Time Moves Backwards and Other Space-Time Headlines

Image Source: New Scientist.

There are several notable, mind-bending and possibly related recent scientific headlines. Four new elements have been confirmed, completing the seventh row of the periodic table (ununtrium (Uut or element 113), ununpentium (Uup, element 115), ununseptium (Uus, element 117), and ununoctium (Uuo, element 118)). The Guardian confirms that these elements are synthetic, created by "slamming lighter ­nuclei into each other and tracking the following decay of the radioactive superheavy elements. Like other superheavy elements that populate the end of the periodic table, they only exist for fractions of a second before decaying into other elements."

Then there is a report that time arises organically and moves in different directions in adjacent multiverses. The idea rests on problems with the way we define time as a function of the behaviour of matter, with the parts we can't explain disappearing into universes we cannot see. The associated notion that time flows backwards comes from Sean Carroll at the California Institute of Technology at Pasadena and Alan Guth at MIT. On 13 January 2016, The New Scientist reported:

Guth and Carroll's work is motivated by a problem vexing physicists and philosophers: why it is that time's arrow points in just one direction. It's true we can only remember the past ... but the laws of physics don't much care which way time flows: any physical process run backwards still makes sense according to those laws.

There's no such thing, at a very deep level, that causes [must] precede effects, says Carroll.

In the absence of other laws to set the direction of time, physicists have settled on entropy – basically, a measure of messiness. As entropy grows, time ticks forward. For example, you can stir milk into coffee but you can't stir it back out again – so neatly separated black coffee and milk always comes first.

“We can't talk to beings in a time-reversed cosmos: they are in our past and we in their past.” Zooming out to the entire universe, we likewise define the future as that direction of time in which entropy increases. By studying the motion of faraway galaxies, we can predict how the cosmos will evolve. Or we can rewind time back to the big bang, when the universe must have had much less entropy.

Try to rewind further and we meet a cosmological conundrum. We can't proceed if the big bang was indeed the beginning of time, but in that case, why did it have such low entropy? And if it wasn't the beginning of time – as Guth suspects – we'd still want to know how an eternal universe could have reached such a low-entropy state that would allow for the arrow of time to form.

In an as yet unpublished model, Guth and Carroll explore the latter idea. They drop a finite cloud of particles, each zipping around with its own randomly assigned velocity, into an infinite universe. After a while, arrows of time emerge spontaneously.

The random starting conditions mean that half the particles initially spread outwards, increasing entropy, while the other half converge on the centre, decreasing entropy, then pass through and head outwards. Eventually the whole cloud is expanding, and entropy is rising in tandem. Crucially, this rise happens even if you reverse time by flipping the starting velocity of every particle: ultimately, all particles will end up travelling outwards. If entropy grows either way, who's to say which way the arrow of time should point?

We call it the two-headed arrow of time, Guth says. Because the laws of physics are invariant, we see exactly the same thing in the other direction.

The model shows that an arrow of time arises spontaneously in an infinite, eternal space. Since this allows entropy to grow without limit, time zero could simply be the moment where entropy happened to be at its lowest.

That could explain why the big bang, the earliest moment we can see, has so little entropy. But it also feels a little like a cheat: if entropy can be infinite, anything can have relatively low entropy by comparison.

The point that Alan and I are trying to make is that it's very natural in those circumstances that almost everywhere in the universe you get a noticeable arrow of time, Carroll says, though he admits the model still needs work. Then of course you do the work of making it realistic, making it look like our universe. That seems to be the hard part.

If the model matches reality, it would have implications for more than just our own observable universe. This is intended to describe the whole of existence, which would mean the multiverse, Guth says. In his view, the arrow of time may have arisen in a parent or grandparent universe of our own.

In the next headline, supermassive black holes might be hiding whole universes inside them. The New Scientist:

Black holes may be hiding other universes. A quirk of how space-time behaved in the early universe could have led to short-lived wormholes connecting us to a vast multiverse. If borne out, the theory may help explain how supermassive black holes at the centres of galaxies grew so big so fast. The idea that ours is just one of a staggering number of universes - what cosmologists call the multiverse - is a consequence of our leading theory of how the universe grows: eternal inflation.

The theory holds that during its early phase, space-time expanded exponentially, doubling in volume every fraction of a second before settling into a more sedate rate of growth. Eternal inflation was devised in the 1980s to explain some puzzling observations about our universe that standard big bang theory alone couldn't handle.

But cosmologists soon realised that the inflationary universe came with caveats. Quantum mechanical effects, which normally only influence the smallest particles, played an important role in how all of space-time evolved.

One of these effects was that a small patch of space-time within the larger universe could shift into a different quantum state, forming a bubble. Such bubbles could form at random throughout our inflating universe. [Thus:] “Our universe could even look like a black hole to physicists in some other universe”

In other news, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) which has detectors in Hanford, Washington, and Livingston, Louisiana, USA, may have discovered the existence of gravitational waves, which are ripples in the fabric of space-time. Wired:

Gravitational waves tell the story of the universe’s mass. Every object from black hole to supernova, everything from black hole collisions (the most likely explanation for this potential LIGO discovery) to superfast expansion of the universe has its own gravitational fingerprint. From those swirls, astronomers will be able to learn about spacetime, gravity, and the objects themselves. And no one knows what they’ll find out.

Critics urge caution about this report, which might have more to do with the internal workings of the cosmology profession than the workings of space-time. If true, it could be a huge discovery, proving the "last unproven prediction of Einstein's theory of general relativity." The Guardian:

According to the rumours, [LIGO] scientists on the team are in the process of writing up a paper that describes a gravitational wave signal. If such a signal exists and is verified, it would confirm one of the most dramatic predictions of Albert Einstein’s century-old theory of general relativity.

[Professor Lawrence] Krauss[, cosmologist at Arizona State University,] said he was 60% confident that the rumour was true, but said he would have to see the scientists’ data before drawing any conclusions about whether the signal was genuine or not.

Researchers on a large collaboration like Ligo will have any such paper internally vetted before sending it for publication and calling a press conference. In 2014, researchers on another US experiment, called BICEP2, called a press conference to announce the discovery of gravitational waves, but others have since pointed out that the signal could be due entirely to space dust.

Speaking about the LIGO team, Krauss said: “They will be extremely cautious. There’s no reason for them to make a claim they are not certain of.”

If gravitational waves have been discovered, astronomers could use them to observe the cosmos in a way that has been impossible to date. “We would have a new window on the universe,” Krauss said. “Gravitational waves are generated in the most exotic, strange locations in nature, such as at the edge of black holes at the beginning of time. We are pretty certain they exist, but we’ve not been able to use them to probe the universe.” Einstein predicted that the waves would be produced in extremely violent events, such as collisions between two black holes. As gravitational waves spread out, they compress and stretch spacetime.