How long does it take for a single photon to travel from the sun?

[White Rabbit]

An easy question, anyone can google the answer, but heres a slightly curly version.

If it takes about 8 min+/- for a photon of light to traval from the sun to here going by our clock, how much flight time does the photon experience by it's own clock due to time dilation?

I was reading the latest issue of Sky and Telescope and the cover story is on Superparticles. One of the blurbs in the article said that some of these particles traveling at near the speed of light are coming from light years away., sometime hundreds of light years but that the actual flight time for the particle was 20 min due to time dilation. This got me thinking about the question above.

Any ideas?

[renormalised (Carl)]

Originally Posted by White Rabbit

An easy question, anyone can google the answer, but heres a slightly curly version.

If it takes about 8 min+/- for a photon of light to traval from the sun to here going by our clock, how much flight time does the photon experience by it's own clock due to time dilation?

I was reading the latest issue of Sky and Telescope and the cover story is on Superparticles. One of the blurbs in the article said that some of these particles traveling at near the speed of light are coming from light years away., sometime hundreds of light years but that the actual flight time for the particle was 20 min due to time dilation. This got me thinking about the question above.

Any ideas?

It's a photon....it doesn't experience any time at all, since it's traveling at the speed of light. However from our perspective, it takes 8.3 minutes for the photon to zip across the distance from the Sun to the Earth. But if you take its flight time to Earth from the time the photon is produced in the core of the Sun, it's several million years. That's because of the density of the core and surrounding gases is such that every time the photon is emitted it only travels a minute distance before it's reabsorbed by another atom.

That travel time of 20 minutes for the 12 million Ly between Centaurus A and the earth was for a cosmic ray of extremely high energy. That's all the time it experiences in that distance due to time dilation.

[sheeny (Al)]

Yep. What renormalised said...

Al.

[AGarvin]

I think it goes something like this:

Assuming we're talking within the framework of special relativity, the photon experiencs zero time, in other words, from its point of view it gets there instantly.

It has to do with the concept of spacetime separation as defined by the metric:

dS^2 = c2 dT^2 - dX^2 - dY^2 - dZ^2

where dS^2 is the spacetime separation between two events, c2 dT2 is the time dimension and dX^2 - dY^2 - dZ^2 are the spacial dimensions. This metric will give either a positive, negative or zero result. The time is the proper time experienced by an observer travelling between these two events.

If the time component is greater than the spacial separation, you get a positive result and the two events are causally connected (known as a timelike interval). If you get a negative result, the spacial distance is greater such that not enough time will pass between the two events to allow them to be causally connected (a spacelike interval).

If the answer zero, the events have a lightlike interval, often refered to as null. This would be like two events occuring one light year and one year apart. In the case of the photon from the Sun, the travel time is ~8.3min and the distance equally ~8.3 light min, so the interval is lightlike and the proper time interval experienced by an observer on the photon would be zero.

DOH, hope I've got that right after that ramble.

Andrew

[renormalised (Carl)]

It would be easier to explain the time dilation using the Lorentz transform equation for time...T = T(obs)/SQR(1-(v^2/c^2)), where T(obs) is the time measured relative to an observer of an event, v is the velocity of the moving object and c is the speed of light. Plug in the numbers for a photon and your answer is T = 0.

[janoskiss (Steve H)]

Yep zilch. When you're a photon everything exists at once - no past - no future - just the Isness of it all

[OneOfOne (Trevor)]

Originally Posted by renormalised

It's
That travel time of 20 minutes for the 12 million Ly between Centaurus A and the earth was for a cosmic ray of extremely high energy. That's all the time it experiences in that distance due to time dilation.

I read the same article. They said the particle packed the punch of a baseball travelling at something like 100kph...as I recall? Boy, would that give you a headache. You come in to work in the morning with a black eye and everyone asks you what happened. "I got hit in the head by a cosmic ray!"

[DJDD]

Originally Posted by renormalised

T = T(obs)/SQR(1-(v^2/c^2)), where T(obs) is the time measured relative to an observer of an event, v is the velocity of the moving object and c is the speed of light. Plug in the numbers for a photon and your answer is T = 0.

wouldn't that be T = T(obs)/SQR(0), which is undefined not 0?

