What is Time? | General Physics Discussion

[kateman]

first off, sorry if this is in the wrong place. iam not sure exactly were this topic should be put so i posted it in general physics

anyway, i was interested to know what time is. i mean its used in formulas i know, so obviously its something, but does it really exist.

i was originally thinking that was just some measurement that we pulled out of our hats because the formula's are wrong without it. but then i think about how its possible to go back in time, which makes me think that there has to be something more to it than just some made up measurement.

i would appreciate all your thoughts on time in general :)

 

[cesiumfrog]

I liked your train of thought better before you assumed time-travel was possible.

 

[DaveC426913]

I liked your train of thought better before you assumed time-travel was possible.

We can't really know that traveling backwards in time is impossible until we know what time is.

 

[kateman]

I liked your train of thought better before you assumed time-travel was possible

well more assuming its probable, but not for living beings. more or less for particles instead.


i don't mind opinions but what do we actually know about time?

 

[wysard]

Well...

It relates to space, velocity and mass in a meaningfull way.

Time is not like a switch, that you can turn on or off, or a variable you can zero out. Think of it this way, if an object has mass and any velocity relative to any other arbitrary object (including an external observer) then time is a function of the observation.

Just like a Scottie dog you measure the length of while he/she/it is taking it's leisure at a local fire hydrant gives you a number the same puppy doing the same thing at the same "time" if measured from orbit the puppy is just about a doggy hair shorter not due to a rounding error, but by nature of the fact that in orbit the measurer is moving at several thousand miles per hour and there is a tiny, but predictable, difference.

Read "Einstien's Dreams" and see if you can pick which dream reflects the real world.

 

[kateman]

cheers

 

[W3pcq]

Is it safe to say that time is the speed of light which varies depending on the strength of the gravitational field in GR?

 

[russ_watters]

Um, no.

 

[W3pcq]

Thats what confuses me. Time is said to run slower near strong gravtational fields. Light redshifts to adjust distance vrs. time changes to keep c constant and as a result clocks in different places run at different rates. Are you saying that the speed of light does not slow when it is redshifted, but the atomic clock does slow.

 

[Denton]

Are you saying that the speed of light does not slow when it is redshifted, but the atomic clock does slow.

Im pretty sure that's a yeah. C is constant.

 

[pervect]

Thats what confuses me. Time is said to run slower near strong gravtational fields. Light redshifts to adjust distance vrs. time changes to keep c constant and as a result clocks in different places run at different rates. Are you saying that the speed of light does not slow when it is redshifted, but the atomic clock does slow.

The speed of light is always equal to 'c' if you use local clocks and local rulers.

While the modern approach to GR always uses local clocks and rulers to measure speeds, sometimes you'll see people doing things differently, especially if they aren't doing GR, but something more down-to-earth.

There are at least two different basic sorts of time, but people don't always carefully distinguish between the two sorts. One sort of time is "coordinate time". The other sort of time is "proper time", which is the sort of time a clock measures. So when one asks "what is time", one could mean several things, because the term is used ambiguously. In addition there can be several different coordinate systems used, there are several sorts of coordinate time possible. (Example: TCB vs TCG - this gets technical quicly, but if you are interested in the details, see for example http://aa.usno.navy.mil/publications/docs/Circular_179.pdf).

"Gravitational time dilation", which depends on gravitational potential rather than the gravitational field, really describes the relationship between a certain type of coordinate time, and proper time.

 

[country boy]

...i was interested to know what time is. ... but then i think about how its possible to go back in time, which makes me think that there has to be something more to it than just some made up measurement.

The concept of time comes from our awareness of change. It is most obvious that objects around us are in flux, and our inherent sense of this is necessary for interacting with the world. We have refined our notion of time by finding (or fabricating) objects that undergo cyclical change, and we measure the passage of time by counting cycles. We measure time by measuring change.

From this it is clear that traveling "forward" in time is no problem, because our concept of change (cause --> effect) is a forward progression. But traveling backwards is problematic: What does changing backwards mean if all change is forward by definition?

Next we invent Thermodynamics.

 

[InfinateLoop]

The concept of time comes from our awareness of change. It is most obvious that objects around us are in flux, and our inherent sense of this is necessary for interacting with the world. We have refined our notion of time by finding (or fabricating) objects that undergo cyclical change, and we measure the passage of time by counting cycles. We measure time by measuring change.

