EDIT: Please replace all instance of "time travel" with "time travel to the past"
Spacetime is an abstract methematical space. It isn't what space is. It's the map, and the map is not the territory. Our world is one of space and motion. Worldlines are abstract things that do not actually exist, as are lightcones, as is the block universe. The past present and future do not all coexist. The present exists. The only time is now, whatever you deem it to be. And there is now way no how that you can go to the middle of last week.
Yes. A closed timelike curve is like a looping worldline. In the block universe. Only a worldline doesn't actually exist. And nor does the block universe. These things are abstract mathematical things. And in the real world things do not move in magic loops to obligingly put themselves back where they were yesterday.
I agree that the spacetime manifold is, in a nutshell, a mathematical abstraction of whatever reality is truly is (for the purpose of doing calculations and predictions). But like all models in science, if it matches the experimental findings (such as GPS, the Gravity Probe B experiment, Bending of light from distant stars by the sun, the Michaelson Moray interferometer experiment etc.) and provide good explanations, then it is supported (but not necessary the full picture) and should there is at least one observation that is at odds with the theory, then it would be shot down or modified to account for it.
A closed timelike curve (CTCs) is one of the weird things predicted by this model (General relativity). Like what you said, there is currently no evidence that these (not in the math abstraction describe here, but as some phenomenon that can be model by said math abstraction) exist in some form. However there are also no known evidence that can rule them out (as mentioned in the
Stanford time travel link, if using general relativity, then t
heoretically whatever structure that is mathematically represented as CTCs are not only possible but quite commonplace. However this does not mean they exist. Theoretically possible does not imply they are actually there.
As for time, there are two prevailing camps about the philosophy of time, one is eternalism (the one used in general relativity and special relativity, modelling space and time as one single mathematical space known as spacetime, and that past present and future exists) and presentism (That the only time is
now and that past and future are not there), the one that is basically the idea in the OP.
I guess the reason why some mainstream physicists will analyse about time travel is because they are predicted by the models and yet we cannot (have not?) found any way that any known laws of physics can rule it out, thus is it likely to be possible and worth investigating about it's
theoretical implications (i.e. how it may work if it is there). It's just like how 10 years ago physicists think those who proposed extra dimensions are crank, bogus ideas, but nowadays they embraced the idea because of how it can possibly account for a lot of phenomenon and simplify the maths. This, however does not mean they necessary exist in reality and the experimental guys are still trying to search for them via the high energy experiments
The problem of eternalism, IMO, is that I have no idea how it can give rise the daily experience that time has passed/elapsed as everything on it is basically static and already there since day 1. Presentism, despite how it agrees more on our daily intuition and experience, will require that they is an objective existence of time, but we still have no idea whether time is something intrinsic or an emergent phenomenon EDIT: I realise I do not understand presentism enough to give any sensible comment on it, thus see below
http://en.wikipedia.org/wiki/Presentism_(philosophy_of_time)
Wikipedia said:
Some of the difficulties and paradoxes of presentism can be resolved by changing the normal view of time as a container or thing unto itself and seeing time as a measure of changing spacial relationships among objects; thus observers need not be extended in time to exist and be aware, but rather they exist and the changes in internal relationships within the observer can be measured by stable countable events.
I remember back in the
Time is Dead thread Aqueous Id mentioned how change can be brought about without a change in time, for example the gradient function, which measures how the function value changes with the position. I actually carry a simialr view as yours regarding about time because I just don't get what "change of time" means (so to speak), i.e. for the gradient example, the function value will differ because you go to another position, this result is easily visualise and can be related to something physical (like how we can infer space by the distance between two objects), But for things like velocity (displacement per unit time),
what physically detectable or tangible thing is changing that result in the change in displacement (separation between two things)?
Well the definition of the word "clock" is enough to prove you wrong on that, the way you worded it, but there is more to your claim than that. You are really trying to say something to the effect that clocks just measure motion. It is wrong on two levels:
1. Motion is not what is being measured, periodic events are. Clocks don't record motion, they count events. This is true of all clocks, even those that use a periodic motion to create those events. This error of yours is based on your misunderstanding of what time is*.
