Is time a substance, particle, force or concept? Is it one big “block”, so the future already exists and “now” is subjective? Or is the present the only place that’s real? Are most physicists determinists?

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I have studied a little of this in philosophy (metaphysics, epistemology and ontology) but I really need a scientist (preferably a physicist) to break down what we actually know about the world, as I regrettably have no formal education in physics and can’t work this one out on my own. Thanks 🙂

In: Physics
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Time is a concept, and that concept can break down in some instances.

We only have any evidence of there being one reality, one universe. So whatever happens in our universe is what really happens. Determinism is debatable because the Heisenberg uncertainty principal prevents us from using the current state of the universe to predict the future. I think a key piece of determinism is the ability to predict the future. We can’t know enough about the universe to make an accurate prediction.

There was an experiment done on relative time where two clocks were synched up perfectly and then one clock was flown around the world on a jet plane. They had different times afterwards! They experienced a different amount of time because one of them was in motion. Which clock is “correct”? This is why time is just a concept. Neither clock is truly correct because humans invented the idea of time. It works well for our lives, but it doesn’t hold up in all cases.

The prevailing answer is that time is a geometric axis. Specifically, the geometry is (usually) something called [Minkowski space](https://en.wikipedia.org/wiki/Minkowski_space), more commonly referred to as *spacetime*, and *time* just happens to be one of its four dimensions.

So, time is about as much a “substance” or “particle” as the direction “right” is: a better analogy is that they represent the canvas on *which* things like substances and particles can exist.

> Or is the present the only place that’s real?

One of the most startling, but most well-tested conclusions of Einstein’s theory of special relativity is that the concept of “*the* present” doesn’t really make sense. Depending on your direction of motion, different things are happening at “the same time”. So two observers moving past each other will disagree about what the present looks like, but they *will* agree that each other’s “present” looks exactly like their own “past” in one direction, and their own “future” in a different direction.

So the idea of there only being one *present* that exists, at any given time, is demonstrably incompatible with special relativity. We have to accept, at some level, the simultaneous existence of the past and future.

> Are most physicists determinists?

Depends on what you mean by ‘determinist’, really. If you mean “believes the universe can be fully described by a set of equations and initial conditions”, then yes – that’s practically the core assumption behind all of *physics*, and physicists tend to get extremely worried about this not being the case, something they would call an “information loss paradox”, a “violation of causality”, or any other number of horrible-sounding names.

Of course, this is somewhat at odds with the distinction between *objective* reality and *subjective* reality, since quantum uncertainty means the two concepts can never be equal – even if the universe is *objectively* deterministic, our own subjective perception of the universe will always appear to obey random chance.

[Watch this video](https://youtu.be/UKxQTvqcpSg) for an explanation of spacetime. I happened to be watching it just now and saw this post!