# I What is frequency from a photon's perspective?

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1. May 19, 2016

### nnerik

If a photon does not experience time, how can it change? Without change, how can it have frequency?

2. May 19, 2016

### phinds

"Photon" is what we call the result when a light beam hits an object. Light does not travel as photons and photons are not particles in the classic sense, they are quantum objects. Light does not HAVE a perspective or an experience.

3. May 19, 2016

### ZapperZ

Staff Emeritus
Here's the problem with your question.

In saying "a photon does not experience time", you have explicitly used the concept of Special Relativity (unless you came up with your own personal theory). SR has a set of postulates, which are the starting points of that theory. One of the postulates stated that light has a constant speed in any inertial reference frame. Only then, can you derive all the equations from SR, including the "time dilation" equation which you had used to somehow arrive at your conclusion that a photon does not experience time.

But by saying that, you have assumed that you can be IN a photon's reference frame. This means that light is at rest with respect to you. If you do that, SR's postulates will no longer be valid, and this consequently means that the SR equations are no longer valid. So if they are not, how are you able to use it to draw the conclusion that a photon doesn't experience time? You are using a set of rules in a situation where the rules no longer apply.

Do you see the inconsistencies and contradictions here?

There have been numerous other threads on PF that addressed questions like this where members wanted to know what we see when we travel at light speed, etc. You might want to browse for them.

Zz.

4. May 19, 2016

### nasu

What do you mean by this?
True, there are models where you can describe light propagation without using photons, like classical electromagnetism.

But the QED describes propagation and the other phenomena related to light and electrons in terms of photons, doesn't it?
How would you test the fact that is does not propagate as photons?
The single photon sources and detectors exist. How does light propagates in the case of a single photon source?

At the beginning of quantum physics, many physicists, including Plank, liked to think that photons are just some artifact of the interaction between light and matter.
But I think that the modern theories accept the existence of photons.
I understand that many times it is appropriate to tell the poster struggling with understanding light to forget about photons and stay with Maxwell's theory.
But this is not because photons do not "exist" in the most complete theories we have now.

5. May 19, 2016

### Demystifier

The change and frequency are descriptions of the photon from our human perspective. Physics describes what we can observe, not what photons can observe.

6. May 19, 2016

### phinds

OK, I stand corrected. Thanks. I try to get people to avoid thinking about light propagating as photons because it conjures up images of little tiny balls traveling along and that's not helpful.

7. May 19, 2016

### lox_and_whiskey

Yeah but you're confusing the people more familiar with the topic

8. May 20, 2016

### nnerik

Not really. My point is that the photon will have to "swing" n periods in zero proper time. That is an infinite "proper frequency", isn't it? This works fine in a spacetime diagram, but that is only because you are allowed to draw lines between points with zero separation!

How can anything "happen" between A and B when the invariant "distance" is zero? Don't you agree that this is at least a little bit puzzling?

9. May 20, 2016

### ZapperZ

Staff Emeritus
You still don't get it.

Where do all these concepts of "proper time" and "proper length" and "invariant distance is zero" came from? From your dreams?

What is puzzling here is your use of something in places where they haven't been shown to work.

Zz.

10. May 20, 2016

### nnerik

Not dreams, mostly from books, articles and lectures. A few accessible references:

http://www.universetoday.com/111603/does-light-experience-time/
https://www.quora.com/Does-a-photon-experience-time [Broken]
https://quantumweird.wordpress.com/2007/06/23/does-time-exist/

Last edited by a moderator: May 7, 2017
11. May 20, 2016

### phinds

I got as far as that first reference and found "From the perspective of a photon..." which indicates that he doesn't know what he's talking about. I would not be surprised if the rest are equally pop-science misinformation. These kinds of things can be fun to read and can get you thinking about things, but they are not to be trusted for the actual science.

EDIT: "doesn't know what he's talking about" is a bit harsh. What I mean is that he is not versed in science and unknowingly uses a concept that sounds interesting but really is meaningless. This is common in pop-sci writing.

Last edited: May 20, 2016
12. May 20, 2016

### Staff: Mentor

This is a good time to remind everyone of the Physics Forums policy on acceptable sources - none of these are valid references.

But you may also have missed micromass's point. The proper time, proper distance, and invariant intervals that you mention in your post #8 above are all the results of a chain of logic that starts with an assumption (essentially, one of the postulates of relativity) that there is no frame in which light is at rest. Therefore any attempt to use these concepts to understand a frame in which light is at rest is starting with inconsistent premises, and there is no reason to trust the conclusions you draw from them.

Take a look at demystifier's post above to see why notions of time for a photon are irrelevant to the question of how a photon can have a frequency.

Last edited by a moderator: May 7, 2017
13. May 20, 2016

### phinds

You think it funny, and I can understand that, but you need to realize that we get very frustrated here by people asking questions based on crap science (and this happens over and over) so we don't see the humor in it after about the 10th time, to say nothing of the 100th, and this post is approximately the 25,000th (possibly more). We don't MIND, because dispelling such misconceptions is part of the point of the forum, I'm just saying we don't see it as humorous.

