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.
Hi,
I was thinking about the following.
From a mathematical point of view, SR assumes the following postulate: spacetime is a flat Lorentzian smooth manifold.
From the above and a minimal interpretation (i.e. a minimal set of "rules" to define the correspondence between mathematical objects...
Outside a static spherically symmetric body in vacuum lies the Schwarzschild metric. However, when observing a contractive sphere from a point near the sphere, it no longer appears spherical. Its opposite end appears stretched for an observer stationary relative to the center of the sphere. In...
The current explanation in Wikipedia for the slowing of the light wave phase velocity in a transparent medium considers:
From another discussion I learned that the waves radiated by the "shaken" charges are real waves (not virtual), so they would also leave the medium, as the original wave. In...
TL;DR Summary: Solving a problem regarding a train going past a station using length contraction and the Lorentzian transformation.
I'll dive straight in. I encountered a problem where there is a train travelling at 0.6c going past a station, length 500 m when measured by an observer at rest...
I could find some approximations in a Google search about the speed of visible light in distilled water, but couldn't find much about the speed of X-rays in liquid mercury.
In which medium electromagnetic waves travel faster, distilled water or liquid mercury? Does the wavelength of the...
This is my first (non-professional) post: It seems accepted that space is "created" (otherwise the expansion of the universe would exceed the speed of light). We are talking about "creation" (!!). The question would be: does current physics tell us if this represents a waste of energy for the...
I am a newbie and the last time I did physics was in secondary school. I was researching the topic of pi and came across the euler's constant and was wondering about the relationship between euler's constant and the speed of light specifically ce². I think that works out to be one third the...
I am aware of the explanation for light slowing down. From what I understand the EM field causes electrons in the medium to oscillate interfering with the wave and slowing its progress through the medium. The question I am asking is about a light source (or any source) that is turned on. If I...
If I mount a fast spinning motor - let's say a 250k rpm, inside a cylinder, and use this cylinder as the armature of the second cylinder that sits round the first one and has the same rpm with the angular velocity in the same direction as the first one, and continue building these layers of fast...
The context is the so called warp drive.
In pop-sci articles I've seen the claim that the speed of light limit only applies to objects in space but not the space-time itself, thus claiming that the expansion and contraction of space by a warp drive has no speed limit.
On the other side, I've...
Here is a screenshot from Einstein's 1905 ELECTRODYNAMICS OF MOVING BODIES:My understanding is that here Einstein says that the rod, the 2 observers and the 2 clocks are in the moving system, one observer & clock at each end of the rod. From their point of view they are not moving. They can very...
The methods of measuring the speed of light are based on the transit time of light to and from a certain path.
In this way, apparently there is no way to measure the forward speed and the return speed separately and if these speeds are the same.
Light is an electromagnetic wave located in the...
I have always wanted to ask this question, because there is something that I am missing.
I think it should be conceivable that the speed of light is affected by masses, so that its value within the solar system would be different from its value in interstellar space and even more so in...
I thought of this description recently and I think it's pretty intuitive, but I've gotten some side eye telling it to friends and family (maybe because relativity is screwy, maybe because I'm confused, maybe both?) so I want to get some confirmation that it's reasonable. Here goes:
If you're in...
From what I've read, it is not possible to measure the one way speed of light. We must reflect it off a mirror and then divide its travel time by 2, giving us its round trip average speed. Time dilation makes synchronizing two separated clocks impossible. We just assume light goes at C in all...
I don't know if there are some here familiar with "Veritasium", a YouTube video channel dedicated to science and engineering. It was created and is hosted by Dr. Derek Muller. It has over 8 million subscribers and many of its programs have been watched millions of times.
Today I was watching an...
Hello,
I have a question linked to gravity and speed of light :
According to the Special Theory of Relativity, the speed of light cannot be exceeded because it would need an infinitive energy to accelerate the relativistic mass of a moving objet (a space craft for example) : indeed if the...
If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed?
Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy) :oldconfused:
The constancy of the speed of light is a fundamental principle in modern physics, and it is supported by a wide range of current experimental evidence.
There is no evidence to suggest that the speed of light was different in the past, and the idea that it could have been different is at odds...
1. How much is acceleration of light?
2. Why is speed of light constant if time is relative, to keep speed constant distance must be changed as well?
3.What is time by Einstien? If we state it is not absolute, it is illusion, dont exsit?
4. Can we say ratio between distance and time is...
Is my understanding correct that if we have a moving vehicle moving to the right at speed v, as above, with a light source in center going in both directions, that (upon emitting the light at time T), a detector at D1 & D2 would both detect light reaching it at T2? (even though in the time it...
I once read (though I don’t remember where) that in the same way that you talk about a dimensionless ratio between Y and X in ordinary space, you can conceive of c as a dimensionless ratio between T and X in spacetime.
Do you know where I can find a reliable treatment of that idea?
As...
This is much like the "grain of sand" at near speed of light hiting Earth, but now targeting Jupiter with a needle!
What if...
Any way, which is the correct way to analyze this kind of scenarios?
For example, I think than being the speed of the needle near c, is nonsense to think about the...
