplest and most demonstrable explanation to date is that time can be considered a physical property in some contexts and not one in others.

Is the cat really dead, alive, or in superposition? Is time really a physical property or not? Who cares! We observe what we observe and the rest I leave to the metaphysicians and mystics to debate. A photon doesn't appear to experience time so its perfectly sensible to tell someone time is not a physical property of photons. Clocks measure time so its makes perfect sense to tell someone time is a physical property of clocks. What matters first and foremost is what we observe, and communicating effectively about what we observe.

 

[Mech_Engineer]

Clocks don't "measure" the passage of time directly, they estimate it through the measurement of an oscillating circuit (whether it's electrtical, mechanical, or atomic). This is an important distinction in my humble opinion.

 

[russ_watters]

Yes, clocks count periodic events. But I don't see how that makes the word "estimate" applicable.

 

[Gokul43201]

Sometimes it is.

A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time.

http://tycho.usno.navy.mil/cesium.html

That's not accurate. In fact, the paragraph following the one you quoted provides a better explanation. The frequency that is being matched corresponds to the energy absorbed/emitted during one specific (hyperfine) electronic transition. It has nothing to do with the vibration of the Cs atoms.

Here's a better source: http://hyperphysics.phy-astr.gsu.edu/hbase/acloc.html#c2

 

[Jimmy Snyder]

That's not accurate. In fact, the paragraph preceding the one you quoted provides a better explanation.

A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time.

 

[Mech_Engineer]

Yes, clocks count periodic events. But I don't see how that makes the word "estimate" applicable.

"Measure" indicates in my opinion a direct link between the subject of the measure and the instrument performing it. Measurment of a distance for example can be done between two points using an instrument. "Measurement" of time gets a bit more abstract though, because the methods of measurement being used are one or two degrees removed from the raw subject.

Maybe I'm thinking a little far into the philosophical defenses, but it all goes back to the circular definition of "measuring" time using a periodic event which has (using either a frequency or distance or velocity) time as a fundamental unit in it's own definition:

Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities would result in circularity of definition.

An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event ... leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured.

 

[russ_watters]

I'm not seeing how counting events is removed from time by any degrees.

Is it the inherrent granularity that you take issue with? Does a surveyors wheel measure or estimate distance?

The wiki's complaint about the measurement of time being dependent on motion/distance has it backwards: it is distance that is measured/defined using time, not the other way around. But it really doesn't matter either way: to be overly fair to length, it was a practical decision based on measurement technology and accuracy. If I were to be less fair, I'd say that length is inherrently inferior to time due to the lower accuracy of measurement.

 

[Mech_Engineer]

I suppose the measurment of time isn't really any fundamentally different than measurement of length. Different instruments have different precision, and none give the "real" value, only one subject to their accuracy.

I just seem to find time measurement's traceability to a fundamental standard to be a bit fuzzier than physical measurement...

 

[lisab]

I just seem to find time measurement's traceability to a fundamental standard to be a bit fuzzier than physical measurement...

Me too. In a lab, one measures length by comparing it to another (calibrated) length; it's a direct comparison. But to measure time, there's more involved (time = distance/rate).

Some species of animals are known to use simple tools -- I can almost see such a species understand how to measure a stick length using the length comparison described above. No way can they understand a time measurement, though.

 

[Gokul43201]

Me too. In a lab, one measures length by comparing it to another (calibrated) length; it's a direct comparison. But to measure time, there's more involved (time = distance/rate).

Why can't you measure time by a direct comparison as well? What your doctor does with his hand on your pulse and his eye on a wristwatch is essentially how we all measure time - by a direct comparison to some other calibrated time interval.

Some species of animals are known to use simple tools -- I can almost see such a species understand how to measure a stick length using the length comparison described above. No way can they understand a time measurement, though.

I think most any predator that actively hunts prey (not the kind that ambush) has a pretty good intuitive sense of time.

