(original review, 2004)
"Within each individual [time] slice, your thoughts and memories are sufficiently rich to yield a sense that time has continuously flowed to that moment. This feeling, this sensation that time is flowing, doesn't require previous moments—previous frames—to be "sequentially illuminated."
In "The Fabric of the Cosmos" by Brian Greene
I agree that this is at least as much philosophy as science, though mathematically based philosophy. But what irks me is that for all the pages of science books devoted to this subject, no one has pointed out that for us to experience moments sequentially (assuming those moments don't themselves move) our mind has to move through those moments. And movement entails time. So while time may be a spatial dimension, if Greene (and Godel, etc.) are right, then there must be at least one other dimension of time that allows our minds to move through the different moments that all exist and experience them sequentially.
I'm still not seeing a real explanation for why in any given time slice we have memories of past events, but not memories of future events. There's no reason for one but not the other. Moreover, does Greene address what this does for the idea of causality, which is so central in science? For example, "point mutations plus natural selection results in speciation." If all moments exist simultaneously from a four-dimension point of view, nothing causes anything.
A four-dimensional block universe is temporal. Time is one of its dimensions, so there's no lack of time. Time exists, as events can be ordered by the temporal relations "earlier than," "simultaneous with," and "later than." It's "temporal becoming" which doesn't exist—meaning that things don't really come to be and then cease to be, since every moment exists. Events can stand in causal relations in this kind of universe. Here's an example of what it would mean to stand in a causal relation:
For any entities x and y, x is the cause of y if and only if
(i) If x were not to exist, y would not exist, and
(ii) If y were not to exist, x would still exist.
Regarding temporal relations, they can exist if time is asymmetrical. Time is not the same in both directions. That's an obvious feature of time. The temporal relations "earlier than" and "later than" are surely not arbitrary. If you had to put a series of scrambled frames of a video in the correct order, you would know how to order them. Thus, the temporal relations of "earlier than" and "later than" seem to be meaningful even in a block universe. And still regarding causal relations, I think the description that I gave for such a relation is adequate for causally relating x and y. When you say, "I still don't see why such a description would hold in a block universe," remember that events in a block universe don’t exist independently of other events. If x is the cause of y, then it’s due to the fact that x exists that y exists. X could have failed to exist and thus, y would have failed to exist. There could have been different events in one location of spacetime if matters had been different in another location of spacetime. The reality of every moment in time just shows that there aren’t in fact different events, not that there couldn’t have been different events. An account of causation doesn’t require temporal becoming to be a feature of reality. Causation can be "tenseless."
Professor Greene may have gone a bit overboard trying to simplify things. Special relativity says that observers may not agree on when or where events happen, but they do have to agree that the events do actually happen and that the causal link that leads up to the event is the same. As for the distinction between the past and the future, that too is quite real. It's just that if there's no causal link between events, then it really doesn't matter which happens before which, e.g., a man sneezes in America and a butterfly flaps its wings in Africa. To an observer, the two events might happen at the same time. To another, the sneezing might happen before and to another, the sneezing might happen after. But since the events aren't causally related, it doesn't really matter. But if the man sneezing caused the butterfly to flap its wings then everyone has to agree that sneezing happened before. That is how time is actually determined. By causally linked events.
Much of this book is complete nonsense and lacks a philosophical understanding of relativity. Time is absolutely a real thing at least in the flow sense of it. Things are happening, changing, evolving, and that’s definitely not an illusion. The sense of simultaneity is an illusion and the rate at which different things experience the flow or change of time varies. The future is not a place that already exists and neither is the past. They are gone. The revolutionary war isn't still going on, it ended over 200 years ago. When physicists talk about the possibility of traveling to the future, what they really mean is you are slowing the flow of time for yourself relative to the area you are attempting to travel to in the future. If you traveled on a ship really fast outwards into space at half the speed of light for about an hour and then came back, you wouldn't have traveled to the future. For you it would have only been 2 hours but for earth it would have been much longer because the flow of time was different for you two. When you say things like "time travel" or "traveling to the future" you make it sound like the future place that you're going to travel to already exists. It doesn't. The same is true with the past.
As I wrote above, I suppose the key word here is causality, I believe that without causality there can be no time. mental or physical, without matter and energy there cannot be no time being observed....and where there is no time, well here then this is the illusion. It does appear however that present time is the activity of potentiality, during any instant..(Aristotle), movement in time is not the illusion, but the observation of it, ...time is time because there IS an instance of reality...reality ever changes in the things that are moving in time, this is because there is a cause for any cause, but the perception of time changes because of space time being met with energy...as with Einstein’s theories, by and large atoms and electrons are moving, stars are moving, the sea moves, the earth rotates...all that is a fact about what causes them (Newtonian physics). Every part comes from some other part during the present. I don't think that Einstein was saying that time changes, but rather the perception of it; the nature of light and energy never changes, but rather the "apparent" mass of matter changes or seems to distort reality of our time next to some other distant event, into another "now"? And this seems to be according to the general theory relativity. It would be an interesting idea if atoms have different “now times” traveling at their relativistic speeds from each other.
Physics has partially been in a crisis for a while because there is still a lack of understanding about time and exactly what it is. Sean Carroll and Lee Smolin have written about this extensively. It is possible to generalize science for popular consumption and not have to tell fairy tales.
NB: The proverbial idea of a loaf of bread representing entire spacetime, subject to slicing in different angles depending on the observers' relative motion may lead to conveying an inconsistent picture of reality. This implies that future already exists and all events that could ever take place have already occurred. If this was the reality, we ought to be able to predict the future. As much as I am a fan of Professor Brian Green, I sometimes feel that his enthusiasm of explaining concepts like those of spacetime that emerged from Special Theory of Relativity to ordinary people without the underlying math conveys a wrong picture to people. I applaud his zeal of explaining intricate physics to everyone, but I am afraid such explanations without the accompanying math will not provide the consistent picture that theories like Special Theory of Relativity actually paint.