"Why does time go forwards, not backwards?" by Martha Henriques for BBC Future

I've been meaning to share this fascinating article from BBC Future for a little while now. I am a bit of armchair physicist so this is really interesting to me. And as we have seen, modern physics also incorporates philosophy and spirituality to varying degrees, areas that I am also interested in.

Why does time go forwards, not backwards?

By Martha Henriques
3rd October 2022

The arrow of time began its journey at the Big Bang, and when the Universe eventually dies there will be no more future and no past. In the meantime, what is it that drives time ever onward?

When Isaac Newton published his famous Principia in 1687, his three elegant laws of motion solved a lot of problems. Without them, we couldn't have landed people on the Moon 282 years later. But these laws brought to physics a new problem, which wasn't fully appreciated until centuries after Newton and still nags at cosmologists today.

The issue is that Newton's laws work about twice as well as we might expect them to. They describe the world we move through every day – the world of people, the hands that move around a clock and even the apocryphal fall of certain apples – but they also account perfectly well for a world in which people walk backwards, clocks tick back afternoon to morning, and fruit soars up from the ground to its tree-branch.

"The interesting feature of Newton's laws, which wasn't appreciated till much later, is that they don't distinguish between the past and the future," says the theoretical physicist and philosopher Sean Carroll, who discusses the nature of time in his latest book The Biggest Ideas in the Universe. "But the directionality to time is its most obvious feature, right? I have photographs of the past, I don't have any photographs of the future."

The problem is not confined to the centuries-old theories of Newton. Virtually all of the cornerstone theories of physics since then have worked just as well going forward in time as they do backwards, says physicist Carlo Rovelli of the Centre for Theoretical Physics in Marseille, France, and the author of books including The Order of Time.

"Starting from Newton, and then Maxwell's theory of electromagnetism, then Einstein's work, and then quantum mechanics, quantum field theory, general relativity, and even quantum gravity – there is no distinction between past and future," Rovelli says. "Which came as a surprise, because the distinction is so evident to all of us. If you make a movie, it's obvious which way is the future and which one is the past."

How does a clear direction of time emerge from these descriptions of the Universe, which all lack their own arrow of time? As Marina Cortês, an astrophysicist at the University of Lisbon, puts it: "There's a lot of implications that start with taking seriously the question, 'Why does time pass?'"

Part of the answer lies at the Big Bang nearly 14 billion years ago. Another insight comes from the opposite extreme, in the Universe's eventual death.

But before embarking on this epic journey back and forth along the timeline of the Universe, it's worth stopping off in 1865, just as the first truly time-directional law of physics came hurtling down the tracks of the Industrial Revolution.

GATHERING STEAM

In the 19th Century, when coal was shovelled into furnaces to generate steam power, scientists and engineers hoping to develop better engines embraced a set of principles that described the relationship between heat, energy and motion. They became known as the laws of thermodynamics.

In Germany, 1865, the physicist Rudolf Clausius stated that heat cannot pass from a cold body to a hot one, if nothing else around them changes. Clausius came up with the concept he called "entropy" to measure this behaviour of heat – another way of saying heat never flows from a cold body to a hot one is to say "entropy only ever increases, never decreases."

As Rovelli stresses in The Order of Time, this is the only basic law of physics that can tell apart the past from the future. A ball can roll down a hill or be kicked back to its summit, but heat can't flow from cold to hot.

To illustrate, Rovelli picks up his pen and drops it from one hand to the other. "The reason this stops in my hand is that it has some energy, and then the energy is turned into heat and it warms up my hand. And the friction stops the bouncing. Otherwise, if there was no heat, this would bounce forever, and I would not distinguish the past from the future."

So far, so straightforward. That is, until you start to consider what heat is on a molecular level. The difference between hot things and cold things is how agitated their molecules are – in a hot steam engine, water molecules are very excited, careening around and colliding into each other rapidly. The very same water molecules are less agitated when they coalesce as condensation on a windowpane.

