On March 15, 1955, Albert Einstein learned of the death of his close friend Michele Angelo Besso at the age of 81. An engineer by training, Besso attended university with Einstein and worked with him at the Swiss patent office, serving as a supporter and sounding board to some of the great scientist’s early and most profound ideas. Upon hearing of his friend’s passing, and although he himself was ill with only weeks to live, Einstein sent a letter of condolence to Besso’s family in which he wrote this beguiling passage:
“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.”
Some might dismiss this passage as comforting words to a grieving family. But this was Albert Einstein writing, and Einstein knew a thing or two about time and space. He must have meant something here, perhaps something profound, but what?
By 1955, Einstein had been the world’s most famous scientist for half his life. He made many far-reaching discoveries with leaps of imagination and mathematical acumen that astonish physicists even today. He theoretically demonstrated the physical reality of atoms. He discovered that light, in some circumstances, seems to consist of discrete packets called photons with energy proportional to their frequency. He proposed a new mechanism of light emission from atoms, stimulated emission, that formed the basis of the invention of the laser. He co-discovered a new form of condensed matter. He co-developed the basis of quantum entanglement. Any of these many discoveries were worthy of a Nobel Prize. But the taint of early-20th century antisemitism and general befuddlement about some of Einstein’s most far-reaching discoveries restricted him to just a single Nobel, the 1921 Nobel Prize for Physics for “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”.
But Einstein’s most famous and far-reaching insights arise from his special and general theories of relativity published in 1905 and 1915, respectively. The general theory, a tour-de-force of revolutionary ideas, uses staggeringly complex mathematics that took Einstein nearly a decade to work through. It postulates that mass and energy bend the very fabric of space and time and remains as a rock-solid framework for understanding everything from the motion of the planets to the expansion of the universe.
The Special Theory of Relativity
The earlier special theory of relativity, and the other hand, relies on math understandable by any serious high-school student. It arose from Einstein’s thought experiments about electricity and magnetism, and especially about electromagnetic waves that self-propagate through space at the speed of light. Usually denoted by the letter c, the speed of light works out to about 300,000 km/s and is as fast as anything can move – a cosmic speed limit. Einstein made two assumptions that underpin the special theory. For one, the laws of physics are identical in all non-accelerating reference frames. This means there’s no “absolute” or preferred reference frame – the laws of physics work the same way whether you’re standing still on the ground or moving at a constant speed as in a plane or train or spaceship, even at very high speed, which is in line with everyday experience. Einstein also suspected the speed of light was an inviolable constant of nature regardless of an observer’s rate of motion relative to light itself. For example, if we’re riding in a train at 100km/h and an oncoming train on an adjacent track passes us at 80km/h, we perceive its speed relative to us as 180km/h. Or if the train is riding in the same direction as us at 80km/h, it seems to us that we’re passing it at only 20km/h. Does a beam of light behave the same way? Einstein postulated that it did not. If a light beam moves towards or past a travelling spaceship, for example, regardless of the ship’s speed, the light beam still travels at the speed of light relative to an observer on the spaceship.
Using these assumptions, Einstein worked out the math and predicted several unexpected consequences for moving objects. His theory predicted the phenomenon of time dilation in which time passes slower for a moving clock (or a moving person) relative to one at rest. It predicted length contraction in which moving objects appear shorter in the direction of their motion relative to a stationary observer. It also predicted the mass of a moving object appears to increase relative to a fixed reference frame. These effects apply to all moving objects, but they’re especially noticeable for objects moving at an appreciable fraction of c. While surprising, Einstein’s predictions of special relativity have been verified by experiment and observation to many decimal places. And they have practical application. For example, the effects of special relativity must be taken into account by orbiting GPS satellites in order to register accurate positions on the Earth’s surface. The theory works.
Einstein worked out his special theory of relativity with only pencil, paper, and imagination in an apartment in Bern, Switzerland from 1903 to 1905, and he published his work in two separate papers as a sole author. But in one paper, he acknowledged his friend Michele Besso where he writes, “In conclusion I wish to say that in working at the problem here dealt with I have had the loyal assistance of my friend and colleague M. Besso, and that I am indebted to him for several valuable suggestions.”
So, what does all this have to do with Einstein’s touching note to Besso’s family upon his death in 1955?
The Relativity of Simultaneity
The answer comes with how special relativity helps us understand space and time and the concept of now. We believe we experience now as a special moment, one that passes by forever. Before now is the past, after now is the future, it’s as simple as that, right?
