Eternalism and presentism
What is the nature of time?
The most popular view is presentism which argues that the present is all that exists — everything else only existed or will exist. This fits in strongly with our intuitive view of the world.
Others believe that everything in the past and future actually does exist in some “ontological” sense. Just as Rome exists, just not here (in Sydney), perhaps dinosaurs exist, just not now? These people are called eternalists. One reason why this view has become popular is because within physics it is often convenient to treat time as a fourth dimension.
There is also a result in special relativity that shows different observers will perceive different events to be simultaneous, even if their measuring devices were perfect. There is no way to objectively determine which set of events is really simultaneous, leading some people to go further and argue that such an objective simultaneity doesn’t exist. Of course, this doesn’t necessarily follow, it may be the case that such an objective simultaneity exists, even if we could never determine it.
What is interesting about this question is how hard it is for both eternalists and presentists to define exactly what they mean by the past and future existing. If we can’t even define the question, how can we expect to find the answer?
Here, it is important to remember that both these are simply classes of models. It is entirely possible for two models to make exactly the same predictions about our universe. In this case, we might still prefer the simpler model based on Occam’s Razor, but we can’t say that the other model is false.
It is also possible that only one of these models maps naturally to the universe. For example, if we could prove the existence of time travel, then this would be rather bizarre to attempt constructing a model based on the presentist position.
This is what makes these terms so hard to define — we can see models of the universe that we would clearly label presentist or eternalist, but there are also models where it isn’t clear what this question means, because the choice wouldn’t make a difference. This is the form that many problems of metaphysics take. Once this is understood, many of these problems become tractable.