Here's the problem: when you zoom in to the level of, say, one water molecule colliding and bouncing off another, the arrow of time disappears. If you watched a microscopic video of that collision and then you rewound it, it wouldn’t be obvious which way was forwards and which backwards. At the very smallest scale, the phenomenon that produces heat – collisions of molecules – is time-symmetric.

This means that the arrow of time from past to future only emerges when you take a step back from the microscopic world to the macroscopic – something first appreciated by the Austrian physicist-philosopher Ludwig Boltzmann.

"So the direction of time comes from the fact that we look at big things, we don't look at the details," says Rovelli. "From this step, from the fundamental microscopic vision of the world to the coarse-grained, the approximate description of the macroscopic world – this is where the direction of time comes in.

"It's not that the world is fundamentally oriented in space and time," Rovelli says. It's that when we look around, we see a direction in which medium-sized, everyday things have more entropy – the ripened apple fallen from the tree, the shuffled pack of cards.

While entropy does seem to be inextricably bound up with the arrow of time, it feels a bit surprising – perhaps even disconcerting – that the one law of physics that has a strong directionality of time built into it loses this directionality when you look at very small things.

"What is entropy?" Rovelli says. "Entropy is simply how much we're forgetting about the microphysics, how much we are forgetting about the molecules."

THE BEGINNING AND THE END

If there is an arrow of time, where did it come from in the first place?

"The answer is embedded in the beginning of the Universe," says Carroll. "The answer is because the Big Bang had low entropy. And still, 14 billion years later we are swimming in the aftermath of that tsunami that started near the Big Bang. That's why time has a direction for us."

The extraordinarily low entropy of the Universe at the Big Bang is both an answer and an enormous question. "The thing we understand the least about the nature of time, is why the Big Bang had low entropy, why the early Universe was like that," says Carroll. "And I think honestly, as a working cosmologist, I think that my fellow cosmologists have dropped the ball on this one. They don't really take that problem seriously enough."

Carroll published a paper in 2004 with his colleague Jennifer Chen, in which they aimed to explain why the Universe had such low entropy close to the Big Bang, rather than just assuming or accepting this was the case. "There's plenty of loopholes in the theory, plenty of aspects of it that are not completely baked – but I also think it is by far the best theory on the market," says Carroll. "It doesn't cheat."

Other cosmologists agree that it is indeed time to turn serious thought to this problem of the Universe's low entropy origins. "The likelihood of our current Universe having initial conditions of this kind, and not any other kind, is around one in 10 to the 10 to 124 (1:10^10^124)," says Cortês. (Another way of saying it is that the event had a probability of 0.00…01 – with 10^(10^124) zeroes omitted – a number so large it's awkward to express in conventional maths, Cortês notes.) "I mean I could safely say, this is the largest number in modern physics, outside of philosophy or mathematics."

Simply taking such unlikely low-entropy origins as given is a grand case of "shoving the problem under the rug", Cortês says. "If physicists keep doing this, after a while it's going to be a very big pile under the rug. It's left to us cosmologists to explain why time only moves forward."

Even if we don't yet know why, the Universe's low entropy past is a plausible source of time's arrow. Like most things that have a beginning, the arrow will also have an end. The first person to spot this was, once again, the Austrian physicist Ludwig Boltzmann.

"Boltzmann thought, 'ah, entropy is growing in the Universe and maybe it's going to maximum at some point'," says Rovelli. At that point, heat would be evenly distributed throughout the Universe, no longer flowing from one place to another.

There would be no energy available in a useful form for doing work – in other words, almost nothing interesting would be happening throughout the entire Universe. As astrophysicist Katie Mack describes it, "As that process continues, everything is decaying so much that all that’s left is the waste heat of everything that ever existed in the Universe." This fate is known as the thermal death of the Universe, or heat death.

"Stars will stop burning, nothing will happen anymore. There will be nothing but small thermal fluctuations," says Rovelli. "Suppose this happens – and we don't know for certain if it's going to happen, but suppose it does – should we say that there is no time direction there? Of course there's no time direction, because every phenomenon that happened one way could also go one way or the other. Nothing will distinguish the two directions of time."

