From: inteligencialtda
What is Time?
Defining time is complex, with various definitions spanning from ancient philosophy to modern physics [00:21:59]. Historically, humans first understood time through observable natural cycles, such as the changing positions of the sun and moon, and the progression of day and night [00:22:47], [00:22:50]. This led to time being perceived as a human convention based on counting these cycles that impact our daily lives [00:23:00], [00:23:03].
From a physical perspective, time remains largely an unknown [00:23:41].
Time in Physics: Relativity and Spacetime
The concept of a four-dimensional universe, where time is one of the dimensions, was first introduced by Hermann Minkowski [00:23:50], [00:59:00]. This transformed our understanding from a three-dimensional space to a unified four-dimensional “spacetime” [00:24:09], [00:24:11].
The Arrow of Time and Entropy
One of the key physical definitions of time is through entropy, based on the second law of thermodynamics [00:21:19], [00:24:20]. This law states that entropy, or the disorganization of a system, always increases [00:25:06], [01:37:02]. Most processes in the universe are irreversible, meaning they cannot be undone, similar to a broken glass not reassembling itself or an omelet not turning back into an egg [00:24:28], [00:24:42], [00:25:22]. This irreversibility defines the “arrow of time” – the direction in which physical processes occur [00:25:15], [02:20:00].
While entropy can decrease locally (e.g., a caterpillar becoming a butterfly, which appears more ordered), at the scale of the universe as a whole, it always increases [01:37:57], [01:38:37].
Relativity and Time Dilation
Einstein’s theories of relativity are crucial for understanding how time functions physically [00:56:15].
Special Relativity (Speed)
Special relativity posits that the maximum speed any massive entity can reach is the speed of light [00:56:27], [00:56:35], [00:56:38]. As a particle approaches this speed, time passes differently for that particle compared to a stationary observer [00:56:43]. This phenomenon, known as time dilation, is observable in particle accelerators like the LHC, where accelerated particles exhibit longer decay times [00:56:58], [00:57:01].
For instance, a spaceship traveling at 90% the speed of light to Alpha Centauri (4.2 light-years away) would experience a journey time of only two months for its occupants, while for observers on Earth, 4.2 years would pass [00:58:31], [00:58:44], [00:58:48]. This concept is often confusing because our perception of speed is relative; we only feel acceleration, not constant speed [01:02:51], [01:02:55].
General Relativity (Gravity)
General relativity extends this by stating that gravity also affects the passage of time [01:06:44]. The stronger the gravitational field, the slower time passes [01:30:46], [01:30:52]. This effect is crucial for technologies like GPS satellites, which require relativistic corrections to maintain accuracy [01:06:54], [01:06:57].
In the film Interstellar, characters traveling close to a black hole (Gargantua) experience extreme time dilation [01:30:09]. For them, two hours spent on Miller’s planet equated to 27 years passing on their mothership [01:32:11], [01:32:13], [01:32:16]. This illustrates a journey to the future, as those close to the strong gravitational field age much slower [01:32:22].
The Possibility and Paradoxes of Time Travel
Travel to the Future
Time travel to the future is theoretically possible through time dilation, as demonstrated by the effects of high speed and strong gravity [01:28:50], [01:32:44]. Experiments with astronauts like Scott Kelly, who spent 340 days on the International Space Station, have shown slight changes at an organic level compared to his twin brother on Earth, due to the station’s high speed [01:55:09], [01:55:31].
Travel to the Past
Travel to the past, however, is generally considered impossible within our current understanding of physics, primarily due to the second law of thermodynamics [01:40:08], [01:40:11]. This law dictates the irreversible nature of physical processes, making it impossible to “un-break” a glass or rewind time [01:36:16], [01:36:18]. If travel to the past were possible, it would violate fundamental principles like the continuous motion of motors (perpetual motion machines) [01:40:50], [01:41:05].
Time Travel Paradoxes
Several paradoxes are used to illustrate the impossibility of past travel:
- Bootstrap Paradox: This paradox involves information or an object that has no original source, creating a self-sustaining loop. For example, someone travels back in time to give the author a book, who then publishes it under their own name, making the book’s origin unknown [01:46:27], [01:48:47]. This paradox argues that past travel is impossible because it defies the principle of self-consistency [01:47:04], [01:50:12].
