How Does a Clock Measure Time?

[russ_watters]

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

That is completely wrong and your conveyor belt example shows exactly the error you made earlier with "here" and "now". Both the time and position of the starting point are lost on a conveyor belt. They are only kept track of.

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".

Sometimes yes, sometimes no. Don't fool yourself into thinking that since sometimes you can see both ends of a ruler at the same time that it makes a difference. It doesn't. I'm sure you wouldn't concede the point for time, that both ends of an interval of time that is shorter than human perception happen "now", would you?

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".

Nonsense. The concept of time was developed because it was needed. It was not assumed to exist, it was observed to exist.

Again, the measurement device (the wheel) is in direct contact with the object of measurement; something which is not possible with time.

What do you mean "again"? That's a new objection that has nothing to do with anything. There is no requirement that a distance measuring device be in contact with what it is measuring.

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?

Have you ever measured a distance from the wrong end of a tape measure?

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.

That is pseudophilosophical nonsense. Again, if that were true, which it isn't, the fact that you are always here would mean distance exists only in your head as well. Both time and distance are measured by establishing one end point, then tracking, with an instrument, to another.

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.

Nonsense - that's "not even wrong" (it would have to improve in order to merely be wrong). There is no claim nor requirement for an event that happened in the past to still exist. They did exist. They don't anymore.

Moreover, not only do experiments occur in the past, it is actually more correct to say they all occur in the past -- not to mention, they all occur there, not here! But that's true of both time and distance, so again an irrelevant concern.

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.

Not necessarily true. I once visited the World Trade Center twin towers, but I can't again. Clearly, this shows us that "location" really needs 4 coordinates, not three.

Yes, there is a difference between distance in time in that time flows and distance does not. But who are you to call that a flaw? It's just a difference and when you oversimplify examples to try to make it into a flaw, you typically only show that time is real.

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.

Both webcams and phone calls record in the past and bring the recordings to you in the present. You're arguing against yourself.

 

[ThomasT]

@ mangaroosh,

You ask how a clock measures time. Time isn't some dimension that exists apart from or independent of the objects of our experience. The word time just refers to the indexing of incongruent configurations. We don't use clocks to measure time. Whatever that might mean. We use clocks to index, and communicate unambiguously, the location of incongruent physical configurations in the temporal index. They serve essentially the same function as, say, numerical street addresses. But with the difference that clocks index and communicate relationships, via the clock index, of evolving, changing physical configurations (like you being one place at 5 pm and supposed to be at another place at 6 pm), while street addresses communicate the relative positions of objects in more or less persistent physical configurations. Does this make any sense wrt your way of thinking about time?

 

[wuliheron]

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

You are equivocating. Spacetime is no more an ontological claim then the existence of bacteria. It is a scientific theory that predicts and explains what we observe. Nothing more, and nothing less. As for the block universe, that is not a scientific theory.

 

[mangaroosh]

Thanks for the detailed response cm, I'm not sure that I have fully understood the part relating to quantum decoherence, although I am familiar with the concept.

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?"

[Seth Lloyd says] clocks don’t really measure time at all. “I recently went to the National Institute of Standards and Technology in Boulder,” says Lloyd. (NIST is the government lab that houses the atomic clock that standardizes time for the nation.) “I said something like, ‘Your clocks measure time very accurately.’ They told me, ‘Our clocks do not measure time.’ I thought, Wow, that’s very humble of these guys. But they said, ‘No, time is defined to be what our clocks measure.’ Which is true. They define the time standards for the globe: Time is defined by the number of clicks of their clocks.”

http://discovermagazine.com/2007/jun/in-no-time/article_view?b_start:int=1&-C=

It is one thing to say that the flow of the physical property called "time" is measured by a clock, it is another thing entirely to say that "time" is defined by counting the [above mentioned] events, or that time is defined by what a clock measures.

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.

I'm familiar with the concept of the block universe, but have a number of objections to it. Your analogy was interesting, but I think we can just as easily think about it without using clones and just think in terms of yourself and the memories you have of yourself. For example, according to the block universe, you still exist as an 8yr old, or rather, your 8yr old self still exists. What I wonder though is, given the fact that your 8yr old self would have grown up at the exact same rate as you, such that your 8yr old self should be the age that you are now, how can you still exist as an 8yr old? Is your 8yr old self frozen in what must be an ultimately timless block universe? If all such "nows" are frozen, then how do we experience the illusion of change and motion? The explanation I've heard is that we exist as "worldlines" not necessarily in the human form we appear to have. Of course, this explanation is not without its own set of assumptions; and still requires an explanation of how we experience change and motion.