 

[Mech_Engineer]

And of course time isn't constant- its measurement depends on your speed, further complicating things!

In Test of Relativity Theory, Superaccurate Atomic Clocks Prove Your Head Ages Nanoseconds Faster than Your Feet

In a study published today in the journal Science, researchers at the National Institute of Standards and Technology explain that a one-foot difference in altitude between two clocks caused them to tick at slightly different rates. The optical clocks can even measure changes in the passage of time caused by a 20-mile-per-hour speed difference.

 

[mangaroosh]

We measure it relative to some other process: this process could be mechanical in the form of a clock like using gravity and a pendulum to utilize a measure of periodicity that eventually translates into 'one second' or one 'time unit', or alternatively use a process like that found in the cesium atom way of measuring one second.

From these things we use some kind of change to effectively measure time. The changes can be something as simple as the periodicity of a pendulum in a huge grandfather clock to something a little more complicated like the entropy increase law in thermodynamics as we currently understand it.

Also its important to remember that all of the things that are usually utilized to measure time have a huge dynamic component. In other words if you have some kind of process that does not change, then its really hard to use that process to measure time so naturally we want to get some kind of process that is dynamic with properties that are well understood enough to extract the appropriate information about what kind of unit of time we are looking for.

One final thing: in order to get any universal measurement it is a good idea to use any universal constant(s) that we can use. If we choose things that are not constants we run into trouble getting different answers for the same thing.

Since in our current understanding the speed of light has so far looked to be a constant, this gives us a good candidate for measuring something in a more standardized way since so far it has passed the 'universality' test.

As long as we have some kind of standardization (the pendulum utilizes gravity which is for the most part well understood in the context that it is in in terms of its mechanics), then we can be sure that to whatever appropriate level of accuracy, that there will be the right standardization so that it can be used in many reference frames and therefore have everyone agree on it.

The emboldened sentence above is the assumption that is being challenged. I cannot see how a physical property called "time" is measured; even less obvious is how the temporal and physical aspect of spacetime is measured using a clock. You see, I can't see how we measure "time" relative to some other processes; we measure processes relative to other processes - "time" does not seem to enter the equation, unless we assume that it does.

The use of a periodic cycle simply gives us a common unit of comparison, in which we can express information about a process. We can then compare other processes by expressing them in terms of this common unit. The periodic cycle exists, and the processes that are expressed in terms of the cycle exist, but physical "time" cannot be deduced from that.

 

[mangaroosh]

No, it isn't.

But why is this in General Discussion?

I started a different thread, related to a similar topic, in the philosophy thread, but it was moved to the GD section; so I just presumed to start this one here.

What does the counter in an atomic clock count; and how does an atomic clock measure time?

 

[mangaroosh]

As I've said many times, there is nothing fundamentally wrong with our concept of time for this question to be useful. You wouldn't ask where (or when!) on a ruler the measurement of length occurs, would you? It's meaningless and you've got to stop seeing a problem with time in such questions: the problem isn't with time, it is with your meaningless questions.

"Length", much like "time", is just a concept; what happens when we use a ruler to measure the physical dimensions of an object is, in a simplistic example, we take a standard unit and hold it beside a physical object, and see how many of those standard units can be held beside the object. This allows us to express the physical dimensions of the object in a standard unit, which allows us to compare the physical dimensions of other objects expressed in the same units. That objects have spatial dimensions is self-evident.

The measurement of the temporal dimension is not quite as straight forward - impossible if it doesn't exist - because the attempted temporal measurement of an object can only ever be carried out in the present; that is, the actual time co-ordinate will always be "now". We may of course remember a past state, and project a future state, but those are just mental constructs. The object only ever exists in the present.

