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or
2. Just Read it below,
and
Write one-page article report.
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EXISTENTIAL PHYSICS
A Scientist's Guide to Life's
Biggest Questions
by
SABINE HOSSENFELDER
VIKING publisher
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CONTENTS
Preface xiti
A Warning xvii
One. DOES THE PAST STILL EXIST? 1
Two. HOW DID THE UNIVERSE BEGIN? 23
HOW WILL IT END?
Other Voices #1. IS MATH ALL THERE IS? 43
An Interview with Tim Palmer
Three. WHY DOESN'T ANYONE EVER GET YOUNGER? 51
Four. ARE YOU JUST A BAG OF ATOMS? 79
Other Voices #2. IS KNOWLEDGE PREDICTABLE? 95
An Interview with David Deutsch
Five. DO COPIES OF US EXIST? 105
Six. HAS PHYSICS RULED OUT FREE WILL? 125
Other Voices #3. IS CONSCIOUSNESS COMPUTABLE? 143
An Interview with Roger Penrose
Seven. WAS THE UNIVERSE MADE FOR US? 169
Eight. DOES THE UNIVERSE THINK? 169
Other Voices #4. CAN WE CREATE A UNIVERSE? 191
An Interview with Zeeya Merali
Nine. ARE HUMANS PREDICTABLE? 199
Epilogue: WHAT'S THE PURPOSE OF 217
ANYTHING ANYWAY?
-----------------------------------------------
PREFACE
Can I ask you something?" a young man inquired after learning
shyly. I was all ready to debate the measurement postulate and the
pitfalls of multipartite entanglement, but I was not prepared for the
question that followed: "A shaman told me that my grandmother is
still alive. Because of quantum mechanics. She is just not alive here
and now. Is this right?"
As you can tell, I am still thinking about this. The brief answer is,
it's not totally wrong. The long answer will follow in chapter 1, but
before I get to the quantum mechanics of deceased grandmothers, I
want to tell you why I'm writing this book.
During more than a decade in public outreach, I noticed that phys-
icists are really good at answering questions, but really bad at explain-
ing why anyone should care about their answers. In some research
areas, a study's purpose reveals itself, eventually, in a marketable
product. But in the foundations of physics-where I do most of my
research-the primary product is knowledge. And all too often, my
colleagues and I present this knowledge in ways so abstract that no
one understands why we looked for it in the first place.
Not that this is specific to physics. The disconnect between experts
and non-experts is so widespread that the sociologist Steve Fuller
claims that academics use incomprehensible terminology to keep in-
sights sparse and thereby more valuable. As the American journalist
and Pulitzer Prize winner Nicholas Kristof complained, academics
encode "insights into turgid prose" and "as a double protection against
public consumption, this gobbledygook is then sometimes hidden in
obscure journals."
Case in point: People don't care much whether quantum mechan-
ics is predictable; they want to know whether their own behavior is
predictable. They don't care much whether black holes destroy infor-
mation; they want to know what will happen to the collected infor-
mation of human civilization. They don't care much whether galactic
filaments resemble neuronal networks; they want to know if the uni-
verse can think. People are people. Who'd have thought?
Of course, I want to know these things too. But somewhere along
my path through academia I learned to avoid asking such questions, not
to mention answering them. After all, I'm just a physicist. I'm not com-
petent to speak about consciousness and human behavior and such.
Nevertheless, the young man's question drove home to me that
physicists do know some things, if not about consciousness itself, then
about the physical laws that everything in the universe-including
you and I and your grandmother-must respect. Not all ideas about
life and death and the origin of human existence are compatible with
the foundations of physics. That's knowledge we should not hide in
obscure journals using incomprehensible prose.
It's not just that this knowledge is worth sharing; keeping it to our-
selves has consequences. If physicists don't step forward and explain
what physics says about the human condition, others will jump at the
opportunity and abuse our cryptic terminology for the promotion of
pseudoscience. It's not a coincidence that quantum entanglement and
vacuum energy are go-to explanations of alternative healers, spiritual
media, and snake oil sellers. Unless you have a PhD in physics, it's
hard to tell our gobbledygook from any other.
However, my aim here is not merely to expose pseudoscience for
what it is. I also want to convey that some spiritual ideas are perfectly
compatible with modern physics, and others are, indeed, supported
by it. And why not? That physics has something to say about our con-
nection to the universe is not so surprising. Science and religion have
the same roots, and still today they tackle some of the same questions:
Where do we come from? Where do we go to? How much can we
know?
