- There seems to be a special direction "down", where things fall by default, because we live in the vicinity of an influential object in space, called Earth
- There seems to be a special direction "forward in time", where things happen by default, because we live in the vicinity of an influential event in time, called Big Bang
If we had stuck to the local Earth context and geocentrism, the objectivity of the "down" direction would remain unquestioned. It's when we started modeling other things outside of Earth, it became clear there's no objective "down" direction, just a more general concept of Gravity.
Carroll's argument is that it's the Big Bang, an extremely low-entropy configuration of the universe, that gives rise to the 2nd Law of Thermodynamics and the resulting emergence of an arrow of time in the forward direction. It's purely a statistical phenomenon at larger scales, and attributable to being "next to an influential event", according to him.
It's difficult to see how such a hypothesis could be tested either experimentally or observationally. Sounds more like metaphysics?
Stars shine until they run out of fuel, and the age of a star - the status of its fuel, the buildup of fusion products - happens as time passes. Life on planets taps into the flow of sunlight from the star (and/or the flow of heat and chemical energy from the a hot planetary core) to generate complex structures in defiance of the regular direction of entropy (not violating conservation of energy, though). So... life reverses the arrow of time?
I think that one thing to keep in mind is that we are only capable of observing the passage of time along one vector because our perception relies upon entropic biological processes.
We cannot observe in any subset of possible universes where entropy is not present or is working backwards-Ergo those possibilities are wholly out of our direct perception.
That does not mean that causality cannot run in reverse, however, only that we cannot interact with those mechanisms in a way that would preclude our existence or observation.
But could we observe the effects? Can there be particles which have mass and therefore exhibit gravitation but which are subject to reverse entropy? We would observe these for example as an unexplainable increase in gravitational pull on observable matter without an observable source of that gravitation and without a preceding cause.
Are photons themselves stratling this entropic boundary since they travel at the speed of light and within their own reference frame are not subject to the passage of time?
Wouldn't questioning those pre-existing notions be the first step towards constructing such a machine? It sounds silly to dismiss the idea time could go backwards because we don't have any machines that work that way, since that typically how all ideas work before we implement them.
> It's difficult to see how such a hypothesis could be tested either experimentally or observationally. Sounds more like metaphysics?
Sure, but some claims which cannot be tested experimentally or observationally are nonetheless important. For example, the claim “it’s important that scientific hypotheses be tested experimentally or observationally.”
While I agree that not everything in science can be tested experimentally or observationally, arguably this one can:
Models created using both empirical experimentation and logical reasoning match reality much better than those based on "pure logics" (if that even is possible).
I think that the real question is whether the death of the universe by cool down will happen before the arrow of time is reversed. To me it seems more like it will just slow down and then finally stop.
Or if there ever is a "big crunch" where all black holes crunch together to a critical mass, would there be another big bang and time then run backwards?
The hypothesis I have encountered is that time would reverse when the expansion of the universe peaked and started to collapse back in on itself. The next big bang would start another run of our universe. I'm guessing the randomness in quantum fluctuations would allow this next run to evolve somewhat differently from the one we are experiencing.
Well, of course not :) Entropy as direction of time just means that the chaos on the whole increases. So, to create an ordered structure, like a cell organism, the by-product is more chaos around it.
I’d be shocked if that were the case, even only considering the Earth. The oceans and the atmosphere are full of entropy. So is the liquid outer core. And even if the mantle is not quite liquid, and even if the crust is mostly solid, these are huge in terms of volume and mass, much larger than the sliver of dirt we inhabit on top of them. So yeah, I really doubt we (collectively, all human beings) are changing the Earth’s entropy in any meaningful way.
