How long does it take for a light to traverse distance of one light year?

[Born2bwire]

Originally posted by: Biftheunderstudy
There are many cases where "stuff" can travel faster than light, take for instance evanescent waves, the surface wave can travel faster than the speed of light.

In this case the phase front can move faster than light, but the actual information or energy of the wave will always move at or slower than light. We can define properties of a wave as moving faster than light, but these properties can never reveal any kind of information or carry energy as this would then violate relativity.

 

[thomastaranowski]

From what I understand, the speed of light is constant, period. What changes is the distance it travels. So, when they measured the time it took for light to travel a specified difference from various points in the sky, what they were actually measuring is the distance the light travelled. It seems as though it's a measurement of the 'straightness' of space at different points in the sky.

In the bigger picture, something I never understood is why is light different than everything else? It's a strange special case, and in my normal everyday existance, special cases are inelegant, and usually arise because a general solution has not been discovered, or something is not entirely understood. Are there other such peculiarities that I don't know about? Maybe other features floating around that disobey the rules that everyone else obeys.

 

[Born2bwire]

Originally posted by: thomastaranowski
From what I understand, the speed of light is constant, period. What changes is the distance it travels. So, when they measured the time it took for light to travel a specified difference from various points in the sky, what they were actually measuring is the distance the light travelled. It seems as though it's a measurement of the 'straightness' of space at different points in the sky.

In the bigger picture, something I never understood is why is light different than everything else? It's a strange special case, and in my normal everyday existance, special cases are inelegant, and usually arise because a general solution has not been discovered, or something is not entirely understood. Are there other such peculiarities that I don't know about? Maybe other features floating around that disobey the rules that everyone else obeys.

The speed of light is not constant, the speed of light in a vacuum is constant. The speed of light is dependent upon the medium that it is passing through. In terms of classical EM, which takes into account relativity and I would say is accurate below Terahertz, the speed of light is dependent upon the permeability and permittivity of the medium. When you get into the frequencies starting near the visible region and up, it is generally more accurate to use quantum mechanics to describe the mechanism of light. And so you can think of that the speed of light can be regulated by the medium because the photons are absorbed and emitted by the atoms in the medium.

I do not know if one considers light as a special case. By taking into account relativity, light becomes the limiting case. Light follows the same rules as everything else, it's just that it is the limit that everything approaches but cannot obtain. Light travels via particles called photons that have no rest mass and behave in a wavelike nature. Our understanding of light and relativity is at a point where we can very accurately predict systems. For example, someone in the GPS thread noted that the frequency of the satelite signal has to be slightly off to account for relativity. We can measure and predict the time difference from a cesium clock that spends time flying at high speeds for days in a jet. We can accurately predict the perturbation in the position of stars that occurs when their light passes by massive objects like the Sun (we did this in 1919). The original indications of relativity were discovered by Maxwell in the 1870's.

 

[SuperFungus]

This may be really stupid, if so i'm sorry, but how can you quantify the "speed of light" absolutly? don't your measurements need to be relative to something? If they are relative to earth then won't the earth's orbit have a greater affect on the precieved "speed of light" than any insignificant time dialation or space-time bending from gravity? If two bodies of matter travel away from each other at .5c each are they each traveling at the "speed of light". Also it seems difficult to ask for the level of accuracy you want between this galaxy and earth when the distance between the two is shurely changing in the five years it takes the light to travel. Is the galaxy 5 light years away when you emit the light or when you recieve the light? Anyways you guys seem to know a lot more about this than i do, maybe you've heard of some theoretical point from which everything else is relative (the center of the universe perhaps?), Anyways thats a question i've always had and i'd really appreciate any explanations you may have or may have come across.

 

[Born2bwire]

Originally posted by: SuperFungus
This may be really stupid, if so i'm sorry, but how can you quantify the "speed of light" absolutly? don't your measurements need to be relative to something? If they are relative to earth then won't the earth's orbit have a greater affect on the precieved "speed of light" than any insignificant time dialation or space-time bending from gravity? If two bodies of matter travel away from each other at .5c each are they each traveling at the "speed of light". Also it seems difficult to ask for the level of accuracy you want between this galaxy and earth when the distance between the two is shurely changing in the five years it takes the light to travel. Is the galaxy 5 light years away when you emit the light or when you recieve the light? Anyways you guys seem to know a lot more about this than i do, maybe you've heard of some theoretical point from which everything else is relative (the center of the universe perhaps?), Anyways thats a question i've always had and i'd really appreciate any explanations you may have or may have come across.

