The Speed of Light
Maximum Speed at Which all Energy, Matter, and Information in the Universe can Travel



 
The Speed of Light
Maximum Speed at Which all Energy, Matter, and Information in the Universe can Travel

 
The finite speed of light is important in astronomy. Due to the vast distances involved, it can take a very long time for light to travel from its source to Earth.

For example, it has taken 13 billion (13×109) years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images.

Those photographs, taken today, capture images of the galaxies as they appeared 13 billion years ago, when the universe was less than a billion years old.

The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the evolution of stars, of galaxies, and of the universe itself.



The speed of light (meaning speed of light in vacuum), usually denoted by c, is a physical constant important in many areas of physics.

Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time.


This speed is approximately 186,282 miles per second. It is the maximum speed at which all energy, matter, and information in the universe can travel.

It is the speed of all massless particles and associated fields—including electromagnetic radiation such as light—in vacuum, and it is predicted by the current theory to be the speed of gravity (that is, gravitational waves).

Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer.

In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc2.

The speed at which light propagates through transparent materials, such as glass or air, is less than c.

The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v).

For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s slower than c.

In most practical cases, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light has noticeable effects.

In communicating with distant space probes, it can take minutes to hours for the message to get from Earth to the spacecraft and back.

The light we see from stars left them many years ago, allowing us to study the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip.

Finally, the speed of light can be used with time of flight measurements to measure large distances to high precision. Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io.

Einstein's "Zur Elektrodynamik bewegter Körper" ("On the Electrodynamics of Moving Bodies") was received on June 30, 1905 and published 26 September of that same year.

It reconciles Maxwell's equations for electricity and magnetism with the laws of mechanics, by introducing major changes to mechanics close to the speed of light.


This later became known as Einstein's special theory of relativity. Consequences of this include the time-space frame of a moving body slowing down and contracting (in the direction of motion) relative to the frame of the observer.

This paper also argued that the idea of a luminiferous aether – one of the leading theoretical entities in physics at the time – was superfluous.

In his paper on mass–energy equivalence Einstein produced E = mc2 from his special relativity equations. Einstein's 1905 work on relativity remained controversial for many years, but was accepted by leading physicists, starting with Max Planck.

After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second.

As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.



Travel Faster Than Light

Through the Wormhole

Faster-than-light (FTL) communications and travel refer to the propagation of information or matter faster than the speed of light.

Under the special theory of relativity, a particle (that has mass) with subluminal velocity needs infinite energy to accelerate to the speed of light, although special relativity does not forbid the existence of particles that travel faster than light at all times (tachyons).

On the other hand, what some physicists refer to as "apparent" or "effective" FTL depends on the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations in less time than light could in normal or undistorted spacetime.

Although according to current theories matter is still required to travel subluminally with respect to the locally distorted spacetime region, apparent FTL is not excluded by general relativity.

Examples of FTL proposals are changing the frequency of mass to a higher state by applying high frequency waves of energy, the Alcubierre drive, and the traversable wormhole, although the physical plausibility of some of these solutions is uncertain.

In models of the expanding universe, the farther galaxies are from each other, the faster they drift apart. This receding is not due to motion through space, but rather to the expansion of space itself.

For example, galaxies far away from Earth appear to be moving away from the Earth with a speed proportional to their distances. Beyond a boundary called the Hubble sphere, the rate at which their distance from Earth increases becomes greater than the speed of light.


 
An unknown object has been located in M82. The object is located at several arcseconds from the center of M82. It has an apparent superluminal motion of 4 times the speed of light relative to the galaxy center.

In late September 2011, physicists working at the OPERA experiment published results that seemed to suggest beams of neutrinos had travelled from CERN (in Geneva, Switzerland) to LNGS (at the Gran Sasso, Italy) faster than the speed of light, arriving (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) nanoseconds early (corresponding to about 18 metres in a total distance of 730 kilometres)

These findings have yet to be independently verified and the OPERA researchers say they are going to "investigate possible still unknown systematic effects that could explain the observed anomaly" and "deliberately do not attempt any theoretical or phenomenological interpretation of the results."

Unknown Object in Messier 82

Messier 82 (also known as M82) is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major.

The starburst galaxy is five times as bright as the whole Milky Way and one hundred times as bright as our galaxy's center.

In April 2010, radio astronomers working at the Jodrell Bank Observatory of the University of Manchester reported an unknown object in M82. The object has started sending out radio waves, and the emission does not look like anything seen anywhere in the universe before.

There have been several theories about the nature of this unknown object, but currently no theory entirely fits the observed data. It has been suggested that the object could be a "micro quasar", having very high luminosity, and being fairly stable.

However, microquasars also produce large quantities of X-rays, whereas no X-rays have been seen from the mystery object. The object is located at several arcseconds from the center of M82. It has an apparent superluminal motion of 4 times the speed of light relative to the galaxy center.