Eins(time)
Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.
Einstein on the passing of his friend Michele Besso (1955).
Albert Einstein was born in 1879 in Ulm (German Empire). In 1895 he moved to Switzerland to continue his studies. Soon after, Einstein gave up his German citizenship (purportedly to avoid military service). In 1990 he graduated from the Swiss Federal Polytechnic School in Zürich, but he struggled to find a job. In the end, Einstein was employed at the Swiss Patent Office in Bern through some connections. From 1902 to 1909, Einstein’s job was to evaluate patents. While doing so, he managed to publish four truly disruptive research articles (in this order, with titles translated to English):
On a Heuristic Viewpoint Concerning the Production and Transformation of Light
This study about the photoelectric effect suggested that energy exchange occurs in discrete amounts: “quanta.” This article was essential for the subsequent development of quantum theory and largely responsible for Einstein’s Nobel Prize in Physics in 1921.
In this piece about Brownian motion, Einstein provided the empirical evidence for the atomic theory.
On the Electrodynamics of Moving Bodies
Einstein proposed a theory about how objects move in space that emended some holes in more than two centuries of Newtonian physics. The special theory of relativity was based on two postulates:
1. The laws of physics are identical in all frames of reference (when there is no acceleration).
2. The speed of light in a vacuum is the same for all observers: This is true whether any of the observers (or the light source) are moving or not.
and finally:
Does the Inertia of a Body Depend Upon Its Energy Content?
This article presented “the world’s most famous equation”: E = mc2. The mass-energy equivalence is derived from special relativity theory, stating that a body’s mass (m) is a direct measure of its energy content (E). This formula can be used, for example, to predict the amount of energy released (or consumed) by nuclear reactions. One only needs to measure the mass of all constituents and the products and multiply the difference between the two by the square of the speed of light (c). This equation implies that light speed is the cosmic speed limit. As an object approaches the speed of light, its mass becomes infinite, and it is unable to go faster than light (which is made of massless photons).
Special relativity
Einstein’s research rapidly became accepted and implemented in the scientific community due to its direct application to the new fields of atomic and nuclear physics and quantum mechanics. Special relativity produced a genuine scientific revolution with some bizarre consequences:
Relativity of simultaneity:
There is no absolute universal time. Instead, the time between two events located at different points in space is dependent on a specific reference frame. In other words, two events can occur simultaneously for an observer while happening at different times for another one. To explain this phenomenon, Einstein presented the example of a moving train as seen from two observers’ perspectives: one person inside the train and another looking at the train passing by from the station. Imagine the train car at a point along the track between two trees. If a bolt of lightning hits both trees simultaneously, the person at the station would see simultaneous strikes. However, due to the train’s motion, the train’s person would see the bolt hitting the tree ahead before the other tree.
Time dilation:
Time slows down or accelerates depending on your speed (relative to someone else). For example, imagine the light from a distant star traveling through space at its constant speed of ~1 billion kilometers per hour (c = 1,080,000,000 km/h or 671,000,000 miles per hour). You are standing in your apartment looking at a star. In contrast, an astronaut is traveling in a spaceship towards that star, let’s say at half the light speed (c/2= 540,000,000 km/h). Before Einstein, one would assume that whereas you would observe the light coming at you at its regular speed (1,080 million km/h), the astronaut would see it coming towards her at 1.5 times the light speed (1,620 million km/h). It makes sense, right? If you drive your car on the highway at 50 km/h and another vehicle is coming towards you at 100 km/h, both drivers feel that they are approaching the other at 150 km/h.
The second principle of special relativity states that the astronaut would see the light coming at the same speed as you are from your stationary apartment. How is that possible? It can only be explained if the astronaut’s clock is ticking at a slower rate than yours. Of course, for the astronaut, it will be your clock the one doing strange things.
Length contraction:
Objects moving at speeds close to the speed of light become shorter (along the dimension of motion). At least, this how someone at rest (relative to the moving object) would see it. For example, if our astronaut turns around and comes back to Earth at high speed, you’ll see her spaceship become shorter.
Einstein’s special relativity implies that we live in a space-time continuum. Space and time are relative to each other. Every event that ever happened, that is happening, or that will happen, can be defined by four coordinates in this four-dimensional (4D) world: the three dimensions of the space (left and right, up and down, forward and backward), plus the time.
General relativity
During 1907-1915 Einstein developed the “general theory of relativity,” a more comprehensive version of his theory enriched with others’ contributions. Einstein determined that massive objects distort space-time. We feel it as gravity. Until then, the concept of “special relativity” didn’t exist. Hence, it became necessary to describe a special case of general relativity in the absence of gravity: Minkowski’s “flat” space. Thus, general relativity introduced the concept of a “curved” space-time.
Initially, general relativity didn’t seem as disruptive as special relativity. However, starting in the 1960s, general relativity became an essential tool for modern astrophysics. For example, it provided the theoretical foundation for understanding the concept of black holes, regions of space with such strong gravity that not even light can escape. Within a few years, new cosmic phenomena were discovered: quasars (1963), pulsars (1967), and the first actual black holes (1971). Wormholes—portals to other universes—remain elusive.