When it comes to these questions, physicists have learned a lot in
the past century. Their progress makes clear that the limits of science
are not fixed; they move as we learn more about the world. Corre-
spondingly, some belief-based explanations that once aided sense-
making and gave comfort we now know to be just wrong. The idea, for
example, that certain objects are alive because they are endowed with
a special substance (Henri Bergson's "élan vital") was entirely compat-
ible with scientific fact two hundred years ago. But it no longer is.
In the foundations of physics today, we deal with the laws of nature
that operate on the most fundamental level. Here, too, the knowledge
we gained in the past hundred years is now replacing old, belief-based
explanations. One of these old explanations is the idea that conscious-
ness requires something more than the interaction of many particles,
some kind of magic fairy dust, basically, that endows certain objects
with special properties. Like the élan vital, this is an outdated and
useless idea that explains nothing. I will get to this in chapter 4, and in
chapter 6 I'll discuss the consequences this has for the existence of free
will. Another idea ready for retirement is the belief that our universe
is especially suited to the presence of life, the focus of chapter 7.
However, demarcating the current limits of science doesn't only
destroy illusions; it also helps us recognize which beliefs are still
compatible with scientific fact. Such beliefs should maybe not be
called unscientific but rather ascientific, as Tim Palmer (whom we'll
meet later) aptly remarked: science says nothing about them. One
such belief is the origin of our universe. Not only can we not currently
explain it, but also it is questionable whether we will ever be able
to explain it. It may be one of the ways that science is fundamentally
limited. At least that's what I currently believe. The idea that the
universe itself is conscious, I have found to my own surprise, is diffi-
cult to rule out entirely (chapter 8). And the jury is still out on
whether or not human behavior is predictable (chapter 9).
In brief, this is a book about the big questions that modern physics
raises, from the question whether the present moment differs from
the past, to the idea that each elementary particle may contain a uni-
verse, to the worry that the laws of nature determine our decisions. I
cannot, of course, offer final answers. But I want to tell you how
much scientists currently know, and also where science crosses over
into mere speculation.
I will mostly stick with established theories of nature that are
backed up by evidence. All of what I am going to say, therefore, should
come with the preamble "as far as we currently know," meaning that
further scientific progress might lead to revision. In some cases, the
answer to a question depends on properties of natural laws that we do
not yet fully understand, like quantum measurements or the nature
of space-time singularities. If so, I will point out how future research
could help answer the question. Because I don't want you to hear just
my own opinion, I have added a few interviews. And at the end of the
book, you'll find a brief glossary with definitions of the most impor-
tant technical terms. Terms in the glossary are marked bold when
they first appear in the text hereafter.
Existential Physics is for those who have not forgotten to ask the big
questions and are not afraid of the answers.
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Chapter 1
DOES THE PAST STILL EXIST?
Now and Never
Time is money. It's also running out. Unless, possibly, it's on your side.
Time flies. Time is up. We talk about time ... all the time. And yet time
has remained one of the most difficult-to-grasp properties of nature.
It didn't help that Albert Einstein made it personal. Before Ein-
stein, everybody's time passed at the same rate. Post-Einstein, we
know that the passage of time depends on how much we move around.
And while the numerical value we assign to each moment-say 2:14
p.m .- is a matter of convention and measurement accuracy, in pre-
Einstein days, we believed that your now was the same as my now; it
was a universal now, a cosmic ticking of an invisible clock that marked
the present moment as special. Since Einstein, now is merely a conve-
nient word that we use to describe our experience. The present mo-
ment is no longer of fundamental significance because, according to
Einstein, the past and the future are as real as the present.
This doesn't match with my experience and probably doesn't
match with yours either. But human experience is not a good guide to
the fundamental laws of nature. Our perception of time is shaped by
circadian rhythms and our brain's ability to store and access memo-
ries. This ability is arguably good for many things, but to disentangle
the physics of time from our perception of it, it is better to look at
simple systems, like swinging pendulums, orbiting planets, or light
that reaches us from distant stars. It is from observations on such
simple systems that we can reliably infer the physical nature of time
without getting bogged down by the often inaccurate interpretation
that our senses add to the physics.
