In a previous question I asked what environment on a rocky planet could evolve an energy lifeform. It was then the counter question was raised of what I meant by an energy lifeform. Could it only be an entity made of plasma? Is there another substance it could be made of that exists on the surface of a rocky planet?
All living beings are energy beings.
<cue bongo drums..
What is life? Life is the gathering and application of energy. Life is a striving against the force of entropy, the force of dissolution. To quote Rudolf Clausius
The general struggle for existence of animate beings is not a struggle for raw materials – these, for organisms, are air, water and soil, all abundantly available – nor for energy which exists in plenty in any body in the form of heat, but a struggle for [negative] entropy, which becomes available through the transition of energy from the hot sun to the cold earth. https://en.wikipedia.org/wiki/Entropy_and_life#Early_views
We are energy beings, housed in matter. Without the energy we gather, our matter would cease to strive. We store the energy within the matter of our beings, but energy used to oppose entropy is what makes us live. We are each of us a striving, and that is what makes us alive.
<yeah man. Willk is one deep cat...
Can there be energy without matter? Matter and energy are the same, these distinctions are artificial. For us, matter beasts, trapped in our matter, a photon is energy without mass; a massless particle. Light speed is the only speed for a particle of light. But a photon can interact with another photons if the mood is right.
Matter beasts, my matter beasts, can you grok a photon energy beast, opposing entropy thru the love of other photons? Maybe we could find them in the photon sphere of a neutron star where photons can circle endlessly. And the energy photon beasts strive against the entropy of gravitational death by dancing across the photons, that they might stay in their endless circle of living light.
Energy is any quantity - a number with the appropriate units (in the SI system, Joules) - that is conserved as the result of the fact that the laws of physics don't depend on the time when phenomena occur, i.e. as a consequence of the time-translational symmetry. This definition, linked to Emmy Noether's fundamental theorem, is the most universal among the accurate definitions of the concept of energy.
What is the "something"? One can say that it is a number with units, a dimensionful quantity. I can't tell you that energy is a potato or another material object because it is not (although, when stored in the gasoline or any "fixed" material, the amount of energy is proportional to the amount of the material). However, when I define something as a number, it is actually a much more accurate and rigorous definition than any definition that would include potatoes. Numbers are much more well-defined and rigorous than potatoes which is why all of physics is based on mathematics and not on cooking of potatoes.
Centuries ago, before people appreciated the fundamental role of maths in physics, they believed e.g. that the heat - a form of energy - was a material called the phlogiston. But, a long long time ago experiments were done to prove that such a picture was invalid. Einstein's E=mc2 partly revived the idea - energy is equivalent to mass - but even the mass in this formula has to be viewed as a number rather than something that is made out of pieces that can be "touched".
Energy has many forms - terms contributing to the total energy - that are more "concrete" than the concept of energy itself. But the very strength of the concept of energy is that it is universal and not concrete: one may convert energy from one form to another. This multiplicity of forms doesn't make the concept of energy ill-defined in any sense.
In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed.
Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.
So your energy beings would exist either within electric circuits or fields (electric energy), chemical bonds (chemical energy, spread throughout a gas in the form of the vibration of its molecules or in moving masses (kinetic energy) etc. They would be quite incorporeal and abstract. Such lifeforms would be so alien that we might not comprehend how they see the world.
And I keep thinking that since kinetic energy is proportional to speed, which is always a relative measure, the population of kinetic energy individuals would depend on the observer and their frame of reference!