Is light really a constant?

So after reading these articles about the warp drives (so cool) it got me thinking about a topic I had thought about a long time ago when I read stories about scientists slow light down.

Scientists slow down light

At that time it got me to thinking. We consider light to be a constant, but if we can slow light down does that mean it isn't anymore? And if that's the case is there a good possibility that our calculations on distances via light years is way off?

Secondly, if we are able to slow light down to lower than light speed and then go past it, does that mean that traveling faster than light speed is feasible and possible even if in a loop hole sort of way?

reply to post by powerdrone

not even my realm, but i still like to imagine the possibilities in my head.
I could definitely learn more about light speed, so I'll sit back and wait for an expert

reply to post by powerdrone


Only the apparent speed of light was slowed down. In a medium, photons are absorbed and re-emitted by atoms, and there's a tiny delay before a photon is re-emitted. The denser the medium, the longer the delay. But in-between the atoms, light travels at c.

This is more a science topic than one of space exploration, isn't it?

Originally posted by powerdrone
At that time it got me to thinking. We consider light to be a constant, but if we can slow light down does that mean it isn't anymore?

The speed of light in a vacuum is a constant and nobody has ever slowed that down.

In some media other than a vacuum, light appears to travel slower, when the speed is measured, however even then in reality it still travels at the speed of light, and the apparent slowdown is caused by an complex interaction with the material.

Do Photons Move Slower in a Solid Medium?

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations.....

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material.

Even when headlines proudly proclaim light slowed down, that's not exactly what happened. The details never make it to the headlines.

reply to post by wildespace

A lot of people think that, but see the correct explanation in my previous post.

reply to post by powerdrone


Unfortunately op, the universal speed limit( C ) has still to be proved wrong, even once, even by any fraction.

It is at present "absolute" in all situations at all times.

I hold out hope that we will jump this hurdle at some point, as throuout all of history every "law" of the universe gets broken at some point. However, this one is at the present, so solidly known to be accurate, that they don't even consider any arguement to the contrary.

reply to post by powerdrone


The natural speed of light is constant. Without impedance, it is what it is. I thought many things alter this. Density of the medium it travels, etc.

You can slow particles down? Can't you? Just a fancy trick I thought, why not.

The scale between "I can touch this" and "This I cannot touch!" is all I really know about light, however.

Thanks for the explanations and input! With that said however:

Is there a chance that light traveling through different matters would possibly appear slower and therefore take longer to reach us what with gravity wells and all and therefore mean something is further than what we think it is..?

Originally posted by powerdrone
Thanks for the explanations and input! With that said however:

Is there a chance that light traveling through different matters would possibly appear slower and therefore take longer to reach us what with gravity wells and all and therefore mean something is further than what we think it is..?

The actual speed that light travels is not what gives us the distance of an object. Parallax is how we do that:

Astronomers use the principle of parallax to measure distances to celestial objects including to the Moon, the Sun, and to stars beyond the Solar System. For example, the Hipparcos satellite took measurements for over 100,000 nearby stars.

Stars, Observational History

The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[23]

reply to post by eriktheawful


Interesting. Learn something new every day!

reply to post by powerdrone



just read this thread. it says that using certain loopholes, you can make a ship that appears to travel faster than light but actually bends the fabric of space and doesn't move at all.

i always thought that speed is relative to the observer, so if it looks like its going faster than the speed of light then it probably is.

Originally posted by powerdrone
Thanks for the explanations and input! With that said however:

Is there a chance that light traveling through different matters would possibly appear slower and therefore take longer to reach us what with gravity wells and all and therefore mean something is further than what we think it is..?

Gravity does indeed affect light. Since the speed of light is constant, it never affects the speed, but since gravity distorts space-time, light is blue-shifted as it approaches a gravitational field and is red-shifted as it leaves a gravitational field. Gravity also bends light so we see gravitational lensing. We don't see it this dramatically but this illustrates how light gets bent by a black hole:

en.wikipedia.org...


