# What Is the Speed of Light?

The speed of light is the rate at which light travels. The speed of light in a vacuum is a constant value that is denoted by the letter c and is defined as exactly 299,792,458 meters per second. Visible light, other electromagnetic radiation, gravity waves, and other massless particles travel at c. Matter, which has mass, can approach the speed of light, but never reach it.

### Value for the Speed of Light in Different Units

Here are values for the speed of light in various units:

• 299,792,458 meters per second (exact number)
• 299,792 kilometers per second (rounded)
• 3×108 m/s (rounded)
• 186,000 miles per second (rounded)
• 671,000,000 miles per hour (rounded)
• 1,080,000,000 kilometers per hour (rounded)

### Is the Speed of Light Really Constant?

The speed of light in a vacuum is a constant. However, scientists are exploring whether the speed of light has changed over time.

Also, the rate at which light travels changes as it passes through a medium. The index of refraction describes this change. For example, the index of refraction of water is 1.333, which means light travels 1.333 times slower in water than in a vacuum. The index of refraction of a diamond is 2.417. A diamond slows the speed of light by more than half its speed in a vacuum.

### How to Measure the Speed of Light

One way of measuring the speed of light uses great distances, such as distant points on the Earth or known distances between the Earth and astronomical objects. For example, you can measure the speed of light by measuring the time it takes for light to travel from a light source to a distant mirror and back again. The other way of measuring the speed of light is solving for c in equations. Now that the speed of light is defined, it is fixed rather than measured. Measuring the speed of light today indirectly measures the length of the meter, rather than c.

### History

In 1676, Danish astronomer Ole Rømer discovered light travels at a speed by studying the movement of Jupiter’s moon Io. Prior to this, it seemed light propagated instantaneously. For example, you see a lightning strike immediately, but don’t hear thunder until after the event. So, Rømer’s finding showed light takes time to travel, but scientists did not know the speed of light or whether it was constant. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave that travelled at a speed c. Albert Einstein suggested c was a constant and that it did not change according to the frame of reference of the observer or any motion of a light source. In other words, Einstein suggested the speed of light is invariant. Since then, numerous experiments have verified the invariance of c.

### Is It Possible to Go Faster Than Light?

The upper speed limit for massless particles is c. Objects that have mass cannot travel at the speed of light or exceed it. Among other reasons, traveling at c gives an object a length of zero and infinite mass. Accelerating a mass to the speed of light requires infinite energy. Furthermore, energy, signals, and individual photos cannot travel faster than c. At first glance, quantum entanglement appears to transmit information faster than c. When two particles are entangled, changing the state of one particle instantaneously determines the state of the other particle, regardless of the distance between them. But, information cannot be transmitted instantaneously (faster than c) because it isn’t possible to control the initial quantum state of the particle when it is observed.

However, faster-than-light speeds appear in physics. For example, the phase velocity of x-rays through glass often exceeds c. However, the information isn’t conveyed by the waves faster than the speed of light. Distant galaxies appear to move away from Earth faster than the speed of light (outside a distance called the Hubble sphere), but the motion isn’t due to the galaxies traveling through space. Instead, space itself it expanding. So again, no actual movement faster than c occurs.

While it isn’t possible to go faster than the speed of light, it doesn’t necessarily mean warp drive or other faster-than-light travel is impossible. The key to going faster than the speed of light is to change space-time. Ways this might happen include tunneling using wormholes or stretching space-time into a “warp bubble” around a spacecraft. But, so far these theories don’t have practical applications.

### References

• Brillouin, L. (1960). Wave Propagation and Group Velocity. Academic Press.
• Ellis, G.F.R.; Uzan, J.-P. (2005). “‘c’ is the speed of light, isn’t it?”. American Journal of Physics. 73 (3): 240–27. doi:10.1119/1.1819929
• Helmcke, J.; Riehle, F. (2001). “Physics behind the definition of the meter”. In Quinn, T.J.; Leschiutta, S.; Tavella, P. (eds.). Recent advances in metrology and fundamental constants. IOS Press. p. 453. ISBN 978-1-58603-167-1.
• Newcomb, S. (1886). “The Velocity of Light”. Nature. 34 (863): 29–32. doi:10.1038/034029c0
• Uzan, J.-P. (2003). “The fundamental constants and their variation: observational status and theoretical motivations”. Reviews of Modern Physics. 75 (2): 403. doi:10.1103/RevModPhys.75.403