14. May 20, 2016

### nnerik

Come on, guys! The spacetime interval between two events on a photon's trajectory is 0! This is not up to debate. My philosophical question is what would be the most intuitive way to understand how something can "change" along an interval that measures zero! Why can't we think of all events on a light-like interval as a single point/event? I am not suggesting there is something deep here, I just need help to improve my intuition.

My apologies to all you serious guys for formulating my original question as a completely clueless noob!

15. May 20, 2016

### Staff: Mentor

Because they're two different points. Imagine a photodetector wired up to an explosive device, and consider the two events "Someone standing 100 meters away from the bomb uses a laser pointer to illuminate the photodetector" and "the light reaches the photodetector and sets off the bomb". Those two events are separated by a lightlike interval of length zero, but it makes a big difference whether your worldline passes through one or the other. One way you're shaking someone's hand and saying "nice fireworks"; the other way you're blown to bits.

Mathematically, you can choose an affine parameter other than proper time to label the points on a lightlike curve. That allows you to compute the different intensity of the electromagnetic field at different points on the curve (they are different points - see above). However, if you're thinking specifically about the frequency of a photon, the simplest answer is that "frequency" is the name that we attach to a property of photons defined by $\nu=E/h$ and which has dimensions of inverse time - the assumption that anything except the electromagnetic field is oscillating is unnecessary.

Last edited: May 20, 2016
16. May 20, 2016

### robphy

It might be appropriate to dig up this old (2006) post of mine from the
which tries to use DEFINITIONS and their logical consequences to address issues on this topic.
In some situations, the answer may be less than satisfactory to some...
but that may be that some concepts (modeled by definitions) don't easily carry over from one case (of a particle with non-zero mass) to a photon.
One may try to introduce an alternate set of definitions...but one has to address its logical consequences.

A good starting point for any question in relativity is
to draw a spacetime diagram and
then model the physics as statements of geometry, interpreted according to your definitions and its logical consequences.

17. May 20, 2016

### nnerik

But this should not prevent us from trying to understand the world from a photons perspective. If we can understand a massive particle's perspective, why not a massless particle's? It is often fruitful to try to see things from different perspectives.

18. May 20, 2016

### Staff: Mentor

When you speak of "the perspective of X", that is a convenient shorthand way of saying "from the perspective of an observer colocated with and at rest relative to X". Such observers can exist (at least in principle) if X is a massive particle but not if X is massless.

19. May 20, 2016

### phinds

You are going to have to come up with your own definition of "perspective" in that case, since as has been pointed out repeatedly in this thread, the way you are using it now does not make any sense.

20. May 20, 2016

### nasu

Same reason that prevents us from "understanding" the charge of charge-less particles. :)

21. May 20, 2016

### nnerik

Fair enough, which should imply that unless you are clueless, you would mean something slightly different when the particle is massless. Refer Roger Penrose's reference to the photon's perspective in the Youtube video I linked in a previous post.

22. May 20, 2016

### nnerik

I see that the Youtube link I posted, which pointed carefully to a specific time in the lecture, was converted by the forum to an embedded youtube video starting at the beginning. I will try to give you the URL without making a link or an object out of it so you can go straight to the relevant part:
Code (Text):

23. May 20, 2016

### Staff: Mentor

It's unfortunate that Penrose makes comments like that. They only serve to confuse people. When you get right down into the nitty, gritty details, a photon cannot have a frame of reference in SR.

24. May 20, 2016

### votingmachine

One thing that might help is to consider the Doppler effect.

Say you are moving away from a laser light source. You measure the photons as red-shifted, at a lower frequency than someone at rest relative to the laser. There is a LARGE range (all frequencies???) that you could measure, if you consider all the velocities ... from traveling at near c towards the laser, to traveling at near c away from the laser.

Yet you are measuring the same photons as the person at rest, with different frequencies at different velocities. So your question "what is the frequency of the photon" needs to be considered in that the frequency depends on the reference frame of the measuring person. Asking about the theoretical time "experience" of the photon does not really change anything. It is a form of energy that when you measure it, has a frequency property, that depends as much on you, as the photon.

As an aside: I feel irritated that this response is not quite as clear as I hoped. I typed it and the power went out. I think I was clear the first time, but there is some block that always happens the second time thru. It does not quite say what I wanted to say ...

25. May 21, 2016

### Demystifier

That is true. But then one should not have prejudices that massless particle's perspective should be very similar to the massive particle's perspective. From a massive particle's perspective the spacetime can be split into 3-dimensional space and 1-dimensional time, but in a sense this is not so from a massless particle's perspective. Instead, for photons, the spacetime splits into a 2-dimensional space and two 1-dimensional null-coordinates.