 

[mangaroosh]

No memory = no time. The present is the only reality. How long is the present? For humans it's as long as the psychological smear of past/present we can hold in consciousness, and the vast bulk of that is, in fact, memory. In fact, the present is more like a dimensionless euclidian point: a 'location' with no actual dimensions. (A hypothetical being with consciousness but no memory would perceive the world as a static phenomenon.) We don't know, and I don't think there is a way to know, the authentic rate that point "travels", because we would need some other kind of time to compare it to, and there isn't any.

I would agree to a large extent with that.

 

[mangaroosh]

The best established linguistic theories assert that words only have demonstrable meaning in specific contexts and the newest theories in quantum mechanics suggest this principle of contextualism applies to physical observations as well. Whether the cat is perceived to be dead, alive, or in superposition could be merely a question of the specific context in which we take the measurement. Relativity makes a similar assertion that whether we perceive something as time or space merely depends on the context of our relative motion. Thus the simplest and most demonstrable explanation to date is that time can be considered a physical property in some contexts and not one in others.

Is the cat really dead, alive, or in superposition? Is time really a physical property or not? Who cares! We observe what we observe and the rest I leave to the metaphysicians and mystics to debate. A photon doesn't appear to experience time so its perfectly sensible to tell someone time is not a physical property of photons. Clocks measure time so its makes perfect sense to tell someone time is a physical property of clocks. What matters first and foremost is what we observe, and communicating effectively about what we observe.

The question is about what do we actually observe. Physical theories, make ontological claims about the nature of things like time and space; to have more accurate physical theories we need to see if those claims are justifiable. Indeed, our subconscious belief in time, just as our other subconscious beliefs, can affect our experience of reality, or more pointedly, how we live our lives.

Again, it is the emboldened in the last paragraph that is being questioned.

 

[mangaroosh]

Clocks don't "measure" the passage of time directly, they estimate it through the measurement of an oscillating circuit (whether it's electrtical, mechanical, or atomic). This is an important distinction in my humble opinion.

Yes, clocks count periodic events. But I don't see how that makes the word "estimate" applicable.

Whether it is estimated or not, how does the counting of periodic events allow us to deduce that there is a temporal dimension; bearing in mind that the memory of a past event is just a mental construct, and only events in the present can be said to be real, without assuming that past or future events are real?

 

[mangaroosh]

Why can't you measure time by a direct comparison as well? What your doctor does with his hand on your pulse and his eye on a wristwatch is essentially how we all measure time - by a direct comparison to some other calibrated time interval.

I think most any predator that actively hunts prey (not the kind that ambush) has a pretty good intuitive sense of time.

In the example above, one process is compared to another process - where does physical "time" enter the equation?

 

[Gokul43201]

In the example above, one process is compared to another process - where does physical "time" enter the equation?

In what way is the time interval between two ticks of a clock different from the length interval between two ends of a ruler? (Other than that the first interval is measured along a dimension we call 'time' and the second is measured along a dimension we call 'length'.)

 

[russ_watters]

Me too. In a lab, one measures length by comparing it to another (calibrated) length; it's a direct comparison. But to measure time, there's more involved (time = distance/rate).

That's only true if you use a mechanical clock! As I said above, you (and M_E) have the dependency backwards when it comes to the definition/most accurate ways of measuring: Time is counted, length is calculated from measuring time and converting.

I've seen an awful lot of people saying they just don't like time or feel uncomfortable with it, but I've never seen it be based on a logical reason. It's just a feeling and that's not good enough to really say there is a problem with the concept of time.

 

[russ_watters]

And of course time isn't constant- its measurement depends on your speed, further complicating things!

No, that doesn't further complicate it any more than it complicates measuring length (due to length contraction). You're misunderstanding the point of Relativity.

 

[russ_watters]

The use of a periodic cycle simply gives us a common unit of comparison, in which we can express information about a process. We can then compare other processes by expressing them in terms of this common unit. The periodic cycle exists, and the processes that are expressed in terms of the cycle exist, but physical "time" cannot be deduced from that.

