[londounkm (OP)]

Please excuse me if I am asking a stupid question, but I am trying to understand exactly what time is. When I think about time I immediately link it to light. Astronomers often tell us that they are able to take pictures of the universe billions of years in the past and that this is due to the amount of time it takes for light to travel. As I understand it, if I am looking up into the night sky what I am seeing is actually the past, in the case of the sun light I am seeing approximately 8 minutes into the past where as looking at the stars I could be looking many millions of years into the past. This is where I start to get boxed in with my own limited knowledge and perhaps some of you more knowledgeable people could enlighten me. If there was no light would time cease to exist altogether? I appreciate anyone taking the time to help me try and understand this.

Graham

[Joe L. Ogan]

Time is in three parts:  Future-which never arrives and exists only in ones' imagination.  Present-which exists only for an infinitesimal split second and Past which goes on forever in ones' memory but never returns except as mistakes which we make and are inclined to repeat.  Thanks for comments.  Joe L. Ogan

[Bill S]

Please excuse me if I am asking a stupid question, but I am trying to understand exactly what time is.

Where understanding time is concerned, I think there is no such thing as a stupid question. St Augustine shared our problem and expressed it well: "What then is time? If no one asks me, I know; if I wish to explain it to someone who asks, I know not".

[raghusesha]

Here is what my limited knowledge says:
There was a similar question on sound in one of the naked scientist shows and the question was that if a tree falls in a forest and no one is around to hear the sound of tree fall, can we conclude that the tree did not make a sound while falling? Answer is obvious.
Light consists of particle traveling at a fixed speed (thanks to relativity theory).  If there is nothing happening on stars to produce light, the stars still exist in time and time does pass.
From my very little knowledge of relativity theory, speed of light is a upper bound for the velocity of any particle. Electromagnetic waves travel at this speed.  So, if none of these electromagnetic waves existed, will time exist?  My guess is yes, it will.  Only that a small set of people on a planet like ours may not be able to "see" the rest of the universe.

[Soul Surfer]

A difficult question because it depends a bit precisely what the questioner means.  If you just mean would time cease to exist if you could not see anything and could only be aware of things by touching them.  groping around an drecognising things by feel requires a sense of time so clearly time exists.

However most of the interactions of which we are aware of are mediated by electromagnetic radiation and light is one particular form of electromagnetic radiation and without this no electromagnetic interactions could happen.  This would still leave other sorts of interactions notably gravity by which changes in the environment could be sensed so on the whole Time appears to be a fundamental part of the sort of universe we can understand.

[Murchie85]

Personally I think time is poorly understood simply because its a man made concept and not an actual physical phenomena that can  be captured in its entirety or leave a mark on the universe.

It is relative, it is changing for different observers, it is never agreed upon by all parties and it certainly doesn't have a quantity that can be defined with universal precision (atomic clocks using cesium atoms are not the last say) and may never be.

Time can be considered as a measurement of passing events it is limited only by the measurer and time its self leaves no trace in the physical world. Its not a piece of string in which people consider a time line that can be traced backwards or forwards like a video cassette.

Time is however important, it is connected with entropy in that we know we are going forward and not backwards in time. It helps us compare certain events with other events but in terms of the light, it does not require this to exist although light does require time to exist.

Adam

[Geezer]

not an actual physical phenomena that can  be captured in its entirety or leave a mark on the universe.

However, everything in the Universe is controlled by time.

[Murchie85]

I disagree, time in my opinion is not a real entity, time can not control anything it does not "steer a series of events" it simply is a result we obtain from measuring the passing intervals of certain events for example.

[Bill S]

it simply is a result we obtain from measuring the passing intervals of certain events

So far I agree in general with what you have said about time, but it is interesting to see how assumptions creep in.  E.g. the assumption that "intervals of certain events" are passing, rather than that the events are stationary, and we are doing the passing.

Nit picking? Perhaps; but whole books have been written about this.