A hundred years' worth of observation have confirmed that time
has the properties Einstein conjectured at the beginning of the twen-
tieth century. According to Einstein, time is a dimension, and it joins
with the three dimensions of space to one common entity: a four-
dimensional space-time. The idea of combining space and time to
space-time goes back to the mathematician Hermann Minkowski, but
Einstein was the one to fully grasp the physical consequences, which
he summarized in his theory of special relativity.
The word relativity in special relativity means there is no absolute
rest; you can merely be at rest relative to something. For example, you
are now probably at rest relative to this book; it's moving neither away
from nor toward you. But if you throw it into a corner, there are two
ways of describing the situation: the book moves at some velocity rela-
tive to you and the rest of planet Earth, or you and the rest of the planet
move relative to the book. According to Einstein, both are equivalent
ways to describe the physics and should give the same prediction-
that's what the word relativity stands for. The special just says that this
theory doesn't include gravity. Gravity was included only later, in Ein-
stein's theory of general relativity.
The idea that we should be able to describe physical phenomena
the same way regardless of how we move in Einstein's four-dimensional
space-time sounds rather innocuous, but it has a host of counterintu-
itive consequences that have entirely changed our conception of time.
In our usual three-dimensional space, we can assign coordinates to
any location using three numbers. We could, for example, use the
distance to your front door in the directions east-west, north-south,
and up-down. If time is a dimension, we just add a fourth coordinate,
let's say the time that has passed at your front door since 7:00 a.m. We
then call the complete coordinates an event. For example, the space-
time event at 3 meters east, 12 meters north, 3 meters up, and 10
hours might be your balcony at 5:00 p.m.
This choice of coordinates is arbitrary. There are many different
ways to put coordinate labels on space-time, and Einstein said these
labels shouldn't matter. The time that actually passes for an object
can't depend on what coordinates we chose. And he showed that this
invariant, internal time-proper time, as physicists call it-is the
length of a curve in space-time.
Suppose you go on a road trip from Los Angeles to Toronto. What
matters to you is not the straight-line coordinate distance between
these points, about 2,200 miles, but the distance on highways and
streets, which is more like 2,500 miles. It's similar in space-time.
What matters is the length of the trip, not the coordinate distance.
But there's an important difference: in space-time, the longer the
curve between two events, the less time passes on it.
How do you make a curve between two space-time events longer?
By changing your velocity. The more you accelerate, the slower your
proper time will pass. This effect is called time dilation. And, yes, in
principle, this means if you run in a circle, you'll age more slowly. But
it's a tiny effect, and I can't recommend it as an antiaging strategy. By
the way, this is also why time passes more slowly near a black hole
than far away from one. That's because, according to Einstein's prin-
ciple of equivalence, a strong gravitational field has the same effect as
a fast acceleration.
What does this mean? Imagine I have two identical clocks; I hand
you one, and then you go your way and I go mine. In pre-Einstein
days, we'd have thought that whenever we met again, these clocks
would show exactly the same time-this is what it means for time to
be a universal parameter. But post-Einstein, we know this isn't right.
How much time passes on your clock depends on how much and how
fast you move.
How do we know this is correct? Well, we can measure it. It would
lead us too far off topic to go into detail about which observations
have confirmed Einstein's theories, but I will leave you recommenda-
tions for further reading in the endnotes. To move on, let me just sum
it up by saying that the hypothesis that the passage of time depends
on how you move is supported by a large and solid body of evidence.
I have been speaking of clocks for illustration, but the fact that
acceleration slows time down has nothing in particular to do with the
devices we call clocks; it happens for any object. Whether it's com-
bustion cycles, nuclear decay, sand running through an hourglass, or
heartbeats, each process has its own individual passage of time. But
the differences between individual times are normally minuscule,
which is why we don't notice them in everyday life. They become
noticeable, however, when we keep track of time very precisely, which
we do, for example, in satellites that are part of the global positioning
system (GPS).
The GPS, which your phone's navigation system most likely
uses, allows a receiver-like your phone-to calculate its position
from signals of several satellites that orbit Earth. Because time is not
universal, time on these satellites passes subtly differently compared
with how it passes on Earth, both because of the satellites' motion
relative to the surface of Earth and because of the weaker gravita-
tional field that the satellites experience in their orbits. The software
on your phone needs to take this into account to correctly infer its
location, because the different passage of time on the satellites oh-so-
slightly distorts the signals. It's a small effect, all right, but it's not
philosophy; it's physically real.
