Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: mxplxxx on 23/05/2014 04:37:47
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I am scratching my head to think of a way that the frequency of a photon can be changed. For example how can I changed UV light into Infra Red light and vice versa. I am not talking about the situation where a photon is emitted from a particle, just a photon on its own.
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One option is to release your photon from the other side of the universe. Wait about 14 billion years, then observe the photon on Earth.
If you aren't quite that patient, you could release the photon from a super-fast rocket leaving Earth, but it would have to be really fast, faster than anything we've made so far to make a significant difference.
Much of what is discussed about the speed of light is in a vacuum. When it enters a medium, such as going through water, then:
Frequency remains constant
Velocity reduces
Wavelength decreases.
If the photon is absorbed by a particle at a "low" temperature, then a new photon is emitted, then it will likely be changed from whatever the incoming frequency was to a frequency based on the temperature of the object, often IR.
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I have found a very enlightening and thought provoking web conversation for you
"Changing the Photon Frequency", enjoy :
http://www.newton.dep.anl.gov/askasci/phy05/phy05154.htm
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I am guessing the answer is no, you can't change the frequency of a particular photon. A startling possibility! A photon absorbed and reemitted from a medium is not the same photon obviously. And a photon that has travelled for 14 billion years is in the same category. Basically, we can't detect or alter photons (and gluons etc. basically all bosons) so we can only theorise that they even exist. As for them travelling at the speed of light, ditto, we can only theorise this. Given that recent discoveries around quantum entanglement where the state of a particle influences the state of an entangled particle potentially light years away, instantly, it seems to put our model of bosons on very shaky ground.
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The light that we're seeing from distant galaxies is the same light that was released.
Astronomers look for known spectral lines, and find them shifted redder. If what we were seeing was light that was absorbed and re-emitted, then the spectral lines would be lost.
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The light that we're seeing from distant galaxies is the same light that was released.
Astronomers look for known spectral lines, and find them shifted redder. If what we were seeing was light that was absorbed and re-emitted, then the spectral lines would be lost.
Ok, thanks for that. Seems pretty weird that a photon can travel so far and never be absorbed. What are the odds!
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Red-shift and blue-shift due to the Doppler effect (http://en.wikipedia.org/wiki/Doppler_effect) at high velocity were mentioned above as ways to modify the frequency of a photon (while still keeping the same photon). This effect is used by police radar and laser traps to measure the speed of your car.
Another way is to fire it into (or out of) a gravitational well. The effect is small (but measurable) on Earth, but the effect would be stronger in a more intense gravitational field. This is sometimes called Einstein shift (http://en.wikipedia.org/wiki/Gravitational_redshift).
There are ways to double the frequency of a photon. The generated photon has twice the energy of the original photon, so it takes 2 photons to create one photon at twice the frequency. In a sense, this is the original photon, with some extra energy added.
It is also possible to add or subtract the frequency of photons, by using nonlinear optical materials (http://en.wikipedia.org/wiki/Nonlinear_optics).
Fluorescence (http://en.wikipedia.org/wiki/Fluorescence)takes the energy of a photon (eg a UV photon), and then releases it as a photon of lower frequency (eg a visible photon), after some delay. The emitted photon bears some traces of the original photon (eg polarisation of the emitted photon is related to the polarisation of the absorbed photon), and it uses the energy of the absorbed photon, but you would have to say that it is a different photon.
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The light that we're seeing from distant galaxies is the same light that was released.
Astronomers look for known spectral lines, and find them shifted redder. If what we were seeing was light that was absorbed and re-emitted, then the spectral lines would be lost.
Ok, thanks for that. Seems pretty weird that a photon can travel so far and never be absorbed. What are the odds!
It turns out space is really, really close to being empty! The light that does interact with matter along the way is scattered, and often doesn't make it to us. Most of what we see from space is directly from the source star.
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Yet 14 billion years ago when the universe was just beginning it was incredibly dense (or so I have been led to believe).
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Yes, there is a point before which we really can't see because it was too dense, but I think any time within the last 12 billion years is relatively sparse.