I perfectly understand that we cannot, as of our current methods, determine the one-way speed of light - I was, however, wondering if we could determine a relative speed using electrical transmission. the basis of which would be sending a laser light across a field into a sensor, starting a...
Quote from NASA:
My understanding of dark energy is based on NASA's report: https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy; were NASA state as follows: "It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on...
hello
Einstein assumed the invariance of the speed of light as an hyphotesis, while I was told that :
"The speed of light need not have been postulated as an invariant."
in other words
the invariance of the speed of light could have been proven even regardless of the special relativity
is it...
What is the difference between the GTR vacuum and the vacuum of quantum theory? What is the speed of light in the vacuums, specifically what can be predicted and what must be measured?
hello everyone
According to Einstein's second principle of relativity:
"The principle of the constancy of the speed of light: The speed of light in free space has the same value c in all inertial reference frames"
Does free space mean vacuum? Is this principle not valid in the air and the...
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As our universe is expanding, that is, our vacuum is becoming more and more sparse, I believe it is possible that some key characterics of our vacuum is also variable.
Hello,
I have a problem with the postulate of the invariance of the speed of light.
When we move away from a light source it is redshift, it is the sign that the relative velocity between us and the light source has changed. If a stationary observer observes the phenomenon, he will measure that...
I would like to know the opinions on the paper that I saw about Michelson-Morley experiment.
Michelson-Morley experiment was done in 1887 and had an impact on the future direction of physics. It is taught in schools as an experiment that proves the non-existence of the aether and proves the...
Imagine a plane 1 light minute across. Now imagine 1 person on either and of that plane. Between them, is a thin indestructible bar that is 1 light minute in length. In the center of this plane, a simple device has a wire that leads to a motion sensor on the left side of the bar. If the left...
We can measure the two-way speed of light, but not, apparently, the one-way speed. Light could travel at c in every direction or c/2 in one direction and instantaneously in the other. Nature does not provide us with a way of determining the one-way speed.
I can follow some of the basic...
I get it, it sounds cool. But it is a very misleading and sometimes confusing way to represent c.
The speed of light is not constant. When I say that, I’m talking about the speed of LIGHT. Not c. Cherenkov radiation is a result of particles moving through some material (usually water) faster...
Hello,
Is there a mirror that will reflect light in parallel trajectories ?
If yes, is the reflected light in sync, and will all beams hit a flat surface simultaneously ?
Thank you
The question constantly arises how the speed of light is measured and what does it mean that the speed is constant, including at remote points for the observer, including at points beyond the local frame of reference, as you understand it in general relativity (GR).
First of all, it should be...
The idea is to have 2 clocks at position A and B. The clocks are synchronized by sending a light pulse from position S over 2 equal distances x.
The receiver is at position R at a distance y rectangular to the direction AB and exactly in the middle between A and B and right below S.
for proofing...
Been thinking about this since I was 12. Suppose you had a 1 lightyear long pole, and on the other end was a button 1 inch away that activates a light. The light is powerful enough for you to see it 1 lightyear away. You push the pole 1 inch forward. How long does it take for the light to reach...
One of the reasons to suggest that light might be bent by gravity is the assumption that light is behaving as the other objects that bend their trajectories by gravity. On a similar ground, we could suggest that as the objects are experiencing Gravity Assist, then the light could be also...
I understand the relativistic effects at high speeds, what accounts for the constant speed of light at low speeds?
Example... If I travel towards a beam of light at 25 MPH, I will still measure the light as traveling towards me at the speed of light and not the speed of light plus 25 MPH.
Good evening all,
I had a question on how the standard for the meter is defined. A simple Google search tells us that since 1983, the meter has been internationally defined as the length of the path traveled by light in vacuum during a time interval of 1 / 299, 792, 458 of a second. Many other...
We know that as speed increases, time slows, and at the speed of light time apparently ceases. Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are...
I may be way off, but at some point in the past I understand there was super expansion where the universe expanded faster than the speed of light. If matter expanded with it it's an example of matter in separate areas moving apart faster than the speed of light. If it didn't the universe has...
Recently I have seen a number of General Relativity visualisations that show spacetime flowing towards any mass, similar to water flowing into a sink hole. ScienceClic's video is an example. That model is also used in the "waterfall model" to explain the event horizon of a black hole, as the...
I think infinite speed is unimaginable. If something is moving at infinite speed, we can't find it at all because it has moved to infinity. Furthermore, when the maximum speed is limited, a reasonable inference should be that observers in different reference frames should find the same one speed...
The question is of what does it depend on? We take it for a constant still how do we know that its the exactly the same as in a vacuum in a center or closer to the center of our galaxy for example. Why is the number what it is?
If light goes from air through glass, the reason the light bends is because I'm told that the light travels slower inside the glass. If I change my observer reference to inside the glass, the speed of light inside the medium it look like light is still traveling at the speed of light? if so, the...
If a person was rotating on a verticle axis from head to toe like the Earth or quasar. If nothing can go faster than light, from the person's perspective looking at the stars traveling across the night sky, if you increase the rotation of the earth, stars further than a certain critical distance...