> Life on planets taps into the flow of sunlight from the star (and/or the flow of heat and chemical energy from the a hot planetary core) to generate complex structures in defiance of the regular direction of entropy
It doesn't defy the direction of entropy. Frankly, I'm not sure why you have that notion? Entropy is a measure of system's state of equilibrium. Moving energy from a concentrated source, and distributing it more equally (across life forms and other complex structures), is following the direction of entropy, as is it makes energy more equalized, in contrast to the former state where energy levels are more unequal between highly concentrated areas (like stars, the Big Bang), and areas with less energy.
Entropy only increases on average. You can decrease it locally, for example, making steel from iron ore. You generate heat, so you still get a net increase, but you could easily redefine your boundary to only include the local decrease in entropy (the steel from ore).
Yes, but like you said, that requires re-drawing your boundaries. It doesn't really defy the direction of entropy, it only appears that way from having changed scope.
At least in France we take as fait accompli that causality is non trivial and that objects are non local at best: either you can consider objects as one despite immense distance between them, or you accept information passed between them in reverse time. Both are hard to swallow.
The regular arrow of time from the human perspective, "leaks" changes into the future. The future is constantly being changed by the past.
You can measure those changes by many ways, the 2nd. law of thermodynamics is everywhere just to make sure you see this happening across everything.
So, probably there are other non-discovered underlying mechanisms in the reality that are already signaling changes from the future "leaking" into the past, changing the past right infront of us, without us noticing it.
A probably omnipresent mechanism for observing the future changing the past, valid in the whole cone of light visible to us, is the probability law.
If you have 10 millions of possible events in a probability, one of them is the exact thing it will happen in the future, hence the information of what happens in the future is right here, existing in the past.
Yeah, you can say how would the nature could use this information without powerful processors predicting branches trillions of times per second on Earth right now?
Well, the brain is a pattern predictor, quite several parts of the brain are firmly attached to the physics of the universe, biochemically but what if the pattern prediction mechanisms in the brain are actually an adaptation of the evolution, just like wings to fly, but with those mechanisms the brain uses probability to "see" information from the future, and here in the "present", it changes the past (from the perspective of the future created originally from the past), if the brain "chooses now" to do something different that what originally created the future it "saw" by analizing patterns, essentially evaluating probabilities.
So the Probability Law would be a kind of Inverted 2nd. Law of Thermodynamics.
It would work perfectly as law of the universe: from the future to the past, the revered time of arrow, one single event created "a" future, but from the regular arrow of time, there would a range of probable future outcomes which you cannot precisely say it has "1" (100%) probability of ocurring, right to exact moment till the future becomes the present.
And it was right there all the time, something obvious infront of everybody in the world, like electricity (which was suspected by many across thousands of years, but just "discovered" recently).
You can, but that inevitably leads to far more Boltzman brains than things like the universe, and that's disastrous because then you should expect to be a Boltzman brain yourself, but they're configured randomly so if you are a Boltzman brain you can't trust any belief you have about reality including the maths that says you should be a Boltzman brain.
It's basically a softer version of Russell's paradox but for cognition and reality.
Hard to summarize, but it's well worth anyone interested in this topic. There's a good bit where he ties the sleeping beauty thought experiment with Boltzmann Brains and consciousness; it blew my mind trying to understand the whole thing.
- There seems to be a special direction "down", where things fall by default, because we live in the vicinity of an influential object in space, called Earth
- There seems to be a special direction "forward in time", where things happen by default, because we live in the vicinity of an influential event in time, called Big Bang
If we had stuck to the local Earth context and geocentrism, the objectivity of the "down" direction would remain unquestioned. It's when we started modeling other things outside of Earth, it became clear there's no objective "down" direction, just a more general concept of Gravity.
Carroll's argument is that it's the Big Bang, an extremely low-entropy configuration of the universe, that gives rise to the 2nd Law of Thermodynamics and the resulting emergence of an arrow of time in the forward direction. It's purely a statistical phenomenon at larger scales, and attributable to being "next to an influential event", according to him.
[1] https://www.youtube.com/watch?v=AZsmyTE3j9o