2. The best clocks are atomic clocks and they don't use motion at all, they use quantum mechanical transition events. An additional clock (calendar, since it runs so slow) that uses events directly is radiocarbon dating (and thus any other time measure using radioactive decay).
This is why I sometimes got confused about clocks.
Clocks use periodic changes in events (not necessary motion, as the atomic clock example demonstrates) to count other events of interest, and this is how time is measured. But what causes those periodic changes, and since clocks are measuring time via counting events,
then what does those periodic changes are changing w.r.t. and why are they changing in the first place? (The scenario is like the gradient function, but what is that parameter in that is corresponding to position in the gradient function, since given how clocks measures time by counting events, time itself cannot be the parameter?. That is, given clocks count events by using change in periodic events per unit of <WHAT>?
There are two ways to define the change of entropy of a system:
(1) in terms of a systems temperature and the energy a system gains or loses as heat
(2) by counting the ways in which atoms or molecules that make up a system can be arranged
Yes, that is precisely what I learnt in uni, although 2 seemed to be a generalised version of 1.
But sometimes, I am wondering
whether time is just a function of entropy. Given how physicists treat time as a physical (hence real) parameter,
how will time be measured in the heat death, or does time has an independent meaning without the notion of entropy and the 2nd law? (where entropy is maximized?) (No, do not quote the
spacetime crystal findings, as the jury is still out for that one)
When you show me motion, you show me time flowing. Motion doesn't exist without time: Motion is a measure of change in space and time coordinates.
Me sharing this viewpoint of yours is why I get confused about some recent theories in these 5 years which can completely remove the time parameter from known physics equations.
Because if what those theories said is true (that time does not exist, time is an illusion, time is emergent phenomenon, or time is unecessary), then what caused motion?
You drop one dimension to convert 3D space into a 2D plate in a 3D block universe, then you raise the plate at a uniform rate. The time now is the height of the plate, but then you have to adjust for speed and gravitational potential. The main thing is to appreciate is that spacetime is a mathematical model. It isn't space.
(This question to others as well) But in such model (or abstraction), does time actually elasped, or is it as what you mentioned, it is just another parameter that is not changing by itself (like how the plate does not actually sponteneously rises (I hope I understood this properly)?
I'm afraid some of it is Emperor's New Clothes pseudoscience quackery. What isn't, is the OP. Read it. Understand it. Ask me about anything you're not clear on. If you can find fault with it, please point it out. I don't think you'll be able to. So you'll have to accept it. Then when the penny has dropped about what clocks really do you'll get better at spotting the pseudoscience quackery. Note that I'm a relativity guy. I'm with Einstein. By and large, those "theoretical physicists" are not.
For far fetched ideas like these (the recent theoretical physicist's theories about black holes and entanglement,, and etc.) and are not known to violate or contradict any known laws of physics or past experimental findings, experiments will carry the burden of proof. Until then I will just crossing my fingers and see where they lead to
Plot the events on a spacetime diagram.
View attachment 6882
Just read from
Casual Relationship and the light cone, I understood how the light cone defines causality. But looking at the light cone picture above, no events are moving w.r.t. each other. They are like static points in the spacetime manifold. From a
lecture note, I learnt how the norm of a timelike curve gives the proper time, which is the time measure w.r.t to an observer as he/she moves from one event to another along the timelike curve. But I don't understand
how this give rise to the notion of "time elapsed" (that is, how we seemed to know that time has passed, as in the spacetime diagram, all events are just static points in the diagram, and they don't actually "move/progress forward in time", given how time itself is already a dimension in the spacetime diagram? (setting aside the issue that spacetime is a mathematical abstraction for the moment since in general relativity we use this model to understand how things behave in reality, and it's currently the best model to understand gravitation)) (NB The answer to this bolded question may help me to better understand relativity since I am still kinda preliminary on it and I mostly self learnt form that lecture notes)
Tl.dr:
Focus only on the bolded question if this is the case
