What is Speed of light: Definition and 1000 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.
I keep hearing that the relative speed of light remains constant because time and lengths change with speed, what I believe is called the Lorentz factor. At slow speeds the Lorentz factor is extremely small, so what do people believe accounts for the rest of the change?
If I travel...
The Lorentz factor shows how fast one frame will judge speeds in another frame to be taking into account the relative motion between the two frames.
The speed of light is a factor in the Lorentz factor but I have heard that this is not because the speed of light is fundamental to it.
So...
If the speed of light would change in the universe without any other natural constant changing, would all clocks be affected in the same way by this?
This is implied by Einstein in this paper on page 368
http://myweb.rz.uni-augsburg.de/~eckern/adp/history/einstein-papers/1912_38_355-369.pdf...
I have seen thought problems with an observer on a train or in a station, etc., but I have not seen ones with the observer traveling at relativistic speeds. It seems to me that at sufficient speed he would observe himself exceeding the speed of light due to the slowing of time. This seems like...
Hello. Today I've thinking about limit velocity and speed of ligth. We know that material particles can't achieve that speed, also when the speed of particles increases your own clock walks slowly. In the particular case of ligth your speed don't move anything.
This it a explanation of why...
I am trying to get a better understanding about the constancy of the speed of light which is a well-established axiom of current day physics. for the start i want to understand how it is experimentally established and how these results are interpreted. My difficulty here is that this seems to be...
Hi people!
May be this could be an historical question.
Before Einstein, it is suppoused that speed of light in ether is always the same, I´m right?
I mean, if observer O is at rest respect ether and observer O´ is moving respect ether and O´ send a photon to O, the speed at which that photon...
How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this...
Drive two 15m long optical fibers extended in opposite directions with a 1GHz pulse generator. The recievers at the end of each fiber are now syncronised sources of 1GHz pulses. Connect them to pulse counters, one of which provides a signal whenever a pulse train is present.
Connect this...
Can we truly have a rest frame or should it be a close to rest frame?
Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity.
The Earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph. I'm never in a single...
This is a question I was looking at based on Relativity and John Wheeler's one-electron universe theory.
https://en.wikipedia.org/wiki/One-electron_universe
My question is this. The faster you move towards the speed of light, wouldn't everything in the universe contract to a single particle...
Homework Statement
The problem I have been working with recently has been deriving the speed of light using maxwells equations, however in order to do this I must make two assumptions; there is no net charge or displacement currents in the space in which I am attemptin to derive the speed of...
The speed of light from a moving source of light is usually recognized indirectly, based on various explanations of phenomena. The speed of light is fairly simple directly measurable on the basis of autonomous and separate measurements of the frequency and wavelength of the light.
Is a similar...
Let us imagine a photon circling around a black hole, as the picture shows.
The gravity of the black hole curves the movement path of the photon into the shape of a circle. From point 0, geometric points A and B appear simultaneously with the photon, each in its own direction. The points travel...
Suppose for the sake of argument someone said the outward speed of light is infinite and the return speed is c/2, creating a two-way speed of c.
Wouldn't this violate the conservation of momentum?
p = E/c. That means on the way out, the momentum of light would be zero, but on the way back it...
I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM...
I recently saw this question on a forum thread on The Guardian's website but was unable to follow it up.
Question: Why is the speed of light what it is? Could it have been another velocity?
Hi all
I have struggled with the assumptions that the speed of light is absolute and constant. I have some logic to this which is based on the common assumptions that light behaves both as a wave and a particle. It is also based on light having mass, the effects of heat and vacuum environments...
If a car drives on the highway at a speed of 100 ft per seconds and a gun in the car shoots a bullet forward at the speed of 1000 ft per second the total speed of the bullet will be 1000 + 100 = 1100 ft per second.
If a car travels at the speed of light when you turn on the head lights will...
Hello everyone,
I'll go straight to the question. The gravitational time dilation is equal to tearth = tspace*sqrt(1 - rs/r), with rs = 2GM/c2.
However, the formula for speed of light in gravitational field is equal to v = c(1 - rs/r).
My intuition tells me that these two formulas must be the...
Are there any relationships between the speed of light and the virtual particles in the vacuum?
I mean that, Can I call it as a medium of propagation of a light beam?
As a neophyte when it comes to the relativity solutions I have been surfing the web. I came across something in Science Forums.
<speculative link deleted>
I have now become interested in what gravity does to light. My understanding of the post above is that the reference frame for light...