The fact that the passage of time isn't universal is pretty mind-bending
already, but there's more. Because the speed of light is very fast but
finite, it takes time for light to reach us, so, strictly speaking, we al-
ways see things as they looked a little bit earlier. Again, though, we
don't normally notice this in everyday life. Light travels so fast that it
doesn't matter on the short distances we see on Earth. For example,
if you look up and watch the clouds, you actually see the clouds the
way they looked a millionth of a second ago. That doesn't really make
a big difference, does it? We see the Sun as it looked eight minutes
ago, but because the Sun doesn't normally change all that much in a
few minutes, light's travel time doesn't make a big difference. If you
look at the North Star, you see it as it looked 434 years ago. But, yeah,
you may say, so what?
It is tempting to attribute this time lag between the moment
something happens and our observation of it as a limitation of percep-
tion, but it has far-reaching consequences. Once again, the issue is
that the passage of time is not universal. If you ask what happened "at
the same time" elsewhere-for example, just exactly what you were
doing when the Sun emitted the light you see now-there is no mean-
ingful answer to the question.
This problem is known as the relativity of simultaneity, and it was
well illustrated by Einstein himself. To see how this comes about, it
helps to make a few drawings of space-time. It's hard to draw four
dimensions, so I hope you will excuse me if I use only one dimension
of space and one dimension of time. An object that doesn't move rel-
ative to the chosen coordinate system is described by a vertical straight
line in this diagram (figure 1). These coordinates are also referred to
as the rest frame of the object. An object moving at constant velocity
makes a straight line tilted at an angle. By convention, physicists use
a 45-degree angle for the speed of light. The speed of light is the same
for all observers, and because it can't be exceeded, physical objects
have to move on lines tilted less than 45 degrees.
Einstein now argued as follows. Let's say you want to construct a
notion of simultaneity by using pulses of laser beams that bounce off
mirrors that are at rest relative to you." You send one pulse to the right
and one to the left and shift your position between the mirrors until
the pulses return to you at the same moment (see figure 2a). Then
you know you are exactly in the middle and the laser beams hit both
mirrors at the same moment.
l'igure 2: Space-time diagrams for construction of stmultaneous events. Top left (a): You in your rest frame with coordinates labeled space and time. Top right (b): Sue in your rest frame. Bottom left (e): Sue in her rest frame with coordinates labeled space' and time'. Bottom right (d): You in Sue's rest frame.
Once you have done that, you know at exactly which moment in
your own time the laser pulse will hit both mirrors, even though you
can't see it because the light from those events hasn't yet reached you.
You could look at your clock and say, "Now!" This way, you have con-
structed a notion of simultaneity that, in principle, could span the
whole universe. In practice you may not have the patience to wait ten
hillion years for the laser pulse to return, but that's theoretical physics
for you.
Now imagine that your friend Sue moves relative to you and tries
to do the same thing (figure 2b). Let's say she moves from left to
right. Sue, too, uses two mirrors, one to her right and one to her left,
and the mirrors move along with her at the same velocity-hence, the
mirrors are in rest relative to Sue, like your mirrors are relative to you.
Like you, she sends laser pulses in both directions and positions her-
self so the pulses come back to her from both sides at the same mo-
ment. Like you, she then knows that the pulses hit the two mirrors at
the same moment, and she can calculate just which moment that cor-
responds to on her own clock.
The trouble is, she gets a different result than you do. Two events
that Sue thinks happen at the same time would not happen at the
same time according to you. That's because from your perspective she
is moving toward one of the mirrors and away from the other. To you
it seems that the time it takes the pulse to reach the mirror on her
left is shorter than the time it takes for the other pulse to catch up
with the mirror on her right. It's just that Sue doesn't notice, because
on the pulses' return paths from the mirrors, the opposite happens.
The pulse from the mirror to Sue's right takes longer to catch up with
her, while the pulse from the mirror on her left arrives faster.
You would claim that Sue is making a mistake, but according to
Sue, you are making the mistake because, to her, you are the one who
is moving. She would say that actually your laser pulses do not hit
your mirrors at the same time (figures 2c and 2d).
Who is right? Neither of you. This example shows that in special
relativity the statement that two events happened at the same time is
meaningless.
It's worth stressing that this argument works only because light
doesn't need a medium to travel in, and the speed of light (in vacuum)
is the same for all observers. This argument does not work with sound
waves, for example (or any other signal that isn't light in vacuum),
because then the speed of the signal really will not be the same for all
observers; it will instead depend on the medium it's traveling in. In
that case, one of you would be objectively right and the other one
wrong. That your notion of now might not be the same as mine is an
insight we owe to Albert Einstein.
