What Would a Photon Perceive in the Universe?

[chris.hoops]

If I was a photon...

...time dilation and length contraction would mean that I could travel the entire 'breadth' of the universe in zero time, that the distance required to travel the entire 'breadth' of the universe is zero, but I would not be able to 'see' all of the information across the entire universe because all other photons are still traveling at the speed of light relative to me.

So what would I see?

 

[atyy]

You would still bump into atoms and be absorbed - is that death? When the atom emits another photon - is that a new soul or have you been reincarnated?

I would like to hear from a photon who has escaped a black hole. It was long thought that none who there ventured ever return. But it has now been foretold that a black hole may itself become light.

 

[chris.hoops]

Are you trying to make the point that the experience of a photon and the experience of a human being are incomparable in every sense?

I guess I am interested to know - a photon, as far as it is concerned, exists at all points along its path of travel simultaneously (sic). Whereas we perceive it to 'move' from one point to another, over a certain period of time. What is the reason for this discrepancy?

 

[atyy]

Are you trying to make the point that the experience of a photon and the experience of a human being are incomparable in every sense?

Dang it, no - that was a very serious answer.

I guess I am interested to know - a photon, as far as it is concerned, exists at all points along its path of travel simultaneously (sic). Whereas we perceive it to 'move' from one point to another, over a certain period of time. What is the reason for this discrepancy?

Well, I'm not sure about the photon existing at all points of its path simultaneously.

The fundamental reason for the discrepancy you are asking about is that photons all travel at the same speed, and you can never catch them.

Let's say first you travel at 0 m/s. You measure the speed of a car passing by you to be 1000 m/s, and you measure the speed of light to be c.

If you now travel at 999 m/s, the car will now seem to you to be moving more slowly - but you will still measure the speed of light to be c. This is true for light of all wavelengths and frequencies.

Now, there is no reason for this - it is just an experimental fact. What is lucky is that we have a way of describing this strange behaviour mathematically. In this mathematical description, spacetime has a peculiar geometry. Usually, we think that spacetime is divided into only the past and the future. In relativity, past and future still exist, but there is a new region of spacetime which we may call "elsewhere". Spacetime divides into these three regions for you and for me. But the way it divides for each of us is not the same - your past could be my future. For a person, the significance of the past and future divisions of spacetime is that events occurring within those divisions could potentially affect him. In contrast, an event that occurs "elsewhere" can never affect him.

The crucial role of light is that it defines the boundary between "past and future" and "elsewhere", and the mathematical definition of this boundary is it is a trajectory of zero "proper time". We on the other hand, have non-zero "proper time". This is often said to mean that while we experience time, light doesn't.

The idea that light does not "experience" time can actually make sense in a precise, but limited way. For example, light moves relative to us, so we can use the successive peaks and troughs of a light wave moving past us to measure time. In this sense, we "experience" time. However, the speed of light is the same for light of all frequencies, so light does not move relative to light. If two light waves of different frequencies set out in the same direction, any particular peak in one of the waves will not move relative to the nearest peak or trough in the other wave. It will never experience the peaks and troughs of the other wave moving past it. In this sense, a light wave does not "experience" time.

While light does not move relative to light, it certainly moves relative to us - just as we move relative to light. Furthermore, when light encounters atoms in its path, it may get absorbed. A photon may also be "born" when it is emitted by an atom. From this point of view, things do change for light!

 

[chris.hoops]

Sometimes i think physicists must be on acid or something.

This is also a serious point- many of the principals of modern physics require heady leaps of counter intuition to fully grasp. I am obviously not sufficiently evolved.

 

[atyy]

Sometimes i think physicists must be on acid or something.

This is also a serious point- many of the principals of modern physics require heady leaps of counter intuition to fully grasp. I am obviously not sufficiently evolved.

The important thing is these laws agree with experiment. These ideas are counter-intuitive for everyone. I'm sure most physicists would choose simpler laws if they could! Isidor Rabi famously asked "Who ordered that?", when told of the discovery of a new particle he hadn't expected. Everyone acquires more intuition about the laws the longer they wrestle with the concepts. Einstein himself first conceived the main ideas in his General theory of relativity in 1907 - but it took him another 8 years to arrive at the correct equations - he took many wrong turns along the way, and needed help from quite a few others like Minkowski, Grossmann, Levi-Civita and Hilbert to reach the right result. Furthermore, there were actually many right answers consistent with what people at that time knew. So if Einstein had used logic, he would have come up with more than one answer. It was a matter of luck that Einstein's answer was right. Einstein had other excellent competitors who found the other logical possibilities. Our choice between those possibilities is based, not on Einstein's genius, but on experiment. So certainly no one should feel insufficiently "evolved" just because they have the same difficulties as everyone else!

 

[atyy]

...time dilation and length contraction would mean that I could travel the entire 'breadth' of the universe in zero time, that the distance required to travel the entire 'breadth' of the universe is zero, but I would not be able to 'see' all of the information across the entire universe because all other photons are still traveling at the speed of light relative to me.

BTW, what book were you reading? It sounds pretty interesting!

 

[chris.hoops]

That was just me, after reading some stuff on wikipedia.

 

[atyy]

Well, that was pretty accurate if you had just encountered it.

