# The Speed of Light: It's the Ultimate Cosmic Speed Limit!

How fast does light move? It seems to be faster than we can follow, yet this force of nature can be measured. It's the key to a great many discoveries in the universe.

### What is Light: Wave or Particle?

The nature of light was a great mystery for centuries. Scientists had trouble grasping the concept of its wave and particle nature. If it was a wave what did it propagate through? Why did it appear to travel at the same speed in all directions?

And, what can the speed of light tell us about the cosmos? It wasn't until Albert Einstein described this theory of special relativity in 1905 it all came into focus.Einstein argued that space and time were relative and that the speed of light was the constant that connected the two.

### What is the Speed of Light

It is often stated that the speed of light is constant and that nothing can travel faster than the speed of light. This isn't entirely accurate. What they really mean is that the fastest that anything can travel is the speed of light in a vacuum. This value is 299,792,458 meters per second (186,282 miles per second). But, light actually slows down as it passes through different media. For instance, when light passes through glass, it slows down to about two-thirds of its speed in a vacuum. Even in air, which is nearly a vacuum, light slows down slightly.

This phenomenon has to do with the nature of light, which is an electromagnetic wave.

As it propagates through a material its electric and magnetic fields "disturb" the charged particles that it comes in contact with. These disturbances then cause the particles to radiate light at the same frequency, but with a phase shift. The sum of all these waves produced by the "disturbances" will lead to an electromagnetic wave with the same frequency as the original light, but with a shorter wavelength and, hence a slower speed.

Interestingly, matter can travel faster than the speed of light in different media. In fact, when charged particles from deep space (called cosmic rays) penetrate our atmosphere, they are traveling faster than the speed of light in air. They create optical shockwaves known as Cherenkov radiation.

### Light and Gravity

Current theories of physics predict that gravitational waves also travel at the speed of light, but this is still being confirmed. Otherwise, there are no other objects that travel that fast. Theoretically, they can get close to the speed of light, but not faster.

One exception to this may be space-time itself. It appears that distant galaxies are moving away from us faster than the speed of light. This is a "problem" that scientists are still trying to understand. However, one interesting consequence of this is that a travel system based on the idea of a warp drive. In such a technology, a spacecraft is at rest relative to space and it's actually space that moves, like a surfer riding a wave on the ocean. Theoretically, this might allow for superluminal travel. Of course, there are other practical and technological limitations that stand in the way, but it's an interesting science-fiction idea that is getting some scientific interest.

### Travel Times for Light

One of the questions that astronomers get from members of the public is: "how long would it take light to go from object X to Object Y.?" Here are a few of the common ones (all times approximate):

• The Earth to the Moon: 1.255 seconds
• The Sun to Earth: 8.3 minutes
• Our Sun to the next closest star: 4.24 years
• Across our Milky Way galaxy: 100,000 years
• To the closest  spiral galaxy (Andromeda): 2.5 million years
• Limit of the observable universe to Earth: 13.7 billion years

Interestingly, there are objects that are beyond our ability to see simply because the universe IS expanding, and they will never come into our view, no matter how fast their light travels. This is one of the fascinating effects of living in an expanding universe.

Edited by Carolyn Collins Petersen