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# Light Speed

## Introduction

In our world, the fastest thing are objects able to move at the **Speed of Light**. The speed of light in vacuum, commonly denoted *c*, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour). According to the special theory of relativity, c is the upper limit for the speed at which conventional matter or energy (and thus any signal carrying information) can travel through space. This is a visual representation of how fast someone would be able to travel around the earth at light speeds.

**What Travels at Light Speed?**

All forms of electromagnetic radiation, including visible light, travel at the speed of light. For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. Lasers also can travel at the speed of light, though whether they are accepted or not falls under our laser/light requirements. Massless particles and field perturbations, such as gravitational waves, also travel at speed c in vacuum.

**What Effects does Light Speed Have?**

Any starlight viewed on Earth is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When communicating with distant space probes, it can take minutes to hours for signals to travel. In computing, the speed of light fixes the ultimate minimum communication delay. The speed of light can be used in time of flight measurements to measure large distances to extremely high precision. Games such as *A Slower Speed of Light* showcase what the field of view of approaching light speed would be like.

## Time Dilation

*This article is where we got our information from*.

Time dilation refers to the fact that clocks moving at close to the speed of light run slow. Consider two observers, each holding an identical clock. These clocks work using pulses of light. An emitter bounces light off a mirror, and the reflected pulse is picked up by a detector next to the emitter. Every time a pulse is detected, a new pulse is sent out. So, the clock measures time by counting the number of pulses received; the interval between pulses is the time it takes for a pulse to travel to the mirror and back.

If our two observers are stationary relative to each other, they measure the same time. If they are moving at constant velocity relative to each other, however, they measure different times. As an example, let's say one observer stays on the Earth, and the other goes off in a spaceship to a planet 9.5 light years away. If the spaceship travels at a speed of 0.95 c (95% of the speed of light), the observer on Earth measures a time of 10 years for the trip.

The person on the spaceship, however, measures a much shorter time for the trip. In fact, the time they measure is known as the proper time. The time interval being measured is the time between two events; first, when the spaceship leaves Earth, and second, when the spaceship arrives at the planet. The observer on the spaceship is present at both locations, so they measure the proper time. All observers moving relative to this observer measure a longer time, given by:

In this case we can use this equation to get the proper time, the time measured for the trip by the observer on the spaceship:

So, during the trip the observer on Earth ages 10 years. Anyone on the spaceship only ages 3.122 years.

It is very easy to get confused about who's measuring the proper time. Generally, it's the observer who's present at both the start and end who measures the proper time, and in this case that's the person on the spaceship.

Carrying on with our example of the spaceship traveling to a distant planet, let's think about what it means for measuring distance. The one thing that might puzzle you is this: everything is relative, so a person on the Earth sees the clock on the spaceship running slow. Similarly, the person on the Earth is moving at 0.95c relative to the observer on the spaceship, so the observer on the ship sees their own clock behaving perfectly and the clock on the Earth moving slow. So, if the clock on the spaceship is measuring time properly according to an observer moving with the clock, how can we account for the fact that the observer on the ship seems to cover a distance of 9.5 light years in 3.122 years, which would imply that they're traveling at a speed of 3.04c?

That absolutely can not be true. For one thing, one of the implications of relativity is that nothing can travel faster than c, the speed of light in vacuum. c is the ultimate speed limit in the universe. For another, two observers will always agree on their relative velocities. If the person on the Earth sees the spaceship moving at 0.95c, the observer on the spaceship agrees that the Earth is moving at 0.95c with respect to the spaceship (and because the other planet is not moving relative to the Earth), everyone's in agreement that the relative velocity between the spaceship and planet is 0.95c.

So, distance is velocity multiplied by time and we know the velocity and time measured by the observer on the spacecraft is 0.95c and 3.122 years. This implies that they measure a distance for the trip of 2.97 light-years, much smaller than the 9.5 light-year distance measured by the observer on the Earth.

This is in fact exactly what happens; a person who is moving measures a contracted length. In this case, the person on the Earth measures the proper length, because they are not moving relative to the far-off planet. The observer on the spaceship, however, is moving relative to the Earth-planet reference frame, so they measure a shorter distance for the distance from the Earth to the planet. The length measured by the moving observer is related to the proper length by the equation:

In this case we can solve for the length measured by the observer on the spaceship:

This agrees with what we calculated above, as it should.

One important thing to note about length contraction: the contraction is only measured along the direction parallel to the motion of the observer. No contraction is seen in directions perpendicular to the motion.

## Faster Than Light

**Faster-than-light** (also FTL, superluminal or supercausal) travel and communication are the conjectural propagation of matter or information faster than the speed of light (c). The special theory of relativity implies that only particles with zero rest mass (i.e., photons) may travel at the speed of light, and that nothing may travel faster.

Particles whose speed exceeds that of light (tachyons) have been hypothesized, but their existence would violate causality and would imply time travel. The scientific consensus is that they do not exist. "Apparent" or "effective" FTL, on the other hand, 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 ("undistorted") spacetime.

As of the 21st century, according to current scientific theories, matter is required to travel at slower-than-light (also STL or subluminal) speed with respect to the locally distorted spacetime region. Apparent FTL is not excluded by general relativity; however, any apparent FTL physical plausibility is currently speculative. Examples of apparent FTL proposals are the Alcubierre drive, Krasnikov tubes, traversable wormholes, and quantum tunneling. Mostly, FTL proposals find loopholes around the theory of relativity, such as by expanding or contracting space to make the object appear to be travelling greater than c.

**Some Misconceptions of Things Faster Than Light**

**Exceeding the speed of light with a laser:**This misconception comes from pointing a laser beam at the moon and flicking ones wrist. The spot of light from the laser beam will travel across the moon's face in about half a millisecond, which means it's traveling at 20 times the speed of light. This simply is the spot itself changing positions however and not moving at a speed, thus there's no actual speed here.