Speed of light
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The speed of light, usually denoted by the letter c, is 299,792,458 m/s, exactly, since the meter is defined to be the distance light travels through vacuum in 1/299,792,458 of a second.[1]
The speed of light is considered to be an "ultimate cosmic speed limit". This is because massive ("massive" as in "having mass", not the colloquial "really fuckin' big") particles and objects can attain speeds that approach light speed, but never actually reach it. As the photons which make up light do not have any mass, they not only can travel at this speed, but must travel at this speed - hence the name "speed of light" to refer to c. According to special relativity and the experiments that back it up, the speed of light is the same for all inertial observers.
When light passes through materials, such as water, air, or glass, the photons (light quanta) do travel slower, and in certain materials can even be brought down to a mere walking pace. However, it should be emphasized that this has nothing to do with a change in c itself. When traveling through matter, photons are absorbed by the intervening atoms, raising the atoms to a higher energy state and this energy is quickly released again as photons (and some heat). The slowdown of the light's effective speed is caused by the delay between these absorption and re-emission processes.
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[edit] The modern science
[edit] Definition
In 1983, the Conférence Générale des Poids et Mesures (General Conference of Weights and Measures, or CGPM) defined the meter as the following[2]:
| “ | The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. | ” |
Since the meter is defined in terms of the speed of light, this definition fixes the speed of light at 299,792,458 meters per second (m/s).[3]
[edit] Constancy of the speed of light
A natural question is, "What gives the CGPM the right to define the speed of light?" The answer is that the speed of light in vacuum is a universal constant; it has the peculiar property that all observers measure the speed of light as c. This property makes light very different from, for example, a baseball. Suppose a baseball pitcher is standing on a train moving at 90 miles per hour relative to the ground. The pitcher throws a 90 mile-per-hour fastball towards the back of the train. While the pitcher and anyone else on the train would measure the speed of the baseball as 90 miles per hour, an observer on the ground would measure the baseball's speed as 0 miles per hour - the motion of the ball against the train cancels out as far as the observers on the ground are concerned. That is, the baseball would appear to hang in midair, until the back wall of the train caught up to it. Similarly, if the pitcher threw the ball in the other direction, at the same speed, the people on the ground would see the ball travel at an impressive 180 miles per hour, as the ball would gather momentum from the train and the speeds would combine. However, if the pitcher shines a flashlight toward the back of the train, he would measure the speed of the light as c...and so would the observer on the ground.
This constancy of the of speed of any light beam as measured by any observer has enormously important implications, and the resulting physical theory describing this is relativity.
Some of the earliest experimental clues that c must be constant for all observers were derived from Maxwell's Equations. These mathematical formulations, uniting electricity and magnetism, predict the existence of electromagnetic waves that travel at a certain speed. That speed can be deduced from the equations by measuring certain physical constants, and unlike classical mechanics and the example of the baseball pitcher described above, the equation says nothing about what this speed is measured relative to. Light can travel in a vacuum, and Maxwell's equations simply say what the speed is, and are perplexingly silent on the "medium" that it is measured relative to - although some did think that this would be the fabled aether. The electromagnetic waves predicted by Maxwell's equations turned out to be light, and those equations are considered to be one of the greatest triumphs of mathematical physics.
Attempts to unravel the mystery contained in Maxwell's equations by making experimental measurements of the speed of light in different directions were negative. The Michelson-Morley experiment was the first confirmation of this. The apparent paradox of a universally constant speed was found to be true.
[edit] An ultimate speed limit
The speed of light is considered to be an ultimate speed limit--massive objects can obtain speeds arbitrarily close to the speed of light, but can never reach it. Relativity predicts that an infinite amount of energy would be required to accelerate an object of any mass to the speed of light - particles without mass, however, can travel at the speed of light.
The limit placed on physical objects by the speed of light can also be justified with a simple thought experiment:
Suppose Alice observes a light beam. She must therefore be able to observe oscillating electric and magnetic fields, since that's what light is. Now suppose that she notices Bob traveling at the speed of light alongside that light beam. Bob does not observe oscillating fields; since he's traveling at the same speed as the oscillations, he would see static fields. Without oscillating fields, there is no light, so the light beam does not exist. But we have postulated that Alice sees a light beam, so it must exist. We therefore have a contradiction, and must abandon one of the following:
- a.) Alice can observe light;
- b.) Bob can travel at the speed of light.
We can observe light, so we drop the idea that Bob can travel at the speed of light. Thus, travel at light speed is not possible.
The existence of some faster-than-light particles, such as tachyons, has been suggested. Tachyons, if they existed, would be confined to the "other side" of the light-speed barrier; they would be restricted to speeds faster than the speed of light.[4][5]
[edit] Pre-modern conceptions of the speed of light
[edit] Ancient Greeks
Theories about light and its speed date back to ancient Greece, one of the most famous ideas to come from Greece at the time was the Emission Theory, which state that vision was created by rays of light being beamed from a person's eyes. Empedocles of Acragas (492-432 BC) apparently believed that the speed of light was finite; however, Aristotle (384-322 BC) was not convinced and argued for an infinite speed.[6] Like most Greek thinking at the time, both arguments were based on pure reason and not empirical science. Nevertheless, Aristotle's position became the accepted viewpoint, and it went unchallenged for nearly 2000 years.
