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, 2014 at 3:01
  • @Kerr:I mean upto all theoretical limitations in knowing the past. Dec 30, 2014 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, 2015 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. Feb 28, 2015 at 14:43

1 Answer 1


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.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .