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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...

Nearly, yes. The only problem is that time is not physical. Therefore there is no physical cause for the LT. The LT follow logically from the 2 postulates of the RT. No forces, no fields, potential, nothing. Neither. For you time passes normally. Your reading is in bad faith, you should know that. Of course I am observing the black hole from a big and safe distance. For me processes near the black hole are going slower. I know that, so ask me different: what is the relevance pro or contra my viewpoint? I think nothing of what I said changes anything in the LT. It only gives a different viewpoint on the concept of time: that it is a tool to describe processes. Let's try it in another way. Classical physics believed that there was a kind of universal time and space. You only had to define the origin of your coordinate system, but lengths and durations were universal. RT did away with that. But instead of that, you define now that every inertial system has its own time and length. But again, isn't that an unnecessary metaphysical assumption? Why not stick to what we can observe: that lengths seem shorter, and durations longer. Why is it called length contraction, and not space contraction (OK, I know, it is only one dimension.) But shouldn't we say that we have 'duration dilatation' instead of 'time dilatation'? We see how clocks go slower, how all processes go slower. But we do not see time going slower. I don't know if this parallel helps: if we change the clocks at the start of daylight saving time, what do we in fact do? Change the time? Or are we doing everything one hour earlier? Is the 'same time still flowing' as before? But I agree that the point is not very important, but if somebody asks 'what is time?', then my answer would be still this: it is an abstraction of change. It is definitely not the other way round.

I know: So what is the point?

I mean that apart from change we have no sources to know that time exists. I think that then we can strike time from our metaphysical vocabulary. What is the metaphysical difference between saying that 'time slows down near a black hole' and 'changes go slower near a black hole'? If you observe the surroundings of a black hole, what do you observe: slow time, or slow changes? It has to do with the extreme generality of 'change'.

Not yet. Now I am. BUT: the point stays that there is some other access to e.g. the Higgs field. Was that not all the importance about finding the Higgs boson? Assuming that the Higgs particle would not have been found in any of the possible energy ranges, would that not have counted as falsification of the Higgs mechanism? No, no, I know. It was just a suggestion to make the difference between time and the Higgs field. There is no other access to time then through change. This rightly suggests that time is just an abstraction of that what really physically exists: change. And of course it is a very useful abstraction. We all use it daily, not just physicists.

I said that Paul tells about an event in his life that happened about 5 years after Jesus' execution. And I provided the basis for that assumption. Yes, we already got that. Galatians is one that is authentic. I am not doing that. I give the other criteria independent percentages, and as you see, the end result is, as expected, less than the 95%. So what is your assumption that we have several sources, 2 Christian (Mark/Matthew, Galatians) and one none-Christian (Josephus). Why would both Mark, Matthew and Paul mention this, when it would not be true? Don't you think that any other hypothesis is way less probable? No. It fits perfectly in the agenda of the preacher, so zero points. It is just not true. But you like to believe it.

Well, I am not pedantic enough to suggest that I understand the Higgs mechanism. But only one thing: the Higgs particle is discovered, isn't it? That is independent access to the Higgs field, no? Do you expect we will find the 'chronon' one day, the elementary quantum of time?

Yes. But my point is that the measurement is never more than comparing it with another change. Yep, I assume it was clear that I did not mean that. Yes. Therefore I think we nearly agree. I just wanted to stress that time is not an empirically given fact. Change is. The difference is that I always find a cause of the electric field: a charge, or an electromagnetic wave. There are different modes of access to these phenomena. In the case of time I am standing 'for a wall of change'. Sorry for becoming poetic... But I never find a cause of change, except another change. In relativity, time does not cause processes to slow down: changes just are going slower. I notice this misunderstanding when trying to explain SR to absolute beginners. They think I will reveal some cause why 'time slows down'. They are disappointed when it turns out that the explanation is just a logical consequence of the relativity principle and the constancy of speed of light. They think I tricked them. The abstraction 'time' does not exist in the same way as physical objects exist. And I would not exactly say that it makes physics work; it makes physics easy. (OK, comparatively easy...). But I know what you mean, and more or less agree.

I do not exactly know where you got your numbers, but just look at the sources: Paul mentions he met "James, the Lord’s brother". Josephus mentions 'so he assembled the sanhedrin of judges, and brought before them the brother of Jesus, who was called Christ, whose name was James'. Mark 6:3 (and literally the same in Matthew 13:55): Is not this the carpenter, the son of Mary, the brother of James and Joses, and of Judas and Simon? And are not his sisters here with us? I see no reason why the relationship ' .. is brother of ..' would not be symmetrical.

