Light Speed

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Light Speed

Many science fiction writers feel that humans will only be able to feasibly explore the Milky Way galaxy (and beyond) when it is possible to travel at, or above, the speed of light. Unmanned probes have already been sent to explore the solar system and beyond. NASA's Voyager I and Voyager II blasted off from Earth in the late 1970s to explore the outer planets and are now far beyond them.

The Light-Year

The dimensions of the universe are so enormous that they overwhelm conventional units of distance (such as the meter, kilometer, or mile). Therefore, a much larger unit distance is needed—the light-year. A light-year is the amount of distance that light travels in vacuum in one Earth year. Astronomers have found light-years to be a convenient distance when measuring the distance between stars and other celestial bodies.

In one second, light travels approximately 186,000 miles. To extend this out one year, multiply 186,000 miles times the number of seconds in a minute (60), times the number of minutes in an hour (60), times the number of hours in a day (24), and times the number of days in a year (365). As a result, light travels approximately 5,865,696,000,000 (more than 5.8 trillion) miles in one year—the distance in one light-year.

To better grasp the distance involved in traveling in outer space, imagine flying NASA's space shuttle from Los Angeles to New York City. This journey would take approximately 20 minutes at a speed of about 17,000 mph (miles per hour). At that speed, a journey to the Sun would take about 228 days. Beyond the Sun, the next closest stars to Earth are those of the triple star system of Alpha Centauri A, Alpha Centauri B, and Proxima Centauri at 4.3 light-years away, more than 25 trillion miles distant. This flight on the shuttle would take about 170,000 years!

A voyage from Earth to the center of the galaxy, a distance of about 30,000 light-years, would take about 1.2 billion years. Even the two Voyager spacecraft, the fastest machines ever launched from Earth, are now traveling at only 10 miles per second, not even one ten-thousandth the speed of light. These spacecraft would take 78,000 years to reach the Alpha Centauri star cluster.

As one can see, it would take extremely long periods of time to travel from Earth to other stars at conventional speeds, which is why the prospect of faster-than-light space travel has become so popular. It seems to be the only way to travel throughout the universe.

Is Traveling at the Speed of Light Possible?

Albert Einstein (1879–1955) developed his special theory of relativity in 1905. It declared that any material object can approach the speed of light, but it is impossible to go at or above this cosmic speed limit. The speed of light (denoted as c ) in a vacuum—an approximation of what actually is found in interstellar space—is a fundamental constant of physics and nature. The speed of light is as basic as gravity, which Einstein tackled in his 1915 general theory of relativity.

According to Einstein, if one could travel at the speed of light, then time would stretch to infinity and distances would be abolished altogether. Yet one obstacle to traveling at the speed of light is that matter attempting to attain light speed requires more and more energy but with very little resulting additional speed. At a speed above the speed of light, an object theoretically would be going "backwards in time," an occurrence viewed by many scientists as impossible.

Interstellar space travel appears to be extremely, if not prohibitively, expensive, even if future technologies could make it possible. All the propulsion systems proposed so far for faster-than-light voyages, such as warp drives, would require huge amounts of energy—more energy that is even conceivable to produce. On the one hand, there are many trivial ways in which things, in a sense, can be going faster than light, and there may be other more genuine possibilities. On the other hand, there are also good reasons to believe that real faster-than-light travel and communication will always be unachievable.

Ways of Traveling Faster than the Speed of Light

One way to (apparently) travel faster than light is to make light travel slower. Light in a vacuum travels at a speed c, which is a universal constant, but in a dense medium such as water or glass, light slows down to , where n is the refractive index of the medium (for instance, n = 1.0003 for air and n = 1.4 for water). It is possible for particles to travel through air or water at faster than the speed of light in the medium. Therefore, going faster than the speed of light really means exceeding the speed of light in vacuum c, not in a medium such as air or water.

Another way to (apparently) travel faster than light is to misrepresent speeds. If spaceship A is traveling away from a point at 0.7c in one direction, and another spaceship B is traveling away from the same point at 0.8c in the opposite direction, then the total distance between A and B may be thought to be increasing at 1.5c (derived from 0.7c + 0.8c ). However, this is not what is normally meant by relativistic speeds.

The true speed of spaceship A relative to spaceship B is the speed at which an observer in B observes the distance from A to be increasing. The two speeds must be added using the relativistic formula for addition of velocities:

where

v = 0.7c is the speed of spacecraft A,

u = 0.8c is the speed of spacecraft B, and

c is the speed of light.

After inserting the appropriate values into the equation, the relative speed w is actually about 0.96c (that is, 0.96 times the speed of light) and, therefore, not faster than the speed of light.

