What Is the Origin of Time?

Where does time truly originate? This is a common inquiry I encounter when people discover my background as a physicist. While there isn’t a concise answer, exploring the arrows of time can shed light on the topic.

This concept, emerging in the 1920s, is grounded in the principles of physics governing energy, heat, and entropy. Entropy tends to increase over time, signifying a shift from a low entropy state to a high entropy state—this represents the direction of the “arrow of time.” Often misunderstood as merely obstacles, entropy is better understood as the count of larger configurations, or macrostates, that can arise from smaller ones, known as microstates.

For example, a macrostate with mixed cutlery signifies higher entropy than one where forks are on one side and spoons on the other. Opening a drawer only to find mixed cutlery implies the arrow of time has transitioned from the past to the future.

Yet, a significant issue arises when applying cutlery analogies to the universe. Why did a pristine, low-entropy state ever exist?

This is referred to as the “past hypothesis,” and physicists are generally skeptical of it. When theorizing backward through time, they envision a universe in a state of very low entropy. Given the rarity of such conditions, the existence of such a state raises questions. Moreover, it prompts speculation on whether this state aligns with the Big Bang, the universe’s genesis.

Compounding the mystery is the fact that the laws of physics at scales far smaller than the entire universe—like subatomic particles—are entirely reversible. As Pablo Arrighi from Paris Clair University puts it, this presents a major paradox within physics.

“The laws of physics are reversible, yet our daily experiences contradict this,” he notes. Arrighi and his team set out to create a simplified “toy universe” to better understand this phenomenon.

They discovered that the arrow of time is an inevitability if this toy universe mirrors our own and exhibits constant expansion. In this model, they also discard the need for past hypotheses. The Big Bang can occur without special conditions, while the arrow of time consistently moves forward.

Interestingly, Arrighi highlights that his findings challenge previous notions, such as the potential “big crunch,” where the universe might cease expansion and collapse into a singular point.

Surprisingly, in this constructed universe bound by reversible laws, the Big Bang need not represent a singularity; rather, it opens the door to entropy-driven extensions—conceptually, an alternate universe. “Our existence stems from their emergence. Our challenges are tied to their past,” Arrighi explains regarding the imagined universe beyond the Big Bang.

Though radical, the idea of two universes expanding in opposite directions, each with its own time flow, has fascinated scientists. For instance, in 2014, independent physicist Julian Barbour and his colleagues supported this theory, using gravity studies as a foundation. Unlike Arrighi’s approach, which easily lends itself to simulation, Barbour’s model focuses more on computational arguments. Others, like Sean Carroll from Johns Hopkins University, have previously proposed moving beyond past hypotheses.

Returning to our initial question, can the answer emerge from anywhere, or perhaps from a non-specific place? Philosopher David Albert from Columbia University emphasizes careful consideration of the term “special.” He casts doubt on the assumption that the low-entropic state of the past is inherently special.

“Many believe all physical states should be equally probable. However, when viewed this way, low-entropy states appear quite improbable,” he argues. “My stance is that it’s unreasonable to determine odds a priori.” He advocates for deriving probabilities through observation instead.

Albert favors omitting past hypotheses from fundamental physics concepts, believing that adhering to superior laws is essential. However, he stresses that these insights should rest on observational evidence. The gap between systems studied meticulously, such as gas particles within boxes, and the entire universe is vast. He cautions scientists to be vigilant about the assumptions made when extrapolating from small-scale observations to universal principles.

“Nonetheless, I find it intriguing to explore whether we can derive outcomes without relying on past hypotheses. If that’s feasible, it would be a remarkable discovery,” remarks Albert.

After concluding my conversation with Albert, I plan to reconnect in a year to assess how our understanding of time evolves. Even if I can’t precisely articulate the origins of time, the arrow undoubtedly propels me towards a future rich with exploration and dialogue.