I decided to read up on the chapters we didn’t cover in first year Physics from my textbook, and decided to start with general relativity since it was in the same section of the textbook as the last topic we covered (that topic was physical optics - not lenses- photons and the double slit...
Using Newton's 2nd law F=ma,
If you provide a constant force of 1mil Newtons then an object will accelerate at 100m/s.
Using V = U + AT
I can say that (speed of light) 299,792,458 = 0 + 100T thus T = 2997924.58 seconds or I can achieve speed of light in 35 days or so.
Why is this not...
Just wondering how a photon reacts when it is affected by gravity of an object in space like a star.
Does the gravity actually bend the light/photons?
Say you have a series of photons in a perfect line, all travelling, well, at the speed of light toward a massive object(x).
The photons in the...
Hello,
If the speed of light is the maximum speed limit in our universe, how was the big bang event possible because surely the expansion would have been constrained by the speed of light?
Hello - I'm posting this here because of a discussion I got into on another forum following the recent death of Stephen Hawking. I should stress that I am by no stretch of the imagination a physicist (though I did do modules on special relativity and quantum mechanics as part of my maths degree...
A rocket is in constant velocity. The velocity of the rocket is 150Mm/s (or 0.5 of the speed light, or 150 million meters per second) relative to us (we as observer).
We observe two lights, one moving in parallell with the rocket, another is moving in the opposite direction.
Below I have made...
Respected physicists and members
(I am not a physicist)
I have a little doubt that i want to clarify.
If I am sitting in a stationary train, having a ball in my hand, the ball will remain stationary relative to my hand and plateform. Now if the train starts moving, again the ball is stationary...
Is it possible that the speed of light exists because we cannot move faster than our particles? I.e. the speed of electrons that create the electromagnetic force that hold matter together.
Hello,
I came to this site, with a question in mind.
The speed of light. Nothing travels faster than the speed of light. This is a phrase, most of us are familiar with, to some extent.
The word "Nothing" has two syllables, "No" & "Thing". There is no thing, that can, travel faster than the...
If you were to connect a string from one star to another (assuming everything remains still), and then you were to pull one side of the string towards you, the other would be pulled immediately, thus the "communication" between the two edges is technically faster than the speed of light? Does...
The universe seems to be expanding since the farther away an object is, the faster it is moving. However, because of the finite speed of light, the farther away we look in distance, the further back in time we look. Does that mean that galaxies were moving faster in the past and are now slowing...
10 seconds somewhere can be one on Earth due to time dilation, right? So In the case of distance wouldn't light have traveled more than 186,282 MPS since it had 10 seconds to travel relative to the 1 second on earth. So if light traveled at 186,282MPS for 10 seconds it would have traveled...
Really silly question, but if we assume that our current science is correct, is it plausible that we can move faster than light in a vacuum? Say, for example, can we make the mass of something less than a photon so it then can it move faster than light in a vacuum.
I know this sounds like a...
In school, I was taught that the position of an electron relative to the nucleus is only an approximation. It is very likely that the electron will be close to the nucleus and where it is 'expected' to be, but it may in fact be anywhere in the universe.
Is this correct? And does this allow...
So I followed the derivation of the Macroscopic Maxwell's equations by averaging the fields / equations and doing a taylor series to separate the induced charges and currents from the free ones. But why does light now "suddenly" travel slower in dielectric media? I mean, sure, it comes out from...
Is there any relationship between the Speed of gravitational waves and the Universe's "local" expansion rate?
Speed of gravitational waves is supposed to be equal to the speed of light. Gravitational waves don't travel faster than light.
But we can observe far galaxies moving away from us with...
Hi,
What is people measuring when they measure the speed of light (c) actually? the photons or the waves? or some kind of manifestation of light?
I ask because I was reading about photons and that they "might" have a very small mass and therefore are not traveling at c. If photons have a...
If the speed of light is ~300 m/s in a vacuum, can the electric and magnetic fields of the wave be condensed such that it travels faster? The idea being that an outside force condenses these fields and lenghtens the wavelenght with no loss of energy thus increasing the directional speed. The...
Why the speed of light is constant for every observer ? Is it a special thing for photons cause I guess there's no other elementry particle that can move with c ?
Also I want to investigate the constant of c for every observer, in the perspective of particle relationship.
For example, can I...
Can anyone help me with a very basic question please.
Is it really not possible for anything to travel faster than the speed of light, as dictated by relativity theory ?
I am a complete beginner to Physics and only have very basic mathematics knowledge and I'm trying to understand the basics of...