Why not!? Length is measured using a proxy as well. There is no difference.

"Length", much like "time", is just a concept; what happens when we use a ruler to measure the physical dimensions of an object is, in a simplistic example, we take a standard unit and hold it beside a physical object, and see how many of those standard units can be held beside the object. This allows us to express the physical dimensions of the object in a standard unit, which allows us to compare the physical dimensions of other objects expressed in the same units. That objects have spatial dimensions is self-evident.

The measurement of the temporal dimension is not quite as straight forward - impossible if it doesn't exist - because the attempted temporal measurement of an object can only ever be carried out in the present; that is, the actual time co-ordinate will always be "now". We may of course remember a past state, and project a future state, but those are just mental constructs. The object only ever exists in the present.

That is completely wrong. Both distance and time are only measured in a single reference at a time, using some anchor to another reference. For distance, the obvious example is with a tape measure, where it is fixed at one end and read at the other. For longer distances, you can completely lose the starting reference and still have an accurate measurement (such as with the odometer in your car).

Here and "now" are the same concept.

Whether it is estimated or not, how does the counting of periodic events allow us to deduce that there is a temporal dimension; bearing in mind that the memory of a past event is just a mental construct, and only events in the present can be said to be real, without assuming that past or future events are real?

That is an assertion/assumption without basis and one that is wrong for the same reason as the last. How, for example, can you say all the places you visited in your car, recorded by your odometer, are real? They only exist in the past for you, both in terms of time and space.

 

[wuliheron]

The question is about what do we actually observe. Physical theories, make ontological claims about the nature of things like time and space; to have more accurate physical theories we need to see if those claims are justifiable. Indeed, our subconscious belief in time, just as our other subconscious beliefs, can affect our experience of reality, or more pointedly, how we live our lives.

Again, it is the emboldened in the last paragraph that is being questioned.

LOL ontology is metaphysics, not science, and Relativity can describe time entirely by its relationship to other phenomena without any ontological explanation or reference to metaphysics whatsoever. Reality, existence, being, and the subconscious are for mystics and philosophers to debate. Science can state unequivocally that rainbows are optical illusions, but not whether they exist or are real in any ontological sense.

 

[lisab]

That's only true if you use a mechanical clock! As I said above, you (and M_E) have the dependency backwards when it comes to the definition/most accurate ways of measuring: Time is counted, length is calculated from measuring time and converting.

In my lab, we measure properties just like I said in my previous post. Nearly all of the clocks we use are mechanical clocks. We use tape measures and metersticks to measure distance, most of the time. Sometimes a caliper. When measuring the depth of a glulam beam, we don't ever invoke the length of the path traveled by light in vacuum in 1 ⁄ 299,792,458 of a second.

I work in an engineering lab that tests building materials. It ain't rocket science.

 

[Mech_Engineer]

Well Russ you can explain till you're blue in the face, I still find the absolute definition of time to be a bit of a wishy-washy subject That's not to say I can't utilize it in equations or understand HOW it can be utilized, I'm just saying it's got a certain smell about it...

It's probably just a lack of fundamental understanding of where there are/aren't circular arguments. If I was asked to prove the accuracy of a CMM or a micrometer or even the flatness of an optical surface that's probably not a problem (given access to NIST references haha). But if someone asked me to prove the accuracy of an atomic clock, not sure where I would go other than to say "well it's got cesium in it, and this definition of a second says it should be right..."

 

[collinsmark]

The question is, as per the title of the thread, how exactly does a clock measure time?

Since we've established (I hope) that the flow of time is now defined by counting the periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom in atomic clocks, perhaps one should ask, "Okay, what makes the atomic clock 'tick.' What drives the electron to regularly, seemingly change state in the first place?"

Without thinking, my first thought might be the evolution of the wavefunction, of course. But that leads to a sort of chicken-or-the-egg problem here. The wavefunction is modeled by the time-dependent Schrödinger equation. And the t in the i \hbar \frac{\partial \Psi}{\partial t} term is based on the outcome of the evolution. So that doesn't really help us much here in this particular case.