As it is usually the case, Einstein’s new perspectives were built on the shoulders of previous works from others, such as Albert Michelson (speed of light), Hendrik Lorentz and Henri Poincaré (transformation equations in which special relativity was based), or Hermann Minkowski—Einstein’s former professor who conceived the idea of a 4D space-time. Furthermore, there is evidence that Einstein collaborated with his first wife, Mileva Marić, on the development of some aspects of special relativity. Subsequently, Einstein’s work drew the attention of prestigious scholars, such as Max Planck, who, starting in 1906, helped spread the word about Einstein’s “relative theory,” and later on, developed the quantum theory based on Einstein’s works on the photoelectric effect. However, it is remarkable that Einstein published his four groundbreaking studies in just one year while he was a full-time employee at a patent office. That same year, Einstein (age 26) was also awarded his Ph.D. by the University of Zurich. 1905 was Einstein’s “annus mirabilis.”
Theories of special and general relativity provide the foundation of a better practical understanding of our universe. For example, GPS satellites (the same giving us driving instructions) spin around Earth twice a day at around 14,000 km per hour (!). The relative speed difference causes time dilation to make the satellite clocks tick 7 microseconds slower than ours here down on Earth. In addition, these satellites are around 20,000 km above Earth. Hence, they experience slightly less gravity, making them tick slightly faster (~45 microseconds/day). The net differences are minimal (the satellite’s clocks are ~45 microseconds faster each day). However, this is enough to cause wrong geo-positioning if the engineers don’t adjust for the “time difference.”
Regarding general (theory of) relativity, some phenomena predicted by its warping have been already confirmed. These are two real examples:
Gravitational lensing: The light around a massive object (e.g., black hole) bents causing it to act as a lens showing things behind it. Astronomers use this effect to study galaxies hind beyond these massive objects.
Distortion of space-time around rotating bodies: The spin of a large object twists and distorts the space-time around it (imagine yourself spinning a spoon in a jar full of honey, and the latter starts to swirl). The slight drift over time of the gyroscopes inside NASA’s Gravity Probe B seems to confirm the space-time around Earth is constantly warping as our planet spins around its axis.
Researchers are figuring out another practical application derived from general relativity:
Time travel
According to general relativity, a 4D curved space-time implies that traveling through time is possible, at least in one direction. According to Einstein’s equations, traveling back on time would not be possible because it would require a spaceship traveling faster than light (which should not be possible in any object with mass > 0). However, one could consider that time dilation as a one-way ticket to others’ future. An astronaut traveling a relativistic speed (with acceleration), or through the effects of intense gravity (e.g., getting close to a black hole), would feel as hours what Earth dwellers would experience as years. Einstein would say yay to Interstellar, but nay to Back to the Future. (Most likely, he would love both but tweak the script of the second.)
What exactly is time?
Einstein’s “eternalist” view of time is challenged by the “presentist” perspective that time is a mathematical measure of the change in 3D space (as measured by clocks). In other words, time is not a physical dimension of space through which one could travel into the past or future. A step further, the timeless universe view defends that time is a human-created illusion caused by our obsession with ordering events. Human ingenuity relies on the Sun, Earth, and Moon’s regular motions to artificially quantify days, months, and years. We go on creating a linear archive of past events in “deep time.” We look for critical historical and geological events to describe different periods that we can then arrange in a linear timescale. From the Big Bang to the fall of the Roman empire, all we are measuring is but the number of Earth’s revolutions around the Sun.
Time might only exist in our heads, but we all experience the flow of time. Besides, the pace of life also seems to speed up as we age and the number of new experiences happening in our lives. The “holiday paradox” explains why vacation days fly by, and in retrospect, they seem like a long experience. Our minds use events as signposts to quantify time. When nothing happens, time passes very slowly (you know). Neurobiology studies suggest that our brains use multiple clocks to process the duration of different events in a similar way to Einstein’s special relativity.
Are we calling “time” what in reality are different things?
Understanding what time is and how it works is still a work in progress. Physicists continue to argue about the possibilities of time travel. Meantime, the rest of us keep going back on our timelines (our memories), others’ timelines (their biographies), even the timeline of Earth. Working with fossils is nothing but the real deal of time travel to the past. We can also imagine our future, perhaps, even get there.
Einstein’s duties at the patent office involved examining documents related to electric signals transmission and electromechanical synchronization of time (e.g., a patent for an electromechanical typewriter). This non-academic job forced him to use his imagination to do thought experiments about fundamental aspects of space and time. In retrospect, the stars aligned for Einstein in 1905, and relativity theory resulted from a perfect storm. Or relativistic speaking; Newton, Lorentz, Minkowski, Einstein, … They all provided their piece to the puzzle in a single moment of the eternal now.
The cover image shows Einstein accepting a U.S. citizenship certificate from judge Phillip Forman in 1940. Einstein had unexpectedly settled in the United States during a visit in 1933 due to Hitler’s rise to power in Germany. Credit: WikiCommons.
Thanks to David Alba, Santiago Catalano, and Robert Arcos Villamarín for reading the previous versions of this text.