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I am a student of class 12th and as far as i know when anything reaches about 99.99% of the speed of light it starts traveling in time or time for it slows down so that it don't breaks the speed barrier. So according to this light photons are immortal. Please clear my doubt.

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marked as duplicate by Qmechanic Feb 24 '13 at 8:25

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Possible duplicates: physics.stackexchange.com/q/54162/2451 , physics.stackexchange.com/q/27794/2451 and links therein. –  Qmechanic Feb 20 '13 at 15:51
    
@Qmechanic - I think the second link (27794) is an answers all the questions raised by the OP. –  Kitchi Feb 20 '13 at 18:06

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welcome to physics.SE

A photon is immortal if it does not meet some matter on its way, or even another photon. If it meets a particle, it will interact with some probability given by quantum mechanical calculations. In that sense it is not immortal, because in interacting it may be completely absorbed into new particles. It's average lifetime will depend on the density of matter, which is very small in outer space, so we see light that has traveled thousands of years, and high if it hits a wall.

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You are right and i am bit satisfied with this but still i wanted to know that if it travels in vacuum then what will happen , will it continue to travel forever or it decay or something else will happen to it. –  Akash Feb 20 '13 at 16:10
    
If the photon does not hit anything it will travel forever. However the expansion of space will reduce it's energy. –  John Rennie Feb 20 '13 at 16:15
    
@Akash Imagine that there are a family of particles in the universe. A can decay into B, and there are multiple pathways, etc. Now, you could make a loop, A->B->C->A, but that would require energy input. So if you look at only exothermic (or increasing entropy) transitions, there is a direction, some will be higher than others, and ultimately one particle will have to be the highest entropy of all of them. Without energy, that particle can't transform into anything else. This final "particle" is a photon or a black hole. –  Alan Rominger Feb 20 '13 at 16:16
    
that means it will continue to go forever if it dont crashes to any other particle. –  Akash Feb 21 '13 at 5:55
    
Yes, except "crash" in this case means "to be close enough so that the interaction couplings are strong enough to deflect or absorb it". The probability it will go forever in our known universe is very small, it will meet a dust particle at least on its way sometime, so the "identity" of a given photon will be lost after a very long time. –  anna v Feb 21 '13 at 6:05

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