The speed of light is the constant between any reference frame. So it does not matter if I am moving toward a light source at .9c, the light I measure coming toward me is going to have a speed of c. The fact that light moves at the same velocity relative to any reference frame was the groundbreaking result of the Michelson-Morley experiment. As to quantifying the speed of light, the speed of light is the constant, it is the definition. Our concept of distance is now defined by the distance that light travels in a certain amount of time. The actual standard distances are defined by light-years (well, more like very small fractions of a light-second). So when we are saying that the object is 1 m long, we are really saying that the object is about 1/300,000 light-seconds long.

As for the fact that a star emitting light is moving away from us at a significant speed can be accounted for as well. One of the consequences of light being constant between any reference frame is that if the observer is moving toward or away from the light source, it causes a shift in the wavelengths of the light in the form of the Doppler effect. If a galaxy is moving toward us, the wavelengths become compressed and the light undergoes what is known as a blue shift. If we are moving apart, the light under goes a red shift. We can detect these shifts in frequencies because light emitted from a source is not a continuous spectrum. There are absorption lines, generally specific to the elements of the materials emitting the light. By looking at these absorption lines, we can generally tell what elements are being consumed in a star, or you can do the same with the light from a flourescent bulb and see what gas is inside the bulb. Since we know what these absorption lines should be, we can then compare the baseline to what is detected as being emitted by the source and measure the frequency/wavelength shift and have an idea of the movement of the source relative to ourselves.

EDIT: A more everyday example of the Doppler effect on light are radar guns. Radar guns emit waves of a known wavelength and then detect the shift in the wavelength of the reflected waves. This very tiny shift tells the gun how fast the target is moving.

 

[SuperFungus]

Thanks alot for the explanation Born2bwire, incredible stuff. I have a couple of other questions, related but maybe a little off topic. If two bodies where either moving together or apart very rapidly while emmitting light at each other would the emmited light move accordingly to other parts of the electromagnetic spectrum (i would assume so)? Are there theoretical caps to the frequency of gamma? It seems like at some certain, prodigous frequency the wave could be essesially the same thing as a wave of a certain diminuitive frequency. Would they both be approaching some constant pulse and behave similarly? I apologize if maybe this is too off topic but this is a too good an oportunity to get some questions answered for me to pass up.

 

[Born2bwire]

Originally posted by: SuperFungus
Thanks alot for the explanation Born2bwire, incredible stuff. I have a couple of other questions, related but maybe a little off topic. If two bodies where either moving together or apart very rapidly while emmitting light at each other would the emmited light move accordingly to other parts of the electromagnetic spectrum (i would assume so)? Are there theoretical caps to the frequency of gamma? It seems like at some certain, prodigous frequency the wave could be essesially the same thing as a wave of a certain diminuitive frequency. Would they both be approaching some constant pulse and behave similarly? I apologize if maybe this is too off topic but this is a too good an oportunity to get some questions answered for me to pass up.

The light would move accordingly with respect to how you observe it in relation to the source. Half of the physics with relativity is correctly defining your frames of reference. Since the planets are moving away from eachother, an observer on either planet will see a red shift in the light being emitted by the other. And yes, if the shift is significant enough, you could see a monochromatic blue lift shift towards the red spectrum, to say yellow. Generally though, these shifts are observed by looking at where light is not being emitted than where it is. Take for example a flourescent bulb. The bulbs are permeated with a vapor, generally it is mercury but you can also have sodium. Both of these vapors emit light over the visible spectrum, but not continuously. Mercury tends to have gaps in the reds while sodium is generally yellow in appearance. The gaps in the light being emitted by the vapor is a specific signature of that element. So most scientists have a very clear idea of what kind of light absorption spectrum that we should see from celestial objects.

I do not know if there is a theoretical limit to frequency. My area of research is classical electromagnetics, which generally stops in the Terahertz region. This is not a big problem because a huge amount of products and research is still limited to the microwave spectrum. My understanding has been that when you approach the visible light spectrum and above, classical EM is not sufficient to accurately describe all of the mechanisms and one needs to go up to the quantum EM level. If I recall correctly, generally the frequency of light being emitted is indicative of the amount of energy that the photon carries or the amount of energy emitted. So I would expect that there is probably a realistic limit to the amount of energy that we can use up to excite a single photon and thus have somewhat of a frequency limit to real world situations.

