Energy is a complex topic. There are many forms of it, and many people have trouble fully understanding the concept. For people who are just starting out, there are many unanswered questions. For example, does energy have weight?
Kind of. Every form of energy has a mass (and light is no exception). It follows that energy has mass, but not the kind you can measure with a standard kitchen scale. Energy does not have weight in the form of resting mass, but it does have momentum.
By the time you’re done reading this article on the physics of energy, you’ll be able to impress over half the people you associate with, so let’s get on with it!
Does Energy Have Weight?
Through the special theory of relativity, we learn that E=mc2. By putting extra energy into “creating” a rest mass, particles without rest-mass (such as photons) can transform into particles with rest-mass (such as electrons).
For more information about this theory, watch this video below:
To interact with a gravitational field is to have “weight,” or mass. Is it possible for particles with zero rest mass to have gravitational interactions?
In the case of gravitational lenses, where light’s course is altered by the tremendous gravitational force of an object (such as a planet), the answer is “yes.” A black hole is a more extreme case because it is so enormous that light cannot travel away from its gravitational pull.
Yet, you must use caution. What we commonly refer to as “weight” is actually the steady rest mass. So, no, not in that way. Kicking a football, for instance, doesn’t alter its rest mass just because you’re putting energy into it.
Does Light Have Weight?
The simple answer is “no,” but it’s a conditional “no” since the question can be read in different ways that allow for the “yes” option to be justified.
As light is made up of photons, we might wonder whether they have mass. As the photon lacks mass, the answer is unquestionably “no.” Theory predicts that it will have energy and momentum but no mass, and experiments support this prediction.
Light has always been understood to have momentum and to exert pressure on a surface, long before its particle nature as photons was discovered. Momentum can persist even in the absence of mass; therefore, this is not proof that anything has mass.
A photon possesses energy E = hf, in which h is Planck’s constant and f is the frequency of the photon, which has led several people to argue that perhaps the photon has mass.
They argue that energy equals mass because of Einstein’s famous theory, E = mc2.
Photons, they argue, have momentum, and the equation p = mv links mass and velocity. They’re referring to something called “relativistic mass,” an old idea that can lead to misunderstandings. A particle’s energy E, as measured by its relativistic mass, varies with its speed.
Semantically speaking, it is incorrect to argue that the photon has mass in this sense because relativistic mass is not typically referred to as the mass of a particle in modern physics.
If you want to be technical, you could argue that the photon possesses relativistic mass. A photon has zero rest mass, which is the current definition of mass.
While energy can be thought of as identical to relativistic mass, the phrase “relativistic mass” is rarely used today. In today’s scientific understanding, “mass” refers only to the portion of a body’s total energy that is not in the form of kinetic motion. In contrast to energy, mass does not depend on speed.
Can Energy Lose Mass?
Mass is not destroyed but rather converted into energy.
There is a net loss of mass due to the conversion of the object’s rest mass (m) into energy. If the object absorbs more energy, it will convert some of that into mass, increasing its overall mass.
Surely, you must have observed this as you were sitting up on the Ferris wheel. The sensation of weightiness is due to your body’s collecting potential energy when the wheel accelerates above the ground from the bottom, which is an extra source of energy—the surplus mass results from the conversion of potential energy into mass.
Accelerating downward, however, converts this potential energy into kinetic energy, and since this acceleration is in the direction of gravitational force, the body has a sensation of weightlessness.
Upon falling back to Earth, some of the mass is transformed into energy, resulting in a relative lightening of the weight.
Which Form of Energy Has Mass?
Energy is considered to have mass when it’s put to use for something, and the matter’s mass and momentum are both maintained.
The energy will be transformed into another form of energy; for instance, mechanical energy will be transformed into kinetic energy and afterwards thermal energy as a result of abrasion and friction; finally, the energized particle with mass will radiate the kinetic and thermal energy in order to maintain the total amount of energy.
Sound is just another form of energy that, as it goes through a medium, causes vibrations in the molecules there. There is mass associated with the vibrating molecule that transmits sound waves.
Does Energy Take Up Space?
In the grand scheme of things, mass is no different from any other kind of energy. Mass, by design, occupies space.
In Einstein’s famous equation, E=mc2, one joule (energy) equals one kilogram (mass) multiplied by 299 792 458 meters per second (the speed of light). It’s possible that energy does occupy space, but because of the vast disparity between mass and other types of energy, we don’t identify it as such.
Light has energetic properties. Although the particles that make up light, known as photons, do not have any mass, does this entail that they do not occupy any space? It’s hard to picture them taking up space because they have zero dimensions (or one, if string theory is right), yet all that exists in the universe is composed of zero-dimensional particles, and they do.
A unique property can identify each type of energy; for example, sound is the vibration of molecules, electricity is the motion of electrons, and mass is the element that occupies space. So, the phrase “taking up space” simply refers to how much mass something has.
That said, only one type of energy—mass—occupies physical space.
To conclude, energy does not have weight in the conventional sense. Energy can add weight to an object. And the only type of energy that does have weight is mass, since those two are related via the theory of relativity.