What is Light speed: Definition and 320 Discussions
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any signal carrying information can travel through space.
Though this speed is most commonly associated with light, it is also the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation (of which light is a small range in the frequency spectrum) and gravitational waves. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonzero rest mass can approach c, but can never actually reach it, regardless of the frame of reference in which their speed is measured. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence, E = mc2. In some cases objects or waves may appear to travel faster than light (e.g. phase velocities of waves, the appearance of certain high-speed astronomical objects, and particular quantum effects). The expansion of the universe is understood to exceed the speed of light beyond a certain boundary.
The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves in wire cables is slower 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 ≈ 200000 km/s (124000 mi/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 (56 mi/s) slower than c.
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. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light also ultimately limits the data transfer between the CPU and memory chips in computers. 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 travels at a finite speed (non-instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism.
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (983571056 ft/s; 186282.397 mi/s) with a 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 / 299792458 of a second.
Can someone explain how it is possible that the speed of light is constantly 670 million mph irrelevant of relative speed? how is it that you can be going 669 million mph yet light still accelerates toward you at the same rate??
[SOLVED] Zero to Light Speed - Atom's Point of View
Atoms in a light bulb filament with house current are heated to the point that they readily emit visible light photons.
Three Questions:
1. What is the expected (or virtual) speed of the ELECTRON that emits a photon just as that photon is...
Some day I was driving my spaceship that can go as faster as light velocity. (in year 2175 all of us believe Einstein was wrong). Suddenly, a dumb driver appeared in the galactic highway with a old fashioned spacecraft , and when I reach him I stood behind him wanting to express my angry. To my...
Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?
i have a somewhat elusive question (for me) about light. when you drop a bouncy ball, after it hits the ground, right before it bounces back up, its velocity is zero. correct? it becomes troubling when the ball is replaced with light. when light is reflected straight back to its source, it...
It seems to me that Einstein's equivalence of gravitational acceleration and non-gravitational acceleration, when combined with his constancy of light speed in vacuuo, should yield a constancy of gravitational speed in vacuuo.
the other thread got the lockdown before i got a chance to gather all i could from it <sad>
so i guess i have to start a new one.
when people say "The speed of light is constant in a vacuum at C or 299,060 kps" (or whatever the freaking number is) what exactly do they mean?
from the...
Why does the time on the planet left not appear to slow from the spaceship as it is accelerating at great speed away from the spaceship, relative to the spaceships perception. I mean, could traveling at close to light speed also be viewed as slowing down the spaceship more and more so that the...
Does anyone know whether the expansion of the universe ever exceeded the speed of light taking the early accelerated phase into consideration?
If not then the early photons must have bounded back and forth within the boundary of the early universe and a few or maybe many of all photons we are...
Light speed relative to universal speed...
HI guys,
Just a simple request for confirmation that I have it right.
It may seem so obvious to you but to me I just need to confirm with currently held thought ( this forum ) that the photon travels at relativistically zero velocity with the...
Assume that c is the maximum velocity of electromagnetic radiation in an inertial frame of reference centred upon the source of the elecromagnetic radiation.
Assign an interface (boundary layer) between adjacent inertial frames of reference such that the velocity of the electromagnetic...
Has anyone an idea why speed of light is constant ? Has this something to do with lack of mass of a photon because I cannnot imagine it to be possible to add speed to something that has no mass. As light emerges from energy fall of an electron, does speed of light have anything to do with...
Like we have learned in school, the ultimate speed is a light speed. Do you believe that? We may be wrong. Let's take an example. There are two objects. They start in the same place and they start moving away from each other. They are traveling in a speed of a light. But isn't the speed they are...
Where does the speed of light (~300000km/sec in vacuum) come from
Is this the rotational speed of an electron. Up to Einstein a photon has no mass once it is in movement.
Is the absence of mass the reason why speed of light cannot be modified ?
I say NO. If energy can be converted to Mass and Mass can be converted to Energy, then, being a layman, what would prevent any object from being propelled to the speed of light without, as I have heard, requiring all the energy in the Universe to do so?
One of the main problems of quasars is: How can an energy equivalent to 100 times that of a normal galaxy, be produced continuously in a region smaller than the Solar System?
To solve this puzzle, it was suggested that quasars were not at cosmological distances, but much closer, even within...
I stumbled on an article entitled, “Crash of relativity after Lorenz-Einstein” at: www.worldspace.nm.ru/en/articles/pdf/lor_e.pdf It is written by A.V. Rykov, Ph.D., chief of lab. Of Seismometry at the Russian Academy of Science.
What interested me was his assertion that the speed of light...