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david345, Colour of course. When I want to know the time, I look at the color of my clock, and I know what time it is. Don't you? This is the most sensible answer I can give to your questions.

I assume you want to say I don't. Feel free to say what is wrong. Wikipedia on logical tautologies: Wikipedia on Definitions:

Stuff? Can we detect it? How? Your belief without any argument doesn't bother me. Yes, I did. Physicists also work with all kind of idealisations, like black bodies and isolated systems. So why shouldn't I? But more to the point is: would you accept a definition which contains words you do not know? Give another one, that does not fit the above. Yes, I noted that. I also noted that you give no arguments for it. There are two possibilities: a (real) definition in words, which is logically necessary circular (even if it is over a long chain of other words). Or you give an operational definition, e.g. you point to something ('this here, this is red'). That is methodologically fully OK, but it is exactly what you cannot do directly with time. You cannot point to time: but you can point to change, e.g. a clock. But it hardly makes for a description or definition of 'change'. Of course you can say 'Change is change in time', but then you have used the word change itself. I'll give it another try: time is the most general abstraction of all changes. Now I am also not completely satisfied with this, but at least it shows that you cannot turn it around. How can individual changes be abstractions of something that is more general? Right. You show how physicists use the Lagrangian to solve problems of mechanics. But I asked for a definition, first of the Lagrangian, and then of kinetic energy and potential energy. Obviously you immediately refrain from defining. 'Defining' is something else than knowing how to follow a recipe. Let me help you, with the dimension of energy: [J] = [kg . m²/s²] What does the kg stand for? Sorry, you are just wrong. Take the definition 'a bachelor is a man who never married'. Now I substitute the meaning of 'bachelor' in the definition. I get 'a man who never married is a man who never married'. Isn't that a tautology? (Maybe you are confusing tautology with one of its subclasses, the propositional tautology (A or not A).) Except that you did not follow the rules. Because it is an abstraction. You are like the middle age philosophers that discussed if 'whiteness', or 'horseness' existed, or even may be more real then a white plate or the horse standing in front of me. There is nothing to know about time except as a comparison between different processes. And of course the definition is pretty useless for a physicist. But the question of the thread is 'What is time?'. It is not 'Why all physics is completely wrong because they do not understand what time really is'. How do you know time is passing? Without you being there, breathing, your heart beating, and if you want to be precise, without a clock?

Yes, please. What is time.

OK, I will change it specially for you: A bachelor is a man who never has been married. OK? Can we go on, please?

Studiot, I did not say 'all unmarried men are bachelors'. I gave a definition of the concept bachelor. Given that definition, is there a possible world where I can meet a married bachelor? So if something is true in all possible worlds, then it is a tautology:

Ok. All definitions are expressed in words. Let's assume that a definition is not circular. This means I get at a concept for which there is no word in the dictionary, so which is not defined. But this contradicts the assumption that the dictionary is complete. Applied on physics this means that one of the following must be true: Physics uses concepts that cannot be expressed, because they cannot be defined (btw, this would include a mathematical dictionary of physics). Definitions of concepts are circular. See e.g. here: (Italics by me). The gist of the argument is that normally you stop defining when you use words that are unproblematic in the context.But in fundamental questions you are problematising, you want exact definitions: but then you will inevitably run in circles from some point. The point I am making is that change is more fundamental as reference than time, because we can observe change, but we cannot see time, except through changes. Really? That is just because you stopped. please define: kinetic energy and potential energy to begin with. For the moment I leave you path, the integral over time, and stationary. No, I am confusing nothing. Definitions are logical tautologies. If not, then there would be a possible world in which the definition does not apply. Say I define a bachelor as an unmarried man. Now, is the sentence "a bachelor is an unmarried man" a tautology or not? Given the definition of the concept 'bachelor': is it possible to imagine a world where you meet a married bachelor? If it really is a closed system, then I can say nothing about it. If I can say something about it, then I am, as observer, part of the system. The example you gave does not follow the rules. I have said that the only way we can determine time is by comparing with another process. So I introduced my clocks. Edit: typo

Oh yes, if you decide to stick to defining you will run in a circle, how many links your definitional chain might have. Of course you can stop defining at some point ( ) and say 'this is immediately clear, what do I have to define further', but then it is still exactly that: you stopped defining.

