The Naked Scientists

The Naked Scientists Forum

Author Topic: How old are photons?  (Read 8437 times)

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« on: 26/08/2009 21:25:52 »
If I have this right (which I probably don't!) photons travel at the speed of light in a vacuum (well, I suppose they'd have to, seeing as they are light!)

Now, I think that means that, from the photon's perspective (not that it really has a perspective - maybe it does?) it arrives as soon as it leaves. It experiences no time between it's departure and arrival, so, from its perspective it is the same "age" when it arrived as the age it was when it left, in other words, it did not age. - Phew!

However, when light travels through anything other than a vacuum, it slows down (I think). If that is true, then the photon does age when it is not travelling in a vacuum.

Does this ageing effect the photon in any way, or does it have no effect at all?

« Last Edit: 26/08/2009 22:03:49 by Geezer »


 

Offline PhysBang

  • Hero Member
  • *****
  • Posts: 588
  • Thanked: 14 times
    • View Profile
How old are photons?
« Reply #1 on: 26/08/2009 22:15:17 »
Good question!

I think the overall answer is no, because there is no opportunity for transition at light speed. This is why we currently believe that neutrinos have some mass; we know that some neutrinos change aspect between here and the sun, which means that they are travelling at less than light speed, which means that they have mass.

In the case of the different media, I have to think on that myself. When a photon enters a medium, there is a definite event, so that makes a difference. It could be that the medium includes the absorption and re-emission of light as well, events that can change the nature of what we identify as the same light.
 

lyner

  • Guest
How old are photons?
« Reply #2 on: 27/08/2009 18:52:29 »
I think it is safer to say that time has no 'meaning' for a photon. But, as it has no consciousness, then the word 'meaning' has no meaning in that context.

Whilst the sort of analogies used to illustrate some aspects of Physics can be useful, it is important to go back to the original theory, rather than to try to extrapolate using an over simplified idea before jumping to conclusions.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
How old are photons?
« Reply #3 on: 27/08/2009 19:11:55 »
If I have this right (which I probably don't!) photons travel at the speed of light in a vacuum (well, I suppose they'd have to, seeing as they are light!)

Now, I think that means that, from the photon's perspective (not that it really has a perspective - maybe it does?) it arrives as soon as it leaves. It experiences no time between it's departure and arrival, so, from its perspective it is the same "age" when it arrived as the age it was when it left, in other words, it did not age. - Phew!
We already had a thread on this subject some months ago. What you say is wrong, if you don't specify with respect to what "it arrives as soon as it leaves". For example, if we are talking about galaxies, that is to say, very distant objects *with respect to our terrestrial frame of reference* then yes, moving at almost c with a spaceship would mean to arrive there in a few seconds or fractions of second. But this doesn't mean that we wouldn't experience time inside the spaceship, our clock would run at our biological processes' speed and so we could travel for many years and become old accordingly. Only that, if we came back to Earth, we would find it extremely older (or non existing at all anylonger).

Quote
However, when light travels through anything other than a vacuum, it slows down (I think). If that is true, then the photon does age when it is not travelling in a vacuum.
Does this ageing effect the photon in any way, or does it have no effect at all?
It's a completely different situation: the void does not represent an absolute frame of reference, while a material medium does, so you cannot say that relativity equations change accordingly to the new light' speeed.
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
How old are photons?
« Reply #4 on: 28/08/2009 00:26:13 »
I think it's unwise to assume that the rules that apply to objects with non-zero rest-mass also apply identically to objects with zero rest-mass.

Objects with non-zero rest-mass cannot achieve 'c' but time-dilation is only observed in objects that have non-zero rest-mass and although the rate of time approaches zero for a non-zero rest-mass object as its speed approaches 'c', we also know that it can never actually achieve 'c'.  Thus, the assumption that no time passes for a zero rest-mass object traveling at 'c' is flawed, imo - it is based upon the characteristics and rules of something totally dissimilar.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #5 on: 28/08/2009 01:12:38 »
LeeE,

Point taken. Then, if electrons had some decay characteristic (and, before everyone jumps down my throat, I'm not suggesting they actually do  :)), if I understand you correctly, the amount of decay would in some way represent the "age" (for want of a better word) of a photon?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #6 on: 28/08/2009 01:40:27 »
I think it is safer to say that time has no 'meaning' for a photon. But, as it has no consciousness, then the word 'meaning' has no meaning in that context.