Another way to (apparently) travel faster than light is to observe how fast a shadow can move. If you project a shadow of your finger using a nearby lamp onto a far away wall and then move your finger, the shadow will move much faster than your finger. It can actually move much faster than this if the wall is at some oblique angle. If the wall is very far away, the movement of the shadow will be delayed because of the time it takes light to get there, but its speed is still amplified by the same ratio. The speed of a shadow is therefore not restricted to being less than the speed of light.

These are all examples of things that can go faster than light, but they are not physical (material) objects. It is not possible to send information on a shadow, so faster-than-light communication is not possible in this way. Faster-than-light travel cannot logically be deduced just because some "things" go faster-than-light or appear to do so. This is not what is meant by faster-than-light travel, although it shows how difficult it is to define what is really meant by traveling faster than the speed of light.

Proposals for Faster-than-Light Travel

One proposal for traveling faster than the speed of light is to use wormholes. A wormhole is a four-dimensional shortcut through space-time in which two regions of the universe are connected by a narrow passageway. The wormhole would permit matter/energy to proceed from one spot in the universe to another in a shorter time than it would take light otherwise. Worm-holes are a feature of general relativity, but to create them it is necessary to change the topology (or the physical features) of space-time.

The complete wormhole geometry consists of a black hole, a white hole, and two universe-regions connected at their horizons by a wormhole. According to Einstein's theory of gravitation, empty space is actually a tightly woven fabric of space and time. Massive objects warp the space-time fabric, just as a bedsheet would be pushed down into a deep valley if a whale were to lie on it. Anything that comes near the valley naturally rolls in, and that "falling" is the force perceived as gravity. If the whale twists around on the bed, its motion carries the bedsheet along.

If Einstein's theory is correct, space-time should likewise be dragged around massive objects. Black holes (like whales on a bedsheet) are objects that are so massive and dense that immense gravity warps space around the core, not allowing light or anything else to escape. A white hole, however, is a black hole running backward in time. Just as black holes (supposedly) pull things in, white holes (supposedly) push things out. This so-called naturally made warping of space in the form of wormholes could provide a means of quickly traveling to distant regions of space.

A warp drive, sometimes called hyperspace drive, is a (theorized) mechanism for warping space-time in such a way that an object could move faster than light. The most famous spaceship to use warp drive (at least in science fiction) is Star Trek's U.S.S. Enterprise. Its concept expands space-time behind the spaceship and contracts space-time in front of the spaceship. The warp in space-time makes it possible for an object to go faster than light speed.

But the problem with developing a warp drive is the same problem with formulating large wormholes. To construct it, one would need a ring of exotic negative energy wrapped around the spaceship. Even if such exotic matter can exist, it is unclear how it could be deployed to make the warp drive work.

A more likely scenario for deep-space travel makes use of Einstein's special theory of relativity. For objects moving at relativistic speeds (speeds near but below the speed of light), there is an observable stretching out of time relative to an observer in a stationary reference frame. For example, say a spacecraft travels at a constant speed of nine-tenths the speed of light (0.9c ) from Earth to Alpha Centauri, a distance of 4.3 light-years. According to time dilation , the time observed on Earth would be faster than the time onboard the spacecraft according to the equation

where

t * is the time indicated by the spacecraft clock,

t is the time indicated by Earth's clock, and

c is the speed of light.

Therefore, according to Earth's clock, the spacecraft took 4.8 years to complete the trip (4.3 light-years divided by 0.9c ). However, onboard the spacecraft, the journey takes only , or 2.1 years.

A Future Reality

With present technology, it is possible to (theoretically) produce spaceships that could slowly accelerate to near the speed of light so that a trip to the nearer stars would (in Earth time) take perhaps a few hundred years. However, in the spaceship, the entire trip, due to time dilation, would perhaps take a few dozen years, depending on the time it took to accelerate up (and then accelerate down) to and from light speed and how closely to light speed the spacecraft traveled.

It is rather difficult to define exactly what is really meant by faster-than-light travel and communication. Many things such as shadows can go faster than light speed but not in a useful way that can carry information. There are several serious possibilities for real faster-than-light mechanisms that have been proposed in the scientific literature, but technical difficulties still exist.

The Heisenberg uncertainty principle tends to stop the use of apparent faster-than-light quantum effects for sending information or matter. In general relativity there are potential means of faster-than-light travel, but they may be impossible to actually construct. It is highly unlikely that engineers will be building spaceships with faster-than-light drives in the fore-seeable future. It is curious, however, that theoretical physics, as presently understood, seems to leave the door open to the possibility.

Just because today something seems impossible, that does not mean that it will never become feasible.

see also Cosmos; Einstein, Albert; Space Exploration; Universe.

William Arthur Atkins with

Philip Edward Koth