We just established that two observers who move relative to each
other don't agree on what it means for two events to happen at the
same time. That isn't only odd, but it entirely erodes our intuitive
notion of reality.
To see this, suppose you have two events that are not in causal con-
tact with each other, which means you cannot send a signal from one
to the other, not even at the speed of light. Diagrammatically, "not
in causal contact" just means if you draw a straight line through the
two events, the angle between the line and the horizontal is less than
45 degrees. But look at figure 2b again. For two events that are not in
causal contact, you can always imagine an observer for whom every-
thing on this straight line is simultaneous. You just need to choose the
observer's velocity so the return points of the laser pulses are on the
line. But if any two points that are not causally connected happen at
the same time for someone, then every event is "now" for someone.
To illustrate the latter step, let us say the one event is your birth
and the other event is a supernova explosion (see figure 3). The explo-
sion is causally disconnected from your birth, which means the light
from it hadn't reached Earth at the time you were born. You can then
imagine that your friend Sue, the space traveler, sees these events at
the same time, so they happened simultaneously according to her.
Suppose further that by the time you die the light from the super-
nova still hasn't reached Earth. Then your friend Paul could find a
way to travel in the middle between you and the supernova so he
would see your death and the supernova at the same time. They both
Figure 3: Any two causally disconnected events are simultaneous for some observers. If all observers' experiences are equally valid, then all events exist the same way, regardless of when or where they are.
happened simultaneously according to Paul. I swear that's it for intro-
ducing imaginary friends on spaceships!
We can then put together everything we learned. I believe most of
us would say the clouds exist now, even though we can see them only
as they were a fraction of a second ago. For this, we use our own, per-
sonal notion of simultaneity that depends on how we move through
space-time-that is, usually much below the speed of light and on
the surface of our planet. Therefore, we all pretty much mean the same
thing by "now," and it doesn't normally cause confusion.
However, all notions of "now" for observers who move elsewhere
and potentially close to the speed of light-like Sue and Paul-are
equally valid, and in principle they span the entire universe. And be-
cause there could be some observer according to whom your birth
and the supernova explosion happen simultaneously, the supernova
exists at your birth according to your own notion of existence. There-
fore, because there could be another observer according to whom the
explosion happens together with your death, your death exists at
your birth.
You can advance this argument for any two events anywhere in the
universe at any time and arrive at the same conclusion: the physics of
Einstein's special relativity does not allow us to constrain existence to
merely a moment that we call "now." Once you agree that anything
exists now elsewhere, even though you see it only later, you are forced
to accept that everything in the universe exists now.
This perplexing consequence of special relativity has been dubbed
the block universe by physicists. In this block universe, the future, pres-
ent, and past exist in the same way; it's just that we do not experience
them the same way. And if all times exist similarly, then all our past
selves-and grandparents-are alive the same way our present selves
are. They are all there, in our four-dimensional space-time, have
always been there, and will always be there. To sum it up in the words
of the British comedian John Lloyd, "Time is a bit like a landscape. Just
because you're not in New York doesn't mean it's not there."
More than a century has passed since Einstein put forward his
theories of special and general relativity. But here we are today, still
struggling to understand what it really means. It sounds crazy, but the
idea that the past and future exist in the same way as the present is
compatible with all we currently know.
Eternal Information
The notion that the present moment has no special relevance can be
scen another way. All successful theories in the foundations of phys-
ics require two ingredients: (1) information about what it is that you
want to describe at one moment in time, called the initial condition,
and (2) a prescription, called an evolution law, for how to calculate
from this initial state what happens at another moment of time.
I want to caution you that the word evolution here has nothing to do
with Charles Darwin; it merely means that the law tells us how a sys-
tem evolves-that is, changes in time. For example, if you know the
place and velocity of a meteorite entering Earth's atmosphere (initial
condition), applying the evolution law allows you to calculate its place
of impact. And because we are introducing terminology already, the
technical expression for "that which you want to describe" is system.
No, seriously. While system has a rather specific meaning in other dis-
ciplines, among physicists it can mean anything and everything. That's
very convenient, so it's also how I will use the word.