Instead, perhaps there's another approach to understanding this. Consider a very long hallway. In this hallway there are many, many clones/copies of yourself, all lined up one after the other. Each of these clones has a big box containing a stack of loose-leaf papers, in varying levels of organization. The first clone in the line represents some copy of your much younger self. That clone's box of papers is fairly well organized. The box belonging to the next clone in line is identical to the first's, except one of the papers has been moved out of order. As the line continues back, the corresponding stack of papers belonging to that clone are slightly more disorderly than the preceding clone's stack. This goes all the way back to the last clone in line, an older version of yourself who's stack of papers is fairly disorganized.

Now suppose you go up to one of these clones, and ask, "are you in the past, present or future?" The answer that you will invariably get is: "I am in the present." And it doesn't matter which clone you ask. They all think that they are in the present. Each clone thinks that it is he/she that is the one in the present, and all the others are either past or future.

If you haven't figured out my analogy yet, the line of clones represents a line on the "time" dimension of spacetime. And on this 4-dimensional (or 10 or 11 dimensional, whatever) chunk of spacetime, all versions of you equally exist. No version is more valid or less valid that any other. And each version is fooling himself/herself into thinking that he/she is the only version in the present. What's really the case is that each version is in its own present, and is stuck there.

Remember that stack of papers? Each stack of papers represents a particular quantum state of the universe. And each version of yourself is associated with one and only one quantum state. Each version of yourself is able to recall/look at notes/look at records, etc. regarding versions ahead of it, because those versions are almost identical to the clone in question, except they have stacks of paper that are more orderly, not less.

What I'm getting at is this: time, whether one considers it an illusion or not, is suspected to be related to entropy. And the arrow of time is always in the direction of increasing entropy.

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There's more to it than that still. Take an atomic clock, put it in a refrigerator and let it cool down. The entropy of the atomic clock as decreased, and yet it certainly does not tick backwards. As a matter of fact, it doesn't even slow down or change its rate at all. So there's more to it than the entropy of only the atomic clock itself.

At the heart of the atomic clock is that caesium 133 atom which contains an electron which emits radiation as it transitions states. And when a photon of radiation is detected by a detector, an electron in an atom in that detector becomes entangled with the original electron in thecaesium 133 atom. Almost immediately both of those electrons become entangled with other particles in the detector, and then the apparatus holding onto the detector and then with the all the atoms in the atomic clock, then room containing the atomic clock, then the Earth, and then throughout the universe. This is the process of decoherence: quantum state leaking out into the universe via quantum entanglement, becoming entangled with ever more and more particles.

Decoherence happens fast. And it makes the wavefunction appear to collapse [almost] instantly. And the wavefunction collapses simultaneously (whether treated as instant or "almost" instant) across all space.

Decoherence is akin to the second law of thermodynamics acting at its most basic level. And I speculate that resulting quantum entanglement between the caesium 133 atom and the rest of the universe has a large role in evolution of the wavefunction, and the eventual caesium 133 atom's changing of states again (even if that is the 'illusion' of time by talking to a different clone in that hallway analogy: moving to a different quantum state within spacetime).

(Tangent: Historically, this idea of instantaneous and simultaneous-across-space of the wavefunction collapse has caused many debates and experiments. According to Einstein's relativity, there is no such thing as absolute simultaneity. Events that are simultaneous in one frame of reference are not simultaneous in another frame. Since then the arguments have been mostly worked out by realizing (a) the wavefunction "collapse" should not be thought of as a classical event: it's not valid to describe it that way. And (b) any real events involving a given wavefunction collapse cannot be brought together for comparison faster than the speed of light. With those realizations in mind, special relativity and quantum mechanics are not in conflict. But it's these realizations that have given birth to various interpretations of quantum mechanics.)

