Island Of K
nowledge
,
Part II
PROFESSOR: From Newton to Einstein, what we see is an incredibly accelerated growth of our
understanding of the universe. This growth was possible because measuring tools became
increasingly sophisticated: Telescopes that could see farther and farther out
into the universe;
microscopes that could see further and further into the nature of matter and of life itself. Side by
side, with those tools, incredible mathematical developments and sophisticated experiments
allowed scientists to understand nature in un
precedented ways.
The so
-
called three pillars of the classical physics world view were solidified in the 19th century.
First, mechanics, the study of motion and the laws of gravity. Then, electromagnetism, the study
of electric charges and magnetic fields
, and how electric charges in motion can generate
magnetic fields. And, finally, thermodynamics, the study of heat that led to steam engines and
the Industrial Revolution, and to the laws of conservation of energy and the growth of entropy or
disorder.
A
scientist of the late 19th century saw the universe very differently from Galileo, Kepler, and
Newton, and, certainly, very differently from the Greeks. As science advanced, world views
changed. The closed cosmos of the Greeks became the open cosmos of New
ton, where every star
is a sun and could, in principle, have planets orbiting around it. New planets were discovered,
Uranus and Neptune.
The cosmos appeared to be a very ordered machine, an accurate clockwork mechanism. It
became clear that, as science a
dvanced, the way we saw the world and our place in it changed.
To illustrate this, consider the Island of Knowledge metaphor. Imagine that all that we know
about the world fits in an island, the Island of Knowledge. The more we know about the world,
the mo
re the island grows.
However, as with any good island, the Island of Knowledge is also surrounded by an ocean, in
this case, the ocean of the unknown, of what we don't know about the world. You may think that,
as we learn more and more, the island would o
ne day cover all the ocean of the unknown. That
one day, science would have answers to all questions. But that is not what happens. Because, as
the island grows, so do the shores of our ignorance, the boundary between the known and the
unknown.
We know th
is already. For example, consider astronomy before and after the telescope. The new
tool allowed scientists to develop a new world view, that with the sun in the center. And it
allowed scientists to ask questions they couldn't have even imagined before. Th
is happens often
in the history of science. New tools and new discoveries solve some problems, but also bring out
new ones.
The island of knowledge grows, but the ocean of the unknown is potentially infinite, at least as
long as we keep asking questions a
bout the world and developing new and more powerful tools
to study it. Even more dramatically
--
as we'll see soon
--
in the ocean of the unknown, there are
regions of the unknowable. There are questions that we can ask about the world that science
cannot an
swer, unless we break the laws of nature as we know them today.
There are two main reasons why our knowledge of the world is finite. First, as we have seen, our
tools can only see so far. They can only probe so deep into nature. And second, nature itself
limits what we can see and observe. Perhaps the most obvious example of this in the context of
cosmology is the fact that the speed of light is finite. It's very fast, but it's still finite. In empty
space, or the vacuum, light can travel at about 300,000
kilometers per second, or about 186,000
miles per second.
If you blink your eyes, light goes 7 and 1/2 times around the earth. If we now consider that
modern cosmology tells us that the universe had a beginning about 13.8 billion years ago, this
means tha
t, from our perspective here on Earth, we can only see things
--
or receive information
from things
--
that are at a distance smaller to how far light traveled in '13.8 billion years. This
distance is huge, but it's not infinite.
Remember, that we get infor
mation from the universe from collecting light. Not just visible light,
but many kinds of light. Or better, electromagnetic radiation. And all of these travel at the speed
of light. So the farthest point that we can see or get information from is an object
which is as far
as light has traveled in 13.8 billion years. This is what we call our cosmic horizon.
In astronomy, we like to use light years as a measure of distance. As the name says, a light year
is the distance that light can travel in one year. To
give you an idea, the distance between Earth
and Pluto is about 5 and 1/2 light hours, or 327 light minutes. So this means that if someone sent
us a message from Pluto, traveling at the speed of light, like the spectacular photos from the New
Horizons prob
e, it would take 5 and 1/2 hours to reach us.
[CLOCK TICKING]
Moving farther out, the distance between the Sun and the nearest star to Earth, called Alpha
Centauri, is about 4 and 1/2 light years. You see how far stars are from one another. Now, let's
mo
ve further out still to consider our galaxy, the Milky Way. The Milky Way has a diameter of
about 100,000 light years. If I turn a flashlight on at one edge of the galaxy, it will take 100,000
years for light to reach the other end.
If we keep moving outw
ards, the nearest galaxy to the Milky Way is the Andromeda galaxy at
about two million light years. Typically, galaxies are tens or hundreds of millions of light years
away from one another. Now we can go back to our question: How far would light travel in
the
age of the universe of 13.8 billion years? A simple answer would be 13.8 billion light years. But
that's not quite right, because the universe is not static. The universe is expanding. Galaxies are
moving away from one another at enormous speeds.
Thi
s expansion adds to the distance that light can travel, like a surfer riding on a wave. So the
actual distance that light travels since the Big Bang is about 46 billion light years. Again, a huge
distance, but not infinite. This has a very, very important
consequence to us. It means that we
live in a cosmic bubble of information. We can only know what's going on in the universe within
this bubble of information, the distance that light has traveled since the Big Bang.
The universe may continue beyond this
point, just like when you are standing at the beach and
you look at the horizon. We know that the ocean continues beyond the horizon, but we can't see
what's out there. Well, it's about the same with the universe. The universe may continue beyond
our cosmi
c bubble of information, our cosmic horizon, but we can't know what's beyond it. So if
you ask me "What's going on with the universe outside our cosmic bubble?" We can speculate,
and we can say that, probably, it looks very much the same as around here. Bu
t we can never
know for sure, because we cannot get any information from objects that are outside our cosmic
horizon. This is an example of what I call an unknowable question in science. You can ask it, but
we can't answer it.