From this it is clear that traveling "forward" in time is no problem, because our concept of change (cause --> effect) is a forward progression. But traveling backwards is problematic: What does changing backwards mean if all change is forward by definition?

Next we invent Thermodynamics.

I'd like to learn more about the culture of time in the manner this post was establishing.

 

[lightarrow]

I'd like to learn more about the culture of time in the manner this post was establishing.

I would say time could be defined as a series of irreversible processes, all of which as more similar to the others as possible.

 

[dbecker215]

To grasp a little bit more on what we know, so far, about time you must first establish which time you are talking about. There are two main views that seem to try to explain time; the philosophical that says that time is nothing more than an illusion of consciousness, and the scientific that says that time is measurable. All other descriptions tend to branch from these two ideas.

This forum isn't designed for the philosophical so I won't speak much about it but to sum it up they say that time is an illusion of motion, progression, cause and effect, etc. Everbody has differing reasons for why it's an illusion, but agree that it's an illusion of some sort. Basically time from this perspective boils down to nothing more than being the conscious observation of the relationship between two or more objects in repititious motion: ie the Earth around the sun = a year.

The scientific approach is more detailed and complicated but still does not give a concrete theory of time. (I would suggest further reading "About Time" by Paul Davies. It simplifies a lot of the complexity of the scientific time.) One of the big questions about time that must be figured out is whether time can be measured in static blocks, because we can't currently measure units time to a high enough accuracy this question is left to theorists and tends to border science and philosophy. Some believe that time is the sequence or transition of these static events.

The idea of time being used for calculations was embedded by Newton. But he believed in a constant time, one that he derived from his theological beliefs. This was an assumption that lasted until Einstein who threw relativity at the concept of time into the picture. This where the possibility of time travel comes into play. This is another area where science meets philosophy b/c it raises the paradoxical question of if you go back in time can you kill your grandfather and therefore not exist? I personally don't think so but there is yet to be scientific or mathematical evidence for why you can't.

 

[jcsd]

Time is about ten past nine where I am :D

 

[dhiraje2002]

We can't really know that traveling backwards in time is impossible until we know what time is.

As far as i know, "time" is the extra dimension that we need to add in our usual 3-D geometry to explain gravity in relativity. As i think, the backward travel in time is prohibited by the sets of lorentz transformation (its improper transformation-that makes v>1, weinberg approach (c=1), since proper homogeneous lorentz transformations always have v< 1). The main problem i find is this-How do we relate the time we use in daily life with that in General Relativity theory?

 

[dhiraje2002]

the man who sees the world in his 50s the same when he was in 20s has wasted his 30 years.

 

[Shooting Star]

We can't really know that traveling backwards in time is impossible until we know what time is.

Why not? We may hit upon it by accident. Do we have to understand what goes on in our bodies to live?

well more assuming its probable, but not for living beings. more or less for particles instead.

Why so?

I've heard many people say that time dilation is valid for atomic time, not for biological time, whatever that means. They simply cannot accept the fact that humans may age at different rates.

 

[Troels]

The British Science writer Michael Hanlon has made som very interesting comments on this question in his book "10 questions science can't answer (yet)"

 

[DLuckyE]

The idea of time being used for calculations was embedded by Newton. But he believed in a constant time, one that he derived from his theological beliefs. This was an assumption that lasted until Einstein who threw relativity at the concept of time into the picture. This where the possibility of time travel comes into play. This is another area where science meets philosophy b/c it raises the paradoxical question of if you go back in time can you kill your grandfather and therefore not exist? I personally don't think so but there is yet to be scientific or mathematical evidence for why you can't.

If time is nothing more than a 'layer' of the current spatial state of the universe then it wouldn't matter if traveling back in time was possible. Since it'd be just like rewinding a tape, everything goes backward, and same things happens again going forward. Actually then nobody would even notice time was going backwards...

For all we know time goes back & forth all the time ;)

But then again, if that was true, time dilation wouldn't make any sense...

