actually all particles are basically energy in the sense that they were created by adding energy to a quantum field. This applies whether you're creating a photon, an electron, a quark or whatever.
So in creating a photon we added energy to its quantum field.
If energy is not even a photon, so how can we view energy? Is there any physical view to it?
If energy is not even a photon, so how can we view energy? Is there any physical view to it?
Man is a thinking animal and observes his/her environment, a pattern recognition program is built in our thought structure.
When man started classifying nature, the first observation was that there were two sets:
a) Objects and b) the behavior of those objects classified in attributes: large, heavy, moving fast etc.. in the primitive level.
At present time we have explored the microcosm to the point where the "objects" we measure are fundamental particles as given in the standard model of particle physics. This is a quantum mechanical framework. In this framework "energy" is an attribute that particles carry, together with their mass and their quantum numbers that distinguish them from each other.
Energy is an attribute for elementary particles as an electron, a neutrino , a photon. Elementary means that they cannot be split further so , in contrast to complex particles which may decay to lower mass constituents according to special relativity rules.
Quantum mechanics is a mathematical theory that describes the microcosm of particles with accuracy and has allowed us to build the technological world we live in, including this computer we are communicating with. The mathematics of Quantum Mechanics can be separated into "first quantization", Schroedinger equation etc, and "second quantization", Quantum Field Theories, QFT. The quote from John Rennie's answer comes from the second quantization tool box. Second quantization cannot be understood without going through a course of first quantization.
So in creating a photon we added energy to its quantum field.
The interaction that created a photon gives it its energy; for example bremsstrahlung the radiation of a photon by an electron interacting with an atomic electric field, the photon takes the energy from the electron, which , by conservation of energy has less.
Bremsstrahlung produced by a high-energy electron deflected in the electric field of an atomic nucleus
The energy of the electron is the kinetic energy $1/2mv^2$, and its momentum is $mv,$ making up the four momentum vector of the electron
$$\mathbf p= \begin{bmatrix}E\\ p_xc\\p_yc\\p_zc\end{bmatrix} = \begin{bmatrix}E\\ \mathbf{p} c\end{bmatrix}$$
After the interaction the photon takes away energy and momentum from the electron, described by its own four vector.
[In the second quantization mathematical formalism this interaction is creating a photon with energy $h\nu$ by acting on a photon field at the point of interaction, and this photon propagates with consecutive annihilation and creation operators on the photon field obeying energy and momentum conservation].
If energy is not even a photon, so how can we view energy? Is there any physical view to it?
Energy and momentum are an attribute, describe the behavior and condition , of a particle. By the very definition of energy it cannot be a particle.
Here is a physical view of energy. The energy of a particle specifies how it oscillates as it moves thru time. The amplitude to detect a particle of energy E oscillates with a frequency of $\omega =\frac {E}{\hbar}$ [radians/sec] as the particle is translated in time.
Energy is also useful because it is conserved as time advances.
What is physical significance of energy?
Energy is physical. Matter is made of it. Have a look at Einstein's E=mc² paper (where he wrote L instead of E) and note this:
"If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²".
Radiation is a form of energy. Energy is not merely an attribute of a photon. A photon is energy, in a particular form. And it can change form. This is why there's multiple forms in the hyperphysics image:
John Rennie said "actually all particles are basically energy in the sense that they were created by adding energy to a quantum field. This applies whether you're creating a photon, an electron, a quark or whatever".
I agree that all particles are basically energy. But "they were created by adding energy to a quantum field" is an answer according to the Standard Model, and there are issues with it. For starters, it's at odds with Einstein, and not just with E=mc². See this 1929 article where Einstein was talking about electromagnetic and gravitational fields: "It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds". According to Einstein, a field is a state of space. But the Standard Model makes the claim that space consists of a whole set of quantum fields, all overlaid on each other, with some mystical mechanism for transferring energy from one to another. Surely there surely can't be ten different states of space at one location? See what anna said here about that: "creating this aether of all fields in mathematics does not mean a real physical existence". And what exactly is this energy that is added? Just an attribute? How can it make sense to say a photon is created because you added energy to the photon field, and then say energy is merely an attribute of the photon? Did some zero-energy photon already exist? I think not.
So in creating a photon we added energy to its quantum field.
See above. It's a non-answer I'm afraid. A "lie to children". In creating a photon we liberated some energy from matter. We changed the form of that energy.
If energy is not even a photon, so how can we view energy?
As the fundamental thing that can be neither created nor destroyed, from which other things are made. We can create photons, and destroy them. Ditto for electrons and positrons and quarks. They aren't really fundamental. But they really are made of energy.
Is there any physical view to it?
Yes, there is actually. It's related to the way people talk about the energy-density of space. See the stress-energy-momentum tensor which "describes the density and flux of energy and momentum in spacetime":
Public domain image by Maschen, based on an image by Bamse see Wikipedia
Note the shear stress. That tells you space is modelled as something like a gin-clear ghostly elastic solid. Google on Einstein elastic and note the energy-pressure diagonal. Then consider this analogy: you have a large block of crystal-clear elastic jelly, with grid lines in it so you can see what's going on. You then insert a hypodermic needle into the centre of the block, and inject more jelly. This represent the concentration of energy that is a massive star, and it creates a pressure gradient in the surrounding jelly. Stress is directional pressure, the stress-energy-momentum tensor is modelling the way elastic space is affected by the the energy you added. But note that you added jelly to represent energy, and that the jelly is also representing space. Ergo at some deep fundamental level, the physical view is that energy and space are two aspects of the same thing.
