Lizzie L

OK, so what are the units on the axes

My son loved those!

Does to me

"Science" doesn't develop "practical quantitative models", people do. And they are mental models. What else would they be? They are the output of human minds. The cool thing about scientific models is that they can be communicated very precisely, and thus shared. It doesn't. They aren't. Well, can you give me an example of a mathematical model of time that doesn't use units of time? In response to your edit: I don't say that "my" mental model "takes precedence over scientific models". I say that scientific models are the output of minds, and are as much mental models as any model. The special thing about scientific models is that they can be communicated accurately, and shared - and tested against data by independent observers. I should also just say, as I am new here, that I am not proposing some effete post-modern view of science whereby everything's subjective and one woman's truth is as good as another woman's, even when they are directly contradictory. As I said, I'm a scientist, and having come to science from the humanities, I love the fact that in science what matters is not whether your idea is original or brilliant but whether it's a good fit for the data. Its objectivity, in other words. And the fact that my branch of science is psychology/neuroscience doesn't mean that I'm sloppy about math, nor even mathematically ignorant (although I'm no mathematician!) It doesn't mean I'm sloppy about physics either - I collaborate with physicists all the time, and it's the output from their models that go into mine. Just thought I'd mention that, in case anyone had the idea that psychology was all about potty training and how you feel

Well, I'm not so sure it doesn't. It may not have anything to do with the math, but it has something, I suggest, to do with what the math models. I'm saying (or at least suggesting) that the mathematical model of time assumes uses units of time, and that those units of time are defined in terms of something changing.

Well, no, it doesn't really. Well, I'm disputing the assertion that because time is a dimension in GR (which I do not dispute) it cannot be is related to the concept of change (which I do). And as far as I am aware, I do understand the concept of a dimension, and I didn't say that time involved change. I said that that it is my position (not my "belief") that only makes sense in relation to change. Perception and conception. Both are ways in which we model the world. Physics is about modeling the world. No problem I have a hide like the proverbial rhino. Or duck maybe.

I know what you mean. I'm moderately clinically depressed (or would be if I wasn't taking citalopram) and death to me seems like the ultimate deadline I'm in danger of not meeting! *shudders*

Yes, that's an excellent point. I am happy to accept your emendation I think Schneibster's point about hyperbolic time is essentially about light cones. Sure, but the equations have c in them, so any diagram of any of the equations is going to have time and space units, isn't it? And while we can express space in terms of light and time, we still need some kind of time unit. So aren't we back to clocks? Yes, I think I get that. I Indeed. And it's a short further stride to imagine an existence when all a person knows is the future, and it's the past that has to be guessed at. People with bilateral hippocampal lesions effectively live in such a world. Well, isn't that just because we have an easy translation from time into space via the speed of light? So we can define length in terms of time, but then we are stuck with a definition of time. Although I suppose we could reframe the question as: a unit of time is how long it takes for light to travel some unit of length, but what is length? I'm not sure that the reason we don't ask that one more often is because we can "see" length. It might simply because at scales at which it matters (astronomical scales) the tradition is to talk of length in light-years, rather than time in light-lengths. Yes, I agree that time doesn't pass because things change. My original claim was that time only made sense in relation to change. I still think that's more or less true - for a universe in which nothing changed (as I understand you can get from some solutions to Einstein's field equations - I can't remember who said that, and I'm not in a position to argue it! ETA it was Strange) time wouldn't actually make a lot of sense within that universe, just for us, outside that universe, with clocks that do depend on change. As a toy example, if we plot the location of a stationary object over time, we get a horizontal line, while for a moving object we get a nice curve. From outside the world in which the object is stationary, time is meaningful - and we can compare its non-moving trajectory with the trajectory of something moving. But if All That Is is stationary and unmoving, then time ceases to have any meaning, I'm saying, within that universe - only for us, contemplating it from a changing one. Well, I think it's what permits us to exist, and therefore for beings capable of modeling time as a dimension to exist! Even of systems in which time, within that system, would not. Heh. I try.

petitio principii?

I haven't seen any evidence presented on this forum that contradicts it. Can you cite where you think there was some? Well, I don't think that characterises my position - and I think your evidence to support the hypothesis that it does is weak As for your last - because I like science. I'm even a scientist. Just to disclose where I'm coming from on this - I'm a cognitive psychologist/neuroscientist, not a physicist, and I did my PhD on the subject of disordered time-perception (actually temporal-order perception). So I'm interested in how we perceive time and indeed the direction of causality - but that is not a million miles from the issue as to how we conceive time, I suggest.

Well, I'm not trying to prop up any "belief" - and I don't doubt that in GR time and space are dimensions of a four dimension pseudo-Riemannian manifold. Or at least I don't have the expertise with which to doubt it. But GR is a model of reality, and I'd like to know what is being plotted on the time axis. Once I know that, I also know what is being plotted on the spatial axes, because I can define it in terms of how far light can travel in some time unit. And my more philosophical point, I guess, is that our models are, obviously "man made", we exist in a world that changes, and we have clocks based on changing things, whether it is a rotating planet, a swinging pendulum or periods of radiation between the hyperfine levels of cesium 133. So, so far, I remain unpersuaded that the OP is wrong, except that I would substitute the more general world "change". But it's not a "belief"! It's just something that I shall continue to regard provisionally as true until someone persuades me otherwise

Well, how are they defined? You've defined length in terms of time, but how do you define a second? The only way I know is by a clock of some kind - i.e. something that changes.

