What is Lightspeed: Definition and 48 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.

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  1. misterously

    I Can someone falsify this analogy contradicting time dilation?

    A spaceship travels at the speed of light from the sun to the earth and back, infinite acceleration and deceleration. Sunlight takes about 8 minutes to reach earth. On the trip to earth, the sun seems to be frozen. On the trip back, sun activity appears to resume and is seen at 2x speed. The...
  2. MrYes

    I What was the role of frequency in the Michelson-Morley experiment's null result?

    The Michelson-Morley experiment (MMX) was set up to detect an effect associated with the luminiferous ether, a substance that doesn't exist. On the other hand unmoving space does exist in the sense that a spectrometer can be placed in a position where it doesn't detect Doppler redshifts or...
  3. I

    B Fundamentals of Lightspeed: Questions & Answers

    Hey there, I'm aware this is a bit of a stupid question, and I think that I understand the principle fundamentally, however, my intuition is still having a little trouble catching up, and I'm trying to figure out if it is because of an important detail that I have missed/misinterpreted. I think...
  4. RosutoTakeshi

    B How many times faster than light is this?

    Ok so I'm having trouble understanding how to calculate this If an object is moving so fast that it would take... "Hours for light to catch up to it" .. how fast would it have to be moving? Let's say it would take 2 hours for light to catch up to it. What kind of speed are we looking at...
  5. Johnnyallen

    Why is the speed of light what it is?

    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?
  6. Scott_10438

    Equation for plasma wave/light speed travel?

    Hi guys I'm finishing up some promo art for my original comic book. You're seein it here first. But its missing something- an appropriate equation. I would like to integrate a math equation into the art. I am attempting to depict FTL travel, using qualities similar to an LWFA, the plasma being...
  7. U

    LightSpeed & Time: What's the Relationship?

    What exactly is the relationship between the speed of light and Time? or in other words will Time still exists if an object was to travel faster than light speed? Thanks :)
  8. M

    Uncovering the Truth: Recording Lightspeed with a Trillion FPS Camera

    Had anyone already watched this? He said they successfully recorded the speed of light using a trillion fps camera that they had created. But isn't light already there even before someone started to record it? Not to mention that light is not actually accelerating. Thoughts?
  9. T

    Near Lightspeed Space Exploration - Query

    I mostly just imagine stuff. For some years I've had imaginings of what might be possible if we could fire a laser like that of the N.I.F. into a magnetic field like that of the L.H.C. Recently, Hawking has announced his initiative to explore space at near lightspeed with "micro-ships"...
  10. TheQuietOne

    FTL Collisions in Star Wars: Fact or Fiction?

    I am a big Star Wars fan and I was just wondering what would happen if two Star Destroyers collided during lightspeed? could it be possible to go so fast you could go through something without destroying it?
  11. mileymo

    Travel to Light Speed: Acceleration & Deceleration

    How long/what distance would it take a spaceship (with a hypothetical propellant-less engine) to accelerate to near light speed, and secondly, how low long/what distance would it take to decelerate back to zero again?
  12. abrogard

    Observed speed of light shone back from 0.9c forward motion

    been answered numerous times i guess but i couldn't find it. i am an ignoramus. if spaceship doing 0.9c shines light forward we - from another inertial frame - see that light as proceeding forwards at 0.1c, do we not? But 'they' see it as proceeding forwards at c. fine. But if they shine a...
  13. GiantSheeps

    Is a warp drive possible? How long until we develop one?

    Is a warp drive actually possible to create? If so, how long until we can develop one? Is it at all possible that they could be developed during our lifetimes? I'm asking because of this article...
  14. seb7

    Measuring possible one-way anistropy of light speed

    wiki says "Experiments that attempted to directly probe the one-way speed of light independent of synchronization have been proposed, but none has succeeded in doing so" Here's a proposed experiment which I could not find any evidence of this being performed before.. Central light source...
  15. P

    Faster Than Light Speed: Exploring the Limits of Physics | A Random Thought

    Just a random thought or question. Why can't something go faster than the speed of light? It almost seems like some laws of energy or physics do not agree with light speed. Consider this; imagine Earth pays to construct a light speed ship, and sends me up with a baseball. Once at...
  16. A

    Traveling at Light Speed: Question on Intergalactic Travel

    Hey all, I've been reading my fair share on physics and I would like to find a solution to a question that has been eating away at me. Look at the following extract from a book I read a while ago. "If we managed to travel at 99.99999999 percent of light speed, then we could travel out of...
  17. D

    Understanding the lightspeed limit

    I'm trying to understand the special theory of relativity. All answers I've found to the problem below seem to provide an inadequate answer. Could anyone answer this question? A rock in space at position B explodes into two halves (A & C) in such a (hypothetical) way that each half moves...
  18. A

    How could a photon travel at lightspeed if it has mass?

