Astronomy.com: The struggle to find the origins of time

Quantum entanglement — when the states of two particle are intertwined by the laws of quantum mechanics — has long vexed physicists. But the phenomenon may also hold the key to understanding how time emerged.  Jurik Peter/Shutterstock

St. Augustine said of time, “If no one asks me, I know what it is. If I wish to explain to him who asks, I don’t know.” Time is an elusive concept: We all experience it, and yet, the challenge of defining it has tested philosophers and scientists for millennia. 

It wasn’t until Albert Einstein that we developed a more sophisticated mathematical understanding of time and space that allowed physicists to probe deeper into the connections between them. In their endeavors, physicists also discovered that seeking the origin of time forces us to confront the origins of the universe itself.

What exactly is time, and how did it come into being? Did the dimension of time exist from the moment of the Big Bang, or did time emerge as the universe evolved? Recent theories about the quantum nature of gravity provide some unique and fantastic answers to these millennia-old questions.

Just glancing at these diagrams makes it immediately obvious which scene came before and which one came after. That’s because of entropy, which observes that the amount of disorder in the universe increases over time. Curiously, there is currently no good explanation for why entropy was lower in the past than it is now — the laws of physics describe a pile of blocks springing up to form a tower just as well as they describe a structure of blocks falling down.
Astronomy: Roen Kelly

Our world features an arrow of time where entropy increases with time. This accords with our sense of time as a one-way street, from past order to future disorder. Yet, there is no basis for the arrow of time in microscopic physics — the realm of quantum mechanics. Those equations are just as valid when time runs in reverse. Therefore, some scientists think the arrow of time exists because the universe must have started out in an incredibly orderly and unlikely state. This is called the Past Hypothesis.

Time and space-time

Of course, scientists want to understand how we experience time in mathematical terms that can be tested through experiments. In relativity, the three dimensions of physical space are combined with the one dimension of time into a four-dimensional space-time. The basic elements of space-time are events and worldlines. Events are points within four-dimensional space-time at which some physical interaction or phenomenon takes place, such as two particles colliding or a particle emitting a photon. Worldlines are the paths objects trace through space-time along a sequence of events.

Quantum gravity

The Standard Model is our fundamental theory of how three of the forces of nature — electromagnetism and the strong and weak forces — operate on a collection of 12 different matter particles (and their antimatter twins). This model describes quantum fields that exchange particles that mediate forces (bosons) between matter particles (fermions) and produce complex structures such as atoms.

The Standard Model is so successful that experiments at the Large Hadron Collider at CERN, operating at energies up to 14,000 GeV, have been unable to find any significant deviations from calculated predictions. But we know that the Standard Model is incomplete because it has no room for several phenomena we observe. These are dark matter, the invisible stuff that glues galaxies together; dark energy, the mysterious repulsive energy driving the ever-faster expansion of the universe; and any mechanism to explain either cosmological inflation, the exponential expansion of the universe in its early stages, or the fact that we live in a universe dominated by matter instead of equal amounts of matter and antimatter.

The bottom line

Our experience of time may be subjective and limited to a sense of now, but on the cosmic scale, time seems to be a feature of entangled relationships between objects and not a feature from outside our universe. The arrow of time is a consequence of the increasing entropy of an expanding universe since the Big Bang. It appears this precludes us from remembering the future. But at least we have our memories, courtesy of the steady march of entropy, which allows us to recover past events and stitch them into a consistent story. Lucky for us, our universe seems to have a consistent story to tell in the first place!

Astronomy

The figure showing before and after events agrees with our experience and common sense: systems progress naturally with the passage of time from specified arrangements to random, disordered arrangements–never the other direction.

Scripture agrees with concepts derived from physics theory and observation: time had a beginning. “We declare God’s wisdom, a mystery that has been hidden and that God destined for our glory before time began.” (I Corinthians 2:7, emphasis added)

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