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.
Do laws of physics apply below
the event horizon? It appears as if
black holes had such gravity as to have an
escapr velocity higher than c, which means that
things are pulled inwards at higher speeds than
the speed of light. Or am I overlooking something?
I'm still trying to understand the concept of the slower passage of time at or near the speed of light. Say for example we had the technology to travel this fast and wanted to send astronauts to visit the newly discovered planet Proxima Centauri b four light years away.
My understanding is that...
I assume that, via scattering processes, the speed of light slows from that in a vacuum close to the centre of a black hole to zero at the event horizon. How is the gradient in its speed defined throughout this volume? Is an analogy with a sound wave reaching an interface appropriate?
E=mc^2 states that when you speed up matter to the speed of light, it becomes pure energy, of mc^2 joules. Now, if that is true, can you reverse the equation? Wouldn't energy speed up to the negative speed of light(-c^2), turn into matter? Or is that the wrong balance?
A spaceship traveling at speed of light close to speed of light (wrt inertial reference frame) sending some data every second on their clock to people who are stationary (wrt inertial reference frame). At what time these people would receive this data on their own clock?
Let's say for a second...
In quantum field theory, we use the universal cover of the Lorentz group SL(2,C) instead of SO(3,1). (The reason for this is, of course, that representations of SO(3,1) aren't able to describe spin 1/2 particles.)
How is the invariant speed of light enocded in SL(2,C)?
This curious fact of...
Let's say a spaceship zooms around the Earth repeatedly at 99.99% of the speed of light for a period of time that is 50 years for observers on Earth. How much time will the passengers on board feel has elapsed after these 50 years?
And, if someone on Earth was somehow able to remotely observe...
According to this video I was watching Maxwell was looking for the speed of EM waves and just divided coulomb's constant by the magnetic constant and then took the square root and that was the speed of light.
√(Ke/μ0)=√(9e9/1e-7) = 3e8m/s
So why is this a thing? I just don't understand why it...
I know there is a modern paradigm that states that nothing, including information, can travel faster than the speed of light. (With exception to quantum entanglement, but I'm not going to pretend like I know the exact details of that subject).
Thought experiment.
Imagine there are a series...
I learned in Analytical Mechanics: "Emmy Noether's theorem shows that every conserved quantity is due to a symmetry".
The examples I learned where conservation of energy as symmetry in time and conservation of momentum as symmetries in space.
Now I wonder, do universal constants are also due to...
Hey guys this question is a bit basic but I always get caught up on the idea, I'm not in school for physics so I don't really have a prof to ask and most of the internet always skips ahead of this.
Say I am in an inertial frame traveling close to C speed, the only thing I see is a photon...
So there is lots of information on the Internet specifying that the speed of light from the headlights of a car traveling at 30mph is C and not C+30 because it is impossible to travel faster than light but what about the speed of light from the brake lights pointing backwards? Am I right in...
Hi
I have a question about current start signals in a simple circuit such as explained in http://amasci.com/elect/poynt/poynt.html.
Turning on a switch somewhere in the circuit, sphere of influences, i.e. motions of electrons in wire and generation of Poynting vector around the wire, start from...
Currently, I'm doing the CREST award for physics, and my topic is about different methods of finding out the refractive index of a medium. I have already tested out few ways, and now i am thinking about a method where a formula n=c/v will be used(n-refractive index, c-speed of light in vacuum...
Hi all. New to PFs. I deal in (exotic) human biology, which happens to have put me at the intersection of fields and, in this instance, conceptually more challenging physics. I'm presently involved in a comparative analysis of biological versus physical time contraction-dilation. I happen to...
Dear PF Forum,
Before I get a NO answer, I'd like to ask a few question.
https://en.wikipedia.org/wiki/Hubble%27s_law#Observed_values
As of 13th July, 2016 Hubble flow is 67.6 km/s per mega parsec.
Or 67km/s per 31 trillion * 1 million km or per 3.26 million light year.
Say a galaxy 11.68 giga...
I understand that c is the "ultimate speed" and that it is a result of the fundamental constant, the permeability of free space. But if this is a constant only to a "stationary" frame of reference, how can we accurately measure it from Earth? Since Earth is moving relative to a supposed "fixed"...
Hello! I have a question that has been bothering me since I first started learning about Special Relativity:
Given only the Minskowskian metric and/OR the spacetime interval, how can one reach the conclusion that the speed of light is invariant for every observer and how can one conclude that it...
Dear PF Forum,
What is the limit of acceleration?
I've been reading old threads, and I found this.
G = 6.673 x 10-11 N m2/kg2
Solar mass = 1.989 * 1030kg
And I tried to plug in some numbers...
In a distance 30 km from a 10 solar mass object the acceleration is...
