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vivitar02
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If electron-positron annihilation creates a photon, with "mass-energy", how long will the photon live for? It will travel at the speed of light until when?
Sojourner01 said:Photons, as far as we know, don't decay to anything. Several fundamental quanta are thought to be the same, in particular, electrons, protons and neutrons. There's an experiment looking for proton decay which would have expected to find one by now if it ever happens, and they haven't. All the others are thought to be even less likely. The photon is included.
JesseC said:Free neutrons do decay, with a half life of about 13 minutes.
vivitar02 said:If electron-positron annihilation creates a photon, with "mass-energy", how long will the photon live for? It will travel at the speed of light until when?
Phrak said:Since when do photons propagate at the velocity c? QuantumElectroDynamics makes no such assumption, and off shell photons have a range as a function of their velocity.
mrspeedybob said:From a cosmological standpoint it's easy to see that if photons do decay it takes a loooooong time. We're still seeing cosmic background radiation from 14 billion years ago. In another sense though they decay constantly. The expansion of space causes photons to redshift / decay to lower energies.
vivitar02 said:1. They decay to lower energies, but remain as photons ?
TheTechNoir said:To my understanding the decay to lower energies is the red shift as mentioned, or specifically the change in the wavelength. That is why the microwave background radiation which I believe was once actually visible light has left the visible light spectrum and is now only visible in microwaves because of the amount of energy change aka amount of redshift/stretching of the wavelength.
Phrak said:I think this thread needs some rewinding.
Since when do photons propagate at the velocity c? QuantumElectroDynamics makes no such assumption, and off shell photons have a range as a function of their velocity.
negru said:Any "off-shell" (otherwise known as virtual) photon you'd measure becomes on-shell, hence must move with speed c, as all massless particles. Quantum electrodynamics=quantum mechanics+special relativity, so it'd better respect special relativity.
negru said:Any "off-shell" (otherwise known as virtual) photon you'd measure becomes on-shell, hence must move with speed c, as all massless particles. Quantum electrodynamics=quantum mechanics+special relativity, so it'd better respect special relativity.
inflector said:Do you have some references for this assertion? Specifically that:
- Photons don't travel at c
- Photons have variable velocity
- Photons have a range determined by that velocity
Phrak said:Not at all. Measurement or detection has nothing to do with time evolution of fields.
Phrak said:I suppose you could look up QED in the wikipedia.
DrChinese said:Looking at the existence of a photon: can a free photon truly exist? That is, a photon which is just flying through free space awaiting something to interact with? I am thinking of a photon created long ago, and traveling through space today. If the photon was formally an excitation of a field - and not an independent entity itself - then this wouldn't happen... would it? There would need to be a start point and an end point. The end point being some spot in the distant future in which there is an interaction.
This would mean that if a hot body were to exist in a hypothetical, otherwise empty universe it would never cool off because there would be no other matter in the universe for the energy to be radiated to.
DrChinese said:Looking at the existence of a photon: can a free photon truly exist? That is, a photon which is just flying through free space awaiting something to interact with? I am thinking of a photon created long ago, and traveling through space today. If the photon was formally an excitation of a field - and not an independent entity itself - then this wouldn't happen... would it? There would need to be a start point and an end point. The end point being some spot in the distant future in which there is an interaction.
mrspeedybob said:If I understand you correctly you are postulating that a photon could not exist without definite beginning and end points. This would mean that if a hot body were to exist in a hypothetical, otherwise empty universe it would never cool off because there would be no other matter in the universe for the energy to be radiated to. In our universe it means that If a hot body were to be located in a part of the universe so devoid of matter that the nearest object was receding at the speed of light, that body could not radiate.
Given that we exist in a part of the universe where, if a free photon could exist, it would have a certain probability of eventually interacting and a certain probability of never interacting, and that probability spread will shift towered never interacting as the universe expands and our visible portion of it gets less densely populated. It should be possible to construct an experiment that would compare the radiation rate of two hot bodies at different times and determine if there was a difference due to the expansion of the universe. If there is a difference then free photons cannot exist. If there isn't then either free photons can exist OR we live in a universe that will eventually stop expanding giving every photon an opportunity to interact and therefore exist.
Has anybody done an experiment like this?
The lifetime of a photon is considered to be infinite because it does not decay over time. This is due to the fact that photons have no mass and therefore do not experience the natural decay process that is seen in particles with mass.
Electron-positron annihilation is a process in which an electron and a positron (the antiparticle of an electron) collide and are converted into two or more photons. This process is governed by the laws of quantum mechanics and is often used in particle accelerators to create high-energy gamma rays.
The speed of light is a fundamental constant in the universe and is the maximum speed at which any object can travel. Photons, being massless particles, travel at the speed of light. This means that the lifetime of a photon is directly related to its speed, as it will continue to travel at this speed for an infinite amount of time.
No, the speed of light cannot be exceeded. According to Einstein's theory of special relativity, as an object approaches the speed of light, its mass becomes infinite and it would require an infinite amount of energy to continue accelerating. Therefore, it is impossible for any object to travel faster than the speed of light.
The speed of light is measured using various experimental methods, including the use of lasers, interferometry, and time-of-flight measurements. The most accurate measurement to date is 299,792,458 meters per second, which was determined by the National Institute of Standards and Technology (NIST) in 1983.