[Phillip Boyle]

Phillip Boyle  asked the Naked Scientists:

Hello,

My question for Ask TNS:

Does time 'stop' for photons travelling at the speed of light?  

So...If a photon was emitted from earth and reflected back to earth from
a mirror one light year away, how much time would have elapsed on earth
from the photon's perspective?  In other words, what is the limiting
case for an astronaut who leaves earth and travels at high velocity and
then returns to earth?

Thanks, I really enjoy your show.

Phillip,
Wellington, NZ.

What do you think?

[lyner]

Not only would time cease to exist but its wavelength would go to zero (another consequence of Special Relativity). How would it know what frequency it was supposed to be going at?

[lightarrow]

Not only would time cease to exist but its wavelength would go to zero (another consequence of Special Relativity). How would it know what frequency it was supposed to be going at?

What do you mean?

[lightarrow]

Phillip Boyle  asked the Naked Scientists:
Does time 'stop' for photons travelling at the speed of light? 

So...If a photon was emitted from earth and reflected back to earth from
a mirror one light year away, how much time would have elapsed on earth
from the photon's perspective?  In other words, what is the limiting
case for an astronaut who leaves earth and travels at high velocity and
then returns to earth?

Maybe you would consider this nitpicking, but it's not: one thing is to talk about "the photon's perspective", which doesn't exist and another thing is to talk about "the limiting case for an astronaut" which instead can be easily discussed, since is the well known twin paradox. As sophiecentaur said, the time elapsed for the astronaut which comes back, goes to zero as his average speed goes to c.
You could be interested to know that you could go around *all the universe* in *zero* time, if you travel at light speed. Of course, when you come back, you won't find your relatives or friends anylonger, not even Earth and probably not even the solar system...

[yor_on]

And this is it, isn't it;)
The photon always 'travel' at 'c'.
Minimizing 'times' influence on it.

But on the other tentacle it's also mass less.
which makes times influence negligible.

Do 'time' has anything to say to 'objects' without mass?

--------

And no.
To me 'empty space' seems not empty.
And photons are also p ..ersons ,ah sorry ..articles:)

So now I think it's time to start whistling as I unconcerned strolls away:
At a fast pace...

[kancha]

According to the relativity, photons i.e light have special power of determining Time. Imagine there was an absolute dictator in a country. Now all the rules of the country would depend on him but they wouldn't be valid for himself. Just as the photos whose velocity would be the basis of time for others but no time would exist for themselves.
Now imagine close family members and friends of the dictator. To them although the cruel rules of the dictator would exist but in very small amount. They could break some big laws yet get minimal penalties for doing so. The same thing happens to the objects that move at a velocity near to the velocity of light, i.e the effect of time is less on them. In other words time would slow for them. So if an astronaut would travel at a very high speed, then the time would slow down for him and he would still be young compared to a man of same age on earth who would be traveling in a relatively small speed.

So for the photos no time would elapse during its entire journey.

[Bikerman]

I don't really think that analogy is useful.
A far better one is to think of movement through spacetime. You can move through space and/or time. The total of your movement is always the speed of light. If, therefore, you are relatively stationary then all your movement is through time. If, on the other hand, you are moving very fast (relatively) through space, then you are moving correspondingly slower through time.

[yor_on]

Phillip you asked " If a photon was emitted from earth and reflected back to earth from
a mirror one light year away, how much time would have elapsed on earth from the photon's perspective? "

From the photons perspective as it is expected to travel at 'c' there can be no time.
So if we now had a 'observer' dressed up with the characteristica of a photon 'traveling'
I would assume that our universe wouldn't exist time wise.
Not until 'impact' which is when it interact with 'matter' or some other light quantas.
That moment will be the beginning, and the simultaneous end of its interaction 'in time'.

Then you continue with asking
"In other words, what is the limiting case for an astronaut who leaves earth and travels at high velocity and then returns to earth?"
But that is a question relating to matter, not light.

The difference here being that matter will never be freed from interacting with spacetime and therefore never be 'timeless'
(like Elvis but not really:)
So your astronaut can never reach 'c' no matter how much he tries.
Its a little like wondering what an irresistible force will do to a immovable object.
But as he is a part of 'time' no matter how fast his acceleration / velocity is the universe outside his spacecraft (reference frame) will rapidly age as seen from his perspective.

He 'belongs' at all time 'inside' our spacetime as compared to a photon that somehow becomes 'released' from it while 'traveling'.

Light is always a part of spacetime as seen from our perspective, but treated from its own 'reference frame' internally it does not follow time as I see it.
And that is a mystery needing to be solved, as we do know its speed and can define in time its journey from our sun to our Earth.
And also as we can see it act particle-like (matter).
Also it has a momentum that, when enclosed in a perfectly reflecting medium (ball/box), will produce an added weight.
Some see that as a proof of mass.