Here's a video I uploaded about this back when we could upload videos:
Alex Filippenko on gravitational bending of light

Originally posted by eriktheawful
The actual speed that light travels is not what gives us the distance of an object. Parallax is how we do that:

Parallax is one of the best methods to determine distances because it's reliable and accurate where we can use it, but unfortunately, for most of the universe, we can't use it. It only works on relatively close objects, cosmologically speaking.

The way we determine distances is described in this link and it really takes some study to learn and understand all these different methods, and as the link shows, parallax is one method but it only works on "close" objects:

Cosmic distance ladder

If you really study the uncertainties of these measurement methods which aren't as accurate as the parallax method, indeed some objects may be closer or further than the estimate, but we also try to estimate the measurement uncertainty.

Thanks so much for taking the time to explain all this!

reply to post by powerdrone

You're welcome. Constant light speed is counter-intuitive, so it's natural that many questions arise regarding this topic. The first puzzle I had trouble solving was how, if you stand on top of a train going 1000 kph, and shine a flashlight ahead of the train, can the light not be going the speed of light plus 1000 kph as measured by an outside observer? The light is leaving the flashlight at the speed of light, right?

When you can wrap your mind around the answer to this question, you will have made great progress, but for most people this is a hard thing to do. It took me a while to understand.

Originally posted by Arbitrageur
reply to post by powerdrone

 

You're welcome. Constant light speed is counter-intuitive, so it's natural that many questions arise regarding this topic. The first puzzle I had trouble solving was how, if you stand on top of a train going 1000 kph, and shine a flashlight ahead of the train, can the light not be going the speed of light plus 1000 kph as measured by an outside observer? The light is leaving the flashlight at the speed of light, right?

When you can wrap your mind around the answer to this question, you will have made great progress, but for most people this is a hard thing to do. It took me a while to understand.

whats the answer, whats the answer??

is it similar to driving in a car 100 mph and throwing a baseball forward out the window?

the light has the same starting momentum when it is first produced as it always does?,,, and is only traveling relative to/with the motion of the train,?

Originally posted by ImaFungi
whats the answer, whats the answer??

is it similar to driving in a car 100 mph and throwing a baseball forward out the window?

That's the analogy, and since light doesn't behave like the baseball, that's why it's so unintuitive for most people.

The answer is that speed is distance per unit time, and time isn't passing at the same rate on the train as it is on the ground, according to relativity.

Originally posted by Arbitrageur

Originally posted by ImaFungi
whats the answer, whats the answer??

is it similar to driving in a car 100 mph and throwing a baseball forward out the window?

That's the analogy, and since light doesn't behave like the baseball, that's why it's so unintuitive for most people.

The answer is that speed is distance per unit time, and time isn't passing at the same rate on the train as it is on the ground, according to relativity.

okok,..,., what is light itself? all light comes from a transfer of energy in an atomic interaction? everytime a transfer of energy takes place is light emitted, or only in certain conditions? everytime light is emitted is it the same "kind" of light,, is it composed of the same stuff ( i know there are different bands and wavelengths) and it travels at light speed? the light from a flashlight,, the light from a campfire?

Originally posted by ImaFungi
( i know there are different bands and wavelengths) and it travels at light speed? the light from a flashlight,, the light from a campfire?

Light falls within a narrow band of electromagnetic (EM) radiation. All EM radiation travels at the speed of light in a vacuum, not just the light frequencies.

Light speed is a contsant in our current physics.

I tend to think that light has an 'event horizon' as nothing in the universe could have never ending power supply. I think it is somewhere near the 13-14 billion light year mark. It would break the laws of physics if light could go on indefinitely, as entropy is another key principle...

I don't think you could 'hold' a beam or photon, I tend to think of it more as an elements half-life, and maybe there is a 6-7 billion year half life, making galaxies at this distance appear 'red shifted' and moving away from us. Rather than the universe expanding, maybe the light is 'fading', if you catch my drift...

Pred...