Lizzie L

Right. So the irreversibility of the time dimension reflects the irreversibility intrinsic to our concept of causation, is what I'm getting at. I wouldn't bank on it I suppose what I'm trying to nut out is whether our space-time model has the characteristics it does because of our conceptualisation of causality as irreversible, or whether causality is irreversible because of something intrinsic to the nature of time. To use an analogy, the answer to the old chestnut: why do mirrors reverse left and right but not top and bottom? hangs on the fact that left and right are not actually the same kinds of animal, as it were, as up and down - left and right are intrinsically rotational concepts whereas up and down are translational. I'm suggesting, I guess, that there is something analogous between that difference and the difference between spatial and temporal dimensions: that causality is intrinsic (but not obviously so) to the way we define the temporal dimension, and not to the way we define the spatial dimensions.

Thanks. I'm not very fluent at reading mathematical explanations but I'll stare at it a little harder But if I'm following the gist, then a key difference between time and space as dimension is causal linkage, and this is what effectively makes time a non-reversible dimension, whereas the spatial dimensions are, right?

So that suggests that c is the upper speed limit for information transfer?

Out of causal order, presumably. But non-causally related events can be witnessed in different temporal orders, right?

So causality actually defines an absolute temporal order?

Could you explain what you mean by this? And I'm not arguing otherwise. What I'm saying is that if Alice assumes (on the basis of misleading evidence) that B caused A, she will report B as preceding A, even though a more objective measure gives A as preceding B. That IS an "illusion" brought about by an erroneous assumption of causality, just as the moon looks bigger at the horizon than the zenith because of an erroneous assumption of distance (although, ironically, less erroneous than the assumption that makes the zenith moon look smaller!)

Well, I may not have been making such an excellent point as you credited me with! I did mean "illusion" i.e. people report a different temporal order to events to that indicated by objective measures (or for that matter, manipulated by an experimenter) just as the "moon illusion" is when people report that the moon at the horizon is "bigger" than the moon at the zenith, when it can be readily demonstrated that they subtend the same angle. In contrast, I don't call "free will" and "illusion" (as some do) - it's a perfectly good model. I'd say the difference between an "illusion" and a perfectly good model lies in the degree to which it conflicts (makes different predictions than) predictions based on more objective (i.e. measured by independent observers) measures. But I do think it's interesting - because it tells us that our association of temporal order and causality runs both ways - we think that if something precedes something else, it can have caused it, whereas if it follows it, it can't. However, it turns out that if we think A caused B, that can fool us into thinking A preceded B, even though it didn't (by more objective measures).

Temporal order illusions are also possible.

Or even better:

"Charge current" isn't a term I've heard before. Your instruction might have been using it to refer to the flow of electrons, rather than the flow of ions, as in "ionic current". But it's possible s/he was referring to wave of depolarisation that travels down an axon. It's also possible that your instructor was confused!

Well, yes, but don't misunderstand me - I'm not naively thinking that objective measures of time should map directly to subjective temporal perception. Even psychologists talk about "veridical" time as opposed to "subjective" time. I do think that we need to be aware that objective models of time arise from minds, and that our ideas about causality, for instance are rooted in the way we make sense of the world - in our intuitive models.

If "scientifically" means "empirically" the answer is probably no. But I can't prove that.

hmm Unitless?

OK, so how would you operationally define time, independent of the method we use to measure it?

Definitely a different beast. Which is why I've been arguing that we operationally define time, for scientific purposes, in terms of clocks. There doesn't seem to be a more fundamental definition than that.

What does appear to be true is that we tend to perceive time as going more slowly when the event rate is more rapid. Which means that rather than perceiving the speed of time as the rate of change, we have a normative idea of the rate of change, so that when things happen rapidly we perceive time slowing down. And when nothing happens, time seems interminable. The classic example is when people report their experience of a car crash as of things happening "in slow motion".

Well, all our words are ways of modeling the world. Some models are more precisely defined than others.

Well one of my underlying points, I guess, is that our models are only ever as good as their fit to the data, and the data is only as good as our measurements. So if we are going to have equations with measurable units in them (like seconds) we need at some point to operationalise that construct as a measure.

OK. Well, that actually makes sense And to be fair on myself, isn't a million miles (light-seconds?) from what I thought in the first place. Cool. Sanity returns. Thanks.

OK, that makes sense. Schneibster posted something similar here. Although I still don't get the thing about hurtling through time at the speed of light!

Just because it's bugging me - would you like to settle the argument as to whether a second and a light-second are the same thing in General Relativity? And if the answer is yes - why ? Thanks! right. Gotcha.

Yes, it would be, and it's not one I'm making Indeed, but as I understood it from an earlier post, such a world does emerge as a solution to Einstein's field equations. Did I misunderstand? Yes. Nice

Yes indeed Nice to talk to you too.

Well, that all sounds wonderful Schneibster, but you have not persuaded me that a light-second is the same as a second; that the speed of light is a second per second (as opposed to a light-second per second), that there is a "speed of time"; or that we are travelling through time at lightspeed! But all the other stuff sounds fine.

Well, I'm finding I'm laboring the point so much it's maybe coming over as less trivial than it actually is! I'm not saying that change is time or time is change. I'm saying that time only makes sense in terms of change - if nothing changes, there's nothing to happen along the time axis! Furthermore, our current working definitions of time units seem to be in terms of actual physical oscillators. So if we were to imagine a world without change, it would also be a world without time. Although looking at it from our changing world, it would be just a very boring world! But the reason it might be important it seems to me is that change can be reversible or irreversible. And if time is linked in some deep sense to change, the irreversibility of some changes would also give time a single direction. Which it seems to have. At least from our decaying world Yes, I know it's a dimension. And I agree it is a dimension of the same thing (spacetime) as the spatial dimensions are dimensions of, just as the up down dimension of a box and the back front dimension and the side-to-side dimension are all dimensions of the same thing - the box. But the time has different units, is what I'm saying. We have units of length (e.g. light-seconds) on the spatial axes and units of time (e.g. seconds) on the time axis. Are you disputing this? If not, then we're cool. If you are, then we disagree.