[renormalised (Carl)]

Originally Posted by DJDD

wouldn't that be T = T(obs)/SQR(0), which is undefined not 0?

Yes, but undefined is close enough to zero to be zero....it's an example of an infinity (a weird one nevertheless) creeping into an equation.

[DJDD]

Originally Posted by renormalised

It would be easier to explain the time dilation using the Lorentz transform equation for time

I looked up Lorentz Transformation on Wikipedia- no wonder I dropped Physics after second year and maths, as well. Chemistry was always easier...well, except for "theoretical chemistry" but quantum stuff was only for one semester, so...

[renormalised (Carl)]

Originally Posted by DJDD

I looked up Lorentz Transformation on Wikipedia- no wonder I dropped Physics after second year and maths, as well. Chemistry was always easier...well, except for "theoretical chemistry" but quantum stuff was only for one semester, so...

Relativity isn't all that hard....involved, yes, but there's a lot harder maths and physics than relativity. Although, I'd much rather deal with the final equations than try to derive them from 1st principles. That's when you really need to be good at maths.

[AGarvin]

I guess it depends on how you look at it. The Lorentz factor itself goes up to infinity, not down to zero, as you approach the speed of light.

Although, I'd much rather deal with the final equations than try to derive them from 1st principles. That's when you really need to be good at maths.

Absolutely, and mine ain't that good

[renormalised (Carl)]

Originally Posted by AGarvin

I guess it depends on how you look at it. The Lorentz factor itself goes up to infinity, not down to zero, as you approach the speed of light.



Absolutely.

True, however when you try to get the sqrt of 0 (the result of solving the LT for a photon) and you get an undefined answer, that indefinite answer is in itself an infinity and might as well be zero. The equations are renormalised to rid the infinity from the result.

[Zuts]

Hi,

The Lorentz equations only apply to things with mass travelling at less than the speed of light. If you plug in V=c, then the equation is undefined for this value which means it does not give any usefull values.

Basically photons experience no time, maybe thats why they never decay into other particles, but are only absorbed and re-emitted. They dont experience any duration so have no time to decay?

Paul

[renormalised (Carl)]

Originally Posted by Zuts

Hi,

The Lorentz equations only apply to things with mass travelling at less than the speed of light. If you plug in V=c, then the equation is undefined for this value which means it does not give any usefull values.

Basically photons experience no time, maybe thats why they never decay into other particles, but are only absorbed and re-emitted. They dont experience any duration so have no time to decay?

Paul

Precisely. You can't define the SQRT of 0...it's a meaningless answer.

[Zuts]

Originally Posted by renormalised

Precisely. You can't define the SQRT of 0...it's a meaningless answer.

I shouldnt be so pedantic but i've had a few , you can define the SQRT of zero, it's just zero. The value of 1/SQRT(0) though is undefined or infinity.

Paul

[renormalised (Carl)]

Originally Posted by Zuts

I shouldnt be so pedantic but i've had a few , you can define the SQRT of zero, it's just zero. The value of 1/SQRT(0) though is undefined or infinity.

Paul

That's what I meant to say

[sjastro]

Originally Posted by renormalised

Relativity isn't all that hard....involved, yes, but there's a lot harder maths and physics than relativity. Although, I'd much rather deal with the final equations than try to derive them from 1st principles. That's when you really need to be good at maths.

The mathematics behind Special Relativity is nothing more than simple algebra. Deriving the Lorentz equations is a straight forward exercise.

The maths for General Relativity, tensor analysis and Riemannian geometry on the other is not so simple.

Regards

Steven
http://users.westconnect.com.au/~sjastro/small

[renormalised (Carl)]

Originally Posted by sjastro

The mathematics behind Special Relativity is nothing more than simple algebra. Deriving the Lorentz equations is a straight forward exercise.

The maths for General Relativity, tensor analysis and Riemannian geometry on the other is not so simple.

Regards

Steven
http://users.westconnect.com.au/~sjastro/small

I agree. I'd much rather deal with Special Relativity than General. Although compared to some branches of Quantum Theory, even GR is rather simple.

[Brian W (Brian)]

Moderator, I think this generally fits into this discussion so here goes; It has been decades since my high school math. Could anyone here point me towards on line resources that will enable me to upgrade my math skills so I can at least follow the above discussion? Has to be online as there are no bookstores nor universities near me.
Brian