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My belief is that the universe is a type of finite state machine. Bosons (a photon is a type of boson) in this system appear to be state modifiers based on previous states (events if you like - a boson is a memory of a past event). Evolution would doubtless arrange for bosons to be extremely difficult to change (if this is indeed possible) to retain the integrity of the system. Lets face it, the only thing that can change a boson is another boson and given that bosons don't seem to be able to interact with one another (maybe at extremely high energies they can?) my original premise that a boson's frequency cannot be altered is likely to hold true.
Red shifts seem to occur when the photon is originally created, not to a previously created photon.
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Red shifts seem to occur when the photon is originally created, not to a previously created photon.
The Earth orbiting the Sun causes a cyclic red-shift in the light from stars, as the Earth moves towards or away from the star of interest.
The motion of the Earth can be measured by high-resolution spectrographs hunting for extrasolar planets orbiting other stars, and must be subtracted from the redshift of the distant star.
- Redshift caused by the gravity of the extrasolar planet certainly applies at the time the photon is generated.
- But the redshift caused by the Earth occurs at the time the photon is detected. It may be detected on an instrument that did not exist when the photon was generated.
Doppler shift is an effect of both source and detector.
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The redshift from stars occurs because the universe is expanding. The EM waves from stars are being lengthened because of this. i.e. their frequency is less than what they would be if no expansion was happening. Pleas explain how this effect happens at detection point.
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The redshift from stars occurs because the universe is expanding. The EM waves from stars are being lengthened because of this. i.e. their frequency is less than what they would be if no expansion was happening. Please explain how this effect happens at detection point.
You can observe doppler shift of electromagnetic radiation on a very small scale,. The Mossbauer effect is measurable at relative speeds of a few cm per second in a laboratory, and doppler laser and radar systems are common techniques for measuring mesoscopic speeds (cars, aircraft...). Not the same as gravitational red shift, but a change in the received frequency of a photon nonetheless.
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The redshift from stars occurs because the universe is expanding.
It is true that Cosmological Red Shift (http://en.wikipedia.org/wiki/Cosmological_redshift) (as identified by Edwin Hubble) is due to the general expansion of the universe. It only really applies between clusters of galaxies, not between stars in our own galaxy.
Galaxies within our local cluster are bound together by gravitation, and are in a complicated orbit around each other. In fact, the Andromeda galaxy (http://en.wikipedia.org/wiki/Andromeda_Galaxy#Future_collision_with_the_Milky_Way) is moving towards our galaxy at about 110km/sec, and this shows as a blue shift.
Stars within our galaxy are bound together by gravitation, and are in various orbits around the center of the galaxy (the Sun moves at about 220km/sec). These stars can show a red shift or a blue shift.
Very high-resolution spectrographs were developed for hunting for extrasolar planets, and they are able to easily measure the orbit of the Earth around the Sun, which moves at about 30km/second. They are even able to measure the movement of a star due to the tug of an orbiting planet, which amounts to a movement of just 4km/h (http://en.wikipedia.org/wiki/High_Accuracy_Radial_Velocity_Planet_Searcher#Characteristics) (a leisurely walking pace).
The Doppler effect occurs within our Solar System - in fact data from a space probe was nearly lost during a critical planetary flyby, because Doppler Shift caused the frequency of the radio signal to move outside the acceptable range. Fortunately, mission control discovered this before the event, and changed the trajectory of the spacecraft to compensate.
The Doppler Effect (http://en.wikipedia.org/wiki/Doppler_effect) also occurs on the street - every time you hear a police siren pass by, and the pitch drops as it moves away from you ("redshift" for sound). Police radar also uses the Doppler Effect (redshift for cars moving away, blueshift for approaching cars).
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Can a photon bounce off a particle without being absorbed and lose power (frequency) in the process?
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Gamma Ray photons can bounce off electrons in air molecules, which reduces the gamma ray energy and decreases the gamma ray frequency.
See the Compton effect (http://en.wikipedia.org/wiki/Compton_scattering).
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Gamma Ray photons can bounce off electrons in air molecules, which reduces the gamma ray energy and decreases the gamma ray frequency.
See the Compton effect (http://en.wikipedia.org/wiki/Compton_scattering).
Thx evan_au