This is perhaps the strangest thing about the arrow of time: "It only lasts for a little while," says Carroll.

It's very hard to picture what might happen if the arrow of time eventually vanishes. "When we think we produce heat in our neurons," says Rovelli. "Thinking is a process in which the neuron needs entropy to work. Our sense of time passing is just what entropy does to our brain."

The arrow of time that arises from entropy brings us a long way closer to understanding why time only goes forward. But there may be more arrows of time than this one – in fact there is arguably an entire volley of arrows of time pointing from the past to the future. To understand these, we have to step from physics into philosophy.

HUMAN TIME

The ways that we intuitively understand and experience time shouldn't be taken lightly, says Jenann Ismael, professor of philosophy at Columbia University, New York. If you think about your own experience of time, you may soon be able to recognise several of the psychological arrows that form a core part of human experience. One of these arrows is what Ismael terms "flow".

"If you look out at the world, you don't experience a purely static representation of the instantaneous state of the world," she says, like in a movie made up of a number of static frames every second. "We see directly that the world is changing."

This experience of the flow of time is built into our perception. "Vision isn't like a movie camera at all," says Ismael. "Actually what happens is your brain is collecting information over some temporal period. It's integrating that information so that at any given moment, what you're seeing is a computation that the brain has done. So that you not only see that things are moving, you see how fast they're moving, the direction in which they're moving. So the whole time, your brain is integrating information over temporal intervals and giving you the result. So you see time, in a way."

There's a second feature of time that Ismael distinguishes from flow, which she terms "passage".

The idea of passage is closely bound up with time-oriented experiences such as memory and anticipation. Take the example of a wedding, or any much-anticipated life event. Our experience of these moments has many layers – from the fractious planning stages, to the intensity of the day itself, to recollections that stay with us for years. There is a directionality to these different experiences: the way we anticipate an event in the future is fundamentally different from how we remember it when it's passed.

"All of that is part of what I think of as the experience of passage, this idea that we experience every event as anticipated from the past, experienced in the present, remembered in retrospect," says Ismael. "It's kind of Proustian in its density."

These aspects of the directionality of psychological time – as well as many others, like the sense of openness we have about the future but not the past – could all trace their roots back to the arrow of time born of the Industrial Revolution.

"I think it does all come back to entropy," says Ismael. "I see no reason now to think that the kinds of arrows that are involved in human psychology are anything but ultimately rooted in the entropic arrow. But it's an empirical question. This project to understand human experience in relation to the entropic arrow, I've no reason to think it's going to fail."

That project is what Carroll hopes to do, taking several features of our experience of time and relating them back to entropy. His first target is causality, another element of the arrow of time, as causes happen before their effects.

To say the least, this project is a major undertaking for all physicists and philosophers involved. And still, lurking in the shadows behind all such efforts, there remains that nagging question about why entropy was so low in the earliest Universe.

"I think we understand why we have this sense of flowing," says Rovelli. "We understand why the past seems fixed to us that the future seems open. We understand why there are irreversible phenomena, and we can reduce all that to the second law of thermodynamics, to the rise of entropy.

"It's very much related to the fact that if we trace it back, back, back, to fact that the Universe started very small, in a very peculiar situation. Then somehow, it's falling down from that peculiar situation.

"But of course there's one question open, I mean, why? Why did it start in that particular way?"

Link to original article here:
https://www.bbc.com/future/article/20221003-why-does-time-go-forwards-not-backwards

Proof of The Big Bang

A huge scientific breakthrough was announced yesterday. The twisting gravity waves from the Big Bang have been detected and seen for the first time! This is being hailed as Nobel worthy and will certainly add to the knowledge base for a Unified Theory of Everything! VERY exciting!


Cosmic inflation: 'Spectacular' discovery hailed
By Jonathan Amos Science correspondent, BBC News

Scientists say they have extraordinary new evidence to support a Big Bang Theory for the origin of the Universe.

Researchers believe they have found the signal left in the sky by the super-rapid expansion of space that must have occurred just fractions of a second after everything came into being.

