What is a photon?

And if you say "energy", what is energy?

This is the best I could find.

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles). The photon has zero rest mass and always moves at the speed of light within a vacuum

Are we Ok with the term energy without really questioning what energy really is?

Perhaps this is the key to understanding the material universe, since we are all supposedly made up of or from photons.

Could a photon simply be a way to convey information that makes things?

@whodey said
Are we Ok with the term energy without really questioning what energy really is?

I don't know what you mean by "questioning what energy really is".
When we say what something 'is', whether a physical thing or not, we state its intrinsic characteristics and/or its relationships it has with other things. Whether its a physical thing, or a mathematical thing, or a thing of the mind, or purely a conceptual thing, or whatever kind of thing it is, providing its something meaningful and not just complete gibberish, that is ALL we can do to state what something 'is'. There is nothing 'beyond' that we can do to state what something 'is'; at least nothing meaningful or that makes any sense. 'Energy' is no exception to that. In the case of energy, science has done a good job of telling us some of its intrinsic characteristics (for example, the energy conservation law) and its relationships it has with other things (for example, one of its relationships with matter is known as the mass-energy equivalence). Science has told us much about what energy 'is' in that only meaningful sense and there is no other sense we can be told or know what it 'is'. This is why I don't know what you mean by "questioning what energy really is"; its because there doesn't appear to be any meaningful question I know of about what energy 'is' that science hasn't already answered.

@whodey said
And if you say "energy", what is energy?

This is the best I could find.

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles). The photon has zero rest mass and always moves at the speed of light within a vacuum

Are we Ok with the term energy without really questioning what energy really is?

Physics tells us what energy does but not what energy is, beyond the ability to cause change. If you continue to ask what ability to cause change is, you will find that the answers either get vaguer and vaguer or simply repeat the question in other terms.

Regard the photon as a minimum unit of change with regard to light. Same applies to "electron" with regard to electromagnetic phenomena.

@whodey
It is electromagnetic fields and photons come in a huge array of sizes. There is a photon of radio waves, say a radio wave of 1 meter wavelength, it is the exact same thing as a green light photon but a lot larger physically.

The odd thing to me is the ability of a massless photon to impart kinetic energy which is the basis for the solar sail.

It is well known the idea photons can power spacecraft by using the sun as if the light from the sun is like wind in the atmosphere and steering is similar to sail movements, tacking and so forth to angle at different direction.s.

A magnetic field imparts kinetic energy to magnetic materials like iron.

It also can impart kinetic energy to conductors too if the field is varying.

Electric fields can also impart kinetic energy, accelerating charged particles like in my job as an ion implanter field service engineer.

So mix the two fields together, you have the electromagnetic field and the combination imparts kinetic energy to most anything a photon strikes, conductor or insulator, the conduction state doesn't matter much except for the production of surface plasmons which is a whole other story.

@whodey said
And if you say "energy", what is energy?

This is the best I could find.

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles). The photon has zero rest mass and always moves at the speed of light within a vacuum

Are we Ok with the term energy without really questioning what energy really is?

The definition of photon you typed in above is correct. Energy is the canonically conjugate variable to time.

@sonhouse said
@whodey
It is electromagnetic fields and photons come in a huge array of sizes. There is a photon of radio waves, say a radio wave of 1 meter wavelength, it is the exact same thing as a green light photon but a lot larger physically.

The odd thing to me is the ability of a massless photon to impart kinetic energy which is the basis for the solar sail.

It is well kno ...[text shortened]... tate doesn't matter much except for the production of surface plasmons which is a whole other story.

Which is a better statement, we are made of photons/atoms?

I assume atoms are "created/manufactured" after the photons "transition".

@whodey said
Which is a better statement, we are made of photons/atoms?

I assume atoms are "created/manufactured" after the photons "transition".

The way I look at it everything is comprised of energy. Mass is basically congealed energy, but requires a minimum amount of energy in order for it to exist as mass. So a single photon can exist and exert a force, and at the same time have no mass.

@lemon-lime said
The way I look at it everything is comprised of energy. Mass is basically congealed energy, but requires a minimum amount of energy in order for it to exist as mass. So a single photon can exist and exert a force, and at the same time have no mass.

How can we be sure it has no mass?

Obviously, when transitioning to atoms mass is either created or simply slows down so that it can be measured

Considering the laws of science, we must opt for the last possibility.

@whodey said
How can we be sure it has no mass?

Obviously, when transitioning to atoms mass is either created or simply slows down so that it can be measured

How can we be sure it has no mass?

Do you mean how can we be sure a single photon has no mass?
A better question might be, how many photons would it take all tightly scrunched together to form the lightest (least heavy) unit of mass.

@whodey said
How can we be sure it has no mass?

If a photon had mass then, given the rest mass it must have according to the energy-mass equivalence law, relativity tells us it would either have infinite mass in our frame of reference (but still finite REST mass) and infinite kinetic energy if it travels at c in a vacuum, which wouldn't make sense, or it would have finite mass BUT travel less than c in a vacuum (which we know isn't true). Here is the relevant equation to explain why:

https://en.wikipedia.org/wiki/Lorentz_factor

THAT is at least one way we know a photon has no mass.

Obviously, when transitioning to atoms mass is either created or simply slows down so that it can be measured

NO, NOT the latter; a photon has no mass to measure.

Considering the laws of science, we must opt for the last possibility.

WHICH "laws of science" implies this? No such "law" exists.

@humy
Turning light into matter:

https://phys.org/news/2014-05-scientists-year-quest.html

If light can be turned into matter, wouldn't the mass come from interaction with the Higgs field?

@humy

The law I refer to is that you cannot create or destroy matter.

So do you believe that photons transitioned into matter or did matter create the photon?

Chicken or the egg?

@whodey said
How can we be sure it has no mass?

Obviously, when transitioning to atoms mass is either created or simply slows down so that it can be measured

Considering the laws of science, we must opt for the last possibility.

There are experimental upper bounds on the mass of a photon, the largest mass it can have is tiny. The empirical evidence is consistent with the photon being massless. If the photon had a mass it would mean it propagated at less than the speed of light, which isn't quite the contradiction it sounds like, the speed of light as it appears in Einstein's formula is just the upper bound on speeds at which only massless excitations can travel. If photons have a mass then they travel at less than that upper limit.

What we normally think of as matter consists of particles called fermions, they have the property that only one of them can be in a given quantum state at a time. So electrons tend to stack in atoms rather than just all dropping to the ground state - this is good news for us as otherwise life could not exist as chemistry depends on this.

Atoms consist of electrons orbiting a nucleus containing neutrons and protons. The nucleons each consist of smaller particles called quarks. These particles are all fermions and all have mass.

Mass is not a conserved quantity. Energy is conserved, mass is not. One can create arbitrary amounts of matter providing the various conservation laws in play are respected. CP violating processes mean that fermion number is not conserved and so matter can be created. This explains why there is matter in the universe and it doesn't just consist of radiation.

@whodey said
@humy

The law I refer to is that you cannot create or destroy matter.

NO NO that law is ONLY supposed to apply when there is NO energy-matter transformation. When there IS energy-matter transformation, that law simply doesn't apply. There are MANY so-called 'laws' in science that are like that in the sense that they come with a number of exceptions, often implicit, when they simply don't apply. The laws of physics are no exception. Another example of that is the second law of thermodynamics that states that the total entropy of system can never decrease over time BUT that law doesn't always apply to a system that isn't isolated.