    I've heard that photons have a mass. If a photon were to be stationary, would it have mass? If not, then where does the mass of a photon come from? I know that if an object has stationary mass (forgive me if this isn't the correct term), then it takes an infinite amount of energy in order to...
  19. E

    The effect of the formation of electron-positron pairs on lightspeed

    Hello physicsforum, As the topic name says, I would like to know if a photon forming electron-positron pairs spontaneously could or does affect it's speed. Correct me if I'm wrong, but photons do form these pairs (and heavier?) even without the correct amount of energy, right? And if they do...
  20. V

    Any in depth explanations on why near lightspeed affect aging?

    The time dilation effect on a lightclock moving horizontically is very easy to understand. But how does the speed affect our biological processes? "Because time slows down!" you might say. But specifically, what does make say the cells move slower? If they are moving in the same direction of a...
  21. G

    Lightspeed - Spaceship on a lever theory.

    I've been thinking about the speed of light, and if it is possible to exceed it. So, I came up with my spaceship on a lever theory. Essentially, you have a long lever with a fulcrum near one end. At the other end, you attach a spaceship. Next, you apply pressure at the short end of the...
  22. T

    Lightspeed and entanglement 'phone' question

    All, I'm new to the forum and registered as I have a scenario which has me confused (not difficult!) The scenario goes as follows and involves:(and please correct me if anything I'm suggesting is incorrect) -person A journeys to a nearby star and travels near the speed of light...
  23. B

    Measured lightspeed in general relativity and moving through space.

    Hey guys I was just wondering the following. Since time slows down when traveling faster, and we move around 1.3 million miles per hour, using the CMBR as a frame of reference. Does this mean our perceived lightspeed is wrong? How fast would we measure time to travel if we managed to come to...
  24. R

    RELATIVE Space/Time and Lightspeed question

    Hey everyone, First off I am not a physics major or a math guru. Physics has always been some thing that I have been interested in though. Physical science and the way things work has always interested me. I mention this, because I do not want to come off sounding like an idiot. I know most...
  25. A

    References for a Lightspeed Ship

    I searched this site and could not find any referances for what a good lightspeed ship would look like. Does anyone know of any good books or references on how to design ships that travel near light speed? Here's a few things I do know people speculate on: It would be sleek. Stationary...
  26. S

    How Does Gravity Affect Light with Zero Mass?

    So, an object can move with c if and only if it has 0 mass.So howcome that light is bended by gravity? How can smth with 0 mass be bend by gravity? Ty
  27. K

    Faster Than Light Travel: Exploring Hypothetical Technology & Causes

    From what I understand, space (not matter) can expand at a speed faster than that of light, and when it does, it goes in all directions. Would it be possible to create a controlled expansion of space behind a vehicle along a single path, like a corridor, that would change your position in space...
  28. I

    Help a rookie understand lightspeed

    Although I do have a degree in chemical engineering, I am studying relativity/cosmology on a beginning level just as a hobby. I'm currently attempting to wade through Einstein's relativity equations. However I remain troubled by the underpinning of the whole thing - the constancy of the speed of...
  29. D

    Higgs free space travelling, close to lightspeed possible ?

    I was thinking if the Higgs part would be found next year, is it true if it could be removed from materials, that you could build spaceships from such materials, you could travel with speeds close to the lightspeed, so Star Track would be a real possibility in the future ? Or do I forget...
  30. C

    Hypothetical situation of planck length object and lightspeed

    Initially known theories: 1. General consensus: Planck length is smallest measurable unit of length 2. Objects approacing light speed contract in length Scenario: Some hypothetical mass measured to be a Planck length is accelerated infinitely close to light speed. Questions: Can a...
  31. J

    2m car near lightspeed fall into 1m hole?

    will a 2m(rest length) car moving near lightspeed fall in a 1m hole?
  32. A

    Reasons why lightspeed travel is impossible

    So, as something with rest mass gets closer to the speed of light, it gains more mass, making it require greater force to accelerate it making it gain even more mass and so on. So the reason we use to say nothing can travel at or faster than light is because it gains more mass, and requires more...
  33. T

    Traveling at Lightspeed - Time Stops

    If an object were to travel at the speed of light from Point A to Point B, then wouldn't the speed of light for the person on the object be v = d/0? Because when you travel at the speed of light in space, your time stops. So when the object reaches point B, even if it is 1 billion lightyears...
  34. R

    Basic conceptual description of lightspeed having a limit

    To be able to explain in a nutshell to my daughter why you can't go faster than the speed of light does this work: The universe exists now for us based on the expansion from a point. assume this expansion is occurring at a fixed rate. This runs the risk of assuming the answer I know. If...
  35. L

    Measuring Light Speed - Is Light Massless?