 

[Geniere]

Originally posted by: Born2bwire

I do not know if there is a theoretical limit to frequency. My area of research is classical electromagnetics, which generally stops in the Terahertz region. This is not a big problem because a huge amount of products and research is still limited to the microwave spectrum. My understanding has been that when you approach the visible light spectrum and above, classical EM is not sufficient to accurately describe all of the mechanisms and one needs to go up to the quantum EM level. If I recall correctly, generally the frequency of light being emitted is indicative of the amount of energy that the photon carries or the amount of energy emitted. So I would expect that there is probably a realistic limit to the amount of energy that we can use up to excite a single photon and thus have somewhat of a frequency limit to real world situations.

Just to add to this a little. The quantum particle associated with EM radiation is the photon. A red photon carries more energy than a photon emitted by a radio stations antenna. A red photon has less energy than a blue photon that has less energy than the ultra-violet photon; all have less energy than x-rays. X rays have less energy than gamma rays. The greater the photons energy, the shorter is its EM wavelength.

The shortest possible wavelength would be the Planck length, while the longest wavelength would be? Even during the first fraction of a microsecond after the big bang, I doubt there ever existed a sufficiently energetic reaction to produce a Planck wavelength photon. Since the universe was opaque to radiation for the first 750,000 years, we can never have direct evidence of the original photons energy as they were immediately absorbed after being emitted.

As the early universe expanded, its density decreased so there was less energy in a given volume of space, in other words the expansion cooled the universe. As the temperature fell to about 3000K (5000F), photons were no longer immediately absorbed and suddenly the universe became transparent. These first ?free? photons are now visible to us as the Cosmic Background Radiation. Because of the expansion, the CBR photons have had their wavelengths stretched and now have the equivalent temperature of about 3K (minus 455F).

The longest possible wavelength is hard to determine or even think about. To create EM radiation (emit a photon), it is only necessary to accelerate a charge. If I hold a length of copper wire (copper has free electrons) and wiggle it, it will emit low frequency photons. How slow can you wiggle a wire?

Lastly re: speed of light. Due to quantum effects a photon may spontaneously decay into an electron and an anti-electron (positron). These particles than collide and annihilate each other emitting a photon identical to the original. Electrons and positrons have rest mass and cannot travel at the speed of light. The photons velocity seems to decrease due to the lost time while in the electron-positron state.

 

[Eeezee]

Originally posted by: scott
As for the speed of light, isn't that now thought to be variable, depending on local magnetic conditions where the light is passing through? I vaguely remember news a few years ago about some lab slowing light in a magnetic bottle.

Edit: Add this link.

If a particle of matter can't attain light speed then
the dual theory (light is both a wave and a photon particle) has an obvious paradox. A photon can't travel at the spped at which it travels.

Isn't gravity thought to be faster than light?

And doesn't that theory about entanglement (I don't know much about it) say "communication" occurs faster than light between two particles that were first closely situated, then moved a wide distance apart?

1) It's a particle with mass that can't attain light speed. Therefore there is no paradox, because light is massless.
2) Gravity is thought to travel at the speed of light, although experimentally this is almost impossible to prove. We haven't even detected a particle responsible for gravity yet.
3) There are some weird quantum mechanical / optical tricks you can do that will seem to break the speed of light boundary, yes. However, massive particles do not themselves move faster than the speed of light. This has not been disproven.

 

[Eeezee]

Originally posted by: thomastaranowski
From what I understand, the speed of light is constant, period. What changes is the distance it travels. So, when they measured the time it took for light to travel a specified difference from various points in the sky, what they were actually measuring is the distance the light travelled. It seems as though it's a measurement of the 'straightness' of space at different points in the sky.

In the bigger picture, something I never understood is why is light different than everything else? It's a strange special case, and in my normal everyday existance, special cases are inelegant, and usually arise because a general solution has not been discovered, or something is not entirely understood. Are there other such peculiarities that I don't know about? Maybe other features floating around that disobey the rules that everyone else obeys.

Light isn't different from everything else. In fact, there are several particles that are massless and travel at the speed of light.

For example, water is a special case of the solid form taking up more volume than the liquid. It doesn't mean that it's not well-understood or that it's inelegant, it's just a special case.

 

[WildHorse]

Quoted from Eeezee:
1) It's a particle with mass that can't attain light speed. Therefore there is no paradox, because light is massless.

Interesting nuance. I've never considred, nor even heard that before.

So a photon qualifies as a "particle" but has no mass, which I thought was part of the definition of a particle. A "massless" particle! ???
If ever there was a paradox, that sound like one to me!

