I've recently learned about the theory of relativity and time. I'm trying to understand it more.

My (basic) understanding is that there is no distinction between past present and future and that all of time already exists.

If this is true, what prevents us from seeing the past and the future? And/Or what causes us to only be able to see the "present"?

  • $\begingroup$ The restaurant at the end on the universe. Actually, I don't think you are talking of relativity, but rather some exotic theory of a multiverse. Or science fiction. $\endgroup$ – Eubie Drew Nov 8 '15 at 4:46
  • $\begingroup$ There are lots of sources of information on relativity, good and bad, and at all sorts of levels. Where have you been learning about it? $\endgroup$ – James K Nov 8 '15 at 11:01
  • $\begingroup$ Time begins where space ends, and it reaches all the way to here in the center where the future is created. $\endgroup$ – LocalFluff Nov 8 '15 at 11:04
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    $\begingroup$ The reason we can't see 1 year into the past is because all the light from 1 year in the past is 1 light year away. We can only see (assuming we're looking at things close to us) what happens very very very recently. The past does still exist in a sense, from a very distant perspective, for example, the stars we see in the sky are years in the past. That's probably kinda obvious, but that's my thoughts on this. $\endgroup$ – userLTK Nov 8 '15 at 12:15
  • $\begingroup$ @JamesKilfiger This video (and similar documentaries) is where my questions are stemming from. youtube.com/watch?v=vrqmMoI0wks $\endgroup$ – batman Nov 8 '15 at 18:23

Your question is difficult to answer exactly, because it touches the question of consciousness, which is problematic in general. But the answer is basically this:

Theory of relativity describes the space-time as four-dimensional space that exists as a whole. You are described by a world-line, which contains all states you go through from birth to death. There is many slices, which describe "you" in different times. What you can see is given by what information reached your sensory inputs (your senses) and how they are processed by your brain. The brain creates a model of reality based on your sensory inputs, which is stored in space-like slice of your world-line in the space-time, which we call present. This is what "you" can see. So the reason basically is in how the brain processes the data it is getting.

Along you world-line, there is many versions of you at different ages and each version feels its own present and the versions more in the future also have memories of the past (because the brain stores the past sensory inputs). There is no single "present time" in the theory of relativity, only the space-time as a whole.

But you should't forget that the theory of relativity is still only a very well working model. Even within the theory of relativity, the question whether space-time exists as a whole, or whether there is "present state", which evolves in time, is rather philosophical than scientific, because there is no observable difference between the two.


Time isn't quite like space. enter image description here

The first difference is that we can't stop moving in time.

In this picture time is on the vertical axis, and space is horizontal. You have a history and a future. if you don't move you will travel straight up the vertical axis. Even if you do move you can never escape the cone, which shows the path that light takes in spacetime.

You can receive information from anything in the past light cone, and you can send information to something in the future light cone. Points in spacetime outside the cone can never give or get information of you.

Secondly, there is Entropy: Entropy increases with time. This gives a direction to time. This prevents us from remembering the future.


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