Just ignore me, I'm rambling, but still to me a 'static' universe seems to be the most logical even though that would mean free will is just something our mind makes us believe we have ;)

 

[nanoWatt]

One of the big questions about time that must be figured out is whether time can be measured in static blocks, because we can't currently measure units time to a high enough accuracy this question is left to theorists and tends to border science and philosophy. Some believe that time is the sequence or transition of these static events.

Isn't this the same as trying to quantize time?

 

[Kurdt]

Philosophically a theory of time is unresolved. Most philosophers agree that time does exist but they haven't yet been able to determine what it exactly is. Most of what is known comes from the use of time in well established physical theories such as QM and GR and the nature of time is explored by looking at what these theories demand of the time variable. Perhaps it would suffice to say that time is a set of relations between events.

Here is a good article you may like to read.

http://www.iep.utm.edu/t/time.htm#H3

 

[dbecker215]

That was a good article, thanx. A lot of philosophy for one night though...

It's easy to see why time is still unresolved when so many won't even come to a common ground when trying to discuss possibilities. Philosophy's endless circles are daunting.

 

[Shooting Star]

Clever old A.E. cut through the gordian knot by saying, "Time is what you measure with a clock...". It turned to be quite fruitful, too.

 

[lightarrow]

Clever old A.E. cut through the gordian knot by saying, "Time is what you measure with a clock...". It turned to be quite fruitful, too.

So, I could use any clock, for example a sand glass? I don't think that's a good way to answer (infact he knew that was not an answer).

 

[Jonathan Scott]

So, I could use any clock, for example a sand glass? I don't think that's a good way to answer (infact he knew that was not an answer).

A sand glass isn't a self-contained clock on its own, but a sand glass combined with a suitable planet to complete the mechanism would work as a clock for that purpose.

 

[Kurdt]

I think what Einstein omitted from that quote was the word ideal or accurate.

 

[lightarrow]

A sand glass isn't a self-contained clock on its own, but a sand glass combined with a suitable planet to complete the mechanism would work as a clock for that purpose.

Absolutely

 

[lightarrow]

I think what Einstein omitted from that quote was the word ideal or accurate.

And how do you establish if a clock is more accurate than another if you still have to define what time is?

 

[Shooting Star]

So, I could use any clock, for example a sand glass? I don't think that's a good way to answer (infact he knew that was not an answer).

Even I know that's not the answer, taken literally. But if you think about it, since we don't know much about time, this utilitarian approach is the best perhaps.

A sand glass isn't a self-contained clock on its own, but a sand glass combined with a suitable planet to complete the mechanism would work as a clock for that purpose.

Sand glasses or pendulum clocks or anything which works on gravity is inadmissible as a clock in GR.

After many years of research, we have come to the consensus that atomic (or maybe subatomic) vibrations keep the best time. That's why we use those phenomena to measure time; or has it simply become a matter of definition that those keep the right time?

 

[InfinateLoop]

I thought time was successive intervals?
How does that not sum everything up?
Is it because of GR?

I love this topic!

 

[Rade]

... Most philosophers agree that time does exist but they haven't yet been able to determine what it exactly is...

According to Aristotle, Physica, Book VI, he claims the following about time:

...time is (number of motion of a physical thing that is counted) that is intermediate between indivisible moments...​

Suppose three moments, A, B, C and a physical thing {T} in motion in "space" in relation to them. From my understanding of Aristotle, the moments are indivisible and have neither motion nor rest, thus they are outside of time, yet they are the limits of past and future time, such that:

the past ~ ---- time 1 ----> |A|--- time 2 -{T}--> |B| ------- time 3 ---{T}----> |C| ---- ~ the future​

In the above diagram consider the relationship of time 2 & 3 to moment |B|. There exists infinite numbers of motions of the thing {T} in space that can be counted in time #2 leading to |B| (the past) and in time #3 leading away from |B| (the future) and these motions that can be counted are "time". Thus, moment |B| is both a limit of past time (time#2) and future time (time #3), and while it can be said that |B| is in this sense, as a limit, a part of time, there is no time within the moment|B|, since time is always divisible while |B|, because it is a moment, is always indivisible. And see that while time #2 is the past of moment |B|yet also is time#2 within the future of moment |A|.

Thus, as the concepts "odd" and "even" are within number, so the concepts "past" and "present" and "future" (moments) are within any time. So, time will never fail to exist if there is motion of a thing in space that exists, for where there is motion, there is always a beginning to time.

