Light glides gently through the vast space and maintains an unwavering pace, covering a constant distance of 299,792,458 meters every second – no more, no less, reports sciencealert.com.
However, the story undergoes a dramatic change when this electromagnetic wave encounters the electromagnetic fields surrounding matter particles. As we navigate this complicated terrain, the speed of light can slow down significantly.
We witness this phenomenon when light refracts as it passes through a glass of water or when the countless colors of a rainbow spread out.
Physicists have long used 19th-century equations to explain this phenomenon in the context of light and electromagnetism. Yet they still struggle to fully understand the sudden change in the speed of light as it passes between different media in terms of physical waves.
A trio of physicists from the University of Tampere have proposed a possible solution to this riddle, but not without reevaluating some fundamental principles regarding the journey of a light wave through the dimension of time and space.
“I actually found a very convenient way to derive the standard wave equation in 1+1 dimensions,” says the study’s first author, Matias Koivurova, now at the University of Eastern Finland.
“The only assumption I needed was that the speed of the wave is constant. Then I thought to myself, what if it isn’t always constant? This turned out to be a very good question.”
The speed of light – or c to use its abbreviation – is a universal limit for information moving through a vacuum. Although matter can effectively slow down a particle’s overall journey, special relativity says that this fundamental property cannot actually change.
Yet physics sometimes requires a flight of imagination to explore new areas. So Koivurova, along with his colleagues Charles Robson and Marco Ornigotti, set this uncomfortable truth aside to consider the implications of a standard wave equation where any light wave can accelerate.
Initially, their solution didn’t make much sense. It wasn’t until they added a constant speed as a frame of reference that the pieces clicked together.
Send a spaceship into the depths of space at high speed, and the passengers will experience time and distance differently than the observers watching their journey from afar. This contrast is due to the theory of relativity, a theory that has been successfully tested time and time again at various scales.
By framing an accelerating wave at a constant speed of light, the strange effects of the team’s new solution to the standard wave equation looked exactly like those imposed by the theory of relativity. Their realization had profound implications for a debate over whether the momentum of a light wave increased or decreased as it entered a new medium.
“What we’ve shown is that from the wave’s perspective, nothing happens to its momentum. In other words, the momentum of the wave is maintained,” says Koivurova.
Whatever the wave, whether it is in an electromagnetic field, a ripple in a pond, or a vibration along a string, measures of relativity and the conservation of momentum must be factored into the equation as speed increases. This generalization would have another rather remarkable, if somewhat disappointing, consequence.
Whether it’s our intrepid space travelers zooming toward Alpha Centauri at a fraction of the speed of light, or their relatives slowly aging back on Earth, each of their respective clocks is ticking away at what is considered the correct time. The two times may not agree on the length of a second, but each is a reliable measure of the passage of years within its own framework.
If all waves also experience the proper time concern for relativity, the physicists argue, any physics governed by waves should have a strict temporal direction. One that cannot simply be reversed for a specific part.
So far the equations have only been solved for a single dimension of space (and time). Experiments would also have to be conducted to see if this perspective of waves is correct.
If so, our collective journey through the universe is truly one-way.
This research was published in Optics.
