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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.”