It takes the form of a distinctive twist in the oldest light detectable with telescopes.

The work will be scrutinised carefully, but already there is talk of a Nobel.

"This is spectacular," commented Prof Marc Kamionkowski, from Johns Hopkins University.

"I've seen the research; the arguments are persuasive, and the scientists involved are among the most careful and conservative people I know," he told BBC News.

The breakthrough was announced by an American team working on a project known as BICEP2.

This has been using a telescope at the South Pole to make detailed observations of a small patch of sky.

The aim has been to try to find a residual marker for "inflation" - the idea that the cosmos experienced an exponential growth spurt in its first trillionth, of a trillionth of a trillionth of a second.

Complete article from BBC News here:
http://www.bbc.com/news/science-environment-26605974
and the BICEP2 findings here:
http://bicepkeck.org/index.html#papers

Older and Slower! Now With More Dark Matter!

(CNN) -- How cute was our universe as a baby? We now know better than ever: The picture of our early universe just got sharper and tells scientists with greater precision many important facts about how the universe evolved.

This new photogenic moment, released Thursday, comes courtesy of the European Space Agency's Planck space telescope, which detects cosmic microwave background radiation -- the light left over from the Big Bang. Scientists used data from Planck to create an artificially colored map of temperature variations across the sky in the early universe, in more detail than ever before.

"It's a big deal," said Charles Lawrence, Planck project scientist at NASA's Jet Propulsion Laboratory, in a news briefing. He added, "We can tie together a whole range of phenomena that couldn't be tied together so well before, and the sum total of that, the impact, is felt in many, many ways."

The light is technically from 380,000 years after the Big Bang, but that's still infancy when you consider that, according to the new data, the age of the universe is about 13.8 billion years.

"By the matching observations from Planck to predictions from models, we can assemble a surprisingly detailed picture of the universe as it was one nano-nano-nano-nanosecond after the Big Bang," said Marc Kamionkowski, professor of physics and astronomy at John Hopkins University.

Kamionkowski compared the Planck map to the Human Genome Project in terms of its importance for cosmology.

After analyzing the new data, scientists now believe that the universe is about 100 million years older than they thought.

The universe's light started out as a white hot glow and would have been blindingly bright if anyone had been around to see it, Lawrence said.

But since the Big Bang, that hot light has cooled significantly, and the universe itself has expanded by a factor of 1,100. The light has cooled so much that we can't see it, but Planck can detect subtle variations in temperature, which give scientists a wealth of information. By subtle, we mean about one-hundred-millionth of a degree.

The colors in the temperature map image that scientists released Thursday were arbitrarily chosen to show these intensity variations, Lawrence said. Red means a little bit warmer than average, blue means cooler than average, and white is average.

Planck data also suggest that our universe has more dark matter than previously thought. A full 26.8% appears to be dark matter, an invisible phenomenon that scientists have only been able to detect indirectly; experiments both in space and at the Large Hadron Collider are hoping to pin it down.

It appears that ordinary matter -- all of the stuff that we can see, such as planets and stars -- makes up only 4.9% of all the universe.

The rest of the universe is an even more mysterious phenomenon called dark energy, which has also never been detected and appears to be in less abundance than researchers thought.

Scientists said the rate at which the universe is expanding, based on these observations, is 67.15 kilometers per second per megaparsec, a unit of vast distance in space (1 megaparsec = 3.3 million light years). That's significantly less than what had been calculated previously (73.8 km/sec/Mpc). This number, known as the Hubble constant, describes the acceleration of the stretching of spacetime.

The discrepancy between these Hubble constants will likely attract a lot of attention in the scientific community and is one of the most exciting parts of the new data, said Martin White, a scientist with the Planck mission based at the University of California, Berkeley.

"The hope would be that this is actually pointing toward some deficiency in the models, or some extra physics that we're not aware of, and maybe spark a whole new research direction," White said.

One theory that could be explored is that the nature of dark energy, which scientists think is causing the accelerated expansion of the universe, is different from the simplest human-calculated models. Is dark energy increasing with time over some volume of space? That's a radical theory, though, White said, and there are other possibilities.