Whilst the sort of analogies used to illustrate some aspects of Physics can be useful, it is important to go back to the original theory, rather than to try to extrapolate using an over simplified idea before jumping to conclusions.

Sophiecentaur,
I think it would be enormously helpful if you would kindly refrain from including pejorative statements like "over simplified" and "jumping to conclusions" in you postings.
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
How old are photons?
« Reply #7 on: 28/08/2009 02:28:16 »
LeeE,

Point taken. Then, if electrons had some decay characteristic (and, before everyone jumps down my throat, I'm not suggesting they actually do  :)), if I understand you correctly, the amount of decay would in some way represent the "age" (for want of a better word) of a photon?

Well, if you think in terms of space-time, it's movement could represent its age.  I'm not saying that it does, just that it's possible in some theoretical models.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #8 on: 28/08/2009 04:42:02 »

Well, if you think in terms of space-time, it's movement........

If I was able to think in terms of space-time, I probably wouldn't need to ask any questions! ;D
Thanks for the help.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #9 on: 28/08/2009 09:16:29 »
....which reminds me of a very old joke, perhaps even older than JimBob.

A gent goes into a department store. He is approached by a floor walker who asks the gent what he is looking for, to which the gent replies, "Talcum powder".

The floor walker says, "Walk this way sir", to which the gent replies;

"If I could walk that way, I wouldn't need the bleedin' talcum powder!"
 

lyner

  • Guest
How old are photons?
« Reply #10 on: 28/08/2009 09:29:39 »
Geezer
Please don't take offense. I was just making the point that extrapolating using an incomplete model can lead to wrong conclusions. When you're dealing with very sophisticated and unfamiliar ideas then one uses analogies. Those analogies, although useful, cannot be relied on to remain faithful parallels to 'reality'.
Caution is no a bad thing; and,, honestly, do you claim that the ideas are 'complete'?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #11 on: 28/08/2009 18:09:48 »
Sophiecentaur,

If you recall, I took a considerable amount of time recently to answer a couple of your questions on a subject where you lacked some knowledge.

Did I insult you by suggesting that you were "jumping to conclusions" and using "a simplistic model"? No, I took the time to provide, what I considered to be, a helpful response, and I hope it was.

If you continually act like an adjudicator, you are going to continually offend people. I post on this forum to try to gain some knowledge and share some knowledge in a light hearted manner so that, hopefully, it's mildly entertaining. I don't post on this forum to have people tell me I'm a twit (unless it's oviously a joke). My wife is quite capable of taking care of that without any additional help.
 

lyner

  • Guest
How old are photons?
« Reply #12 on: 28/08/2009 23:11:48 »
So how is one supposed to point out when a statement seems to be inadequate?
Stop taking offense and try to justify what your wrote. It would be a more fruitful line of argument.
If you had told me I was working to a simplistic model then I would have looked again at my model and not been offended.  The last thing I would have done would have been to feel 'insulted'. I'm far too interested in improving my understanding.
Do you not take my point about the risk of over extending analogies?
Perhaps you could give me a form of words which would have got the idea across without getting you so cross. I'm sorry;  I really didn't mean to.
« Last Edit: 28/08/2009 23:13:55 by sophiecentaur »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #13 on: 28/08/2009 23:50:37 »
So how is one supposed to point out when a statement seems to be inadequate?
Stop taking offense and try to justify what your wrote. It would be a more fruitful line of argument.
If you had told me I was working to a simplistic model then I would have looked again at my model and not been offended.  The last thing I would have done would have been to feel 'insulted'. I'm far too interested in improving my understanding.
Do you not take my point about the risk of over extending analogies?
Perhaps you could give me a form of words which would have got the idea across without getting you so cross. I'm sorry;  I really didn't mean to.
If you are really interested in my feedback on this subject, I will do my best to respond, but it will take me a little time develop a constructive and sincere response. On the other hand, if you don't think anything I can say will make any difference, please don't make me waste my time. What is your recommendation?
 

lyner

  • Guest
How old are photons?
« Reply #14 on: 29/08/2009 10:40:15 »
I will get back to this when I can. Today, I am out visiting. I will try to formulate a question.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
How old are photons?
« Reply #15 on: 29/08/2009 13:03:40 »
I think it's unwise to assume that the rules that apply to objects with non-zero rest-mass also apply identically to objects with zero rest-mass.