 

[chris.hoops]

I wonder if there is any such thing a 'truth', that all our inquiries will only ever produce ever new interpretations of the universe, that the very nature of perception itself is subjective. In fact, I'm not entirely sure if anything exists outside my head today. Makes it kindof difficult to concentrate on my work.

 

[atyy]

If you're wondering why I said a black hole may become light, look up "Hawking radiation." It was a big surprise when it was discovered that black holes radiate, and can even "evaporate" totally into photons.

 

[chris.hoops]

Wrong forum?

 

[atyy]

I wonder if there is any such thing a 'truth', that all our inquiries will only ever produce ever new interpretations of the universe, that the very nature of perception itself is subjective. In fact, I'm not entirely sure if anything exists outside my head today. Makes it kindof difficult to concentrate on my work.

Yes, that's very important - all our theories are only useful interpretations, useful approximate models of reality. I think physicists 50 years ago thought they were discovering the "truth" - even Feynman thought that a day would soon come when we would have discovered all the laws of physics. But the attitude has changed since then, in physics quite a bit because of Kenneth Wilson's work on the renormalization group and Weinberg's interpretation of quantum field theory as only a low-energy effective theory.

 

[atyy]

Wrong forum?

Huh? You know I wasn't angry right? =

I have nothing to do with running these forums anyway!

 

[chris.hoops]

Ah well, at least we know we'll never get bored.

 

[atyy]

Ah well, at least we know we'll never get bored.

Exactly! Actually, even in everyday physics where we've know the laws for ages, people still make new discoveries. One of my favourites is Denis Gabor's invention of the hologram - very simple, and all based on laws that had been known and routinely taught in universities for almost a hundred years even at that time - but still absolutely revolutionary.

 

[matheinste]

Hello atyy.

Quote:-
----For a person, the significance of the past and future divisions of spacetime is that events occurring within those divisions could potentially affect him. In contrast, an event that occurs "elsewhere" can never affect him.----

I think the last part of this statement is untrue, or i have misread it. Take an observer (event), who we consider to be at the apex of a light cone. Consider another observer (event) outside this lightcone. The second event (observer) is said to be in the "elsewhere" of the fiirst and they are joined by a spacelike vector. Neither observer can experience both events. However the two events can affect each other as sublight speed siganals from each will eventually fall within the light cone of the other .

As an example in everyday experience, if you and another person are in inertial zero relative motion, then in your common frame you are spacelike separated, that is in each others elsewhere, yet you can speak to each other or throw things at each other, or if you are close enough touch each other, or if miles apart send signals to each other.

Matheinste.

 

[chris.hoops]

What happens to the light cone(s) when one observer is in relative inertial motion?

 

[atyy]

Hello atyy.

Quote:-
----For a person, the significance of the past and future divisions of spacetime is that events occurring within those divisions could potentially affect him. In contrast, an event that occurs "elsewhere" can never affect him.----

I think the last part of this statement is untrue, or i have misread it. Take an observer (event), who we consider to be at the apex of a light cone. Consider another observer (event) outside this lightcone. The second event (observer) is said to be in the "elsewhere" of the fiirst and they are joined by a spacelike vector. Neither observer can experience both events. However the two events can affect each other as sublight speed siganals from each will eventually fall within the light cone of the other .

As an example in everyday experience, if you and another person are in inertial zero relative motion, then in your common frame you are spacelike separated, that is in each others elsewhere, yet you can speak to each other or throw things at each other, or if you are close enough touch each other, or if miles apart send signals to each other.

Matheinste.

Yes, that seems right. What is the correct statement?

 

[matheinste]

Hello atyy.

Sorry i have to dash off for a little time. Please don't think i am ignoring you. Someone will probably get back to you before i do.

Matheinste.

 

[atyy]

Hello atyy.

Sorry i have to dash off for a little time. Please don't think i am ignoring you. Someone will probably get back to you before i do.

Matheinste.

Thanks! I've got to go too. Hopefully, someone will put the correct causal interpretation of the light cone up - anyway, it should be easy to find somewhere else (elsewhere?).

 

[DrGreg]

Hello atyy.

Quote:-
----For a person, the significance of the past and future divisions of spacetime is that events occurring within those divisions could potentially affect him. In contrast, an event that occurs "elsewhere" can never affect him.----

I think the last part of this statement is untrue, or i have misread it. Take an observer (event), who we consider to be at the apex of a light cone. Consider another observer (event) outside this lightcone. The second event (observer) is said to be in the "elsewhere" of the fiirst and they are joined by a spacelike vector. Neither observer can experience both events. However the two events can affect each other as sublight speed siganals from each will eventually fall within the light cone of the other .

As an example in everyday experience, if you and another person are in inertial zero relative motion, then in your common frame you are spacelike separated, that is in each others elsewhere, yet you can speak to each other or throw things at each other, or if you are close enough touch each other, or if miles apart send signals to each other.

Matheinste.

A light-cone is relative to one specific event in the observer's history -- let's call that event "now" -- so we just need to insert the word "now" in the appropriate places.

"For a person, the significance of the past and future divisions of spacetime is that events that occurred in the past could potentially affect him now. Events that will occur in the future could potentially be affected by his actions now. In contrast, an event that occurs "elsewhen" cannot affect him now or be affected by his actions now."

(You can't say anything about affecting past or future events in the observer's history, except by considering other light-cones relative to those events.)