- Grandfather Paradox: This involves traveling to the past and altering an event that would prevent your own existence (e.g., killing your grandfather before your parent’s birth) [01:47:23], [01:47:25]. Physics suggests that if such travel were possible, attempts to alter the past would fail, ensuring the original timeline’s consistency [01:50:51], [01:50:56].
- Twin Paradox: Often misinterpreted as a paradox, this is a real phenomenon predicted by special relativity [01:53:30], [01:53:35]. It states that of two twins, the one who travels at relativistic speeds (close to the speed of light) will age slower than the one who remains on Earth [01:53:41], [01:53:50]. The resolution lies in acknowledging that the traveling twin undergoes acceleration, distinguishing their reference frame [01:58:02], [01:58:04].
“Every paradox… is not an element of a theory, it is an element to criticize a theory.” [01:47:11]
Beyond Our Perception: Theoretical Concepts of Time
Block Universe Theory
The philosophical “Block Universe” theory posits that past, present, and future all exist simultaneously [01:17:19], [01:17:22]. It envisions spacetime as a fixed, four-dimensional block, where our consciousness merely moves through it [01:16:55], [01:17:30]. While not a physics theory, it suggests that all possibilities and events are already “done” [01:17:47].
Gödel Universe
Kurt Gödel’s universe, a theoretical solution to Einstein’s equations, describes a rotating universe where time is cyclical [02:00:09], [02:00:27], [02:00:56]. In such a universe, it might be possible to return to the same point in time, even though the “arrow of time” (due to entropy increase) still moves forward [02:01:49], [02:02:04]. This theory, however, presents several problems regarding causality and entropy [02:33:36].
Wormholes (Einstein-Rosen Bridges)
Wormholes are theoretical “tunnels” through spacetime that could connect distant points in space and time [02:09:21]. Predicted by general relativity, they could allow for faster-than-light travel by “folding” spacetime [02:09:40], [02:36:51]. However, their existence would require exotic matter (negative mass) and they have not been observed [02:09:55], [02:14:47]. While a wormhole might shorten paths in space, it would not allow a traveler to return to a time before they entered it, thus respecting the second law of thermodynamics [02:14:02], [02:14:11], [02:14:17].
Observation of the Past
When we observe the night sky, we are inherently seeing the past [01:14:10]. Because light travels at a finite speed, the light from distant stars and galaxies takes time to reach us [01:14:51]. For example, light from the Sun takes 8 minutes to reach Earth, meaning we always see the Sun as it was 8 minutes ago [01:15:02], [01:15:06]. Similarly, looking at a galaxy 50 million light-years away means observing it as it was 50 million years in the past [01:16:41], [01:16:44].
Measuring Time: From Ancient Observations to Modern Science
The measurement of time has evolved significantly throughout human history [01:39:39].
Ancient Methods
Early humans marked time by observing natural cycles, particularly the phases of the moon, which are easier to track than the sun’s subtle changes in position [01:35:28], [01:35:30], [01:35:55]. Evidence suggests that as far back as 30,000 years ago, humans used bones marked with lunar phases as a form of calendar [01:35:58], [01:36:01].
Other ancient methods included:
- Sundials: Used for a very long time, these devices track the sun’s shadow to mark the passage of time [00:44:02]. Many ancient monuments were, in fact, large sundials [00:44:07].
- Lunar Calendars: The Chinese calendar, based on lunar cycles, is considered very effective, with the year beginning on the first new moon [01:33:17], [01:33:20].
- Stars and Constellations: Ancient civilizations like the Egyptians used constellations to track seasons for agricultural purposes, such as knowing when to plant along the Nile [01:38:21], [01:38:39].
- Body Measures: Simple estimations of time could be made by stretching out one’s hand and using the apparent movement of celestial bodies across the sky [01:46:27], [01:46:31].
- Astrolabes and Sextants: More advanced instruments used for navigation and measuring time by angles and positions of stars [01:49:08], [01:49:18].
Modern Measurement
Today, the standard unit of a second is defined by the decay time of a cesium-133 atom, specifically 9 billion vibrations [01:41:40], [01:42:16]. More recently, this definition has been refined to the distance light travels in a certain amount of time [01:42:41], [01:42:45]. Advances in technological advances in timerelated research, such as high-speed lasers and high-precision atomic vibration equipment, allow for measurements of incredibly small time intervals, like zeptoseconds [02:50:40], [02:50:43], [02:51:12].