On Feb. 26, 2020, Leveneleven Brewing filled with people young and old, all excited to hear UNCG physics professor Joel Shaw speak about special relativity. The event was hosted by Greensboro Science Pub, a donation-funded organization that aims to promote science conversation within the community—no science background necessary.
“I’ve been to a lot of scientific talks, and I’ve even taken a few physics classes,” said Chase Crihfield, a UNCG senior studying biology. “But honestly, I’ve never sat down at a physics seminar with a beer in hand and expected to hear jokes about the Death Star. It was both informational and fun, and really what science should be.”
Shaw began with the basics; he showed pictures of famous physicists from the past few centuries and poked fun at their beards.
Despite their interesting facial hair choices, however, he explained that the ideas posited by these men shaped the theory of special relativity as it is known today. One such idea is that the laws of physics are expected to be the same everywhere.
“So far, observations seem to bear this out,” said Shaw. “When we look at other galaxies, other stars, they all seem to be obeying the very laws of physics that we’ve come up with here in our solar system, on our rather ordinary planet.”
He said that because everybody experiences the same laws of physics, the speed of light is constant for everyone as well. That’s a big deal, and different from anything else experienced in daily life.
“There is very little else, when you’re talking about speed, that is an absolute constant,” said Shaw. “Your car can go different speeds and baseballs can be thrown at different speeds, but photons of light, when they’re moving through the vacuum of space, they’re always going to go the same speed—approximately 300 million meters per second.”
Shaw asked the audience to imagine a spaceship flying by Earth. Inside the spaceship a photon is emitted from the floor, reflects off the ceiling, and is absorbed back into the floor. From inside the ship, it appears as though the photon is moving straight up and down. From Earth’s point of view, however, the photon moves up and down but also laterally as the ship flies by.
The result is a path that looks like an upside-down ‘V’.
“The people on the ship have to see this photon moving at approximately 300 million meters per second, but the people on Earth see the photon moving at that same speed,” said Shaw. “In the Earth’s frame [of reference], the photon has traveled a greater distance because it didn’t just go up and down, it also moved laterally. So it’s traveled a greater distance.”
Speed is equal to distance divided by time, which is why a car’s speedometer uses units of miles per hour. If the photon traveled a greater distance within Earth’s frame, but both frames saw the photon as traveling the same speed, it must mean that the time between the photon being emitted and absorbed is greater in the Earth’s frame.
Shaw called this time dilation. Time dilation says that the elapsed time between two events, in this case the photon being emitted and absorbed, is different for different frames. The shortest possible elapsed time will always occur in the frame in which the two events happen at the same location.
“We never really experience this because we’re all moving rather slow compared to the speed of light,” said Shaw. “None of us are moving at relativistic speeds…. You have to be moving at ten percent of the speed of light, or more, before it becomes something we can notice. And ten percent of the speed of light would be 30 million meters per second.”
Special relativity often comes into play in science fiction, such as the ‘hyperdrive’ used in Star Wars, but Shaw said humans are still far from traveling at speeds fast enough to factor time dilation into the equation.
One application of special relativity that is used everyday, however, is GPS.
“GPS uses the geosynchronous satellites, which means they’re always above the same point on Earth,” said Shaw. “And for that orbit, they have to actually be quite a ways away from Earth. As a consequence, they’re moving pretty fast. Now they’re not moving at what we would consider relativistic speeds, but they’re moving fast enough that you have to account for the slight time dilation that occurs.”
Signals are sent from Earth to satellites, and then sent back. Because of time dilation, the time stamp that satellites give each signal must be adjusted very slightly.
“It was really interesting learning about popular topics in physics,” said Jonah VanDoren, a UNCG student and self-proclaimed number one fan of the movie “Interstellar”. “The analogies that [Shaw] gave were easy to grasp, even after a few beers!”
The next Greensboro Science Pub event will focus on how traumatic brain injuries affect speech. More information can be found at their website, www.sci-pub-greensboro.org.
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