Although putting an atomic clock in a refrigerator won't cause it to change its rate of time, relativity will. Put two atomic clocks in airplanes and let one fly around the world toward East, and the other West, and when they return they will show a difference. Put an atomic clock in a gravity well, and it will slow down.

That last point I find quite interesting. Consider a simple, non-rotating black hole. The volume of space contained within the event horizon of a black hole is at maximum entropy. One simply cannot put more entropy into a black hole without making it bigger (btw, the entropy of a black hole is proportional to the event horizon's surface area). Now consider carefully lowering an atomic clock such that it hovers just above the event horizon (assume you the observer are a safe distance away). Or just let the clock fall toward the black hole, whatever. When the atomic clock approaches the event horizon, its rate of 'ticking' approaches zero. Of course its slower rate of time can be explained with general relativity. But is there a connection with entropy and how general relativity affects entropy, and thus time, or is that just a coincidence?

I'm betting one would have to find more connections to quantum entropy, quantum entanglement, decoherence, and how they are affected by special and general relativity, to really understand how a clock measures time.

Further reading:
http://arxiv.org/abs/quant-ph/0203033
http://www.fqxi.org/data/essay-contest-files/McGucken_Dr._Elliot_McGucke_7.pdf
http://fqxi.org/data/essay-contest-files/Kiefer_fqx.pdf
http://lmgtfy.com/?q=quantum +entropy+relativity#

 

[mangaroosh]

In what way is the time interval between two ticks of a clock different from the length interval between two ends of a ruler? (Other than that the first interval is measured along a dimension we call 'time' and the second is measured along a dimension we call 'length'.)

The interval between events is always in the present though.If we caricature the operations of a caesium clock: as a man sitting on a chair counting the events (or whatever the more specific term is) as they appear in front of him. One pops into view, he counts one, and it disappears again; then another one pops into view, he counts two, and it disappears again; and so on.

Here, the temporal "relation" between events is only an imagined one. The man observes them in a sequence, but each one ceases to exist after he counts it. To say that one event exists in the past is simply to imagine that it does.Alternatively, if we imagine that each event is on a conveyor belt such that, as it comes into view he counts it and it continues along the conveyor belt (with or without disappearing, or ceasing to exist). Again, there is no past/present/future relationship, other than in the imagination of the counter. As an event occurs i.e. as he counts the object on the conveyor belt, it continues to exist in the present, but he retains the memory of it having passed him and designates is as being "in the past" - again, this is only imagined, because the object continues to exist in the present and the physical relationship between it and the other events is entirely spacial, not temporal. The temporal relationship is imagined, on the basis of his capacity for memory.

 

[chiro]

Thankyou for that detailed response collinsmark: very insightful.

 

[ThomasT]

I would say that time is merely a concept, with no physical existence ...

Time refers to changes in physical configurations.

... this doesn't appear to be how time is treated in physical theories, however. Einsteinian relativity appears to treat it as being physical and dynamical.

Right.

I'm wondering how a clock measures time in this context, without simply assuming that it does; or how a temporal dimension can be deduced from the processes of a clock.

A clock is a more or less periodically changing physical configuration. The underlying assumption is that any and all physical evolutions are ultimately due to some fundamental physical dynamic(s). So, counting the vibrations of a quartz crystal, or counting the revolutions of the Earth around the sun, or counting the ticks of a mechanical wound up clock, or counting the full moons, etc., are all ways of indexing the evolution of our universe.

But just to try and stick to the strict topic of the thread; if we consider the processes of a clock, where is the physical entity called "time" actually measured?

Time is the changing, the evolution, of configurations, from the largest to the smallest scale. So, one can measure time on virtually any scale.

The counter of an atomic clock counts the number of events (or oscillations) in the clock i.e. it measure the number of events; where does the measurement of the temporal (and physical) element of spacetime occur?