But measurement of time always is comparing it to some standard of time! That means: compare it with something that changes. What do you want to say with 'well-tested'? I am not questioning RT. I am only questioning if time exists as an independent entity. And as said, Einstein uses clocks, and 'light explosions' (events) to build his SRT (at least in the beginning). You seem to think that every inertial frame drags his own time with it. What I can say of an inertial frame is that if I would have a clock in it, and I am in another inertial frame that moves against the other, that the clock would seem to go slower than exactly the same clock in my inertial frame. Same with another clock, put in the same inertial frame as the first clock. That is the meaning of 'the same time everywhere in one inertial frame'. And don't forget, an inertial frame is an abstraction too... Edit: it of course means that an other, identical clock in my own inertial frame changes just as fast as mine. And again you are using the phrase 'time proceeding'. Things change in time, or said differently, processes occur in time. Proceeding can only be defined by using the concept of time. That's not far from asking what is the speed of time in my own inertial frame. (My answer: it is 1, a dimensionless constant... It turns out it is exactly the same as the volume of space, which is also the same dimensionless 1.)

Aha, maybe I see the misunderstanding. I am not saying that time is turned on and off depending on if there is change or not. I am saying (nearly your words!) that time is an abstraction of change. As it is with abstractions, they have no self-existence. They are certain aspects that have certain objects in common. Now change is everywhere in the universe. It is simply one of the broadest aspects that everything has in common. And even if I am observing a system that does not change during my observation, my observation in itself is a process (maybe accompanied with my standardised change-device to see how much it changed during my observing the not changing system). As for gravity: it is always connected with mass (or energy if you want). So my tests for gravity will not always be positive. But everywhere I want to measure a duration, there is change: I need a clock of some kind. Some people say that the time is the necessary condition for change: without time, change would be impossible. But this is of course an untestable theory. I cannot turn time off, and look if there is still change. Time is not the cause of change. Time is only given to us through change. Time is not accessible for us, except through change. So why should I say that this abstraction of time exists? Why is change not enough? As an empirical science, physics cannot tell us if time exists. Physics can only tell us if something exists if it causes something. But as said, time does not cause change. Changes are caused by other changes. And physics describes how changes happen, depending on the kind of objects involved and the initial conditions.

It is not prejudiced. You showed again and that scientific arguments do not bother you. So I call you a believer. I hoped at least to help you a little with language, but that seems to be in vain too. (Of course, I am not a native speaker, so I am pretty sure I make a lot of errors too. 'Criterion - criteria' is just one of my favourite errors. Especially when I once read that somebody said there were 4 criterias...)

Would you mind to read the thread from the point on where I chimed in? Otherwise I have to repeat everything. From a single muon itself you will never be able to conclude that time has passed. Only when compared to a standard-changer device. Just for the ease, say a muon has a halftime of 1 μs. I give a muon to you, and after 1ns it decays. Can you now tell that the muon must have lived already a while, because it decayed so fast? Say I repeat this a thousand times. I give you a muon and you measure the decay times. Do you think you can conclude that all my muons are 'older' because you find a shorter halftime? Sorry, but that is not what I meant. I don't think I am right because of the many reactions. I was saying that people seem to care, as elfmotat says, about how many angels can dance on the head of a pin. Read better, in context.

I don't think I missed it. I already said before that time in QM is as difficult to understand as is location. I also implied some postings before that taking a single radioactive nucleus is not a good device for e.g. confirming the twin paradox. You should take at least a huge amount of them. The change is in the number of muons, that decreases slower when flying close to the speed of light. More I have never said or implied. Maybe I was not so precise, but I thought that wasn't needed here. Maybe I am wrong?

So it seems that yo have an answer to the question: for you there is at least one angel is dancing on the pin of the head. I don't bother. But somebody asked and I gave my answer. And as you see this provoked some strong reactions. Isn't that funny? And just for the record: I do not belong to the 'new age' category of people who say that time is an illusion. Even if I would say something like that, but would immediately followup with "but change isn't", I would be spit out as a shallow materialist. (which I am not. I am a deep materialist... ) And, btw, there was some famous physicist, who sent the widow of his beloved friend Besso, as comfort, that time is an illusion. No, no, that is not fair. It should be about space then. Length can be easily defined by comparing it with standard lengths, as duration can be defined by comparing it with standard durations, like clocks.