[londounkm (OP)]

Firstly, thank you all very much for your thoughts and ideas it is much appreciated.

Time is in three parts:  Future-which never arrives and exists only in ones' imagination.  Present-which exists only for an infinitesimal split second and Past which goes on forever in ones' memory but never returns except as mistakes which we make and are inclined to repeat.  Thanks for comments.  Joe L. Ogan

What a lovely and concise description of time. I read in the paper this morning that the first observed galaxy to appear out of the big bangs white soup has been imaged by Hubble and confirmed by the VLT at appx 600 million years P.B.B(Post Big Bang).

Where understanding time is concerned, I think there is no such thing as a stupid question. St Augustine shared our problem and expressed it well: "What then is time? If no one asks me, I know; if I wish to explain it to someone who asks, I know not".

Haha brilliant, St Augustine has almost exactly described how I feel about time. Only when I really applied thought to time did I realise how little I understood it. It is certain to me that individualism and separation in society has all but destroyed the long conversations that used to take place on issues such as this. Websites like the Naked Science Forum are a blessing to those who wish to expand there understanding and knowledge of the things around them.

A difficult question because it depends a bit precisely what the questioner means.  If you just mean would time cease to exist if you could not see anything and could only be aware of things by touching them.  groping around an drecognising things by feel requires a sense of time so clearly time exists.

However most of the interactions of which we are aware of are mediated by electromagnetic radiation and light is one particular form of electromagnetic radiation and without this no electromagnetic interactions could happen.  This would still leave other sorts of interactions notably gravity by which changes in the environment could be sensed so on the whole Time appears to be a fundamental part of the sort of universe we can understand.

In all honestly I believed that light would exist even if light did not, because as you quite rightly pointed out if the light were "turned off" in the Universe we would still be able to grope around touching and feeling. In asking the question I guess I was trying to understand the nature of time. I understand that time is not a physical thing, such as a particle but rather a measurement applied to universe by an observer. When I read that an image was taken by Hubble appx 600 millions years after the big bang, what they really mean by that is that an image was taken over a distance of X billions of miles relative to our position in space and due to the time take for the light to reach us what are seeing is actually X number of years in the past. I also understand that whether and observer is observing or not time still exists as it is simply the measurement of before, now and next.

I think I am going to stop there for now and think some more on this topic. I really appreciate all your comments and suggestions and once again would like to say thank you very much.

[Bill S]

A few years ago, when I was trying to get my head round the idea of time travel, I found I was going round in circles.  My wife suggested I should write every thing down.  The following is a small extract, which might be relevant.

    One of the much discussed aspects of time is that of its apparent motion.  We talk of an arrow of time which we observe as progressing steadily from future to past, but does time actually progress in any direction.  We do not talk of an arrow of space.  We perceive space as static, and ourselves as moving through it.  Perhaps our perception of this would be different if our only experience was of being aboard a space craft, many light years away from our nearest celestial body; as we travelled steadily through empty space, for us, the only static object would be the space craft.  As long as our craft maintained a steady direction, we would, with time, observe the Universe drifting slowly past us.  Perhaps we would reason that space had a directionality of movement - an arrow of space.  The fact is that our experience, as far as space is concerned, is that we have many points of reference, the things about us which we have always regarded as static, so it is natural that we see ourselves as doing the moving.  Although we may consciously accept that these things, collectively, are in relative motion, for example, the Earth is moving round the sun, so everything on the Earth is in motion; our senses tell us that the things around us are stationary. We can see that individual objects are stationary relative to other objects.  Intuitively we know that when we move from room to room in our house, we are doing the moving, not the house.  When we get in the car and drive to town we know it is the car that is moving along the road, not the other way round.  Reason, as well as intuition, tells us that this is right,  because if it were space that was moving, our roads would have to move in many directions and at a range of different speeds at the same time in order to accommodate the observed flow of traffic.  Such is not the case with time; we have no static reference point, therefore we are able to perceive ourselves as static, and time as moving.  Relativity says that in the case of space, any point from which an observation is made has as much right as any other to be regarded as stationary.  If the same is true of time, it is natural that we would regard ourselves is stationary, and time as flowing.  This is reinforced by our perception of time flowing in exactly the same way past everyone and everything around us.  It makes equally sound sense, though, to regard time as a static entity, and to see ourselves as moving through time.  Unlike space, however, we all seem constrained to move through time in the same direction.