Now, Aristotle also holds "space" = "that which is intermediate between existents". Thus, it is possible to suggest that Aristotle would claim that;

"space-time" = that which is intermediate between moments of existents.​

Applying the above diagram, suppose two existents {E1} and {E2}, and they are in motion at two different moments |A'| and |B'| in space-time:

the past ~ ---- time 1 ----> |A|--- time 2 -{E1|A'|}--> |B| ------- time 3 ---{E2|B'|}----> |C| ---- ~ the future​

So, here we see that "space-time" is that which is intermediate between {E1|A'|} and {E2|B'|} as these two existents relate to the three moments A, B, C and their respective concepts of past and future time.

This is my understanding of what Aristotle may claim about the philosophic question..."what is time" as relates to physical things that exist that follow the laws of nature.

 

[pervect]

OK, given that we are now talking about Aristotle, and given the title of the topic, I think it's time (probably past time) that I move this to the philosophy forum.

 

[Shooting Star]

Thank you, Pervect.

I was about to suggest to a mentor to lock the thread, but you, being the expert, have taken better care of it. A bit rough on Aristotle, who, by the standards of his day and two millennia after, is considered to be one of the greatest minds in recorded history.

 

[Kurdt]

I think in summary then Rade, Aristotle would consider time as the interval between events. Its interesting his use of the term indivisible moments. Does this imply Aristotle thought there was a minimum 'duration' (i.e. interval between events) in which one could not have any smaller duration. In essence a quantum theory of time.

 

[ripcurl1016]

Time is not the changing of events but the change between events to one another. An event itself cannot change, an event is definite. Now if an event is definite it has certain characteristics about it that cannot change either. An event cannot be past, present, and future at the same time. Event A starts out as a future event, it then becomes present, and goes on to past. This is how we know time, from future to past. So A "has been" future, "is" present, and "will be" past. "Has been" only being distinguished from "is" by being existent in the past, and "will be" is only distinguished by being existent in the future. So if the future is the presents past, and the past is the presents future, how much sense does time being real make?

 

[epenguin]

I say that time is what clocks measure, and that is all it is. In other words time is measured by events. There is no room in physics or philosophy for anything else. There is no time thing which 'flows' and oh you can measure it with clocks.

Classically all proper clocks would be thought of as agreeing regardless of time, space and movement, in relativity that is changed in an understood way, but different true clocks traveling together have to agree.

 

[DaveC426913]

...different true clocks traveling together have to agree.

Well, they agree because they're calibrated to agree.

 

[epenguin]

The best ones we call standard have to agree with each other surely? To within a certain amount then we say our best clocks have measure time that accurately. With those we callibrate other less satisfactory ones. It is not that any clock is as good as any other. Once the rotation of the Earth was our clock, so it would then have made no sense to say the Earth was slowing, for that we must have a clock we know is better, it is not arbitrary.

 

[lightarrow]

I say that time is what clocks measure, and that is all it is. In other words time is measured by events. There is no room in physics or philosophy for anything else.

Very good. Now you have to define the word "clock".

 

[Shooting Star]

I say that time is what clocks measure, and that is all it is.

Very good. Now you have to define the word "clock".

He has already defined it in his above post...

 

[D H]

So if we take the famous thought experiment of a man on a train traveling near the speed of light and a man on the waiting bay for the train to pass. The man on the bay (lets call him B) saw that the man on the train (lets call him man A)'s time slowed down, i.e. B saw A's time slow down, but also, A saw B's time slow down.
1) Whose right, they both can't be right?

They are both right.

2) Is there some field surrounding the train where time is allowed to slow down or what?

There is no field. That each of the two men see the other man's clock running slower than his own is purely a function of the non-zero relative velocity between the two men.

 

[lightarrow]

He has already defined it in his above post...

Please?

 

[Shooting Star]

I say that time is what clocks measure, and that is all it is.

Very good. Now you have to define the word "clock".

He has already defined it in his above post...

Please?

In effect, he has already defined clock as that which measures time.

This is a circular definition. However, irrespective of how many links you introduce in the chain, ultimately it will be just a big circle. Isn't that why we are still discussing this, on record, approximately over two and a half millennia?