Maybe the concept is beyond capturing in words. Something funny's going on here with that word, "particle" that I don't understand.

Thank you for your explanation, which I appreciate very much.

 

[Biftheunderstudy]

"The shortest possible wavelength would be the Planck length, while the longest wavelength would be? Even during the first fraction of a microsecond after the big bang, I doubt there ever existed a sufficiently energetic reaction to produce a Planck wavelength photon. Since the universe was opaque to radiation for the first 750,000 years, we can never have direct evidence of the original photons energy as they were immediately absorbed after being emitted."

Actually the whole theory of inflation for the early universe happens at a time that is the planck time (10E-43 seconds) and when the light produced was at the planck limit.
Also, light doesn't spontaneously transform into particle/anti-particle pairs, it needs to have a high enough energy to do this, 'exactly' twice the rest energy of the particly to be created. When the particles annihilate, 2 photons are produce each with the rest energy of the particle. If you supply extra energy to the original photon, the resulting pair will have some kinetic enery and will start with some angle other than 180 degrees.
There are times when energy for this process can be borrowed from nowhere to make particle pairs, however usually they are instantly annihilated so energy is conserved overall, this process is the basis for Hawking Radiation.

Particle physics is really freakin strange sometimes, apparently there is a discrepancy between the mass of a particle and its anti-particle, when they annihilate there is a 'residue', this residue is the matter in the known universe left over from the big bang->we're the residue!

Photons are indeed massless, there are other particles thought to be massless as well, some we're not sure about, there is a certain type of neutrino that is thought to be massless as well as being able to travel backwards through time

 

[Born2bwire]

Originally posted by: scott

Quoted from Eeezee:
1) It's a particle with mass that can't attain light speed. Therefore there is no paradox, because light is massless.

Interesting nuance. I've never considred, nor even heard that before.

So a photon qualifies as a "particle" but has no mass, which I thought was part of the definition of a particle. A "massless" particle! ???
If ever there was a paradox, that sound like one to me!

Maybe the concept is beyond capturing in words. Something funny's going on here with that word, "particle" that I don't understand.

Thank you for your explanation, which I appreciate very much.

One of the results of relativity is the idea of equivalence between mass and energy. People often refer to the photon as having no "rest" mass. Still, despite having no rest/intrinsic mass, a photon still has momentum. The photon's momentum is purely a result of it's kinetic energy.

E != mc^2, this only occurs when the particle is at rest. The relationship is actually E^2 = c^2p^2+m^2c^4 where p is momentum. So despite having no intrinsic mass, there is no discrepency with the photon. We can still derive how it is able to transfer energy and carry a momentum using special theory of relativity. The fact that we can make the equivalence between mass and energy means that there is no paradox here.

If you have time, read Einstein's Special Theory of Relativity. The thesis is within the grasp of anyone with basic understanding of physics and high school math due to his wonderful use of Gedankenexperiments, or thought experiments (the actual German term is Gedankenversuch). Relativity provides for the existence of the massless photon and the rationality for all of which we have been discussing, like the consequences of time dilation and length contraction via the Lorentz contraction, the behavior of momentum at relativistic speeds, and the equivalence of mass and energy.

This is the copy that I own and find it to be more than satisfactory.

 

[Eeezee]

Originally posted by: scott

Quoted from Eeezee:
1) It's a particle with mass that can't attain light speed. Therefore there is no paradox, because light is massless.

Interesting nuance. I've never considred, nor even heard that before.

So a photon qualifies as a "particle" but has no mass, which I thought was part of the definition of a particle. A "massless" particle! ???
If ever there was a paradox, that sound like one to me!

Maybe the concept is beyond capturing in words. Something funny's going on here with that word, "particle" that I don't understand.

Thank you for your explanation, which I appreciate very much.

You hadn't heard that a photon is massless? A particle is defined as pretty much anything that is smaller than an atom. In fact, there are many massless particles. I'm not sure where you see a paradox in this. No one ever said that a particle, by definition, must have mass. In fact, many theorists are quite certain that there is a particle responsible for what we call mass (the Higgs boson).

Think of the photon as the carrier of the electromagnetic force. It is a massless entity that carries out electromagnetism. Similar force-carrying particles are the gluon, the W and Z bosons, and the (theoretical) graviton. However, W and Z bosons are not massless, which is part of what makes them so interesting.

There's also the problem with massive particles radiating energy. An electron accelerated to the speed of light will quickly lose most of its kinetic energy through radiation effects. Photons, being massless, do not experience this difficulty. That's a simple explanation, anyway.