Another anomaly of these results is that temperature fluctuations are not uniform across the sky map. There are more variations in one direction than in another.

"Perhaps we could say that our universe has thrown us a curve ball, and it rarely fails to surprise us," said Krzysztof Gorski, Planck scientist at NASA's Jet Propulsion Laboratory.

Scientists ran 10 million computer simulations and chose from among them the best match to the new data, White said. Out of those, they found a good match describing important statistics about the universe.

The Planck telescope is aboard a spacecraft that launched in May 2009. It is not circling the Earth but orbits a point in the Sun-Earth system called the second Lagrange point.

The Planck mission helps to nail down many of the parameters that other experiments must know to explore aspects of the universe, such as its expansion history, White said.

New analyses are based on the first 15.5 months of data from this mission, which is run principally by the European Space Agency. NASA is a partner of the project.

Planck represents the third generation of attempts to map the cosmic microwave background. The first was COBE, launched in 1989, followed by WMAP, launched in 2001. Comparing the resulting maps shows just how much better the maps have gotten with each successive satellite.

"This is a beautiful illustration of how science works," Lawrence said. "Make a measurement, learn from it, make a better measurement, learn from it."

By Elizabeth Landau
http://www.cnn.com/2013/03/21/tech/innovation/universe-planck-map/index.html?hpt=hp_c3

One Step Closer To Other Universes

Astronomers Find First Evidence Of Other Universes



There's something exciting afoot in the world of cosmology. Last month, Roger Penrose at the University of Oxford and Vahe Gurzadyan at Yerevan State University in Armenia announced that they had found patterns of concentric circles in the cosmic microwave background, the echo of the Big Bang.

This, they say, is exactly what you'd expect if the universe were eternally cyclical. By that, they mean that each cycle ends with a big bang that starts the next cycle. In this model, the universe is a kind of cosmic Russian Doll, with all previous universes contained within the current one.

That's an extraordinary discovery: evidence of something that occurred before the (conventional) Big Bang.

Today, another group says they've found something else in the echo of the Big Bang. These guys start with a different model of the universe called eternal inflation. In this way of thinking, the universe we see is merely a bubble in a much larger cosmos. This cosmos is filled with other bubbles, all of which are other universes where the laws of physics may be dramatically different to ours.

These bubbles probably had a violent past, jostling together and leaving "cosmic bruises" where they touched. If so, these bruises ought to be visible today in the cosmic microwave background.

Now Stephen Feeney at University College London and a few pals say they've found tentative evidence of this bruising in the form of circular patterns in cosmic microwave background. In fact, they've found four bruises, implying that our universe must have smashed into other bubbles at least four times in the past.

Again, this is an extraordinary result: the first evidence of universes beyond our own.

So, what to make of these discoveries. First, these effects could easily be a trick of the eye. As Feeney and co acknowledge: "it is rather easy to find all sorts of statistically unlikely properties in a large dataset like the CMB." That's for sure!

There are precautions statisticians can take to guard against this, which both Feeney and Penrose bring to bear in various ways.

But these are unlikely to settle the argument. In the last few weeks, several groups have confirmed Pernose's finding while others have found no evidence for it. Expect a similar pattern for Feeney's result.

The only way to settle this will be to confirm or refute the findings with better data. As luck would have it, new data is forthcoming thanks to the Planck spacecraft that is currently peering into the cosmic microwave background with more resolution and greater sensitivity than ever.

Cosmologists should have a decent data set to play with in a couple of years or so. When they get it, these circles should either spring into clear view or disappear into noise (rather like the mysterious Mars face that appeared in pictures of the red planet taken by Viking 1 and then disappeared in the higher resolution shots from the Mars Global Surveyor).

Planck should settle the matter; or, with any luck, introduce an even better mystery. In the meantime, there's going to be some fascinating discussion about this data and what it implies about the nature of the Universe. We'll be watching.

From the Physics arXiv blog, on the Technology Review published by MIT
http://www.technologyreview.com/blog/arxiv/26132/