No, says Einstein. The concept of ‘now’ is also relative to an observer. Let’s show this with an example. Let’s say a star 50 light years away from Earth blows up as a supernova. Because light travels at a finite speed, the light won’t reach Earth for 50 years. So, a future young astronomer born today (let’s name her Anne) would be 50 years old and in the prime of her career when the supernova appeared in the sky. But let’s say there’s another observer 25 light years away (let’s name her Grace), halfway between Earth and the exploding star. In 25 years, Grace will see the light from the supernova. And, if she had a very powerful telescope, Grace could look back at the Earth and see Anne being born. To Grace, her ‘now’ says that the birth of Anne and the exploding supernova happened at the same time. Now let’s add a third observer, Elizabeth, who’s located in a position in space where she sees the light from the supernova arrive when Anne reaches her death at a ripe old age. We have three observers with three different versions of what happens in their ‘now’. When the light from the supernova arrives, Anne is 50 years old, Anne is born, or Anne is an old woman at her death. If each observer defines ‘now’ as the time when the light from the supernova arrives, whose ‘now’ is the correct one, Anne, Grace, or Elizabeth? According to Einstein, they all are! This is called the relativity of simultaneity and it’s what Einstein was getting at when he said “the distinction between past, present and future is only a stubbornly persistent illusion.”
Put another way, just as there is no privileged ‘here’ in space, there is no privileged ‘now’ in time. I may be sitting in the Calgary, but Bangkok is over there somewhere in space and I don’t doubt its existence. All of three-dimensional space exists. According to special relativity, it’s the same for time. You may be sitting here in what you see as the present, but someone else’s ‘now’ might be in your past or future. All points in time – the past, present, and future – are equally valid and there’s no absolute reference of time. This all makes more sense if we view the universe not as three-dimensional space with time passing but, but as a four-dimensional construction in space and time – spacetime – in which everything exists at once. And indeed, both the special and general theories of relativity describe the world mathematically as a four-dimensional structure of spacetime. This idea that all of spacetime (and everything in it) exists all at once and contains all the events of the past, present, and future is called the block universe.
A Concept of Immortality
In his letter to Besso’s family, Einstein was trying to comfort his grieving family who believed their loved one was gone forever, that he no longer existed. But to Einstein, Besso wasn’t gone at all. His entire existence, though finite, is embedded into his own little section of four-dimensional fabric of spacetime. In the block universe, no one goes anywhere – the living, the dead, the yet-to be-born, humans, animals, stars, and galaxies – all exist both somewhere and somewhen. This idea may be comforting or appalling, or both, depending on your point of view, and it’s certainly counterintuitive. But it’s as close to immortality as we’re likely to get (according to science) and it’s supported by the rigorously reasoned and tested special theory of relativity. Our bodies may coalesce, grow older, and eventually expire. But our existence, though finite, remains like a raisin baked into the four-dimensional cake of spacetime as the insightful comic below portrays.
Of course, we don’t perceive the universe as a block of spacetime in which the past, present and future exist together. We perceive the passage of time. And we can move back and forth through space, but not back and forth through time. Why not? No one knows. Part of the answer may lie in the limitation of the human brain. We may have the illusion that through our senses and powers of reason we see all the universe exactly as it is. But we most certainly do not. Our brains and bodies evolved for one reason – to help us live and make babies in the local here and now in a challenging and not always hospitable environment. That’s all. There’s very likely much more to physical reality than we perceive. Perhaps we might one day find a way to experience experience past events, lost parents, cherished pets, or the birth of the Sun. But even if we can’t, Einstein’s ironclad special theory of relativity backs up the reality of the block universe. Or as a physicist might say, we need to trust the equations (of relativity) to understand what’s really going on.
We have some inkling of the past, of course, through our imperfect memories, written notes, photographs, and so forth. But the future lies beyond our perception – it seems different than the past. Again, why? No one knows for sure, but they answer may lie with how information and disorder, described by the concept of entropy, changes through time. That’s a topic perhaps for another day.
But if the block universe contains the past, present, and future, does that mean all that happens is pre-determined, that there’s no free will? Quite probably, yes, although we retain the illusion of free will from which we can take some comfort. But some philosophers and scientists continue to argue that the future remains indeterminate, unlike the past, and that the block universe continuously grows as local time speeds by – this is the growing block universe hypothesis.
The implications of a block universe are mind-bending to be sure. Comforting perhaps, counterintuitive surely. But the block universe arises directly from Einstein’s bullet-proof special theory of relativity that establishes a cosmic speed limit, allows us to precisely navigate the face of the Earth, and perhaps offers us a glimpse, however abstract, of immortality.
Further Insight (Videos)
Sabine Hossenfelder – The Afterlife According to Einstein’s Special Relativity.
Arvin Ash – Does the past, present and future exist simultaneously?
Sean Carroll – The Passage of Time and the Meaning of Life.
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