I don't know how it works, but you will have to differ between matter and light, as time seems to do so.

[Mr. Scientist]

Phillip Boyle  asked the Naked Scientists:

Hello,

My question for Ask TNS:

Does time 'stop' for photons travelling at the speed of light? 

So...If a photon was emitted from earth and reflected back to earth from
a mirror one light year away, how much time would have elapsed on earth
from the photon's perspective?  In other words, what is the limiting
case for an astronaut who leaves earth and travels at high velocity and
then returns to earth?

Thanks, I really enjoy your show.

Phillip,
Wellington, NZ.

What do you think?

Yes, it does. 

A photon belongs to a class of zero-time particles called Luxons. For a system to move at the speed of light, would suggest that the spacetime triangle it is moving in (with an imaginary line representing time), would be stretched into infinity. These Luxon particles move along a null path in zero-spacetime. This means from its reference frame, it has no passing of time, or duration of space or distance.

[lightarrow]

/quote]

Yes, it does. 

A photon belongs to a class of zero-time particles called Luxons. For a system to move at the speed of light, would suggest that the spacetime triangle it is moving in (with an imaginary line representing time), would be stretched into infinity. These Luxon particles move along a null path in zero-spacetime. This means from its reference frame, it has no passing of time, or duration of space or distance.

What I coloured in blue doesn't exist.

[Mr. Scientist]

Yes it does exist; depends on how you want to refer to it. It's a zero-time existence, but it still exists. It's a frozen frame of reference.

[lightarrow]

Yes it does exist; depends on how you want to refer to it. It's a zero-time existence, but it still exists. It's a frozen frame of reference.

A frame of reference is a physical object not a chose of coordinates. To have a frame of reference you must be sure that you could put many physical clocks in various points of that frame and that you could physically syncronize them. Are you sure you can do it for a photon?

[yor_on]

To my eyes you are both right:)
It all hangs on how you define it.
With lightarrows definition no proposals can be made for a photon seen internally.
As well as a black hole inside the event horizon.

For myself I allow myself to see a frame of reference as something describing an event.
And events may be inside time or out of it depending on where you look from.
After all spacetime allows for photons and black holes?
http://www.answers.com/topic/frame-of-reference

[lightarrow]

To my eyes you are both right:)
It all hangs on how you define it.
With lightarrows definition no proposals can be made for a photon seen internally.
As well as a black hole inside the event horizon.

For myself I allow myself to see a frame of reference as something describing an event.
And events may be inside time or out of it depending on where you look from.
After all spacetime allows for photons and black holes?
http://www.answers.com/topic/frame-of-reference

http://www.answers.com/topic/frame-of-reference
<<Frame of reference
Home > Library > Science > Sci-Tech Encyclopedia

A base to which to refer physical events. A physical event occurs at a point in space and at an instant of time. Each reference frame must have an observer to record events, as well as a coordinate system for the purpose of assigning locations to each event. The latter is usually a three-dimensional space coordinate system and a set of standardized clocks to give the local time of each event. For a discussion of the geometrical properties of space-time coordinate systems See also Space-time; Relativity.>>

[Bikerman]

I have to agree - you cannot construct a Frame of Reference for a photon - can't be done.

[yor_on]

Although I agree on us using and expecting an arrow of time macroscopically that's not true QM-wise.
There you might have a 'representation' where in time moves 'backward'.
So between those 'extremes' of time there seems to be one wherein time 'theoretically' could be seen as null and nonexistent
And that's why I'm playing those 'movies' inside my head.

"A further aspect of a frame of reference is the role of the measurement apparatus (for example, clocks and rods) attached to the frame (see Norton quote above). This question is not addressed in this article, and is of particular interest in quantum mechanics, where the relation between observer and measurement is still under discussion (see measurement problem).

In this connection it may be noted that the clocks and rods often used to describe observers' measurement equipment in thought, in practice are replaced by a much more complicated and indirect metrology that is connected to the nature of the vacuum, and uses atomic clocks that operate according to the standard model and that must be corrected for gravitational time dilation.[32] (See second, meter and kilogram).

In fact, Einstein felt that clocks and rods were merely expedient measuring devices and they should be replaced by more fundamental entities based upon, for example, atoms and molecules. [33]"

http://www.answers.com/topic/frame-of-reference

-------------

And what happens to them (those atoms and molecules:) when 'slowed down'?
They 'smears out' and in fact loose their 'individuality'.
So yes, to me 'time' is very interesting.
And I also believe that photons are included in spacetime.
As we otherwise wouldn't observe them as coming from 'sources'.
As our sun.