##a =...
Hi everyone,
Recently I randomly thought of a thought experiment of something going faster than light, along the lines of a shadow or some other "non-information carrying object". In this case it would be an empty seat.
The idea is to take 800 million and one chairs of half a meter breadth...
I know that no object can travel faster than the speed of light, but if two objects travel in the opposite direction, both at almost the speed of light, then would one object be traveling faster than the speed of light relative to the other?
How close to the speed of light would you have to travel to be able to traverse the entire span of the known universe(94 billion light years i think?) in a persons 80 year lifetime?
If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct...
I am confused about Einstein's thinking. I understand when he formulated his general theory of relativity, he wanted to incorporate 3 foundations for his theory: The relativity principle, the equivalence principle, and Mach's Principle. He believed that inertia and weight were essentially the...
Are objects beyond the observable universe in some sense moving faster the the speed of light. Given that objects at the edge of the observable universe are "maximally" red shifted, objects beyond that theoretically must be more red shifted, which is impossible. Is it therefore a nonsense to...
According to this video, , if a black hole is large enough you could actually travel for some time within the event horizon without dying because the event horizon is so far from the actual singularity. So, assuming that's true, what would you see while you were inside the black hole?
Here's...
Light travels more slowly through light-transmitting substances such as air or glass, otherwise no lens would refract light. so does this mean that the speed of light is not a universal constant?
I do not have a problem with the concept of the constant speed of light as it has no mass and therefore no inertia and therefore no relationship to any IFR. However it seems to be expressed as constant in all IFR's which I do not understand. This seems to say that if I am traveling at 1/2c and I...
Dear PF Forum,
I've been wondering about how on Earth (and I do mean it, from earth :smile:) that we know there's a galaxy 20 billions light away. Considering that https://en.wikipedia.org/wiki/Age_of_the_universe is 13.8 billions years old. But before I'm asking about Supernova Ia and Hubble...
If we could imagine a medium that could slow down light quite significantly, if a sound wave and a light wave were both passing through this medium, would the sound wave see the light wave passing by at the speed that light passes through that medium or would it see it passing by at the speed...
I believe that the speed of sound is constant in the same medium as is the speed of light. I would like to understand why we need the the theory of relativity to explain the speed of light being constant but I believe it is not used to explain why the speed of sound is constant within the same...
I know that if an object moves at the speed of light, from its persepctive time will stop to a halt. However the smallest unit of time should be the plank time. So my question is, will the object (say a photon) experience throughout its life a single still "frame", or will it experience...
Is it true that the speed of light is only dependent on the size of the universe, if the universe were much smaller would the speed of light be faster?
I was researching variable speed of light theories as alternatives to inflation theory; from Wikipedia I cam across this:
"The idea from Moffat and the team Albrecht–Magueijo is that light propagated as much as 60 orders of magnitude faster in the early universe, thus distant regions of the...
It's said that, speed of light is same in every frame is reference. Consider an ideal situation, if I'm also moving at the speed of light, will I feel light to be at rest or still at the speed of light itself according to my frame of reference?
Is there a simple way to prove to someone that light travels at c in a vacuum? I was having a debate with a friend and he said the speed of light isn't real.
Einstein said that the speed of light is constant for all observers.
When Maxwell derived that the speed of light in the ether was 299,792,458 m/s could he or did he surmise that the speed of light is constant for all observers (regardless of the motion of the source)?
If things such as quantum entanglement and the expansion of space can travel faster than light, then why can't gravitational waves, which are vibrations of spacetime? I thought that only matter cannot move through space faster than light. Also, has it been 100 percent proven that gravity waves...
Alright, so I'm by no means a trained physicist and most of what I know comes from sporadic readings on the internet, but I had a strange theory the other day. I'm more the type to think in visualizations or analogies rather than cold hard math, so I'll explain it the way the idea came to me...
So I've heard from multiple sources that one explanation for why light slows down whilst traveling through mediums other than a vacuum is that the light "takes every possible path at the same time" through the medium.
Below I've drawn my two possible interpretations of what that means. Can...
I'm having a difficult time researching the answer to my question about the speed of light. Now obviously it is a speed not only reserved for light but also all other massless particles/waves. It's obviously a constant property of our Spacetime since we can manipulate th speeds of different...
I was listening to a podcast about the solar neutrino problem, and they discussed how we have deduced that neutrinos are not massless due to the fact that they interact with other particles (even if this interaction occurs rarely). I paraphrase: "a particle traveling at the speed of light is...
Assume a spherical black hole that is eating matter from its surroundings. Then its Schwarzschild radius will increase with a speed proportional to the mass flux that enters the black hole. The question is: is this speed limited by the speed of light in vacuum c?
If the event horizon is a...