Thus, when we want to make a prediction, we take the state of a
system at one time, and then we use the evolution law to calculate
from this one time what the system will do at any other time. But we
can do this in either direction of time. The laws, as we say, are time-
reversible. They can be run forward and backward, like a movie.
In our everyday experience, forward in time looks very different
from backward in time. We see eggs breaking but not unbreaking,
logs burning but not unburning, people aging but not getting any
younger. I have dedicated the entire chapter 3 to the question of why
forward in time looks different from backward in time. But for this
chapter, I will put aside the question why time seems to have a
preferred direction and instead look just at the consequences of the
time-reversibility of the laws.
Time-reversibility does not mean that both directions in time
look the same; that would be called time-reversal invariance. Time-
reversibility merely means that, given the entire information at one
moment, we can calculate what happened at any moment before that
and what will happen at any moment after that.
The idea that all events in the future can in principle be calculated
from any earlier time is called determinism. Prior to the discovery of
quantum mechanics, the then-known laws of nature were deter-
ministic. In 1814, the French scientist and philosopher Pierre-Simon
Laplace conjured up a fictional, omniscient being to illustrate the
consequences.
We ought then to regard the present state of the universe as the
effect of its anterior state and as the cause of the one which is to
follow. Given for one instant an intelligence which could compre-
hend all the forces by which nature is animated and the respective
situation of the beings who compose it-an intelligence sufficiently
vast to submit these data to analysis-it would embrace in the
same formula the movements of the greatest bodies of the universe
and those of the lightest atom; for it, nothing would be uncertain
and the future, as the past, would be present to its eyes.
This omniscient being, Laplace's demon, is an ideal. In practice, of
course, no one has all the information necessary to predict the future
with certainty-we aren't omniscient. But I am here not concerned
with what calculation can be done in practice; I want to look at what
the fundamental laws and their properties tell us about the nature of
reality.
Now, a time-reversible law is also deterministic, but the opposite
is not necessarily true. Imagine a video game that can't be won. You
watch recordings of gamers playing but ultimately always losing the
game. Inevitably, the recording will end with the same screen saying,
GAME OVER. This means if you see only the end screen, you can't tell
what happened previously. The outcome is determined, but not
time-reversible. A time-reversible law, in contrast, results in a unique
relationship between any two moments of time. For the example
of the video game, this would mean that the final screen contains
enough details for you to figure out exactly which moves led to this
outcome.
The currently known fundamental laws of nature are both time-
reversible and deterministic, with the exception of two processes that I
will discuss in the next section. That the future is fixed by the present
in this way seems to severely constrain our ability to make decisions.
We will talk about what this means for free will in chapter 6. For now,
I want to focus on the brighter side of time-reversal invariance, which
is that the universe keeps a faithful record of the information about all
you have ever said, thought, and done.
I use the word information here loosely to refer to all numbers you
need to put into the evolution law to be able to make a prediction with
it. Information, hence, is merely all the details you need in order to
completely specify the initial state of the system at one particular
time. In other areas of physics, information has properties beyond that,
but that's the way I will use the term here.
The evolution law maps the initial state at any one time to the
state at any other time, so it really just tells us how matter in the
universe and space-time reconfigures. We start with particles in one
arrangement, we apply the equation to it, and we get another arrange-
ment. The information in these arrangements is completely main-
tained. To recover an earlier state, all you need to do is apply the
evolution law and run it backward. In practice, this is unfeasible. But
in principle, information-including every oh-so-minute detail about
your identity-cannot be destroyed.
Let us then talk about the two exceptions to time-reversibility: the
measurement in quantum mechanics, and the evaporation of black
holes.
Quantum mechanics has a time-reversible evolution law (the
Schrödinger equation) for a mathematical object called the wave func-
tion. The wave function is usually denoted by V (the Greek capital
letter psi) and it describes whatever it is you want to observe (the "sys-
tem" again). From the wave function, we compute probabilities for
measurement outcomes, but the wave function itself is not observable.
To see how this works, consider the following example. Suppose we
use quantum mechanics to calculate the probability for a particle to be
measured at a particular place. To detect the particle, we use a lumi-
nous screen that emits a flash where the particle hits it. Let us say our
calculation predicts there's a 50 percent chance we will find the parti-
cle on the left side of the screen and a 50 percent chance we'll find it
on the right side. According to quantum mechanics, this probabilistic
prediction is all there is to say. It is probabilistic not because we are
missing information. There just isn't any more information. The wave
function is the full description of the particle-that's what it means for
the theory to be fundamental.
