> Imagine that you could see your life in rewind: You would slide into bed each morning with the ring of your alarm clock. At bedtime, you would gently drift awake. In the evening, you would un-cook your dinner, transforming a plate of Tuesday spaghetti into raw noodles and cold water. Your morning coffee would spontaneously warm in its mug as it sat on the countertop, the steam drawing out of the air and back into your cup. Another day begins, and you’re younger than the day before.
Needless to say, this never happens in our universe.
This reminds me a lot of one of my favorite parts in any book:
It was a movie about American bombers in the Second World War and the gallant men who flew them. Seen backwards by Billy, the story went like this:
American planes, full of holes and wounded men and corpses took off backwards from an airfield in England. Over France a few German fighter planes flew at them backwards, sucked bullets and shell fragments from some of the planes and crewmen. They did the same for wrecked American bombers on the ground, and those planes flew up backwards to join the formation.
The formation flew backwards over a German city that was in flames. The bombers opened their bomb bay doors, exerted a miraculous magnetism which shrunk the fires, gathered them into cylindrical steel containers, and lifted the containers into the bellies of the planes. The containers were stored neatly in racks. The Germans below had miraculous devices of their own, which were long steel tubes. They used them to suck more fragments from the crewmen and planes. But there were still a few wounded Americans, though, and some of the bombers were in bad repair. Over France, though, German fighters came up again, made everything and everybody good as new.
When the bombers got back to their base, the steel cylinders were taken from the racks and shipped back to the United States of America, where factories were operating night and day, dismantling the cylinders, separating the dangerous contents into minerals. Touchingly, it was mainly women who did this work. The minerals were then shipped to specialists in remote areas. It was their business to put them into the ground, to hide them cleverly so they would never hurt anybody ever again.
The American fliers turned in their uniforms, became high school kids. And Hitler turned into a baby, Billy Pilgrim supposed. That wasn't in the movie. Billy was extrapolating. Everybody turned into a baby, and all humanity, without exception, conspired biologically to produce two perfect people named Adam and Eve, he supposed.
The problem with this view is causality. Since things happening as a result of other things is pretty much the definition of "time moving forward", there is no reasonable definition of time moving backwards that still includes causality.
In other words, the "backwards" world he described is exactly the world we live in, since there would be no measurable way to tell the difference. A world where you're getting younger but can only remember the things that happened in the future is pretty much the same as a world where you're getting older but can only remember things that happened in the past.
> things happening as a result of other things is pretty much the definition of "time moving forward"
No, that's the definition of causality. Just because you are used to thinking of casual relationships in the context of forward-moving time, doesn't mean that time's arrow has anything to do with it. That's kind of a meta correlation/causation argument.
In physics causality is more appropriately defined in terms of the propagation of forces/information. E.g. if you are farther than a distance c*t away from me, we cannot be causally connected because no signal moving at the speed of light could propagate (either forwards, or backwards in time) to connect our worldlines.
> No, that's the definition of causality. [...] That's kind of a meta correlation/causation argument.
It's more than that. The only way for us to actually tell which way the arrow of time points is by watching causal events.
> In physics causality is more appropriately defined in terms of the propagation of forces/information.
But that's exactly my point, I'm not sure where the miscommunication happened there. I object to the time-flowing-backwards statement in the article, because anyone who suggests a scenario where you're getting younger while being aware of it has left scientific reality behind.
Moving on beyond the article: You can theoretically assert that the propagation of information could just as well happen backwards, but in the real world this becomes just exceedingly unlikely after just a couple of interactions following an event. So yes, this process has everything to do with the direction and the nature of time.
Causality is not just another thing that mysteriously happens in one certain direction, it's a process that by its very nature can only happen in one direction. I have a feeling you're going to object to this, but in my opinion an information theoretical view of time is just as valid - if not more so - than to hang it on the direction of gravity. It's a construct that helps illustrate why time flowing backwards can never be a perceptible/measurable thing.
> No. We can take arbitrarily spaced "snapshots" of a closed system, and determine which was taken earlier by measuring entropy.
That is not correct. A closed system may by convention still exchange energy with the outside (just not any matter). What you're really talking about is called an isolated system, which is a theoretical construct that does not occur in nature.
Fundamentally, time, causality, and entropy are related, especially considering the idea of entropy as seen through a probabilistic lens with information causality in mind (see Pawlowski et al arXiv:0905.2292).
However, I think causality overall has a stronger, more basic link to the direction of time, and here is why: Entropy can always be reduced locally in an open system without affecting the flow of time at all. Causality, on the other hand, is not subject to reversal under any known circumstances, as long as a reasonable amount of information and time is involved. If, by some yet unknown mechanism, causality were reversed then so would be the direction of time, which in turn means the whole thing could not possibly be measured - not even in principle. Hence the argument that this reversal may not be possible since it's literally something that cannot exist in our universe. No such link exists between entropy and time, respectively.
