*“Excuse me, but what’s the time?”* I’m guessing that you, like
me, are guilty of having asked this question, as if it were obvious that there
is such a thing as *the* time. Yet you’ve probably never approached a stranger
and asked “*Excuse me, but what’s the place?*”. If you were hopelessly lost,
you’d probably instead have said something like “*Excuse me, but where am I?*”
thereby acknowledging that you’re not asking about a property of space, but
rather about a property of yourself. Similarly, when you ask for the time,
you’re not really asking about a property of time, but rather about your
location in time.

But that is not how we usually think about it. Our language
reveals how differently we think of space and time: The first as a static
stage, and the second as something flowing. Despite our intuition, however, the
flow of time is an illusion. Einstein taught us that there are two equivalent
ways of thinking about our physical reality: Either as a three-dimensional
place called *space*, where things change over time, or as a four-dimensional
place called *spacetime* that simply exists, unchanging, never created, and never
destroyed.

I think of the two viewpoints as the different perspectives
on reality that a frog and a bird might take. The bird surveys the landscape of
reality from high “above,” akin to a physicist studying the mathematical
structure of spacetime as described by the equations of physics. The frog, on
the other hand, lives inside the landscape surveyed by the bird. Looking up at
the moon over time, the frog sees something like the right panel in the figure,
“The Moon’s Orbit”: Five snapshots of space with the Moon in different
positions each time. But the bird sees an unchanging spiral *shape* in spacetime,
as shown in the left panel.

For the bird—and the physicist—there is no objective definition of past or future. As Einstein put it, “The distinction between past, present, and future is only a stubbornly persistent illusion.” When we think about the present, we mean the time slice through spacetime corresponding to the time when we’re having that thought. We refer to the future and past as the parts of spacetime above and below this slice.

This is analogous to your use of the terms *here, in front of
me*, and *behind me* to refer to different parts of spacetime relative to your
present position. The part that’s in front of you is clearly no less real than
the part behind you—indeed, if you’re walking forward, some of what’s presently
in front of you will be behind you in the future, and is presently behind
various other people. Analogously, in spacetime, the future is just as real as
the past—parts of spacetime that are presently in your future will, in your
future, be in your past. Since spacetime is static and unchanging, no parts of
it can change their reality status, and all parts must be equally real.

#### A Letter to Einstein from the Future

We don’t believe in time travel, and we’re not into mysticism. But what if we could write a letter to Albert Einstein to tell him about gravity and black holes in a few paragraphs? We’d write something like this letter....**READ MORE**

The idea of spacetime does more than teach us to rethink the
meaning of past and future. It also introduces us to the idea of a *mathematical
universe*. Spacetime is a purely mathematical structure in the sense that it has
no properties at all except mathematical properties, for example the number
four, its number of dimensions. In my book *Our Mathematical Universe*, I argue
that not only spacetime, but indeed our entire external physical reality, is a mathematical structure,
which is by definition an abstract, immutable entity existing outside of space
and time.

The most interesting property of your spacetime tube isn’t its bulk shape, but its internal structure, which is remarkably complex.

What does this actually mean? It means, for one thing, a universe that can be beautifully described by mathematics. That this is true for our universe has become increasingly clear over the centuries, with evidence piling up ever more rapidly. The latest triumph in this area is the discovery of the Higgs boson, which, just like the planet Neptune and the radio wave, was first predicted with a pencil, using mathematical equations.

That our universe is approximately *described* by mathematics
means that some but not all of its properties are mathematical. That it *is*
mathematical means that all of its properties are mathematical; that it has no
properties at all except mathematical ones. If I’m right and this is true, then
it’s good news for physics, because all properties of our universe can in
principle be understood if we are intelligent and creative enough. It also
implies that our reality is vastly larger than we thought, containing a diverse
collection of universes obeying all mathematically possible laws of physics.

This novel way of viewing both spacetime and the stuff in it
implies a novel way of viewing ourselves. Our thoughts, our emotions, our
self-awareness, and that deep existential feeling “*I am*”—none of this feels the
least bit mathematical to me. Yet we too are made of the same kinds of
elementary particles that make up everything else in our physical world, which
I’ve argued is purely mathematical. How can we reconcile these two
perspectives?

The first step is to consider how we look as a spacetime
structure. The cosmology pioneer George Gamow entitled his autobiography *My
World Line*, a phrase also used by Einstein to refer to paths through spacetime.
However, your own world line strictly speaking isn’t a line: It has a non-zero
thickness and it’s not straight. The roughly 10^{29} elementary particles (quarks
and electrons) that your body is made of form a tube-like shape through
spacetime, analogous to the spiral shape of the Moon’s orbit (“The Moon’s
Orbit”) but more complicated. If you’re swimming laps in a pool, that part of
your spacetime tube has a zig-zag shape, and if you’re using a playground
swing, that part of your spacetime tube has a serpentine shape.