If we stick with your analogy however, and try to give it a slight dose of realism; imagine that you are one of the clones and that you can't actually speak to any of the other clones, what then? What if you can't see the long line of clones going back "in time"? There is only you, in your present. Are you frozen in your "now"; why do you experience the illusion that you are not frozen? How does one clone transition to the next clone? Are the "nows" discrete, or do they flow into each other; if so, does picturing a line of individual clones make any sense?

We might be getting a bit ahead of ourselves, however, because I think the concept of "the block universe" comes directly from a theory which utilises a clock to measure time; so we probably need to establish how a clock does that first.

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.

I'm not sure I fully understand this part; I was thinking that the state of disorder of the papers was linked to entropy. If I can deduce correctly, then the association of each clone with one quantum state of the universe simply refers to the actual state of the universe at that given time (from the clones perspective to try and avoid a tangential discussion). The records, I presume, relate to past clones (because I'm not sure it makes sense to have records of future clones), but the notes probably aid predictions about future clones. If I have understood this correctly, then it doesn't change the sentiment that past records (memories) are of things which no longer exist, and future predictions are of things which do not yet exist. Again, the evidence we have is not that there is a long line of clones, rather one single entity which is continually changing.

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.

The notion of increasing entropy is again just related to the notions of "past" and "future", for which there is no evidence. That doesn't mean that systems don't become more disordered, it simply means that there is no evidence that the "past" state of the system continues to exist, and no evidence that the "future" state of the system exists.

Time doesn't have an arrow, human perception does.

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).

Is that essentially all that quantum decoherence is? I thought there was more too it than that; cheers for the explanation, though.

I'm stuggling to make the leap from quantum decoherence, from the electron of the caesium-133 atom, to the existence of a physical property called "time", though.

(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.)

This is an area I have relatively little understanding of, but instead of starting a tangent, I might ask a few questions in a different thread.

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.

An atomic clock uses a laser to detect the changes in the hyperfine state (apologies for the lazy formalism); couldn't the differences in the rate of ticking be equally attributable to the different path lengths that the photon has to travel in each clock? If the eart is actually rotating east to west, then the photon in the clock flying west would have to travel a longer distance to the detector, meaning that it would "slow" down, while the photon in a clock flying against the rotation of the Earth would have a shorter distance to travel and would, therefore, "gain time".

Also, gravity affects light, so the photon in the clock that is "deeper" in the gravity well (experiencing "more gravity") would be expected to be slowed, wouldn't it?

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?

Would we expect the effect of the gravity, on the photon in the clock, to have the same result?

Again though, I'm not sure how a "slower rate of time" can be deduced from a slower ticking clock.

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#

cheers, I'll check those out.

 

[petm1]

A clock counts it own duration as a clock.

 

[mangaroosh]

Time refers to changes in physical configurations.

I would be more inclined to say that the concept of time comes from our observation of the changes in physical configurations; again, a memory of a previous state and the projection of a future state give rise to the ideas of "past" and "future"; but these are just mental constructs. A physical confguration only ever exists "now", in the present.

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.

Again, I can't see from that how a clock measures the physical property of time. Clocks are useful for helping us to index the evolution of the universe, but the experience of all observers - I think it is reasonably safe to say - is that the universe only ever exists in the present. No observation of the "past" state of the universe has ever been made, and again, I think it is fairly safe to say, never will be made. The same can be said of "the future".

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

Why is the changing, evolution of configurations anything more than the chaging, evolution of configurations? How is it also a secondary physical property?

The same point applies again, however, about the present state of a configuration being the only observable one.

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

But the detection of the discrete oscillations is all that is measured, not some secondary physical property.

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.

Time is supposed to be a dimension, however, and it is infinitely more illusive than the three spatial dimensions. The changing configurations of various ponderable objects are only ever observable in the present; there is no evidence to suggest that "past" states continue to exist, or that "future" states exist before they materialise in the present.

It is probably worth pointing out that accepting time as a concept, or as a system of measurement (as opposed to the object of measurement) doesn't mean that we have to do away with the practical uses we have for time; we can continue to use clocks to index the evolution of the universe, but we can recognise that "time" is not a physical property of that universe.

 

[mangaroosh]

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.

This is very true, but again, none of those processes actually measure the physical property called "time"; our concept of time comes directly from our observations of such phenomena, and our time keeping devices evolved from the simplistic method of measuring how much daylight we had and breaking it into more manageable chunks so that we could communicate about things that needed to be done.

If we consider a sundial, then we can see that the sun is physical, the Earth is physical, the sundial is physical, but where does the physical property called "time" come into the equation?

 

[russ_watters]

Ok, this thread has gone on long enough. From not accepting basic physics concepts, now we're on to anti-relativity crackpottery.

mangaroosh, it is ok to be confused and want to learn, but your purpose here seems to be to promote anti-relativity crackpottery, not to try to actually learn what is known about time. That is not acceptable. Please do not start any more threads like this.