    Hi I was reading about the measurement of light years and was suprised to see it is the speed at which light goes through a vaacum. Is that because of the vaacum of space or does light have mass? thanks lneilson
  36. Q

    Lightspeed Wavelengths: Exploring the Standard and Variations of Light Waves

    Hello there, I am new to the sight and am curious to know if light has a standard wave length? And that maybe different spectrum's of light could possibly have different wave length- thus creating multiple speeds of light?
  37. S

    The Mystery of Lightspeed: Exploring the Fundamental Speed of the Universe

    Why does the speed of light equal C? (ie. 3 x 10^8 m/s) Why isn't it 5 x 10^8 m/s, or 7 x 10^5 m/s, or 4 x 10^25 m/s? What makes lightspeed the particular speed it is? What is the underlying basis for lightspeed being what it is?
  38. S

    Light Speed Consistency: Puzzling Questions

    The concept of light speed being constant is puzzling to me. For instance, if a space vehicle is moving through space at near light speed (~c) and emits a light ray at speed c, does the light ray move away from the vehicle at speed c or is the separation speed (c - ~c) ? (Ignore constaints that...
  39. K

    Acceleration up to lightspeed (From the inside)

    Hi, I have a question that has been bugging me for ages now. It concerns acceleration up to the speed of light. I know that if an attempt is made to accelerate a spaceship/particle up to the speed of light from the outside, it is not possible to get it to the speed of light because the energy...
  40. J

    Noob with a question about lightspeed travel.

    I would like to preface this idea by stating that I am a total noob and in no way have any right to be here haha. I was just watching a show on the science channel "Carl Sagan's Cosmos". I am an avid viewer of basically anything on TV about the universe or physics in general. I find...
  41. G

    Is the speed of light really constant?

    Imagine this race. A beam of light is emitted from a planet (point A). Of course this beam will travel in the speed of light relative to the planet until it reaches point B. Starting at the same time as the beam, a spaceship travel from point A to point B (its speed is arbitrary). The speed...
  42. J

    Lightspeed: does the definition of time make it constant?

    this is from electrodynamics of moving bodies einstein's original SR paper.tell me when we defined a common time for A and B didnt we define lightspeed to be constant
  43. T

    Is Lightspeed Determining the Vertical Speed of Light Accurately?

    Is lightspeed defining the speed of light in which it travels a vertical length? Can lightspeed be used to accurately decide how long it will take for light to travel 5 meters straight vertically? Considering that light has wave length that 'should' not be the case. Does this mean that...
  44. M

    Lightspeed fast, then slowed, then fast again.

    I was reading material on light speed and have a question about something I thought may contradict Newtons law of conservation of energy. Light slows down as it passes through water, glass etc.. right? But when it reaches the other side it continues at light speed. Where is it getting its...
  45. T

    Velocity of Light Emission: Lightspeed or More?

    Is the top speed of light emitted by something the velocity of the light emitter + lightspeed or just lightspeed?
  46. R

    Lightspeed Travel: Is It Possible?

    Would this be possible - a spaceship detaches from the fabric or space-time sending a spherical ripple traveling at c which reforms at a diameter's distance bringing the space-ship back into being there. Because the ripple travels a longer distance than the space-ship, the spaceship would travel...
  47. R

    Lightspeed is 0 I will proof that

    Lightspeed is 0...I will proof that.. Hello, I would like to proof that lightspeed should be 0 or 1. the 1 would mean the slowest movement that can exist. It is very simple, If time is zero by lightspeed, movement should then be zero also. Time is like a train going 300.000 km/sec. By moving...
  48. C

    Understanding Relativity and Its Effects on Lightspeed

    imagine two people in deep space, with no frame of reference but each other. if one of them is speeding along at near lightspeed, he might just as well assume he is stationary and the other guy is moving. how can we get around this problem, without something like the ether?? secondly, does...