What is time? Why is it so different from space? And where did it come from? Scientists are still stumped by these questions — but working harder than ever to answer them.
St. Augustine famously said, “If no one asks me, I know what [time] is. If I wish to explain to him who asks, I do not know.” The concept of time has long challenged scientists and philosophers, but it wasn’t until Albert Einstein that we developed a deeper mathematical understanding of time and space to explore their connections. Our internal sense of time is based on the immediate present, which encompasses only about one second and is an illusion created by our brain’s stored memories and predictions for the future. Understanding where time comes from requires us to confront the origins of the universe itself. Recent theories on the quantum nature of gravity offer unique and fantastic answers that challenge our traditional notions of time and space.
The reason we remember the past and not the future has to do with entropy – the amount of disorder in the universe. Our memories and historical records exist because entropy in the past was lower than the entropy of the present, and there are records like diaries, photographs and video records that can corroborate aspects of the past. The universe experiences an arrow of time where entropy increases with time, which is why the future is uncertain and the past is fixed. However, microscopic physics, governed by quantum mechanics, does not provide a basis for the arrow of time. Scientists think the arrow of time exists because the universe must have started out in an incredibly orderly and unlikely state, which is known as the Past Hypothesis. As the universe evolves and expands, the amount of space for possible states increases, resulting in the increase of entropy in the same direction as the universe is expanding. Our subjective experiences take place against this backdrop, and every memory is created because it takes energy to create a memory pathway, which heats up the brain and increases its entropy.
Time and space-time
Scientists want to understand how we experience time in mathematical terms that can be tested through experiments 1. In relativity, physical space is combined with time to create a four-dimensional space-time that consists of events and worldlines. Events are points within four-dimensional space-time where some physical interaction or phenomenon takes place, while worldlines track the paths objects take through space-time along a sequence of events.
Mathematically, space-time is represented as a four-dimensional continuum, but this is not representative of physical space-time. While we work with space-time as an infinite set of points between any two four-dimensional points, most of those points are devoid of anything physical, making 3D space an illusion filled with ghostly, empty points that are not connected to anything in the real world.
Physicist Lee Smolin notes that the world around us is a network of evolving relationships among events that make up the history of the world. These relationships define space, and not the other way around. In relativity, there are at least two views of the nature of objective reality. One view, known as eternalism, holds that space-time is a “block” where all the worldlines of objects exist in their entirety, with past, present, and future events all existing together. However, our inability to perceive future events suggests that the objective reality we experience is an illusion.
The Block Universe is a theoretical concept that suggests that past events still exist 1, and we could potentially revisit them with the right technology. However, this idea conflicts with the concept of free will, as every worldline of every atom and electron in the present and future is already defined. Another idea, presentism, proposes that only the present has any physicality, and the past and future do not exist.
Physicists George Ellis and Tony Rothman proposed the Crystallizing Block Universe, which combines both ideas. In this view, the past of the current moment is fixed and stored, while the future is a cloud of probabilities determined by the laws of quantum mechanics. The present is the boundary between these two regimes, and humans ride this boundary through their perception of their individual nows.
Traveling into the past would be impossible according to this model, as all the worldlines constituting an object’s history are already part of our past universe. While these discussions provide insight into time on different scales, they do not answer the fundamental question of why space-time consists of three spacelike dimensions and one timelike dimension. Through general relativity, space-time is a way of describing the gravitational field, which is subject to quantum mechanics like all other fields.
Quantum gravity
What is the Standard Model?
The Standard Model is a fundamental theory that explains how three of the four fundamental forces of nature – the electromagnetic force, the strong force, and the weak force – operate on matter particles. It describes a collection of 12 different matter particles and their antimatter twins, as well as the bosons that mediate forces between them. The Standard Model has been successful in predicting and explaining particle physics experiments, but it is incomplete in that it does not account for dark matter, dark energy, or the phenomenon of matter dominance in the universe.
What is loop quantum gravity?
Loop quantum gravity is a theory that proposes that space consists of objects called nodes that define three-dimensional space, connected by links to form networks called spin networks. Spin networks can be altered by adding and subtracting links and nodes to form a 4-dimensional network called a spin foam. This theory offers a different approach to understanding space-time from general relativity and may provide insights into the nature of time. However, loop quantum gravity is still a developing theory and has yet to be fully tested or proven.
Based on the search results provided, a spin foam is a 3+1-dimensional structure where the 1 represents the direction along which the changes between spin networks occur 1. It is a key component of the loop quantum gravity theory which proposes that space consists of nodes that define three-dimensional space, connected by links to form networks called spin networks that can be altered by adding and subtracting nodes and links to form a 4-dimensional network called a spin foam. The phenomenon that we recognize as time emerges from the successive changes in these networks, and causality allows us to assign changes in a spin foam to sequences that we interpret as space unfolding over time. Embedded in each spin network is a small collection of nodes and links that can serve as a clock for that network. The concept of time can only exist within the spin networks.
The emergence of time
The physicists Don Page and William Wootters proposed in 1983 that time is an emergent phenomenon from within our space-time and not present outside of it, which they explained through the phenomenon of quantum entanglement. If two particles are entangled, then their quantum states cannot be described independently. The wave function of the partner particle also collapses instantly, even if it has since moved to the other side of the universe. Page and Wootters suggested that an entangled system could give rise to the phenomenon of time, and scientists have begun testing this hypothesis in the lab. In 2013, experiments conducted by physicist Ekaterina Moreva showed that time emerges in a system of two entangled photons. The progression of the universe in the direction of increasing entropy since the Big Bang defines the arrow of time.