Objects with non-zero rest-mass cannot achieve 'c' but time-dilation is only observed in objects that have non-zero rest-mass and although the rate of time approaches zero for a non-zero rest-mass object as its speed approaches 'c', we also know that it can never actually achieve 'c'.  Thus, the assumption that no time passes for a zero rest-mass object traveling at 'c' is flawed, imo - it is based upon the characteristics and rules of something totally dissimilar.
What I'm trying to understand from your answer is if you are aware of the fact that, for a non-zero mass object, time dilation is relative to another object and not absolute. For example, I ask you if you are aware (but probably you are, it's only in order not to confound less aknwoledged people who are reading us) that our planet Earth, in this very moment, is travelling at almost c with respect to some other object in space or in our galaxy or in the universe, and that so we are "time-dilationing" ;) with respect it.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #16 on: 29/08/2009 16:56:01 »
Lightarrow, that is a very interesting point. A related question: I'm assuming that the relative motion to which you refer is independent of any spatial expansion, or is that a bad assumption? (Don't reply if that might derail the dialogue with LeeE)
« Last Edit: 29/08/2009 16:59:41 by Geezer »
 

Offline Make it Lady

  • Neilep Level Member
  • ******
  • Posts: 4050
  • Hands-on fun for everyone!
    • View Profile
How old are photons?
« Reply #17 on: 29/08/2009 18:55:57 »
I'm actually understanding this physics thread as it is being answered in a more "down-to-earth" way than other threads on the physics topic. I'm really keen to increase my knowledge base in this area so using the kind of language that Geezer is using is extremely helpful to the less informed. After all that is what naked science is all about.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
How old are photons?
« Reply #18 on: 29/08/2009 19:03:25 »
Lightarrow, that is a very interesting point. A related question: I'm assuming that the relative motion to which you refer is independent of any spatial expansion,
Sorry, can you explain better what you mean? If you can, use an example.
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
How old are photons?
« Reply #19 on: 29/08/2009 19:12:22 »
Lightarrow: Yes.  For anything within the universe, the observed degree of time dilation can only be relative to the inertial frames of reference of the observed and the observer.

There is however, the hypothetical 'infinitely distant' observer who is posited to be stationary with respect to the universe and outside it's gravitational influence.  From the hypothetical infinitely distant observer's point of view, who cannot share the same inertial frame of reference with anything he observes (not simply because it is unique but because it's outside the universe and in a gravitational gradient of 0, meaning that space-time is perfectly flat), all observations are absolute.
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
How old are photons?
« Reply #20 on: 29/08/2009 19:26:43 »
Lightarrow, that is a very interesting point. A related question: I'm assuming that the relative motion to which you refer is independent of any spatial expansion,
Sorry, can you explain better what you mean? If you can, use an example.

I think what Geezer means is that does the movement of everything away from each other due to the expansion of the universe count as relative motion?

For example, if the expansion of the universe means that the speed of recession at distance d from Earth is x, and we launch a rocket from Earth that maintains a velocity y against gravity, when it reaches point d is its velocity relative to us x + y?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How old are photons?
« Reply #21 on: 29/08/2009 20:25:54 »
Right! That's what I meant - I think! :)
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
How old are photons?
« Reply #22 on: 30/08/2009 10:33:42 »

I think what Geezer means is that does the movement of everything away from each other due to the expansion of the universe count as relative motion?

For example, if the expansion of the universe means that the speed of recession at distance d from Earth is x, and we launch a rocket from Earth that maintains a velocity y against gravity, when it reaches point d is its velocity relative to us x + y?
If it maintains a velocity y with respect to us, then when it will reach point d or anyother else, its velocity will be... y with respect to us  :).
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
How old are photons?
« Reply #23 on: 31/08/2009 02:04:48 »
Umm... perhaps that wasn't the best way of putting it.

Geezer, I think the answer is shown in the phenomenon of red-shifted distant galaxies.  The red-shift we see from these galaxies isn't because those galaxies have all been accelerated away from us but because they're moving away from us with the expansion of the universe, so if we're seeing the red-shift then we're seeing relative motion due to expansion.
 

The Naked Scientists Forum

How old are photons?
« Reply #23 on: 31/08/2009 02:04:48 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums