Is it still connected to it's source when it arrives at a destination?

In a manner of speaking.

Light experiences zero distance between any two given events.

A photon is a particle which can be considered to be roughly the size of its wavelength, if you need to assign it a size. At the quantum level, things tend to be fairly fuzzy, and the notion of a precise length isn't even meaningful.

Bannor:

Is it still connected to it's source when it arrives at a destination?

No. Let's say you take a laser and create a pulse of laser light. When the pulse hits the destination, you can clearly see that it is no longer at the source.

James:

At the quantum level, things tend to be fairly fuzzy, and the notion of a precise length isn't even meaningful.

Unfortunately, photons are not only a quantum phenomena, but a macro phenomena as well. For example, if I use a laser to create a pulse of light, how short can the pulse be? If I send out a radiowave with a radio transmitter, must the wave be a whole wavelength, or can it be a partial wavelength? Does a photon oscillate at all, or are just multiple photons of constant various electric field strengths created at the source?

Originally posted by Prosoothus
No. Let's say you take a laser and create a pulse of laser light. When the pulse hits the destination, you can clearly see that it is no longer at the source.How do you propose to "see" a photon?

errandir,

How do you propose to "see" a photon?

Let me give a different example:

Say, you're looking up into the sky and are witnessing a supernova explosion that is 100 million light years away. Are you claiming that the light that is hitting your retina is still being emitted by the supernova?

according to relativity, in the poton's perspective, it was absorbed IMMEDIATELY after it was emitted.

A photon experiences a life-span of 0.

Your question sounds like a classic simultaneity question.

from the photon's perspective, no time passes at all, so from the photon's 'point of view' it is simultaneously at every point it passes through.
However, this is not the same as saying it is connected to the source, especially as (I assume) you are taking an external view of the photon. Therefore, as the photon is travelling at the speed of light, it's length has contracted (from your point of view) to the infinitesimal.

It is not well defined to talk about the reference frame of the photon, but since everybody else has been doing it already, I would like to point out the following: the photon requires t = 0 to reach its destination, but at the same time its destination is everywhere, because the entire universe gets length contracted to zero.

That's why usually we take well defined frames of reference.

Bye!

Crisp

There are only two points in a photon's path: the beginning (emission) and the end (absorption). These two points are not separated. To speak of any other point, you will need another photon. Pretty soon, you will find that, in order to establish the physical situation, you will need practically an infinite number of photons, and their individual indentity becomes meaningless (in questions of length and such).

Originally posted by Prosoothus

Let's say you take a laser and create a pulse of laser light. When the pulse hits the destination, you can clearly see that it is no longer at the source.

I'm not referring to a pulse though. I'm referring to an uninterrupted light source.

Originally posted by Crisp
the photon requires t = 0 to reach its destination

How's that?

Bannor,

I'm not referring to a pulse though. I'm referring to an uninterrupted light source.

It doesn't matter. Whether you have a pulse, or an uninterrupted light source, they are both composed of photons. Since a pulse of light is possible, it implies that photons have a finite length. This would mean that the length of the photons in an uninterrupted light source should be finite as well.

How's that?

It isn't. But if we're going to talk inaccurately about a photon's frame of reference, you surely won't mind me saying that it is because of length contraction (also called Lorentz contraction, do a search on this board for 100 threads and explanations on it ;)). The distance for the photon to travel shrinks to zero as v = c. No distance to travel = instantaneous travel.

Bye!

Crisp

From the point of view of the photon, there is no separation between emmission and absorption. However, the photon has a particularly limited point of view.

Try this analogy:
Draw two dots on a sheet of paper. Low look at the paper edge on, so that the apparent separation between the dots is zero. That's the photon's point of view. There is no measurable separation from that point of view, but there is still a real separation from other points of view.

For the original question, the length of a photon can be anything you want it to be, if you design your experiment right.

Just my thoughts on this topic.

A photon has no mass which allows it to travel at c. Travelling at c the photon itself does not experience time. In effect it does not itself move within the time dimension. But it does move in the three spatial dimensions. Also, since time is relative, any observers of the photon (assuming they could observe it without destroying it) would see it traveling through both space and time. This assumes the observer is not moving through space in parrallel with the photon at c also, which is quit impossible. So from the viewpoint of anyone but the photon itself it moves thru space and time and does not have an instantaneous lifespan.

A photon must have a length. It is something. We may think of it as a point particle but this is a convenience. It exists within three dimensinal space (possbily more) so it has to have some spatial volume. Some material I have read suggest that the photon has a length close to the planck length. Along these lines, not only how long is a photon, but how wide is it and how tall is it? Is it spherical shaped, ring shaped or what? Problem is we will probably never know other than through theoretical postulates since we will never be able to probe a photon.

Originally posted by creation-in-motion
(1)Travelling at c the photon itself does not experience time. In effect it does not itself move within the time dimension. But it does move in the three spatial dimensions. ... from the viewpoint of anyone but the photon itself it moves thru space and time and does not have an instantaneous lifespan.
...
(2)
A photon must have a length. ... We may think of it as a point particle but this is a convenience. It exists within three dimensinal space (possbily more) so it has to have some spatial volume.(1) The only thing anything experiences from it's own perspective is movement through time. Furthermore, the proper time of the lifespan of anything is a scalar, so any outside observer would have access to this quantity as well. In a way exchange of photons (or maybe I should be more general and say bosons) is what provides us with a definition for time. You cannot speak of photons and spacetime dualistically like you can with matter and spacetime.

(2) Why? Do points (events in space-time) themselves not exist (unattached an independent to other objects)? I suppose I could entertain that notion. Let me just make a suggestion in favor of the point notion.

If we're talking photons then we're talking QM. Consider an isotropic radiator. When it emits one photon, in which direction does the photon travel fromt the source. I suggest that it travels in all directions. In other words, I suggest that it is in a superposition of position eigenstates, a sphere. This sphere expands in time, but it remains a sphere until absorption, in which case it collapses to the position eigenstate (a point in space) at the point of absorption. If this is done a gazillion times, then the proportion of photons absorbed will be the solid angle of the absorbed subtended at the source divided by 4*pi. If this is done just one time, then this amount is the probability of absorption. For this example it is not necessary to consider an isotropic radiator, but it is simplier for me to consider one.

Now, to concede to the possibility that photons are NOT point particles, I will admit two things. a) emission is probabilistic (and therefore uncertain), and b) position itself is uncertain. I think that both of these issues are related, but the point is that, if space-time is based on the exchange of photons (bosons), then these issues cannot be neglected in the consideration of the existence of points (events in space-time) vs the granularity of space-time.

Crisp,

"....but at the same time its destination is everywhere, because the entire universe gets length contracted to zero."


I do believe this is a bit of over statement. Lorentz Contraction is for relative motion and applies 100% only in the motion vector.

Orthogonally there is no relative motion and no contraction. So there should be a volume of the universe simular in shape to a doughnut around the vector of motion where dimension has contracted to zero decreasing affect around to the orthogonal where there is no affect and then back down to zero in the trailing vector.

Knowing to believe only half of
what you hear is a sign of
intelligence. Knowing which
half to believe will make you a
genius.