# Block universe and entropy increase

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Summary:
I understand “Block Universe Theory” says past, present and future all exist together and time is emergent (and not fundamental).
I wonder how physicists who support this theory explain the increase of entropy?

Dale
Mentor
2020 Award
Summary:: I understand “Block Universe Theory” says past, present and future all exist together and time is emergent (and not fundamental).

I wonder how physicists who support this theory explain the increase of entropy?
What would an increase of entropy have against the block universe?

On my house I have a block of wood trim that the top is well sheltered from the rain and the bottom is exposed to rain. So it deteriorates on the bottom but the top is pristine. So a block of wood can be nice and neat on one side and all messy on the other side.

Similarly a block of universe can have low entropy on one side and high entropy on the other side. Blocks need not be homogenous.

What would an increase of entropy have against the block universe?

On my house I have a block of wood trim that the top is well sheltered from the rain and the bottom is exposed to rain. So it deteriorates on the bottom but the top is pristine. So a block of wood can be nice and neat on one side and all messy on the other side.

Similarly a block of universe can have low entropy on one side and high entropy on the other side. Blocks need not be
What I mean is the 2nd law of thermodynamics gives a direction to time, it can travel only in one direction. Does not this mean time is fundumental and all states can not exist together?
In other words, the block universe makes no difference between past, present and future, it means time is bidirectional and we should be able to go back in time. Am I missing something?

Bandersnatch
What I mean is the 2nd law of thermodynamics gives a direction to time, it can travel only in one direction. Does not this mean time is fundumental and all states can not exist together?
In other words, the block universe makes no difference between past, present and future, it means time is bidirectional and we should be able to go back in time. Am I missing something?
The argument I'm familiar with here is that the distribution of entropy within the block is fundamental, but the direction of time isn't. The perception of which direction time goes is just that - a perception. It follows increasing entropy, because formation of memories involves increasing entropy. As far as the block universe is concerned, it doesn't care about directionality to time - it just is.

Ebi Rogha
Chestermiller
Mentor
If you think of time as a sort of spatial coordinate in a 4D space, then the block of wood can be looked upon as being a 4D object that is continuous in the time direction. In this framework, an entropy value can be assigned to the 3D cross section of the block at each value of t. This value is increasing monotonically with increasing t.

Ebi Rogha
Dale
Mentor
2020 Award
the block universe makes no difference between past, present and future, it means time is bidirectional
That is not a correct inference of what the Block universe means. If I stack a bunch of Legos together then my block of Legos has a definite top and a definite bottom. Being a block in no way implies the symmetry you are asserting.

Demystifier