Right, I did not. I challenged to find out yourself. But again: circularity can be via more than 2 concepts. Remember, I said the dictionary is complete. No it isn't. The problem is that I can't show it just for you. So do it: give a complete definition of all concepts of Lagrangian mechanics. You will see that at some moment you drive in circles. Of course I know what a tautology is: it is a proposition that is true in all possible worlds. Definitions always are tautological. That is a fact that every logician can confirm for you. Yes, and I said how you should do it. With a clock. Say the clock ticks exactly at the beginning of the trajectory, and at the end. Now I take a second clock that ticks one time more, i.e. it ticks together with the ticks of the first clock, but also one tick in the middle. Then I notice that the man in the box is exactly at the half of the trajectory. Etc. Where is your problem? OK, then it is the unusual language that brought me on wrong track. But if I look up the Elements, I do not find this as definition of a line, nor of a point. This is what I find. Now do you see the problem? Now you must define 'breadthless', 'length', and 'part'. So let's try this with 'part'. If something can be divided, it has parts. A point has no parts (per defintion). So a point cannot be divided. Now what is the definition of divided? If something exists of parts, it can be divided. A point cannot be divided. So a point has no parts. How do I get out of this? You must not confuse 'defining' with 'immediate clarity'. If you define a concept, you do it with other concepts. You cannot get out of this circle, except by pointing to something observable. But 'pointing to something observable' is not a definition. If one defines the length of a meter, one compares it with the length of something else. But that is not defining what a length is, it is defining what a meter is (same with the elfleg).

Yes, I have. With the dictionary. You just did not notice it. Just one note of possible misunderstanding: the circle might be composed of more than 2 elements. No, that is not the reason. The reason I stick to Newton is that it is an example of a (simple) theory that perfectly works. So it shows that circularity of concepts does not mean that the theory is wrong, or incomplete or something like that. I am not deep enough in Lagrangian physics (and even less in formal quantum mechanics) to argue in detail about the circularity of their concepts, but you can easily try it yourself: ask for every new concept you introduce what its definition is. You will see that at some place you end where you began, or get at a simple direct circularity as with Newtonian physics. Yes, of course I did! Otherwise it doesn't work! It is absolute essential for Darwinian evolution that organisms live long enough to pass on their genes. If they don't their genes will not be passed on. It seems to me you do not know what a tautology is. Whatever the length iof your leg, your leg would always have a length of 1 elfleg. So in every possible world 'the leg of elfmotat is 1 elfleg long' is true. So it is a tautology. Or taking it the other way round, it is impossible for the length of your leg not to be 1 elfleg. Because you have not compared it to a regular process, like a periodic series of events. I have no idea what you want to say with the rest of your exposé. I see no problem by taking another regular process in which several events fit in one event of the other periodical series of events, and so dived my observing the walking man in finer parts. No, I did not. But I choose not to answer it, because it has more of a bonmot than of a serious definition. How would you mathematically define a line and a point?

OK, if you want it the hard way: every definition of a concept is circular. The length of specifically your leg is not a concept, but an empirical fact. It also does not change the fact that Newtonian physics is circular in the definitions of its basis concepts, and is just as well a perfect useful theory. Circularity is not a death verdict of a scientific theory! It is on one side proof of its consistency, and if some of the concepts can be connected to observations, in such a way that we can attach values to all elements in our theory (or at least to all other observable facts), it is a good scientific theory. This was your definition: Which organisms survive: those that pass on their genes. Which genes are passed on: of those organisms that survive. Well, tell me how long your leg is. In every logically possible world, in which the unit is of length is the length of your leg (whatever the length of your leg), the length of your leg is 1 elfleg. So your definition is tautological. Yes, you are right. I was a bit imprecise there. Let's say that [math]y=A(x)[/math], where [math]A[/math] means 'is abstraction of'. [math]x[/math] is an observable, so [math]y[/math] is the abstraction. Now for every individual change there is a projection on the time axis. But the opposite is not true: you cannot take the projection of a change on the time-axis, and find what the change was. The reason is that time has no independent existence.

yahya515, There are many ways to have scientific looks at religion. But theology does not belong to any of these: Study the function of religion in the life of individuals: that belongs to psychology Study the function of religion in society: that belongs to sociology Study the history of religion: that is history Study on what truth claims in religion are based: that is part of the philosophy of religion Evaluate the truth content of scientific claims in religion: they are mostly falsified by science (this includes physics, astronomy, biology, geology, history etc) Evaluate the ethics of religion: ethics is now mainly a study of the the basis of our ethical thinking, i.e. a part of philosophy. But none of these will lead to some conclusion which religion is right. More the contrary: it shows that most factual claims of religion are false, and that their ethics is based on superstition. So it is perfectly OK to have a religion section on a science forum. But you cannot specially expect a meaningful discussion about theology ('Who has right, Jesus or Mohamed?') in a science forum. If some people here are informed or trained in theology, then that will be accidentally, because it is somebody's hobby or something like that.

yahya515, I am wondering why you try to discuss Christian theological ideas on a forum that is dedicated to science. Why don't you discuss this on some forum where you can expect many theologians being around?

That is not a definition of the concept of length: it is defining a unit of length. Wow, yes, I did not notice. Therefore I said Newtonian physics, and not classical mechanics. So first: you do see that Newtonian physics is circular in in the definition of its fundamental concepts of mass and force, don't you? Why do you think Newton came with such a clumsy definition of mass as 'volume times density'? Could you help him out? And besides this circularity of definitions, does Newtonian physics work? I shall answer it for you: yes. You reference to Lagrangian mechanics is empty: the Lagrangian is defined in terms of energy. Can you define energy for me? (No, not a definition of its unit, a definition of the concept). Except that Darwin had no idea of genes, you just replaced a circular definition with another one, on microlevel. I would suggest you start thinking. What has the recursive definition of a number to do with the circularity of definitions? First: every definition is a tautology. Then, if I say that 'x = y', and add to it: but only 'y' is observable, where is your problem? Every time I want to measure 'time' I do in fact compare some change with another, standardised, change. As there are many kinds of changes, but only one concept of time, I think I am fully justified to say that time is an abstraction of change, and nothing more.

I did already multiple times. Don't you realise that every definition is somehow circular? That is just as true for scientific definitions. But in empirical science, some concepts are directly linked to observation. And then it is simply true: you can't observe time, you can only observe change (and objects). Take Newtonian mechanics as example: to define force, you need mass. But the other way round, to define mass, you need the concept of a force. Newton was well aware of this problem, and defined mass halfheartedly as 'volume times density', to get out of this definitional circle. Or another example, of Darwinian evolution: what organisms survive? Those that are the fittest. What organisms are the fittest? Those that survive. Some creationists bring this as a serious argument against evolution. But if you realise that you can attach observations to these concepts, then the problem evaporates. Just think about a complete dictionary: if all the words of a language are in it, then all definitions of them are circular, per definition. Maybe not so directly as with mass and force, or change and time, but it will be circular. Endless loop: see loop, endless Loop, endless: see endless loop

You were right from the beginning: But 'what is time' was exactly the original question.

A little? The question 'what is time' is metaphysical through and through. Right: we cannot explain change without reference to time. But if I ask you, what do we observe: can you observe time when nothing changes? On the other side: do we observe change? What does that tell us about what is an abstraction of what? Yep, and there is nothing more to say about it. Physical events/objects can be observed because they have causal influence. Time and space don't. Exactly. So I wonder why you did not move the whole thread to philosophy from the beginning... But of course, I would amend your sentence as 'how changes behave' (e.g. as function of velocity and gravity), instead of time.

None. For both you need references to objects. Examples, please. You exactly make my point. But you don't seem to realise it.

I have learned that to change a movement, a force is needed. In the end, in relativity, time does not really slow down. We, in another inertial system, or in a weaker gravitational field, see the clock slowing down. But there is no force doing this. In order to have empirical proof of one event causing another, you must be able to observe them independently. But you can't observe time. You can only observe clock-ticks, or more general, change.

