There are some experiments that are being made in an attempt to prove that retrocausality exists. While retrocausality certainly strengthens the case for Block Time, an acceptance of Special Relativity provides sufficient evidence IMO. The primary objection to block time is the subjective "feeling" of the flow of time, correct? A glance at this diagram highlights the fact that any two spacially-separated events can be described as happening in any arbitrary order depending on the observational frame. If you accept this then you must accept that the "flow of time" is not some universal progression through a 4 dimensional spacetime framework; rather, flow (if we're claiming it to be an objective phenomenon) appears to have a certain "thickness" to it as it "moves", and the cross section of this flow that any observer experiences is dependent upon his relative frame of movement. This "thickness" would presumably be determined by the existing frames. But a "thickness" of what bounds? (none) The problem is that once you accept that flow cannot be an objective 3D "sheet" moving through a 4D framework then you are left with the description of flow taking on the same dimensionality as the framework itself which is the very definition of block time (or at least what I mean when I refer to it). In other words, if all of 4D spacetime is represented as a line 1 meter long, and we want to define flow as the traversal of such a line, yet our traversing mechanism is itself a 1 meter ruler, does traversal actually take place? I say no. Everything happens, has happened, and will happen, "at once". Please Register or Log in to view the hidden image! Thoughts?
I believe the general interest in retrocausality (including those experiments) is related to novel interpretations of quantum mechanics, and really has nothing to do with special relativity. I don't know what Block time is, although I can guess maybe you mean that the universe is eternal and that our perception of time passing is just an illusion? I believe the relativity answer to this is not this Block time, but rather that the flow IS subjective, or frame dependent (i.e. not objective as you stated). In truth this is an interpretation question since sure enough a relativistic spacetime has the whole history of that spacetime laid out mathematically in advance, but so does Newtonian physics really. Perhaps I don't quite understand your point.
kurros: no you understand my point pretty well - if time progression is not an objective phenomenon then it cannot be wielded as an argument against Block Time. Block Time of course presumes Determinism which would be buttressed by Dr. Cramer's positive results. Cheski: A bit of meta-physics, maybe. And I don't mean Planck time, I'm not sure why you think that.
Well I personally don't consider it a particularly strong argument against block time. I mean intuitively it seems that things aren't that way but there is nothing rigorous about the argument. Equally though I don't believe the subjective nature of time in relativity is any argument FOR block time, especially since, although each reference frame experiences time differently, all observers are capable of communicating with each other and calculating plenty of objective properties of spacetime; in particular the causal structure is perfectly objective, although not what we are used to.
What do you mean by causal structure being objective? Do you mean that two observers can agree on the ordering of the cause/effect relationship between two events?
Yes. There is a thing called a light cone, which perhaps you know about, which determines uniquely everything that is in the causal past and future of every event, no matter what reference frame you look at it from. In other words, events cannot shift in and out of the lightcone by changing reference frames. The standard relativity of simultaneity problems involve events outside of each others lightcones, i.e. they are not in causal contact. Such events can indeed switch order in time depending on the observer, but cause and effect ordering is preserved.
Gotcha. I believe that the causality you are referring to is a function of information theory, related to thermodynamics, with ultimately no bearing on time itself IMHO. Or rather, we need to discern between information progression from the perspective of sentient beings vs an actual progression of any objective mechanism in spacetime itself. It may be more appropriate to discuss what it means to be sentient! We have no idea what it would feel like to experience everything at once, yet "now" is always "now". Think about that. What we consider the past vs the future is all based upon what information is available to us from our current "now" perspective.
This is a discussion of philosophy rather than physics. It seems that all the interesting discussions of presentism and eternalism are about philosophical issues, and it doesn't seem to be a controversial thing is the world of physics. Eternalism (block time), as you note, it is inherent in the notion of spacetime, while Presentism isn't awfully useful. So in practice, eternalism is well accepted in the world of physics. As for "retrocausality"... I'm really not sure it would distinguish between presentism and eternalism. Retrocausality would seem to place the past and future on a more even basis and imply some degree of determinism... but the common presentist philosophy of time is that the past is no more real than the future. So, if (non-existent) past events can effect the present, then why not (non-existent) future events? Further reading: This is the newest discussion on presentism and eternalism I could find online. Tom Stoneham (2009). Time and Truth: The Presentism-Eternalism Debate (pdf). Philosophy, 84, pp 201-218 doi:10.1017/S0031819109000187 This one is edifying in its errors. It's essentially a rant against the eternalist philosophy inherent in a spacetime model of the universe. Nothing Can Move in Spacetime! By Definition!
Pete, you may be right that this would've been more appropriately posted in Philosophy. My original post was a branch from another thread that discussed the ramifications of John Cramer's proposed experiment, so you can understand the connection with Physics in my mind - if a Physicist establishes retrocausality it seems to strike a blow to certain philosophies so I still feel this thread is appropriate. Anyway, thanks for the references - I haven't really read much (if anything) on the subject of Eternalism (nor had I even heard of Presentism) so I look forward to reading it.
