If you have attended a night sky outreach event, conducted, for example, by the Black Canyon Astronomical Society or the Western Colorado Astronomy Club, then you have heard mention of the distance to a star or galaxy measured in light years, i.e. how long the light has been travelling to reach Earth. You might wonder “Do the photons get tired or slow down as time passes?”
I date myself when I mention Robert Heinlein, a prolific and very popular science fiction author in the mid-20th century. Some of his notable works include Stranger in a Strange Land, Starship Troopers, and The Puppet Masters. However, I want to focus on Time for the Stars. Published in 1956, the novel revolved around a spacecraft sent to explore distant stars and possible planets. At such great distances, radio communication between Earth and the spacecraft became un-usable, due to the time delay.
The premise of the novel involves telepathic twins, one on Earth and one on the spacecraft, allowing instantaneous communication between Earth and a spacecraft located far from Earth. In addition, to make the journey in a few centuries, rather than thousands of years, the spacecraft was accelerated to a velocity that is a substantial fraction of the speed of light, c. To be clear, such technology does not exist even today.
To shed light on what happens as the velocity reaches these levels, we need to take a closer look at Einstein’s theory of special relativity. He noted that a moving clock ticks more slowly than a stationary clock.
Albert Einstein deduced that an individual travelling at any velocity relative to a stationary observer would age more slowly than the observer, i.e. the stationary observer will be older than the traveler, even if they started at the same age. The age difference is calculated from the time dilation factor, based on the traveler’s velocity relative to c, the speed of light. The closer the traveler’s velocity is to c, the greater the differential in ages. This has implications for our long distance photon.
In Time for the Stars, the Earthbound twin aged and died, while the astronaut twin aged just a few years. In fact, as the novel progresses towards the end, the astronaut twin is communicating telepathically with his great-grand-niece.
Interestingly, this so-called twin paradox was brought into the public eye by NASA, when astronaut Scott Kelly spent a year on the International Space Station while his twin, Mark, remained on Earth. Based on the math, Scott aged 9.9 milliseconds more slowly than Mark due to his velocity during the mission. This could not be physiologicallly confirmed, however. Time dilation has been confirmed in other robust scientific experiments.
Now we know that no object with mass can travel at the speed of light, because the amount of energy needed is infinite. However, a photon has no mass and clearly travels at c.
At the beginning, I mentioned the use of light years as the measure of time from Earth to a distant object in space. That time is relative to Earth’s frame of reference. Here is where special relativity gets weird. The time dilation factor for a photon calculates to zero. Hence, for our long distance photon, the time of travel is zero, i.e. the light has not aged at all!! AH, to be young again!
Music written and produced by Kenny Mihelich. Western Slope Skies is produced by the Colorado Mesa University Astronomy Club, the Western Slope Dark Sky Coalition, and KVNF Community Radio. This feature was written and voiced by Bryan Cashion.