 

[Greddy]

Hi, I am kinda new here. Me and some other dude had a discussion about if Time did really exist.

Instead of making a new thread about time, I though crashing/hijacking this thread instead.

Edit by Evo: Let's keep this thread to the discussion going on here.

 

[Shooting Star]

A tremendous effort, but could you please reduce the length of your essay the next time? Something happens to me after a minute...:zzz:

(Seriously, it's too big, with all those links.)

 

[Greddy]

Hehe, I didn't realized that I was making an long "essay", so I apologize
I made the short resumè of the discussion we had, so you could skip those links.
but then again, It wouldn't have made sense if I leaved out those links.
(Links are there so you wouldn't have to search for relevant post among the troll posts)

 

[epenguin]

In effect, he has already defined clock as that which measures time.

This is a circular definition. However, irrespective of how many links you introduce in the chain, ultimately it will be just a big circle. Isn't that why we are still discussing this, on record, approximately over two and a half millennia?

I don't think it is, at least it is trying not to be. I think we can accept without further analysis that there are observed regularities in nature, and of events, and then of events 'happening together', resisting the temptation to say 'at the same time.' We can count. We can say 'every 365 days' - idea of duration not needed but only of events like a shadow passing a mark - 'the stars go back to the same position'. We do not need the events to be repetitive, in fact the best clock for thinking about it is the Tait clock, which is successive relative positions of some stars which are in free (not being accelearated by attractions) motion. Work it out, that is not circular. What we need is:
more than one such clock and to find that they agree;
a reason to believe that they are simple and understood. For the Tait clock the reason would be that each star is very far from the influence of other bodies.

These things are observable. Time is not a thing, but just what you invent to relate these observations; you could just relate them to each other. You invent 'seconds' as a convenience. It is like you might live buying and selling real things, to do this you can have dollars $ or other currency units in a bank which you never see. Currency units do not need to be real. You are so used to talking about them you may think of them as real. You might prefer the most stable currency, the one with most constant relation with the most important simple desirables. In fact ideally you would make such a desirable your unit of currency.

Then in physics you observe things which are not as regular as your clock, and you then say they are complex and need theories to be invented to explain why and exactly how they are not regular. Equations in tems of t which you think of as real like you think of $ as real, but you are just relating phenomena to your clocks which you think are simple.

 

[Rade]

I think in summary then Rade, Aristotle would consider time as the interval between events.

No, this is not my understanding. Time is an interval between "moments" for Aristotle. An "event" is not a moment, an event is what occurs within the interval called time--a coming to be or a doing away with.

Its interesting his use of the term indivisible moments. Does this imply Aristotle thought there was a minimum 'duration' (i.e. interval between events) in which one could not have any smaller duration. In essence a quantum theory of time.

The term" indivisible moment" is my term, Aristotle defined time as being intermediate between moments, then defined moments as being indivisible, so he should have no problem with the term "indivisible moment" since the opposite for him would be a logical contradiction.

Your second question is interesting--here is what I found.

In quantum theory, there is no time smaller than Planck Time, so what would Aristotle say about this ? I think we find the answer in Physica, Book III, Chapter 6. Recall that for Aristotle time is not a magnitude, but "time is a kind of number" (Book IV, Chapter 11), it is "what is counted" between moments, and in Chapter 12 he states "since time is number". So, it it safe to claim that Aristotle holds that time is number, a kind of number related to what is counted about motion between moments.

Now, in Book III, Chapter 6 Aristotle discusses the infinite. And here he claims this..."it is natural too to suppose that in "number" (quote added) there is a limit in the direction of the minimum". Thus, since time is number for Aristotle, I suggest that he would hold that time also has a limit in the direction of the minimum, and that this concept of the limit of time in the direction of the minimum is what we today call Planck Time. Furthermore, Aristotle makes this claim..."hence number must stop at the indivisible". So, here we see that Aristotle would claim also that "time" must stop at the indivisible, and recall that the indivisibles between time are the moments. Thus I conclude that Aristotle would agree that there exists a smallest time duration between any two moments--what we call today Planck Time--it would appear that Aristotle is the great..great grandfather of quantum theory as relates to time as a number of what is counted in relation to motion.