However, the moment we actually measure the particle, we know
for sure whether it's on one side of the screen or the other. This means
we have to update the wave function from 50:50 to either 100:0 or
0:100, depending on which side of the screen we saw the particle on.
This update is sometimes also called the reduction or the collapse of
the wave function. I find the word collapse misleading because it sug-
gests a physical process that quantum mechanics doesn't contain, so I
will stick with update or reduction. Without the update, quantum me-
chanics just does not describe what we observe.
"But what is a measurement?" you may ask. Yes, good question.
This certainly bothered physicists a lot in the early days of quantum
mechanics. By now this question has, luckily, largely been answered.
A measurement is any interaction that is sufficiently strong or fre-
quent to destroy the quantum behavior of a system. Only what it
takes to destroy quantum behavior can be (and, for many examples,
has been) calculated.
Most important, these calculations show that a measurement in
quantum mechanics does not require a conscious observer. In fact, it
doesn't even require a measurement apparatus. Even tiny interactions
with air molecules or light can destroy quantum effects so that we
have to update the wave function. Of course, in this case, speaking of
a measurement is quite the abuse of language, but physically there
isn't any difference between interactions with a man-made apparatus
and interactions with a naturally present environment. And because
in everyday life we can't ever get rid of the environment, we don't
normally see quantum effects, like dead-and-alive cats, with our own
eyes. Quantum behavior just gets destroyed too easily.
This is also why you shouldn't listen to anyone who claims that
quantum leaps allow you to think your way out of illness or that you
can improve your life by drawing energy from quantum fluctuations
and so on. This isn't just off-the-mainstream science; it's incompatible
with evidence. Under normal circumstances, quantum effects don't
play a role beyond the size of molecules. That they're difficult to
maintain and measure is the very reason physicists like doing experi-
ments at temperatures near absolute zero, preferably in vacuum.
We understand fairly well what constitutes a measurement, but
the fact that we need to update the wave function upon measure-
ment makes quantum mechanics both indeterministic and time-
irreversible. It is indeterministic because we cannot predict what we
will actually measure; we can predict only the probability of measur-
ing something. And it is not time-reversible, because once we have
measured the particle, we cannot infer what the wave function was
prior to measurement. Suppose you measure the particle on the left
side of your screen. Then you cannot tell whether the wave function
previously said the particle should be there with 50 percent probabil-
ity or with a mere 1 percent probability. There are many different
initial states for the wave function that will result in the same mea-
surement outcome. This means the measurement in quantum me-
chanics destroys information for good.
However, if you know one thing about quantum mechanics, it's that
its physical interpretation has remained highly controversial. In 1964,
more than half a century after the theory was established, Richard
Feynman told his students, "I can safely say that nobody understands
quantum mechanics." After another half century, in 2019, the physicist
Sean Carroll wrote that "even physicists don't understand quantum
mechanics."
Indeed, the fact that the wave function can't itself be observed is a
dilemma that has kept physicists and philosophers up at night for the
better part of a century, but we don't need to go through the whole
discussion here. If you want to know more about the interpretations
of quantum mechanics, please have a look at my reading suggestions
in the endnotes. Let me just sum it up by saying that if you don't be-
lieve the measurement update is fundamentally correct, that's cur-
rently a scientifically valid position to hold. I myself think it's likely
the measurement update will one day be replaced by a physical pro-
cess in an underlying theory, and it might come out to be both deter-
ministic and time-reversible again.
I should add that in one of the currently most popular inter-
pretations of quantum mechanics-the many-worlds interpretation-
the measurement update does not happen at all, and the evolution of
the universe just remains time-reversible. I am not a big fan of the
many-worlds interpretation for reasons I will lay out in chapter 5,
but to give you an accurate impression of the current status of re-
search, the many-worlds interpretation is another reason that believing
in time-reversibility is presently compatible with scientific knowledge.
This brings us to the other exception to time-reversibility: the evap-
oration of black holes. Black holes are regions where space-time bends
so strongly that light is forced to go around in circles and can't escape.
T'he surface within which light gets trapped is called the horizon of the
black hole; in the simplest case, the horizon has the shape of a sphere.
Because nothing can move faster than light, black holes will trap every-
thing that crosses the horizon. If something happens to fall in-an
utom, a book, a spaceship-it can't get back out, ever. Once inside the
black hole, it's eternally disconnected from the rest of the universe.
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