# How does a clock measure time?

by mangaroosh
Tags: clock, measure, time
 PF Patron Sci Advisor P: 2,207 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.
 Mentor P: 21,683 Yes, clocks count periodic events. But I don't see how that makes the word "estimate" applicable.
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Quote by Jimmy Snyder
Sometimes it is.

 Quote by the Navy 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.../acloc.html#c2
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That's not accurate. In fact, the paragraph preceding the one you quoted provides a better explanation.

 Quote by The Navy 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.
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 Quote by russ_watters 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:

 Quote by Wikipedia.org 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.
 Mentor P: 21,683 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.
 PF Patron Sci Advisor P: 2,207 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...
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 Quote by Mech_Engineer 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.
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 Quote by lisab 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.
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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
 Quote by PopSci.com 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.
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 Quote by 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.
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.
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 Quote by Gokul43201 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?
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 Quote by russ_watters 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.
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 Quote by zoobyshoe 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.
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 Quote by 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.
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.
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 Quote by 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.
 Quote by russ_watters 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?
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 Quote by Gokul43201 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?
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