How can a photon exists on its own without a mass?

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For example, thermal energy exists and has no mass, but is carried by particles which have mass. A photon is described as a particle - how can a photon exist on its own, travel in space and even push other particles with mass if it has no mass itself?


I am not sure if that thread should be in quantum physics section or not so I post it here
 

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  • #2
davenn
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For example, thermal energy exists and has no mass, but is carried by particles which have mass.
radiated thermal energy is carried by infra-red photons ( more precisely IR Electromagnetic radiation) massless particles <-- and I use that term broadly
 
  • #3
TumblingDice
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mvbn: Welcome to PF!
For example, thermal energy exists and has no mass, but is carried by particles which have mass.
As davenn wrote, electomagnetic radiation is how I would consider thermal energy 'travels'. You may be speaking about kinetic energy when you mention particles that have mass, as in the collisions of molecules in a gas or their vibrations in a solid.
A photon is described as a particle
Photons are best thought of as the minimum amount (quanta) of light that can be transferred. You will read that light/photons exhibit a "wave/particle duality", but the experts will tell you this is an analogy, and it's better to think of photons as neither (they are their own unique, quantum object.)
how can a photon exist on its own, travel in space
Light travels as electromagnetic waves - an excitation of the EM field.

and even push other particles with mass if it has no mass itself?
Light has energy and momentum. The E=mc^2 equation from Einstein that most folks are familiar with explains the relationship between energy and mass. But there is more to the right hand side of the equation that's frequently omitted - a momentum component for massless particles. The equation reduces to the simpler E=mc^2 when the momentum term iz zero. So light does indeed exert a pressure, albeit ever so small, from the momentum it carries.
 
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radiated thermal energy is carried by infra-red photons ( more precisely IR Electromagnetic radiation) massless particles <-- and I use that term broadly
I thought When One vibrating particle(atom) hits another vibrating particle and causes it to vibrate more that is transfer of thermal energy without photons. Am I wrong?.

@TumblingDice If photons are not particles I can't undestrand why it is teached that they are particles. Even wikipedia says "A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation,". It says clearly that it is "elementary particle" and not just have particle properties. It is confusing that I don't know if I should think about it as particle or not.
 
  • #5
A.T.
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I thought When One vibrating particle(atom) hits another vibrating particle and causes it to vibrate more that is transfer of thermal energy without photons. Am I wrong?.
http://en.wikipedia.org/wiki/Thermal_radiation

It says clearly that it is "elementary particle" and not just have particle properties.
Light has particle properties. Photons are particles.
 
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I thought When One vibrating particle(atom) hits another vibrating particle and causes it to vibrate more that is transfer of thermal energy without photons. Am I wrong?.
You are not wrong. This is an example of conduction. There are additional ways to transfer heat. One, as A.T. pointed out, is radiation.
 
  • #7
TumblingDice
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Light has particle properties. Photons are particles.
Well first a correct analysis with photons really needs QED (Quantum Electrodynamics) since the photons are actually excitations of an underlying EM field... ...if they were actually quantum particles in the usual sense, which is the usual way its treated in beginning treatments - its wrong - but we all must start somewhere.
ZapperZ said:
We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.
^ Excerpt from PF FAQ: "Is Light A Wave Or A Particle?"
https://www.physicsforums.com/threads/is-light-a-wave-or-a-particle.511178/
 

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