Hmm... An abstraction of something ideal... That sounds nearly as an abstraction of an abstraction. I just want to remind you again of what you said earlier: And that is of course exactly the difference with a test charge in an electrical field. The test charge 'measures' the electrical field, because the field has a causal influence on the test charge. But time does not cause the clock to tick, so there is nothing that the clock measures.

Then you were bluffing when you said this? An abstraction is always an abstraction of something. In fact, you are saying the same as I did, except that I said what time is an abstraction of. Space, not length: Space is an abstraction of objects and distances between objects. It creates even some nice symmetry between space and time: Space is an abstraction of objects Time is an abstraction of processes. Some philosophers think that the universe is made of processes (Whitehead). In the end objects only reveal themselves in processes, e.g. in the process of observation. Isn't there a similar discussion in QM? That particles are the locations and the moments of interaction between fields? In relativity theory we see that space and time are not so strictly separated as we once thought. So that points in a similar direction. But this is highly speculative. Would you like a cheese sandwich without cheese, or do you prefer a meat sandwich without meat?

No, you will never see that. So in the end it is a metaphysical topic. But you forgot one of my questions: you said, many postings before, that time is an abstraction. Of what? That is even for me nearly a question too far... At least it is difficult to derive any law of physics from observation when there exists nothing that behaves like these law. One could say that the elementary entities that physics knows of, are defined by the way they interact with other entities (of the same or of another kind). When nothing happens, i.e. there is no interaction, then, yeah, maybe one should say that there are no laws of physics. Just be aware: of course I am not talking about some empty space in our universe where accidentally just nothing is happening. So the whole discussion is pretty abstract.

Yes, we can. I can also make an appointment for a time in the future that is not there. But they are always related to an arbitrary origin (arbitrary in the physical sense, not in the practical sense), and a clock (i.e. a standard-changer). For time we have our calendar and clocks, for space flight I assume we take some fixed coordinate system attached to the sun. So there is something there, to which I must relate: the sun, the position of the earth and e.g. the position of a comet. There is no space in itself. I thought we agreed that time is an abstraction? Of what, according to you? I say it is an abstraction of change. Length is an abstraction of objects: their sizes or distances. So if you ask this way: yes, space and time exist, but as abstractions, not as physical objects: not as field, force, potential, mass, energy, or whatever. I'll try another comparison: the laws of physics. Do they exist? I think we have here exactly the same situation: they do not exist in the same way as physical objects do, they are descriptions of how physical objects develop. Laws of physics cause nothing: they describe how certain classes of events lead to others, i.e. they describe how the causal relationship between certain classes of events is. They are abstract in the same way as space and time are. maybe we should not be wondered by that, because space and time play a major role in most laws of physics.

Hmmm. That is a good point. But we must be careful: length is the comparison with other objects. So there is a parallel with time, which is a comparison with one change with another, standardised change. So, no. Length in itself is not a physical object. It also causes nothing. It are objects with certain lengths that causes something. The reason that time bothers so more seems to me is the fact that where we can freely move in 3 dimensions, time seems to have one single direction.

I do. And gave an argument: there is no force, no field, no potential, nothing physical, i.e. no physical cause (no 'mechanism', as Swansont formulated it) for the Lorentz transformations. Lorentz Transformation and Relativity Theory. I think that I know what you mean. You say it is impossible to define change without using the concept of time. That maybe true. But since when can we conclude from a concept at the real existence independent of the concept? For something to exist physically, in my opinion, means that it causes something else. Time doesn't cause anything. You seem to postulate a kind of 'sea of time' in which everything exists. But on the other side everything has its own time, depending on the observer seeing it. Well, if you think that events occurring in time have the same physical status as time, be my guest. Yes, I can tell that time passes: here, have my clock. Look at it, and see where the pointer is when the muon decays. I'll give you my mirror too. All change requires time, yes, as a concept, as an abstraction. You did call time an abstraction. Of what? That puts me in a paradox...