Worth reading article by Andrew Grant theorizing that universe may have no beginning and no end. ( f. Sheikh)
For nearly 140 years, scientists have tried to rule out the backward flow of time by way of nature’s preference for disorder. Left alone, nature transforms the neat into the messy, a one-way progression that many physicists have used to define time’s direction. But if nature prefers disorder now, it always has. The challenge is figuring out why the universe started out so orderly — thereby allowing disorder to grow and time to march forward — when the early universe should have been messy. Despite many proposals, physicists have not been able to agree on a satisfying explanation.
A new paper offers a solution. The secret ingredient, the authors say, is gravity. Using a simple simulation of gravitationally interacting particles, the researchers show that an orderly universe should always arise naturally at one point in time. From there, the universe branches in opposing temporal directions. Within each branch, time flows toward increasing disorder, essentially creating two futures that share one past. “It’s the only clear, simple idea that’s been put forward to explain the basis of the arrow of time,” says physicist Julian Barbour, a coauthor of the study published last October in Physical Review Letters.
It may be clear and simple, but it’s far from being the only idea attempting to explain the mystery of time’s arrow. Many scientists (and philosophers) over the decades have proposed ideas for reconciling nature’s time-reversible laws with time’s irreversible flow. Barbour and colleagues admit that the arrow of time issue is far from settled — there’s no guarantee that their simple simulation captures all the complexities of the universe we know. But their study offers an unusually elegant mechanism for explaining time’s arrow, along with some tantalizing implications. Attacking the arrow-of-time mystery along the lines Barbour and colleagues suggest may reveal that the universe is eternal.
Mixing marbles
Nobody knows exactly why time doesn’t flow backward. But most scientists have suspected that the explanation depends on the second law of thermodynamics, which describes nature’s fondness for messiness. Consider a jar containing 100 numbered marbles, 50 of them red and 50 blue. Someone with way too much free time then takes a picture of every possible arrangement of the marbles (yes, this would take far longer than a human lifetime) and creates a giant collage. Even though every photo depicts a different arrangement of numbered marbles, the vast majority of images would look very similar: a jumble of red and blue. Very few photos would have all the red marbles on one side of the jar and all the blue on the other. A photo picked at random would be far more likely to show a state of disorder than one of order.
Physicists in the 19th century recognized this propensity for disorder by thinking about the flow of heat in steam engines. When two containers of gas are exposed to each other, the faster-moving molecules of the higher-temperature container (think the blue marbles) tend to mix with the slower molecules (red marbles) of the cooler container. Eventually the combined contents of the containers will settle at an equilibrium temperature because a disordered state of blended hot and cold is most likely.
In the mid-19th century, physicists introduced the notion of entropy to quantify the disorder of a heat-shifting system. Austrian physicist Ludwig Boltzmann sharpened the definition by relating entropy to the number of ways that one could arrange microscopic components to produce an indistinguishable macroscopic state. The jar with segregated red and blue marbles, for example, has low entropy because only a few arrangements of the numbered marbles could produce that color pattern. Similarly, there are many combinations of speedy and sluggish molecules that will produce a gas at equilibrium temperature, the highest possible entropy. The fact that there are far more ways to achieve high entropy than low provides the foundation for the second law of thermodynamics: The entropy of a closed system tends to increase until reaching equilibrium, the maximum state of disorder.
The second law explains why cream easily mixes into coffee but doesn’t unmix, and why Humpty Dumpty won’t spontaneously reassemble after his fateful fall. Crucially, the second law also defines a thermodynamic arrow of time. The drive toward maximum entropy is an irreversible process in a universe governed by time-reversible physical laws. The second law suggests that time flows from past to present to future because the universe is progressing from an ordered low-entropy state to a disordered high-entropy one.
https://www.sciencenews.org/article/arrow-time
