Its an obvious observation that we know the past, and in an idealised situation we could know all of the past; however we can never know the future, empirically or in any idealised situation: it has yet to be experienced.

I've seen this as one explanation for the (thermodynamic) arrow of time; but is this in fact a tautology?


What I understand by the thermodynamic arrow of time is that it points from positions of low entropy to higher; it's likely to be more complex than this, but I don't want to go into that here.

When I say 'know' it's shorthand for measurement.

  • What do you mean by "idealized situation"? There are various physical barriers: things outside of our light-cone, uncertainty principle, etc.. – Rex Kerr Dec 30 '14 at 3:01
  • @Kerr:I mean upto all theoretical limitations in knowing the past. – Mozibur Ullah Dec 30 '14 at 10:19
  • You write "I've seen this as one explanation for the (thermodynamic) arrow of time" - can you add a reference? it seems weird, since the arrow of time is a physical concept and human memory does not seem to be a component of theories in modern physics. – nir Feb 28 '15 at 11:05
  • @nir: what I'm asking about has nothing to do with the notion of memory; but with Laplaces idea by measuring what is here, now; we can't measure, in the same way, things on the future. – Mozibur Ullah Feb 28 '15 at 14:43

This is the other way around: the thermodynamic arrow of time is sometimes invoked to explain why we have memories from the past and not from the future.

The thermodynamic arrow of time rests on the idea that past sates are statistically implausible (this is the past hypothesis) and that implausible states have more chance to evolve into more plausible states. Now consider a footstep in the sand: this is a rather implausible state. Chances are that this state evolved from an even more implausible state in the past (someone walking on the sand). So traces are always traces of a more implausible state, i.e. traces from the past. The same would go for our memories, construed as traces from the past.

It is controversial whether such explanation is circular or not. Some (e.g. Maudlin) argue that it already requires a notion of evolution, which will be directed from past to future, so the explanation already assumes an arrow of time. It is also controversial whether the applicability of the notion of entropy, which measures the implausibility of a state, can be extended to these cases which are not strictly speaking in the realm of thermodynamics. However some authors (e.g. Albert) argue that a past hypothesis is available in any case, even if it is not casted in terms of entropy.


Without repeating myself too much, we are pretty sure that memory, as humans record it, is an exothermic chemical process. We cannot know what it is impossible for us to remember. That does not mean it isn't real, but we are not going to be aware of it. And reversal of time would be impossible to remember.

The thermodynamic arrow of time assumes physics is arranged in such a way that time never reverses. I do not think we can ascribe truth or falsehood to that, because we are dependent upon the idea of irreversible time to so high a degree that we seem unable to frame experiments that would test the idea. I think that we can imagine multiple dimensions of time, or even a rather complex topology imposed upon unidirectional time, but not a form of time that is singular but freely reverses. To the degree we can imagine reversible time as a version of multidimensional time, it would appear to be indistinguishable from raw indeterminacy, which we already observe, and to which we ascribe different causes.

I don't trust Popper's notion of falsifiability as a definition of science, but it is a wise position to obey if you are trying to think scientifically: It is inappropriate to make decisions about the truth of something, if you cannot imagine a result that would contradict your assumptions. And I would assert that we are biologically incapable of that particular feat of imagination.

So, I would go for this position being something very much like a tautology: It is necessarily and untestably true dictated not by deductive logic or definition, but instead by other basic structures of human thinking.

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