Irrelevant to what? Well, that's my question. I'm suggesting that units time are units of change. And that units of length are units of how many changes will happen between light leaving one place and arriving at the next

Units might help. Spatial dimensions are measure in units of length. What are temporal dimensions measured in units of?

And I appreciate the welcome

I'd still like to see someone define "time" in a way that doesn't involve something changing (or moving, as the OP suggested).

I don't know the answer to your question, my idea was definitely speculative! But it has always struck me that you (well l) can identify people from their voices quite well, even if they are behind a door, say, and the difference between the muffled sound and the their unmuffled voice is much less than the difference between what I hear when I talk and what I hear on a recording! For a start, I don't think of myself as having any accent particularly (not surprisingly), yet when I hear myself I can hear traces of all the places I've lived - from Scotland to Canada to middle England!

Sure, but that's not a definition. How, for instance, would you define the difference between the spatial dimensions and the temporal dimension?

OK, so can you give me the definition in General Relativity?

Cool. Not sure about that. Can you provide a definition of time that doesn't involve reference to something changing? Certainly our perception of the passage time is dependent all kinds of things, and is often contradicted by clock time. But I think I'll still stick with my original claim OK, let me try to make my point a little clearer: by "conception" I meant, I guess "model", and while we can all have different models of reality, we can also share them, using the language of mathematics, for example. We can "operationalise" our conceptions so that they don't differ, at least for current purposes. And my claim, I guess, is that change is intrinsic to at least all operationalisations of time that I am familiar with. I guess that claim can be falsified by an operationalisation of time that doesn't include change

I was talking about our conception of time rather than our perception of time, although our perception of time is interesting in a related way (temporal order illusions for example shed interesting light on how we perceive time). And I'm saying that our conception of time only makes sense in tems of change. I can't say anything about "time itself" because I'd say our own access to "things themselves" are via our models.

I didn't say that time didn't exist, nor that it was "only motion" or "only change". What I'm saying is that time only makes sense in terms of change. Or, if you prefer, that time is intrinsic to the concept of change and that change is intrinsic to the concept of time. And possibly that that is what the people you are referring to mean. More interestingly (because that seems fairly uncontroversial to me), the irreversible direction of time as we model it is intrinsic to our properties as observers with memories of the past, but only contingent imaginings of the future. As a thought experiment it's interesting (well, I find it so!) to imagine a kind world in which all we know is the present, but in which our computational capacity is such that, given the present, we can compute the future with a high degree of confidence - a kind of cellular automaton world, in which many things could have resulted in its current configuration, but given its current configuration, only one future is possible. In such a world "causality" would seem to be reversed - the future would be knowable with certainty, but the past would be highly speculative. In that sense I like Schneibster's hyperbolic view of time (presumably he's referring to something like Minkowski space time) - with the observer at the apex of a double cone. In my (heh) model, though, the flow of time is simply a function of what the observer can know, given what she can observe at the apex: which is more certain - the changes in the upper cone, or the changes in the lower? I'm saying that her perceived flow of time will be in the direction of the less knowable.

Hi. I found a link to this site on Schneibster's blog, and thought it looked interesting I'm a musician-trained-in-architecture-turned-neuroscientist-late-in-life and I like physics, but don't know much about it other than what I've picked up as part of music/architecture/neuroscience, popsci, and being married to a physicist-turned-neuroscientist (not nothing but not a lot either).

Well, it's an extrapolation based on good evidence about other phenomena in which we discount sensory stimuli that we can correlate with our own actions. Another phenomenon is not noticing the sound of your own footsteps but being very alert if they stop correlating with your own steps. And there's a lot of research on illusions of agency, and also of course on delusions of non-agency (alien hand syndrome, for instance, or auditory hallucinations). So that's the origin of my "hunch". It's testable, and maybe someone has tested it specifically. It's sort of related to my own area of research, which is why I thought of it. And as I've never found the "it's filtered through your head" totally convincing (we seem to be able to recognise other voices quite easily filtered through lots of things), this struck me as potentially a better explanation, and fits with a fair bit of evidence.

Those are the standard answers but I think there's more to it than that (just a hunch). After all, we are quite good at recognising other people's voices even when filtered through doors or over bad telephone lines. I think it may be part of the same phenomenon by which you can't tickle yourself - or think that the other guy punched you harder than you punched them. Our brains are set up (very efficiently) to signal differences between what we expect and what actually happens - the idea is that we make "forward models" of the world, and only "notice" when the model fails to fit the reality perfectly. With our own actions, we have very good forward models, because they include an efferent copy of the motor program we are about to execute. So nothing surprises us about what we hear when the voice is our own. However, when we hear it back on a recording, we don't have that efferent copy, so we hear it as we hear other voices - with all the little departures from expectation that alert us to the characteristics of a particular speaker.