[edit] Measurement attempts
In 1626, Galileo attempted to measure the speed of light using two observers with lanterns placed far apart. Unfortunately, the speed of light is too great to measure with this technique, and the best Galileo could conclude was that light traveled at least 10 times faster than sound.
Fifty years later, in 1676, Ole Rømer used astronomical observations of the moons of Jupiter to determine that the speed of light was finite. He calculated a speed of 214,000 km/s, which is about 70 percent of the current accepted value. The discrepancy is due to Rømer's uncertainty in the distance from Earth to the Sun.[7]
Several other measurements were performed using progressively better and more precise methods (see table). A notable measurement was performed by Albert Michelson in 1877, when he measured the speed to be 299,910 ± 50 km/s. This value was the standard for about 40 years.[7] Modern techniques use lasers and precision electronic equipment, but as the speed of light is now defined precisely and it is the measurement of space that is relative to it, trying to measure the speed of light has become a fairly moot.
| Date | Investigator | Method | Result (km/s) | Uncertainty |
|---|---|---|---|---|
| 1626 | Galileo | Uncovering lanterns | > 10X speed of sound | |
| 1676 | Ole Rømer | Satellites of Jupiter | 214,000 | |
| 1726 | James Bradley | Stellar aberration | 301,000 | |
| 1849 | Armand Fizeau | Toothed wheel | 315,000 | |
| 1862 | Leon Foucault | Rotating mirror | 298,000 | ± 500 |
| 1877 | Albert Michelson | Rotating mirror | 299,910 | ± 50 |
| 1926 | Albert Michelson | Rotating mirror | 299,796 | ± 4 |
| 1973 | Evanson et al | Lasers | 299,792.4574 | ± 0.001 |
| 1983 | CGPM | Definition | 299,792.458 | 0 |
[edit] Abuse by fundamentalists, reactionaries, and lunatics
Many modern fundamentalists have a very hard time accepting that the speed of light is so fast, or is constant, and they concoct elaborate theories to contradict science for the sake of their biblical literalism.
They also have a problem with the fact that we can see light from stars that are billions of light-years away.
[edit] The starlight problem
The "starlight problem" can be stated succinctly as follows: there are visible stars that are known to be more than 6,000 light years away, based on the Cepheid Variable technique.[9] Since the speed of light is constant, light from these stars must have taken over 6,000 years to reach Earth. Ergo, the universe is more than 6,000 years old.
Since one of the main goals of creationism is to spread bad theology as science, this scientific paradox is one that creationism must solve to even flirt with the idea of being "scientific." While creationists make an effort to solve the starlight problem, it is in many ways the "silver bullet" of creationism: in attempts to solve the unsolvable starlight problem, creationists often stumble upon and make some of their most blatantly false arguments - even Jason Lisle, the actual real astrophysics expert employed by Answers in Genesis can't make a decent stab at solving it. By using the starlight problem to push creationists onto the steady, non-controversial grounds of astronomy, some of the most damning underlying flaws in creationism have become apparent.
Nevertheless, a good creationist never admits defeat, even in the face of overwhelming logic to the contrary. Therefore, creationists have come up with some ways to reconcile their cognitive dissonance, by trying to solve the starlight problem.
Their efforts are discussed and summarily refuted below:
- God placed the earth in a time-dilation field during the six-day creation, meaning that time slowed for Earth to only six days, while millions of years passed in the outside universe. Therefore, six Earth days passed in billions of universe-years. The idiocy of this argument is obvious: astronomy is not Star Trek. Further to point, this "argument" is unfalsifiable.
- The speed of light has changed, or decayed, in the past. This "theory" is wrong for a couple of reasons:
- Laser rangefinders and digital clocks, in use since the 1960's, are precise enough to detect even small rates of residual decay, but none is observed.
- The major physical constants have not changed in any appreciable manner.[10]
- The "scientist" behind c-Decay theory, Barry Setterfield, uses poor math and worse scientific methodology.[11]
- Triangulation disproves this
Hardcore creationsts don't seem to dwell too much on the most obvious solution to this: that God created the universe before he created the Earth. This is the view often seen with varients of Old Earth Creationism but isn't too appealling to the more stubborn fundamentalists, possibly because it would imply that humans are not the most important thing in the entire universe.
[edit] See also
[edit] Footnotes
- ↑ Handbook of Chemistry and Physics 72nd ed. CRC Press: Boca Raton, 1991.
- ↑ So say these French people.
- ↑ For those interested a second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." From here.
- ↑ Wikipedia on tachyons
- ↑ [1]
- ↑ http://njsas.org/projects/speed_of_light/empedocles/index.html
- ↑ 7.0 7.1 http://galileo.phys.virginia.edu/classes/109N/lectures/spedlite.html
- ↑ http://njsas.org/projects/speed_of_light/index.html
- ↑ christiananswers.net Star Distances
- ↑ See e.g. this. article on historic evidence of the truly-constant constants.
- ↑ A reader-friendly account of the failure of the c-decay theory is this essay here.
