# The Naked Scientists Forum

#### thebrain13

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« on: 07/07/2006 22:49:30 »
I was thinking about how relative redshifts and blue shifts occur and it seems im missing something. I get the feeling that when somebody answers this Im going to smack myself in the head. But anyways the way I understand it is that, a redshift is different from the dopplar effect using sound, because the speed of light is constant, where the speed of sound is not. So you cant explain redshift the same way as the dopplar effect, which is completely caused by the different speed of sound relative to the observer.

So redshift, as I understand is caused by time dilation, or the emmitter of light is slown due to your relative velocity from it. So, you still view a redshift in spite of the constancy of the speed of light. No problems there, but using this same logic how do you explain a blueshift? Because your relative velocity to the emitter of light causes the emitter to emit light slower. And since the speed of light remains constant, the light can not bunch up in front of you, to raise the frequency like sound does, so why do you view a blueshift instead of a redshift when your moving towards an object?

#### daveshorts

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« Reply #1 on: 08/07/2006 00:19:46 »
There relativistic doppler effect is just the normal doppler effect with a relativistic correction. The reason the pitch is higher as the police car comes towards you is that, it is closer to you each cycle, the same is true of light, just everything has a relativistic conversion factor.

#### thebrain13

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« Reply #2 on: 09/07/2006 01:23:31 »
The thing is though, light is quantized. Red light is going to be red light, regardless of if each individual photon is arriving slower or faster pace. If you slow down my speech, its pitch is lowered.

#### ukmicky

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« Reply #3 on: 09/07/2006 03:33:21 »
As your traveling towards something at relativistic speeds the space in front of you is length contracted  (compressed)  and therefore the wavelengh of the photon travelling through it is also compressed  which shortens its wavelength blueshifting it.

Michael
« Last Edit: 09/07/2006 03:36:15 by ukmicky »

#### ukmicky

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« Reply #4 on: 09/07/2006 03:38:05 »

Michael

#### thebrain13

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« Reply #5 on: 09/07/2006 16:34:20 »
Ive seen this drawing before as an explanation for redshift. However I dont see how this makes sense. A light wave is not a continuous wave. Light is quantized, wether you look at it from a particle, or wave point of view. In other words, each photon has an individual wave of its own.

Consider this, a machine launches photons individualy at a screen. lets say I set the frequency so it launches 1000 photons per second. When you view the screen at this frequency, you see green. If you double the frequency and then look at the screen, you again see green. Cut it down to 1 photon per second you still see green. Regardless of how many photons per second you see, they will always remain the same color.

If I cut down the frequency of sound waves, yes pitch will be affected. But light is not sound, and the above illustration, falsely assumes that simply by slowing down, or speeding up the amount of photons you view, in a period of time, is going to affect the color of the photons. And I know that wont happen.

#### another_someone

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« Reply #6 on: 09/07/2006 18:33:40 »
quote:
Originally posted by ukmicky
As your traveling towards something at relativistic speeds the space in front of you is length contracted  (compressed)  and therefore the wavelengh of the photon travelling through it is also compressed  which shortens its wavelength blueshifting it.

I think the problem is a little more complicated than that, and I suspect that is the issue that thebrain13 was referring to.

It may be simpler with blue shifting than with red shifting; but with red shifting, the space (and hence the wavelength) is stretched, but then time is also dilated, and the thing is to show how the interrelation between time dilation and space expansion interact, and that they will not simply form a compensatory system (i.e. you receive fewer waves if time remained constant, but since time has been expanded out, in your perception of time you will actually have a longer slot in which to receive those waves).

I am sure the numbers will show that they do not cancel each other out completely, but I have not yet done the maths to show it.  Particularly, if the source and receiver are moving at very slight speed relative to each other, the amount of time dilation would be negligible.  The effect would be far more noticeable as the receiver and transmitter are moving relative to each other at speeds that are an appreciable percentage of the speed of light.

George
« Last Edit: 09/07/2006 18:39:26 by another_someone »

#### another_someone

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« Reply #7 on: 09/07/2006 19:07:11 »
quote:
Originally posted by thebrain13
The thing is though, light is quantized. Red light is going to be red light, regardless of if each individual photon is arriving slower or faster pace. If you slow down my speech, its pitch is lowered.

Not quite.

Light is composed of photons just as sound, when travelling through air, is composed of the molecules of air.  A single photon carries no colour information (although it does carry energy, which can be equated to colour).  When we talk about the wavelength of light, we are talking about the wavelength of the probability wave of our finding a photon at a particular point in space, not the the physical attributes of a single photon.

There are all sorts of paradoxes with the notion of wave particle duality that I do not claim to fully understand myself; but if the wavelength of light was a property of a single photon, then you would have difficulty explaining how light diffracts, and how those diffractions vary with the wavelength of the incoming light.

George

#### thebrain13

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« Reply #8 on: 09/07/2006 22:28:26 »
An individual photon still acts as a wave. If you fire one photon at a time, interference patterns are still seen.

#### ukmicky

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« Reply #9 on: 10/07/2006 00:19:47 »
yes ,  The energy of each photon is dependent on its wavelength or frequency. but what are you trying to say.

