When Einstein introduced special relativity in 1905, physics did
a summersault.
But his revolutionary theory also changed the way philosophers
since then have looked at some of the tensions raised by science,
said Christopher Smeenk, an assistant professor of philosophy at
UCLA whose specialty is the history and philosophy of science.
“The theory certainly has implications for philosophy,
insofar as various philosophical views depend on claims regarding
the nature of space and time,” Smeenk said.
Philosophers of science at UCLA study fields such as physics to
see what they suggest about various philosophical views, said
Sheldon Smith, also a professor of philosophy of science.
He told the story of one UCLA philosophy professor, Hans
Reichenbach, who used physics to help found a new approach to
philosophy ““ one in which the philosopher uses an intimate
knowledge of physics in order to probe conceptual problems within
physics.
Reichenbach, who taught at UCLA in the 1940s and ’50s,
studied physics in as much depth as physicists themselves, even
taking a class in relativity taught by Einstein himself in Berlin
in 1919, Smith said.
“Reichenbach is probably the person most responsible for
the influence of relativity on philosophy in the United
States,” Smith said.
Smith said physics’ emphasis on empiricism ““ the
practice of gaining knowledge by direct observation of the physical
world ““ contrasts with some philosophies that focus on
gaining knowledge about the world through simply thinking about
it.
Relativity’s great innovation was that it did away with
the concept that there is an objective “time” outside
of an individual observer’s experience of that time, Smeenk
said.
In classical Newtonian physics, Smeenk explained, time is
constant, ticking on independently of our perception of it, while
all motion takes place within the three dimensions of space.
This view dominated physics for two and a half centuries, to the
point where physicists at the end of the 19th century thought that
they had revealed nearly all the secrets physics could teach them,
said Mark Morris, UCLA professor of physics and astronomy.
But Einstein’s theory of relativity combined the notions
of space and time into a single concept, a four-dimensional
behemoth called “space-time,” Smeenk said.
With this notion, Morris said, Einstein’s theory swept the
ground out from underneath the physics that had been in place for
over two centuries.
“It was the first revolution that shattered physics
forever,” Morris said. “Relativity so upended classical
notions of space and time and motion. … It was clear that the
whole story had to be rewritten from the ground up.”
Relativity forced physicists to reformulate all their theories
to fit the new ideas, Smeenk said.
“It’s hard to find a physics experiment now that
doesn’t have to account for special relativity,” Morris
said.
According to relativity, two people who disagree about whether
one event happened before another could both be right, Smith said,
since their perceptions about what happened depend on their frames
of reference.
“If I’m hurtling through space at half the speed of
light, I can defend the proposition that I’m sitting
perfectly still, thank you, and everything else is rushing by
me,” Morris said.
Relativity also posits that everything from planets to people
moves at a total of the speed of light when one adds up all
movement in the four dimensions of space-time, Morris said.
“You and I are moving at the speed of light through time
“¦ just by virtue of sitting still,” Morris said.
Photons, or particles of light, on the other hand, move through
space at the speed of light, Morris said. But they do not move
through time at all; a particle of light is present everywhere
along its trajectory in space at the same time.
“We perceive (the photon) to be moving only because
we’re moving. It’s really us zipping by the
photons,” Morris said.
Morris said when humans move physically ““ say, at 500
miles per hour in a jet airliner ““ part of their net motion
is taken away from the dimension of time and put into the dimension
of space. This causes them to move more slowly through the
dimension of time, a process known as “time
dilation.”
“Our total motion is still the speed of light; that
doesn’t change. So our motion through time has to be slowed
down. That’s why a person in a moving frame of reference
experiences, from the perspective of observers at rest, time
dilation. They’re not going through time as fast because of
their motion,” Morris said.
This revolutionary theory introduced ideas that people today
still find hard to stretch their minds around, Smeenk said.
“I think it does take some time to get used to space-time
thinking,” Smeenk said. “You think about (space and
time) as being intrinsically separated.”
Because of the difficulty of internalizing this
counter-intuitive concept, Smeenk said most people probably have
trouble incorporating Einstein’s insights through relativity
into their everyday lives.
Though relativity has not changed the way people see the world,
Smeenk said, it does show that some of our natural senses about the
world are inherently wrong.
“The senses that the process of natural selection endowed
us with are perfectly fine for surviving in a Newtonian
world,” Morris said. “But that’s all
they’re good for.”