THE TRUE 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. The value of 299,792,458 meters per second (186,282 miles per second) is the speed of light in a vacuum. However, light actually slows down as it passes through different media. As it moves through space, it encounters clouds of gas and dust, as well as gravitational fields, and those can change the speed a tiny bit. The clouds of gas and dust also absorb some of the light as it passes through. 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. As fast as light moves, its path can be bent as it passes by regions in space with intense gravitational fields. This is fairly easily seen in galaxy clusters, which contain a lot of matter which warps the path of light from more distant objects, such as quasars.

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The True Speed of Light

Gravitational lensing and how it works. Light from a distant object
 passes by a closer object with a strong gravitational pull.
The light is bent and distorted and that creates "images"
of the more distant object.  

Learn About the True Speed of Light and How It's Used

By John P. Millis, Ph.D

Light moves through the universe at the fastest speed astronomers can measure.

In fact, the speed of light is a cosmic speed limit, and nothing is known to move faster.

How fast does light move? This limit can be measured and it also helps define our understanding of the universe's size and age.

What Is Light: Wave or Particle?

Light travels fast, at a velocity of 299, 792, 458 meters per second. How can it do this?

To understand that, it's helpful to know what light actually is and that's largely a 20th-century discovery.

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 his 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. The value of 299,792,458 meters per second (186,282 miles per second) is the speed of light in a vacuum.

However, light actually slows down as it passes through different media. For instance, when it moves 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.

As it moves through space, it encounters clouds of gas and dust, as well as gravitational fields, and those can change the speed a tiny bit. The clouds of gas and dust also absorb some of the light as it passes through.

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.

Interesting, as fast as light moves, its path can be bent as it passes by regions in space with intense gravitational fields.

This is fairly easily seen in galaxy clusters, which contain a lot of matter (including dark matter), which warps the path of light from more distant objects, such as quasars.

Lightspeed and Gravitational Waves

Current theories of physics predict that gravitational waves also travel at the speed of light, but this is still being confirmed as scientists study the phenomenon of gravitational waves from colliding black holes and neutron stars.

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?"

Light gives them a very accurate way to measure the size of the universe by defining distances. Here are a few of the common ones distance measurements:

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: 13.8 billion years

Interestingly, there are objects that are beyond our ability to see simply because the universe IS expanding, and some are "over the horizon" beyond which we cannot see.

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

John P. Millis, Ph.D

Introduction

Chairman, Department of Physical Sciences and Engineering at Anderson University

Associate Professor of Physics

Ph.D. in Physics and Astronomy at Purdue University 

Conducts astronomical research at the VERITAS observatory

Experience

John Millis, Ph.D., is a former writer for ThoughtCo, where he contributed articles on space and astronomy for three years. He has taught physics and astronomy at the collegiate level since 2001 and is currently the chair of the Department of Physical Sciences and Engineering at Anderson University in Indiana. He teaches a wide variety of courses while maintaining an active research program in high energy astrophysics.

John's research focus is on pulsars, pulsar wind nebulae, and supernova remnants. Using the VERITAS gamma-ray observatory in southern Arizona, he studies the very high energy radiation from these dynamic sources to extract information about their formation and emission mechanisms. In 2010, he co-founded a small consulting business, Aurum Consulting, LLC, assisting with biological testing, chemical formulations, and product development. 

Education

Dr. John Millis received a Bachelor of Science in physics, with a mathematics minor from Purdue University. He remained at Purdue for the completion of his Doctor of Philosophy degree, where he focused on High Energy Astrophysics.

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https://www.thoughtco.com/speed-of-light-3072257