GOT TO BE CAREFUL WITH YOU :)

Michael

#### another_someone

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« Reply #10 on: 10/07/2006 00:50:41 »
quote:
Originally posted by thebrain13
An individual photon still acts as a wave. If you fire one photon at a time, interference patterns are still seen.

http://en.wikipedia.org/wiki/Wave-particle_duality
quote:

The photon was the first entity that was seen to exhibit these dualistic properties. And so wave-particle duality is often stated like this: "A photon sometimes acts like a wave, and sometimes acts like a particle, but not at the same time." However, this is slightly misleading, because a photon always acts like both to varying degrees. For example, when shooting single photons through a slit, a detector can detect each photon when it hits a photosensitive screen (its position is recorded) - but over time, the detector will detect the same diffraction pattern as it would if the photons were given off all in one burst. This is because any given trajectory the photon could take has a certain probability that is dictated by the properties of an electromagnetic wave.

In other words, one photon does not a wave make.

George

#### thebrain13

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« Reply #11 on: 10/07/2006 01:11:36 »
I've been trying to say, each individual photon follows a wavelength and frequency all its own, the color of light is not a manifestation of many photons, but a manifestation of one. so merely by speeding up how many waves and crests one sees(using many photons, if you could alter frequency of each individual photon it would change the color) in a period of time does not alter the color, energy level, or frequency-wavelength of photons. Light is quantized, a fact proven by einstein. So by a photon being closer to you when it is emmitted, as compared to the last one, is not an acceptable answer.

#### ukmicky

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« Reply #12 on: 10/07/2006 01:34:56 »
i thought one photon does make a wave it just that you cant observe it unless many photons are fired.

Michael
« Last Edit: 10/07/2006 01:41:14 by ukmicky »

#### Solvay_1927

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« Reply #13 on: 10/07/2006 02:04:13 »
Brains,

I thought that the energy of each photon DOES vary with the speed of the emitting source.

Suppose a person moving with the light source measures the energy of each photon emitted as having the value E=1. Then you will measure the energy of each photon received as having E>1 if the source is moving towards you, and as E<1 if the source is moving away from you.

And Planck’s formula says E=hf  (f=frequency – I don’t know how to get the greek letter ‘nu’ on this forum)

So if the energy of the photon increases because the source is moving towards you, so does the frequency increase.  Hence blueshifting.  (Higher frequency = shorter wavelength.)

And when moving away from you, the energy – and hence frequency – associated with the individual photon is lower, hence redshifting (longer wavelength).

Or that’s my understanding, anyway.

#### thebrain13

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« Reply #14 on: 10/07/2006 02:09:28 »
I again wasn't replying to you anothersomeone, your post just entered first. Interesting that we were talking about the same thing though.

#### another_someone

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« Reply #15 on: 10/07/2006 02:21:51 »
My understanding, and I don't claim to be authoritative on the matter, is as follows.

The colour of the light is denoted (as Paul has said) by its energy.  On the other hand, its frequency, although a manifestation of its energy, cannot be determined by a single photon, because a single photon does not make a complete wave.  Thus the problem arises in the association between colour and wavelength, not between a single photon and its colour.  I know that this actually contradicts what I have said earlier, but it is the picture that I am now understanding of what is going on.

But, as Paul has said, if an object that receives light from a transmitter is approaching the transmitter, then the light he is receiving will have its energy raised (and, if the receiver received many photons of light, then the consequence of this is that the frequency of the waves that describe the probability of a photon at a given location then increase due to the higher energies of these individual photons).

As I said, I am more than willing to be corrected, but it is the best that I could piece together from the various bits that I could look up.

George
« Last Edit: 10/07/2006 02:22:20 by another_someone »

#### thebrain13

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« Reply #16 on: 10/07/2006 03:16:50 »
solvay, I think thats right on the button. I knew my question, had a simple answer afterall. Basically when an object is moving towards you the photon has more energy, hence its frequency is higher. The frequency isnt getting higher because the light emmitting object is getting closer to you. Which was implied by the illustation above. And I figured that could not be the case.

Anyways, to anothersomeone, I interpretted that wikipedia article the opposite way. I believe it was saying that regardless of if you fire photons in bunches or one at a time, they still possess wave qualities.

#### thebrain13

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« Reply #17 on: 10/07/2006 15:12:51 »
I guess though Im still wondering why light loses energy when your moving away from it, and given the time dilation factor, are blueshifts more subtle than redshifts?

#### ukmicky

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« Reply #18 on: 10/07/2006 20:13:51 »
« Last Edit: 10/07/2006 20:21:00 by ukmicky »

#### another_someone

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« Reply #19 on: 11/07/2006 01:59:27 »
quote:
Originally posted by thebrain13

I guess though Im still wondering why light loses energy when your moving away from it, and given the time dilation factor, are blueshifts more subtle than redshifts?

Time dilation factors will only be applicable when the transmitter and receiver are themselves moving at speeds that are an appreciable fraction of the speed of light relative to each other.

The Doppler effect can be appreciable even at very low speeds (otherwise police radar traps would never work, since most of them depend upon the Doppler effect upon a microwave signal of a car moving at a mere 70mph).

George

#### thebrain13

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« Reply #20 on: 11/07/2006 20:19:08 »
They can still work, they just have to be sensitive.