It occurs in the detection of the discrete oscillations. Which are enumerated. And which provide an index for comparison.

Time isn't some mysterious dimension. It's just the changing configurations of various ponderable objects. We, or instruments which augment our senses, record physical configurations. The changing of those configurations is what we call the passing of time.

 

[chiro]

To add to what ThomasT said, back in the ancient days the point of references were things like the position of the Earth relative to the solar system and things like relativity to the stars.

This was before all of the computers, grandfather clocks and all of that: a lot of it was based on understanding various periodic mechanisms that eventually gave us the things like the calenders of which you can find in many ancient documents from all across the globe.

 

[mangaroosh]

Why not!? Length is measured using a proxy as well. There is no difference.

An explanation of how physical time, or a temporal dimension cannot be deduced from counting the events of a periodic cycle is proffered here.

The difference between the physical dimensions of an object - bear in mind "length" is just a concept - and the temporal dimensions, is that it is possible to hold that proxy beside the object being measured and "measure its length". The same cannot be said of time, unless we assume that time exists a priori; we can't hold a clock beside time and measure the physical property of "time".

That is completely wrong. Both distance and time are only measured in a single reference at a time, using some anchor to another reference. For distance, the obvious example is with a tape measure, where it is fixed at one end and read at the other. For longer distances, you can completely lose the starting reference and still have an accurate measurement (such as with the odometer in your car).

Here and "now" are the same concept.

Apologies, I was a little narrow in my explanation; the explanation, as the one above represents a possibility that is distinctly lacking when it comes to time. With regard to the odometer of a car, it must be highlighted again that "distance", like "lenght" and "time" is just a concept. Indeed, what is measured by the odometer of a car is the amount of road in between the two locations. Again, the measurement device (the wheel) is in direct contact with the object of measurement; something which is not possible with time.

That is an assertion/assumption without basis and one that is wrong for the same reason as the last. How, for example, can you say all the places you visited in your car, recorded by your odometer, are real? They only exist in the past for you, both in terms of time and space.

It is not so much an assertion or an assumption as reasoning based on lack of evidence. Have you ever existed in a moment that wasn't the present? Bear in mind that the memories you have of "the past" are just mental constructs of the present moment, which has subsequently changed. Is there any evidence, that doesn't exist in the present, of either "the past" or "the future" - bear in mind that antiques or ancient buildings have always existed in the present, and will continue to do so.

It is safe enough to say that no experiment has ever been, or will never be, conducted outside of the present, for anyone. This means that there is no possible evidence of the continuing existence of past or future events, only present events. To say that they exist is to assume they do.

The difference between the places we visit in our car and events in the past and future, is that we can return to the places we visit in our car, but we can never visit past or future events. We can even set up webcams in the places we visit, or make phonecalls to people located there, but we canntot do the same with past or future events.

 

[mangaroosh]

LOL ontology is metaphysics, not science, and Relativity can describe time entirely by its relationship to other phenomena without any ontological explanation or reference to metaphysics whatsoever. Reality, existence, being, and the subconscious are for mystics and philosophers to debate. Science can state unequivocally that rainbows are optical illusions, but not whether they exist or are real in any ontological sense.

No doubt it can, but things like the existence of spacetime and "the block universe" are ontological claims.

 

[russ_watters]

In my lab, we measure properties just like I said in my previous post. Nearly all of the clocks we use are mechanical clocks. We use tape measures and metersticks to measure distance, most of the time. Sometimes a caliper. When measuring the depth of a glulam beam, we don't ever invoke the length of the path traveled by light in vacuum in 1 ⁄ 299,792,458 of a second.

I work in an engineering lab that tests building materials. It ain't rocket science.

That's fine as long as you recognize that you are using relatively crude instruments that work differently than the official definitions of the units. I wouldn't complain that a sundial doesn't have a second hand!

I just find it so odd for otherwise scientific people to analyze a scientific concept according to feelings.