[Geezer]

I disagree, time in my opinion is not a real entity, time can not control anything it does not "steer a series of events" it simply is a result we obtain from measuring the passing intervals of certain events for example.

You are quite entitled to your opinion of course, but time actually does control all physical and chemical processes, and it has been demonstrated repeatedly that time is not a universal constant. That means that, relative to each other, physical and chemical processes may vary depending on their particular time frames.

[Bill S]

That means that, relative to each other, physical and chemical processes may vary depending on their particular time frames.

Could that not be interpreted a little differently?  E.g. physical and chemical processes may appear to vary, depending on the F of R from which they are observed. Time is one of the things that seems to vary. 

Let's say I drop a small piece of calcite into a solution of HCl. My clock tells me how long it takes to dissolve.  An observer who perceives me to be in motion relative to her will observe a different time for the reaction.  Does this not mean that the reaction is independent of any objective measurement of time?

[imatfaal]

Bill - surely we have formulas which will allow all different frames of reference to come to the same conclusion. 

That is to say that every observer could, if he knew his own velocity relative to the chemical reaction frame, calculate the perceived reaction time of any other observer with a known relative velocity.  So the reaction time is variable - but definitely not subjective.  The practical definition of objective is that independent observers will agree, providing these observers know their velocity relative to the reaction frame of reference, they will all agree on the timing.

[Bill S]

That is to say that every observer could, if he knew his own velocity relative to the chemical reaction frame, calculate the perceived reaction time of any other observer with a known relative velocity.

I have no problem with every observer being able to calculate the time in every other observer's F of R.  My point is that, according to S R, every observer has the right to consider her/his time to be as correct as any other's.  There is no objective time, unless we say that the time measured in the F of R in which the experiment is conducted is given special status.
Does S R allow that? 

[imatfaal]

Not sure I agree with that - SR allows all time to be objective, none are given special status, every reference frame can be transformed to another - if we all agree on something (ie the proper time in somesuch reference frame) then surely it is objective. 

[Geezer]

I always like to come back to the Hafele-Keating experiment because it does seem to confirm that there is no absolute time, and that processes are indeed controlled by local time (assuming I understand it correctly of course).

The multiple observers watching each other discussions (which are, I think, impossible to test anyway) always give me an 'orrible sore head  []

[simplified]

A mass radiates gravitation for  receiving local time. Great gravitation streams   complicate reception of local time a little. A photon does not radiate gravitation therefore does not receive local time. However unknown time operates external movement of a photon. []

[Bill S]

if we all agree on something (ie the proper time in somesuch reference frame) then surely it is objective.

If we all agree about the "proper time" in a given F of R, then, surely, we have also to agree on the "proper time" in all the other frames, and since these are all different, but all right, we have to conclude that time is subjective, in that it depends on the F of R.

[imatfaal]

subjective
...
Formed, as in opinions, based upon subjective feelings or intuition, not upon observation or reasoning, which can be influenced by preconception; coming more from within the observer rather than from observations of the external environment..."

objective 
Of or relating to a material object, actual existence or reality.
Not influenced by the emotions or prejudices.
Based on observed facts."

As all observers will agree about proper time - and it has nothing to do with ideas and bias within the scientist but rather is based on observed facts; I would definitely say it was objective.  It may be strictly reliant on the FoR, but it doesn't depend on the opinion of the observer, it relies on a well defined fact about the observer.  Very few objective facts pop straight out from measurement - they need processing, not to massage the facts, but to bring all the different observations into a comparable form.