Ageless photon. do photons travel through time?

[jrrship]

when a photon from a distant star arrives on earth, it has not aged.

does this mean that it has not traveled through time at all, but only space?

thanks!

 

[Hootenanny]

There have been many threads in the forums on this subject in which the question was extensively answered; I suggest you do a search.

 

[jrrship]

Hello there--I did do a search, but none of them seemed to answer my original question.

Does a photon travel through time? Does it age?

Thanks!

 

[ranger]

Some answers can be found here:

https://www.physicsforums.com/showthread.php?t=51863

 

[Hootenanny]

Thread Title: https://www.physicsforums.com/showthread.php?t=51863"

A few excerpts;

'age' is sort of a meaningless concept for a photon. If you mean does time pass for a photon, the answer is that photons travel along paths of zero proper time in spacetime, so the answer is no. On the other hand, if you mean if we could somehow watch a photon traveling (from a frame that *wasn't* traveling at the speed of light) would time pass for *us* whilst we watch the photon, the answer is obviously yes. So we could arbitrarily assign an 'age' to the photon from our frame, say, it's time begins when it is emitted from the atom, and ends when it is absorbed by one. The problems are firstly that the age would be frame dependent (this is the whole idea behind proper time in the first place) and secondly we can't actually 'see' or observe a photon until it's been absorbed - it's path before measurement has no real meaning as far as we can make predictions about it. Therefore there is no process by which a photon could even be *given* an age, as far as I am aware. Meaningless concept.

Cheerio,

Kane O'Donnell

Yes, but... Astronomers routinely estimate the age of photons coming into their telescopes.

A great deal of what we know about the universe is critically dependent upon the notion that a photon then is just like a photon now, as is the case for electrons and protons, quarks, and ... A photon, when absorbed, is identical to that photon at birth. They do not change in and of themselves. (Strictly speaking, as photons do interact with gravitational fields they can change their frequency, and their polarization. But for the vast majority of circumstances, the gravitational perturbations are very negliable.)

Regards,
Reilly Atkinson

Yeah, sorry about that confusion. Obviously you can detect a photon, record the time and then guess how long roughly it's been traveling for. What I was implying was firstly that 'age' is not something you can intrinsically associate with a photon (it looks the same at all points in time whilst it exists), and secondly that you can't actually watch a photon age - if you see it, it's already gone This is a fairly trivial statement, but I was trying to stop the thread shooting off into la-la land.

Cheerio,


Kane O'Donnell

Edit: Damn Ranger, your quick!

 

[quantum123]

A photon can reach one end of the universe to another in no time.

 

[Hootenanny]

A photon can reach one end of the universe to another in no time.

In no proper time.

 

[jrrship]

So basically a photon does not move through time, but only space.

that is the reality of the photon.

Thanks.

 

[jrrship]

Something keeps bothering me here.

Time is the fourth dimension.

A photon that travels across the universe does not move through time.

Thus a photon that travels across the universe has not moved through the fourth dimension.

How can something move a million light years plus through space, and yet not budge one micron in the fourth dimension?

 

[Doc Al]

So basically a photon does not move through time, but only space.

that is the reality of the photon.

If you are going to pretend to be able to view things from a photon's frame of reference, then go all the way: All distances would shrink to zero, so time and space have no meaning for the photon! (Of course, this is just for fun--photons do not have a frame of reference in special relativity.)

Something keeps bothering me here.

Time is the fourth dimension.

A photon that travels across the universe does not move through time.

Thus a photon that travels across the universe has not moved through the fourth dimension.

How can something move a million light years plus through space, and yet not budge one micron in the fourth dimension?

Again, from our frame of reference (or any real frame of reference in which the speed of a photon is c) the photon most certainly does move through both space and time!

 

[jrrship]

But wait a second here.

If there were a clock on the photon, when we looked at the clock after the photon's long journey, we would have to conclude that the photon had not moved at all through time.

Thus both the people on the photon and we agree that the photon did not move through time.

And too, we would both agree that it moved a vast distance through space, for when the people got off the photon, they would find themselves in a brand new place.

So, at the end of the day, we would all agree.

The photon moved across the universe. It did not age.

Hence it moved a vast distance through space, and not at all through time.

That's what its clock shows.

 

[Doc Al]

But wait a second here.

One second according to whom?

If there were a clock on the photon, when we looked at the clock after the photon's long journey, we would have to conclude that the photon had not moved at all through time.

With the proviso that we are just pretending to view things from the photon's frame of reference (a photon does not have a legitimate frame of reference), everyone will agree that the photon's clock will read zero time.

Thus both the people on the photon and we agree that the photon did not move through time.

No. Different observers will measure different times. Each observer is entitled to his own clock readings.

And too, we would both agree that it moved a vast distance through space, for when the people got off the photon, they would find themselves in a brand new place.

No. From the photon's (pretend) frame of reference there is no "new place". From our frame of reference, the distance is vast; from the photon's frame of reference, the distance is zero. (Note that "people" cannot travel with the photon--that's why assigning a frame of reference to a photon is problematic.)

So, at the end of the day, we would all agree.

The photon moved across the universe. It did not age.

We all agree that the photon did not age in the sense of accumulating proper time. (Since "age" is defined according to the photon's own clock.) We see the photon moving across the universe and taking time to do so; the photon sees "the universe" as a single point--no need to travel, no need for time. It's perfectly OK to ask how long a photon has been traveling according to our clocks.