Time, like space, is a relation. Causality indicates that things act according to their nature. This is how relations change. Change, what we commonly think of as the passage of time, is the inevitable unfolding of an entity's potential to actuality, acting according to its identity.
How would change switch direction, actuality revert to potential? would change still be change? I suggest reading Heraclitus, Parmenides, and Aristotle.
I really do hate to start a reply with a flat-out "no", but in this case it's warranted, sorry to say. There are two ways of looking at this, and none of them permit this conclusion:
Physics as a science does not depend on metaphysics in any way. When a physicist does his/her work, it does not require the invocation of supernatural principles. While speculation and hypotheses are essential to the scientific process because they drive further exploration, metaphysical arguments on the other hand are just postulates about the nature of reality - and they are in most cases either in direct conflict with observational data, or designed to be un-disprovable.
Now for physics, as in the actual universe. This doesn't depend on metaphysics either. In fact, it's the other way around. As the universe develops, intelligent life emerges, humans in our case. These humans can then invent religion and the supernatural. The physical world came first.
Proper metaphysics is very limited and has nothing to do with the supernatural. When physicists do their work they rely on such concepts as existence, identity, causality, the relation of consciousness to these. That's metaphysics.
To state that "the physical world came first" is to assert the primacy of existence in opposition to the primacy of consciousness (e.g. Descartes) -- that's correct and that's metaphysics.
We went off on a tangent there, I suspect we've already bored everyone else in this thread to death ;)
I want to close with the observation that whether you're aware of it or not, you're talking about supernatural claims that have been postulated without any scientific evidence.
To make this distinction between the words metaphysical and supernatural feels a bit similar to fundamentalist Christians who assert that they're not religious - a phenomenon which I suspect happens for the same reasons: because they believe their faith represents the "truth" about the universe, it doesn't count as a religion. But to everyone else, it still does.
Are you claiming there is no evidence for the concept of causality? and that any discussion of existence or identity is the province of faith, not reason? If some people replace metaphysics with religous idiocy, that does not invalidate the field as such -- the same applies to all fields of rational inquiry, from physics to psychology.
I subscribe to the evidence of the senses, logic, and science. If you think any discourse about such topics belongs to the supernatural and religious, that's your problem, not mine.
The relative nature of time is kind of what Mercati is working with (I think). He's trying to describe entropy using only a self-relative framework. That is, without using fixed metrics like temperature or distance in an absolute frame of reference.
There was another article about this on HN previously.[1]
Why is it that the causal relations existing as a result of elementary forces spreading from one particle to another only ever change in one direction in time? When you map the space of the universe onto two dimensions, you see the effects of forces radiating from their origins as circles. But when you reduce all of space to one dimension and add another dimension of time, those forces only appear as cones, all pointing the same direction. What is stopping the forces from flowing backward, or even sideways in time? The math we're using gives no indication why this is the case. That's what has physicists scratching their heads.
> One of the hottest new ideas in physics is that gravity is an emergent phenomena; that it somehow arises from the complex interaction of simpler things.
well, giving that gravity, Lagrangian "minimum action", Schrodinger, etc... all basically describe the evolution of a system exactly along the local gradient of entropy increase one can say that all those are just emergent phenomena :)
> But there is a problem with identifying entropy as the driving force behind the arrow of time. For this to be true, the universe must have started out in a very low entropy state. The puzzle is: why?
It's interesting that simplicity is the goal everywhere else in science, but it's bad when it comes to the initial state of the universe. After all, having a lot of neg-entropy is just another way of saying "takes very little information to describe". The fact that the initial state is surprisingly simple to us is oxymoronic in a way.
I'm really not that well versed, but disorder and complexity seem to be used interchangeably which is incorrect:
"According to this fundamental law, entropy - and with it disorder - should always increase with time. This is manifestly not happening in the universe"
I'm completely baffled that they'd put that in their abstract or what they could possibly be thinking. In what way is entropy and disorder not increasing? And I think here is where they are confusing disorder and complexity. For a given system, as entropy initially increases, so does complexity, and then at some point, complexity peaks and falls.
A system with very low entropy is not complex. It is very easy to describe. Additionally, a system with really high entropy is also not complex, it is also very easy to describe.
Both Becker and Aaronson arrive at an interesting puzzle
> So the real question is not why the entropy is increasing, but why it was ever low to begin with
With regards to SapphireSun's question I am not a physicist either, but this seems to be a good place to start down the shape dynamics rabbit hole: https://www.youtube.com/watch?v=kwqkdvKHTlg
Needless to say, this never happens in our universe.
Thank you.