However, the most interesting property of your spacetime tube isn’t its bulk shape, but its internal structure, which is remarkably complex. Whereas the particles that constitute the Moon are stuck together in a rather static arrangement, many of your particles are in constant motion relative to one another. Consider, for example, the particles that make up your red blood cells. As your blood circulates through your body to deliver the oxygen you need, each red blood cell traces out its own unique tube shape through spacetime, corresponding to a complex itinerary though your arteries, capillaries, and veins with regular returns to your heart and lungs. These spacetime tubes of different red blood cells are intertwined to form a braid pattern as seen in the figure “Complexity and Life” which is more elaborate than anything you’ll ever see in a hair salon: Whereas a classic braid consists of three strands with perhaps thirty thousand hairs each, intertwined in a simple repeating pattern, this spacetime braid consists of trillions of strands (one for each red blood cell), each composed of trillions of hair-like elementary-particle trajectories, intertwined in a complex pattern that never repeats. In other words, if you imagine spending a year giving a friend a truly crazy hairdo, braiding the hair by separately intertwining all their individual hairs, the pattern you’d get would still be very simple in comparison.

Yet the complexity of all this pales in comparison to the
patterns of information processing in your brain. Your roughly 100 billion
neurons are constantly generating electrical signals (“firing”), which involves
shuffling around billions of trillions of atoms, notably sodium, potassium, and
calcium ions. The trajectories of these atoms form an extremely elaborate braid
through spacetime, whose complex intertwining corresponds to storing and
processing information in a way that somehow gives rise to our familiar
sensation of self-awareness. There’s broad consensus in the scientific
community that we still don’t understand how this works, so it’s fair to say
that we humans don’t yet fully understand what we are. However, in broad brush,
we might say this: *You’re a pattern in spacetime*. A mathematical pattern.
Specifically, you’re a braid in spacetime—indeed, one of the most elaborate
braids known.

Some people find it emotionally displeasing to think of
themselves as a collection of particles. I got a good laugh back in my 20s when
my friend Emil addressed my friend Mats as an “*atomhög*,” Swedish for “*atom heap*,”
in an attempt to insult him. However, if someone says “*I can’t believe I’m just
a heap of atoms!’*’ I object to the use of the word “*just*”: the elaborate
spacetime braid that corresponds to their mind is hands down the most beautifully
complex type of pattern we’ve ever encountered in our universe. The world’s
fastest computer, the Grand Canyon or even the Sun—their spacetime patterns are
all simple in comparison.

At both ends of your spacetime braid, corresponding to your
birth and death, all the threads gradually separate, corresponding to all your
particles joining, interacting and finally going their own separate ways (As
seen in the right panel of “Complexity and Life”). This makes the spacetime structure of your
entire life resemble a tree: At the bottom, corresponding to early
times, is an elaborate system of roots corresponding to the spacetime
trajectories of many particles, which gradually merge into thicker strands and
culminate in a single tube-like trunk corresponding to your current body (with
a remarkable braid-like pattern inside as we described above). At the top,
corresponding to late times, the trunk splits into ever finer branches,
corresponding to your particles going their own separate ways once your life is
over. In other words, the pattern of life has only a finite extent along the
time dimension, with the braid coming apart into frizz at both ends.^{1}

This view of ourselves as mathematical braid patterns in spacetime challenges the assumption that we can never understand consciousness. It optimistically suggests that consciousness can one day be understood as a form of matter, a derivative of the most beautifully complex spacetime structure in our universe. Such understanding would enlighten our approaches to animals, unresponsive patients, and future ultra-intelligent machines, with wide-ranging ethical, legal, and technological implications.

This is how I see it. However, although this idea of an
unchanging reality is venerable and dates back to Einstein, it remains
controversial and subject to vibrant scientific debate, with scientists I
greatly respect expressing a spectrum of views. For example, in his book *The Hidden
Reality*, Brian Greene expresses unease toward letting go of the notions change
and creation as fundamental, writing “I’m partial to there being a process,
however tentative [...] that we can imagine generating the multiverse.” Lee
Smolin goes further in his book *Time Reborn*, arguing that not only is change
real, but that time may be the only thing that’s real. At the other end of the
spectrum, Julian Barbour argues in his book *The End of Time* not only that
change is illusory, but that one can even describe physical reality without
introducing the concept of time at all.

If we discover the ultimate nature of time, this will answer many of the most exciting open questions facing physics today. Did time have some sort of beginning before our Big Bang? Will it ultimately end? Did it emerge out of some sort of timeless quantum fuzz into which it will eventually dissolve? We physicists haven’t found the mathematical theory of quantum gravity required to convincingly answer these questions, but whatever this “theory of everything” turns out to be, time will be the key to unlocking its mysteries.

*Max Tegmark is an MIT physics professor who has authored
more than 200 technical papers. Known as “Mad Max” for his unorthodox ideas and
passion for adventure, his scientific interests range from precision cosmology
to the ultimate nature of reality, all explored in his new popular science book *Our Mathematical Universe.