Hence it moved a vast distance through space, and not at all through time.

The "vast distance" is traveled only from our frame; the "no time" is only from the photon's frame.

That's what its clock shows.

Clocks only show time.

(I hope you see why it's problematic to assign a frame of reference to a photon.)

 

[jrrship]

Hey Doc,

Here we go:

An observer gets on a spaceship and travels acorss the universe at the speed of c.

They then get ou of their spaceship to talk with us.

We all agree that their clock's time has not changed since when they started.

We also agree that they are on the otehr side of the uiverse now, with us.

Thus, we all agree that they traveled across the universe.

And also that no time elapsed on their clock on the spaceship.

I know that nothing but photons can travel the speed of light, so in the above example, replace c with .99999999999999999999999999c.

So basically they traveled across the universe and their clock showed almost no change. They did not age.

So it is that something which travels .999999999999999999999999c barely travels through time.

 

[Littlepig]

than i have a question: if photons move at no proper time, we can assume that nothing can move faster than it, otherwise would have "negative time". can we assume that the speed of the photon is the "speed of time" ?

i would like to define "speed of time", but i just don't have words for that...:S is a meditation what I've tought...wondering...

 

[Doc Al]

I know that nothing but photons can travel the speed of light, so in the above example, replace c with .99999999999999999999999999c.

Good.

So basically they traveled across the universe and their clock showed almost no change. They did not age.

Good.

So it is that something which travels .999999999999999999999999c barely travels through time.

Beats me why you keep wanting to say this. What does "travel through time" mean to you? To me, I calculate that it takes you (in that ship) about a year (of my time) for you to travel one light-year of distance (as measured by me). Well... that sounds like you are "traveling through time" to me!

 

[ranger]

than i have a question: if photons move at no proper time, we can assume that nothing can move faster than it, otherwise would have "negative time". can we assume that the speed of the photon is the "speed of time" ?

It is theoretically possible to have objects that travel faster than the speed of light. These are known as tachyons. However we cannot observe them. These particles would have imaginary proper time. And no, if it where possible accelerate beyond the speed of light, time won't go backwards if that's what you mean by "negative time".

 

[ranger]

i would like to define "speed of time", but i just don't have words for that...:S is a meditation what I've tought...wondering...

Of what use would be defining the "speed of time"? Time is relative to the observer and there is no absolute time.

 

[jrrship]

Good.


Good.


Beats me why you keep wanting to say this. What does "travel through time" mean to you? To me, I calculate that it takes you (in that ship) about a year (of my time) for you to travel one light-year of distance (as measured by me). Well... that sounds like you are "traveling through time" to me!

But Doc,

When we wit dwon to lucn with the traveller who just crossed the universe at .9999999999999999999999999999999999999999999999c, we all agree that no time elapsed on their watch, and that they did not age.

thus, we all agree that they did not travel through time on their journey.

 

[Hootenanny]

But Doc,

When we wit dwon to lucn with the traveller who just crossed the universe at .9999999999999999999999999999999999999999999999c, we all agree that no time elapsed on their watch, and that they did not age.

Er.. no we won't. Ever heard of something called time dilation?

 

[Littlepig]

Of what use would be defining the "speed of time"? Time is relative to the observer and there is no absolute time.

as would be inpossible to go faster than it, as is imposssible to go faster than light. let me try to explain: when we are moving faster, time "decreases", so my wonder is:

imagine we have time speed, is the speed that time passes in referencial, and now, when we are still, the time passes at a great speed(at a total speed "c") and as we can't still imagine that how big it was, time become a referencial...

now, what I'm saiyng, is that, with veryfast speeds(relativistics speeds like 0.8c), we can say that time is passing slower and slower, as we aproch speed of light, so, why don't think that speed of light is actually speed of time?? and the explanation for that is the fact that, the more fast we go, the "closer" velocity to time we are, as for the ones who are still, time is passing at a huge speed. so, getting aceleration is only a step to say that we are getting speed in relation to "speed of time"

ok is a little bit anoying, but was a wondering of a 17 years old boy...

 

[ranger]

imagine we have time speed, is the speed that time passes in referencial, and now, when we are still, the time passes at a great speed(at a total speed "c") and as we can't still imagine that how big it was, time become a referencial...

It seems like you're just throwing together words here [and some words that don't exist - referencial; you probably meant "referential"]. Where did you get the idea that time passes at c when we are still?

Like I've mentioned before, time is relative to the observer. There is no absolute speed of time. I think you should read up more on time dilation:
http://en.wikipedia.org/wiki/Time_dilation

Time won't really "slow down". In fact the only way to confirm this is to take a clock that was left on Earth and the other that was moving at those immense velocities. Only then would you know that there is a time difference between the clocks.

 

[Littlepig]

It seems like you're just throwing together words here [and some words that don't exist - referencial; you probably meant "referential"]. Where did you get the idea that time passes at c when we are still?

Like I've mentioned before, time is relative to the observer. There is no absolute speed of time. I think you should read up more on time dilation:
http://en.wikipedia.org/wiki/Time_dilation

Time won't really "slow down". In fact the only way to confirm this is to take a clock that was left on Earth and the other that was moving at those immense velocities. Only then would you know that there is a time difference between the clocks.

ye i know...was like i said, wondering...sorry for the offtopic...really annoying as i said...;)