I am trying to better understand the B theory of time. It doesn’t make sense to me to deny tenses and say that every spot in time exists. Can someone please provide a basis for adopting the B theory, mainly the biggest arguments for it over the A theory?
B theory of time does not deny tenses, only their naive (presentist) interpretation. B-series is a B-series after all, there is an objective before/after order of events that tenses reflect, just as there is an order of points on a geometric line. As for the basis, aside from general relativity, some are impressed by McTaggart's original argument for "unreality of time". However, others consider it fallacious.– ConifoldMay 7, 2022 at 4:31
As @Conifold said, the original argument against the A theory was made in McTaggart's 1908 paper "The Unreality of Time", which can be read in full here. His argument basically turns on the difficulties of statements like "event M is in the present, but in the future, M will be in the past", which he thinks leads to either circularity or an infinite regress, and both seem to force us to the conclusion that "time cannot be explained without assuming time", which he thinks makes the idea contradictory. One way of thinking about this is that the A-theory seems to require some notion of the present moment "moving forward" along the timeline (the B-series), but motion involves a change in position over time, so if we ask something like "how fast does the present moment move forward along the timeline", one answer would be that that it moves at "one second per second", which appears to be circular. Another would be to imagine something like an observer watching an arrow marked "the present" moving forward along the timeline, but this implicitly means treating the observer's own sense of time as distinct from time as measured on the timeline--you might say something like "the observer experiences one second of subjective time go by as the arrow moves from T=0 seconds on the timeline to T=1 seconds". But then if you wanted to ask what it means for the present to move forward along the observer's time dimension you'd need a third time dimension, and so on--an infinite regress.
However, I think it's fair to say that most modern philosophers who reject the A theory don't do so primarily because of McTaggart's argument--instead, the biggest reason the A theory has fallen out of favor with a lot of modern philosophers (at least in the analytic tradition) is the relativity of simultaneity in modern physics. The relativity of simultaneity says that different inertial reference frames have different definitions of whether two events at different locations in space happened "at the same time" or not, and it's a fundamental principle of relativity that no inertial reference frame is "preferred" in the sense that the laws of physics obey different equations when expressed in the coordinates of that frame than they do in the coordinates of other frames (another way of putting it is that if two experimenters were on windowless spacecrafts moving inertially at different velocities relative to one another in deep space, with no ability to define distances or times in relation to anything outside the walls of the ship, then if they each used the same experimental procedures they should get identical-looking results on any experiment done on board their ship, no experiment could determine some notion of an absolute velocity for their own ship).
The relativity of simultaneity first arose in Einstein's theory of special relativity, with all quantum field theories respecting the principles of special relativity. Einstein's later theory of general relativity has an equivalence principle in which the theory is said to reduce to special relativity "locally" when looking at small patches of a larger spacetime, so it too lacks a preferred notion of simultaneity in this sense, and global coordinate systems can have more or less arbitrary definitions of simultaneity, see the entry for "global coordinate system" in the glossary on here.
Strictly speaking the lack of any physically preferred definition of simultaneity doesn't rule out the possibility of some kind of metaphysical preferred definition of simultaneity, but unless relativity is completely wrong such a thing would remain forever invisible to all possible empirical experiments, and there are various philosophical arguments as to why we should consider this a dubious possibility. For example, in the book An Introduction to the Philosophy of Time by Sam Baron and Kristie Miller, they write on p. 107-108:
The idea that some simultaneity class is metaphysically, and not physically, privileged gives rise to two difficulties. First, it may not be in the best interests of the dynamic theory of time to cleave metaphysical from physical privilege. As we saw in Chapter 3, passage theorists hold that we have experiences as of time passing, and that we have those experiences because time does in fact pass. Temporal passage just is the movement of a privileged present. So it would stand to reason that if we experience time’s passing, it must be because we have some experiential connection to the privileged present.
But if the very thing that makes the present privileged is not physical, then it is quite difficult to see how the passage of time could be being experienced. In short, the idea that the present is metaphysically privileged would render the dynamic theory of time susceptible to the makes no difference argument discussed in section 3.7.
Second, it is troubling that the metaphysically privileged simultaneity class is not physically discoverable, at least when that simultaneity class is being used to delimit the present. On such a view it would seem that nature is ‘conspiring’ to keep the privileged present hidden from us. This leads to a sceptical conclusion about the present: for all we know, nothing that is simultaneous with us is present. We may not be in the privileged simultaneity class and thus may not have the privileged perspective on the universe needed to be present. On some dynamic theories of time, this has alarming implications. Presentists maintain that only present entities exist. For the presentist, then, we have no way of knowing whether the things we take to be simultaneous with us even exist!
GR is our most successful theory of space and time. It suggests that all of spacetime exists. And hence is an exemplar of the B theory of time.
I agree that it doesn't make sense to say all of time exists. This is likely to be addressed in a future theory of quantum gravity as quantum mechanics has a very different idea of time. Here, the future is open.