From: mk_thisisit
The concept of time remains an active area of research in physics, with scientists still grappling with its fundamental definition [09:47:00].
Time in Einstein’s Theory of Relativity
Einstein’s theory of relativity is considered the best current theory of space and time [00:11:00], [09:53:00]. In this framework, space and time are intricately woven together into what is often referred to as the “fabric of the universe,” known as spacetime [00:17:00], [09:59:00].
Measuring Time
In Einstein’s theory, the concept of distance on this spacetime surface is measured using a clock [10:08:00]. For example, a perfect stopwatch started at birth and carried through life measures one’s age [10:16:00]. This stopwatch also measures the distance traveled in spacetime over a lifetime [10:29:00].
The age of the universe is defined as the time that would be measured on a freely falling clock that began measuring time at the Big Bang [10:41:00]. This clock freely falls through the universe from the moment it was hot and dense to the present [10:54:00], [13:56:00].
The Nature of Time: Thermodynamic Time
Despite these measurements, the fundamental definition of time remains elusive [10:56:00]. While the laws of nature are symmetrical, allowing for recreation of situations forward to predict the future or backward to see the past, there is still the distinct human perception of past, present, and future [11:20:00].
This asymmetry is closely linked to the Big Bang, which was an “extremely orderly,” or low entropy, event [11:56:00]. The Second Law of Thermodynamics dictates that entropy always increases, meaning the future moves in the direction of increasing disorder [12:10:00]. This concept is referred to as “thermodynamic time” [12:23:00].
Recent research explores the concept of a “minimal clock,” with studies on clocks consisting of as few as three atoms where ticking relates to light emission from electrons jumping between energy levels [12:34:00]. These “thermodynamic machines” are understood to measure the flow of entropy [13:13:00].
Time and the Universe’s Beginning
Understanding the age of the universe leads to questions about what it means for the universe to have a beginning, which in turn requires a deeper understanding of what time itself is [14:21:00].
Our best theory for how the universe developed is called inflation, which suggests that space and time existed even before the universe became hot and dense in the “hot big bang” [14:01:00].
Beyond Spacetime
Other theories, such as quantum gravity theories, propose that space and time themselves emerge from a deeper, more fundamental theory that does not inherently include them [14:29:00]. Such theories, developed particularly in response to research on black holes, suggest that to fully understand if the universe had a beginning, we must first understand what space and time truly are [14:36:00], [14:50:00].
Before the Big Bang
It is not possible to use light to observe what happened further back in time than approximately 350 million years after the Big Bang [16:37:00]. Before this period, the universe was so hot that it was a plasma, a thick soup of hydrogen and helium nuclei and electrons, preventing light from passing through [16:05:00]. The oldest light detectable is cosmic microwave radiation, emitted around 350 million years after the Big Bang [15:51:00]. To look further back, other methods, such as potentially observing neutrons or gravitational waves, would be necessary [16:57:00].