There’s nothing like theoretical physics for making you feel
that words are floating, untethered to any normal experience. Or to put it another way, that a
self-supporting explanation for phenomena is being built without reference to
anything verifiable on an everyday level.
This is at times how I felt on a first head-spinning reading
of Rovelli’s The Order of Time, in which he tries to explain to ‘my dear,
cultivated reader’, as he puts it, what time is in terms of physics. Why does
time move from the past to the future? Does the present exist? Is time an
objective ‘container’ for events which exists independently of them, or is it
measurement of change, so that if nothing happens, time stops?
Well, here is my stab at a summary of the book’s answers to
these questions:
Why does time move from the past to the future?
This is a tough question. Rovelli explains physicist Ludwig
Boltzmann’s idea that the direction of time is due to our ‘blurred’ vision of
microscopic events. If humans could ‘take into account all the details of the
exact, microscopic state of the world’, the difference between past and future
vanishes. What gives rise to our understanding of time passing is that the world
is moving from a state of low to high entropy, meaning that disorder is
increasing. One example of this is the sun, a source of low entropy when it emits photons, after which entropy increases when the earth emits 10 cold photons is
in exchange for every one from the sun. This increasing disorder causes events
to happen, and means traces of the past are found in the present. No? Me
neither, well… maybe a little. This footnote from the book helps:
The point is not that what happens to a cold teaspoon in a cup of hot tea depends on whether I have a blurred vision of it or not.… It just happens, regardless. The point is that the description in terms of heat, temperature and the passage of heat from tea to spoon is a blurred vision of what happens, and that it is only in this blurred vision that a startling difference between past and future appears.
Entropy itself is a result of our blurred vision, as is the
‘particularity’ of our universe which means that it is, extremely unusually,
moving from a state of low to high entropy.
Does the present exist?
Rovelli thinks an ‘objective global present’ does not exist,
since the most we can speak of, post-Einstein, is ‘a present relative to a
moving observer’; time has been shown to be relative to qualities such as speed
and proximity to an object. However, he acknowledges voices arguing for ‘a
privileged time and a real present’.
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| Aristotle and Newton, p. 59 |
Is time an objective ‘container’ for events which exists without them, or is it measurement of change, so that if nothing happens, time stops?
Rovelli explains how the first view was that of Newton, the
second of Aristotle. He brings in Einstein to create a synthesis of them:
space-time is one field among many. (‘Fields’ are substances which ‘constitute
the weave of the physical reality of the world’). Space-time is a field which
exists independently of matter, but ‘stretches and jostles’ with other fields.
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| The 'curved' space-time field, p. 69 |
Connecting physics with other worldviews
Bravely, the book moves outside physics, into biology,
philosophy and religion. Rovelli believes physics allows us to study time free
of ‘the fog of emotion’, but he also celebrates our emotional need for time,
its necessity for making us who we are through memory. Perception of time is
also crucial for survival, since we have evolved neural structures that allow
us to predict the future based on our understanding of the past.
These departures from physics into
different areas seem somewhat disconnected from the physics-based approach of
the rest of the book, and less well-developed. For example, Rovelli thinks our
sense of our identity comes from interaction with others, not introspection.
But have not psychologists been studying this for decades? Perhaps these
attempted connections with more human concerns exist to add interest to what
could otherwise be a very dry book, or to clarify that physics coexists and is
separate from other levels of understanding — psychological, biological,
philosophical. How physics may, or may not, connect with these areas is
something that, for me, awaits another book.
The Order of Time is published by Penguin.