Does Light Always Travel at the Same Speed?

Michael Anissimov
Michael Anissimov

The speed of light in a vacuum is 299,792,458 meters per second or 670,615,343 miles per hour. This is represented by the variable c, which stands for the Latin celeritas, which means speed. There is a consensus among physicists that accepting the general theory of relativity, Einstein's prevailing physics theory, entails accepting the speed of light in a vacuum as a constant. Therefore, any experiment proposing that the speed of light in a vacuum changes over time is viewed in the physics community with great suspicion.

Physicist Albert Einstein concluded that no object with mass could exceed the speed of light.
Physicist Albert Einstein concluded that no object with mass could exceed the speed of light.

It is widely known, however, that the speed of light is variable when it is not moving through a vacuum. The ratio of the speed of light through a given medium and through a vacuum is called the medium's refractive index or optical density. Some media have such a high refractive index that they can slow light to the speed of a person walking, or even bring it to a temporary standstill.

The speed of light varies unless it is moving through a vacuum.
The speed of light varies unless it is moving through a vacuum.

For example, the speed of light through air is very close to its speed in a vacuum. Depending on how dense the transparent media is, it can slow light to a greater or lesser degree. Water and glass can slow it to 3/4 and 2/3 of c, respectively. Different wavelengths also travel at different speeds through different media. For example, blue light travels a different speed than red when passing through a prism, causing the two to separate in a process called dispersion.

In reality, the speed of light never actually slows down. It is just delayed as the photons are absorbed and re-emitted by atoms in the intervening space. When a light beam exits a transparent medium into a vacuum, it continues traveling at the same rate as when it originally entered, without any added energy. This shows that slowdown is merely illusory.

At least two media are capable of slowing down light tremendously: Einstein-Bose condensates, and hot rubidium gas. These have both been used to halt light entirely. This was first achieved on a temporary basis in experiments conducted in 2001.

Different colors of light travel at different speeds when they pass through a prism.
Different colors of light travel at different speeds when they pass through a prism.
Michael Anissimov
Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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Why is it that humans always have to overpower mother nature in order to get something done? When humans went into orbit they had to overpower gravity with more power. If they would have spent all that time and money in trying to work with natural powers like gravity, universal vibration and photon (light) energy they could phase out of this universe and then back into it at a different place and time. The problem is to try and figure out when and where you would end up? It would be a one way trip. Good luck Major Thompson you really made the grade.


A 'faster than light speed' ship is a definite possibility before the end of this century. The initial one may be insanely expensive but like any manufactured high tech item, eventually it will become practical. The theory behind existing technology will be employed to create this 'smart' star-ship.


Just a few things to note. Light is affected by magnetic fields; otherwise we would not see sunspots. Light is affected by gravity: otherwise astronomers wouldn't be troubled by lensing. Light is affected by what I will call the invisible precursors of matter which are present in the normal space between light source and detector. What our scientists call red-shift is actually a measure of light degradation caused by this intervening dark matter. The "big bang" people were not aware of this but Hubble may have been.

Are the "big bang" people prepared to state categorically that light degradation does not occur? These days, the big bang theory and the concept of universe expansion are predicated on red shift and microwave background radiation, both of which have logical alternative explanations. I digress.

Light in space always travels at the same speed. What varies is the length of the path it must travel between two points, and the amount of energy transference to stuff in the medium (wavelength degradation not speed). Am I wrong?


Does light travel through every dimension? And could vibrating inter-dimensional super-strings actually be inter-dimensional light, meaning the super-strings in our dimension is seen as light, but in others it is still light but acts and behaves differently than light.


I agree with the person who said that a black hole is nothing more than a rip in our three-dimensional world, hence why it's black. It's nothingness, because we in our third dimension can't comprehend what is in the fourth dimension. Light has to be able to travel though this though, and for some reason i have a strong urge to believe that light/time/information are all one.


@anon170301: How do we know if there are subatomic particles left undiscovered?

You don’t have to actually see the object to know that something is there. By way of an analogy consider this: original predictions that led to the discovery of Pluto were based on observing slight disturbances in the orbit of Uranus and Neptune. Or another example: some planets outside our solar system have been discovered by detecting variations in light from the star which the planet is orbiting. In the case of subatomic particles try smashing protons together near the speed of light and observe the aftermath. That has led to the discovery of a new subatomic particle called the boson (or Higgs boson) which CERN announced recently (July 2012).


Mother of god, so many mistakes in your comments. First of all, light has no stationary mass but it does have a mobile mass, and Eddington's experiment and many, many other ones have proved that it is affected by gravity. And, to the guy who was talking about two objects moving faster than the speed of light, the speed of light has an absolute value. It is not relative to your speed. Check out Einstein's 'train thought experiment'.


To reach the speed of light an object needs to have no mass. Therefore, light has no mass. If light has no mass than it is hard to believe it can be affected by gravity.

We are not certain what a black hole is. Some scientist will argue that a black hole is light a piece of clothe. With enough pressure the clothe will rip creating a hole.

If a black hole is really a rip in the universe that would explain why the light can't escape it. It would be traveling through a different dimension.


@anon125586: No object can travel faster than light but think about two objects moving incredibly fast away from each other. The speeds they are moving would be added together and it is quite possible that the speed of the two objects together is faster than the speed of light. If this were true and neither of us slowed down than it is possible we would never discover the existence of the other object.


How do we know if there are sub-atomic particles left undiscovered? And if this is true, if I am not mistaken, it will make our measurements of space entirely inaccurate. Yet we have no way of telling how far off the measurements commonly accepted today really are. Everything is a mystery.


why do all wavelengths of light travel at the same speed when traveling in a vacuum?


light is the key. light is gravity.


if we started with a big bang we would all be seeing objects in space in real time because we do not travel away from the source faster than the speed of light.


Making accurate judgments about distant objects in space depends on how you do the measuring.

Using luminosity would be prone to error since dust absorbs light, and nobody knows how much space dust is out there.

These days, I think infrared technology provides better estimates. I find it a little ironic that light is part of the electromagnetic spectrum and yet some say it would not be affected by electric nor magnetic fields.

We know that light does not escape a black hole, so either light is affected by gravity or the space-time fabric is warped to make it seem so. We don't know all there is to know. There are still undiscovered sub-atomic particles and unanswered questions about dark matter that makes up most of the universe.


But light is affected by gravity, and so could it be possible that something we judge to be X amount of light-years away could be drastically different in both location and distance? Especially if it passed near a black hole.


as there are not any charged particles in light, it will not be affected by electric or magnetic fields.


If the speed of light is affected by temperature, magnetic fields, or some other thing, how can we be certain that our measurements in light years are correct? In other words, if I calculate an object to be 10 light years away, that could be wrong if something interfered with the light speed as it traveled through space.

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