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Multiple forms, one goal.
"There is a game. You cannot win it. You cannot break even. And you cannot get out of it."
Allen Ginsberg, humorously summarizing the message of thermodynamics (each portion of the quote references one of the laws, ordered from zeroth to third)
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Energy. That which makes us move, do and exist. The concept of energy is one of the most unifying in daily life. The universe is nothing without it (literally, because matter itself is also a form of energy). And as with many other things in real life, it can bring prosperity and success in the right hands, but also misery and destruction in the wrong ones. For millennia, energy has been respected, feared and exploited by mankind; and for almost as long, we have tried to explain its mechanisms and nature in many different ways, initially from the point of view of superstition, but over time with a more scientific lens. Our understanding on energy became one of the key motivations for the foundation of science as a whole.

Here at TV Tropes, we're familiar with the concept of energy due to how we see it act in fiction. Its portrayal has ranged from realistic to fantastical, with a prolonged spectrum of accuracy in-between. Most of the time, it's easy to tell whether or not an artistic license is at play. A character who is running for a marathon, a team of characters pushing a heavy object, and a Badass Biker performing tricks through a half-pipe are actions we deem realistic and plausible. A superhero using heat vision to melt ice, a monster levitating, and a Magical Girl using a special power to defeat said monster are clearly fantastical actions. However, it's not always readily apparent when an event of situation resonates with reality, at least not until we study how energy really works (never better said) in school, especially in physics classes.

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In this article, we'll start by defining energy and its forms, then the science behind it, and then we'll examine some of the most closely-linked tropes in popular culture; this way, we can tell whether the works using it are being grounded in reality or taking artistic liberties.


So what IS energy?

In layman's terms, energy is a property or attribute present in bodies and material systems, and in virtue of it they can employ an action or modify their initial condition or state, whether in relation to themselves or to act onto other bodies and systems. It is one of the most fundamental attributes of the universe, alongside matter, space and time; and there's indeed a relationship between all four of them.

When we warm a body, we're imbuing thermal energy onto it. Because atoms and molecules are capable of receiving and giving away energy, they'll increase their activity by moving faster when they receive the incoming attribute, resulting in a higher amount of internal energy. And when we put that hot body next to a cold one, the former will gradually share part of its energy with the latter, making it so the two will eventually reach a state of thermal equilibrium. The energy transferred between the bodies is known as heat. It is not to be confused with temperature, which gauges the thermal state of a body or system, and it's that which a body reaches by receiving enough heat. Remember when we mentioned that the energy transfer stops when the two bodies reach a thermal equilibrium? That state is achieved when the bodies reach a common value of temperature, not energy. This distinction is important, because not all bodies reach the same temperature by giving or taking the same amount of heat.

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As we know, energy can manifest in multiple ways, thermal being only one of them. If an object is moving, it has mechanical energy; if two substances are reacting to form a compound, they employ chemical energy; a TV or computer is receiving electrical energy to be turned on; the Sun is providing luminous energy to all planets in its system; if there's any mechanical perturbation within an object or two bodies clash, sonic energy is expelled, which is what our ears receive; all radioactive matter frees nuclear energy due to the perpetual decay of its atomic structure; and so on. But most importantly, none of these manifestations of energy exist independently from each other, because it's possible to convert one form of energy into another. For example, the Sun is converting the nuclear energy of its body into thermal, and the extremely high temperatures turn part of it into light; then, that light reaches the Earth's plants, which convert it into chemical energy via photosynthesis to grow and provide breathable air to the world. Energy not only changes with what we do consciously, but also with what our own bodies do inside, which is why drinking water and eating food is essential — they're the fuel that helps all biological processes keep going.

Before we move onto the next section, here's an important remark: While sometimes the word work is used as a synonym for energy (and the units of measurement is the same for both), they're not the same thing. Work is the application of energy to perform an action, and the efficiency of using it over time is known as power (and it's measured in watts).


The science behind energy:

The study of energy is one of the main goals of physics, though it's also important in many other fields like chemistry, biology, geology and astronomy. As mentioned before, energy can manifest in a variety of forms; however, the same physical laws behind it apply regardless. The laws in question are known as the laws of thermodynamics.

Thermodynamics, usually defined as the field of physics that studies heat and temperature, has a very significant influence in all of nature, not just thermal phenomena. Over the centuries, humans aimed to develop new technologies over time, so the need to increase efficiency of production and reduction of costs arose (the earliest application was with steam engines in the 18th century, coinciding with the Industrial Revolution). To this end, scientists and engineers began to study ways to use energy in their favor to satisfy society's needs. These studies led to the realization that, in order to meet the aforementioned goal, they had to cope with certain limitations.

  • Zeroth law of thermodynamics: "If two systems are in thermal equilibrium with a third one, then those two systems are also in equilibrium with each other". This seemingly-trivial statement is vital, because the universe is full of systems that interact with each other in some capacity; it is impossible to have a system that is fully isolated from all others, so it will always try to achieve equilibrium with them. In the case of a solid body, it will try to receive (if it's cold) or share (if it's hot) energy by way of conduction, like the example with the cold and hot bodies we described above. If we're talking about liquids or gases, the heat transfer will occur via convection; it is different from conduction in that the liquid or gas itself will move mechanically due to the heat, since its atoms and molecules will move more freely in those states than they would in a solid. Lastly, if we have two bodies that are either separated by vacuum or a medium with poor or scarce conduction properties, then the energy will be transmitted via radiation. The systems will only reach true equlibrium when all of them are at the same temperature, and one or more of these transfer mechanisms will be at play for that purpose.note 
  • First law of thermodynamics: "The total sum of energy in a system or set of connected systems remains the same". This is the basis of the famous "energy cannot be created or destroyed, only converted" statement. If an object slides through a floor and there's friction between the two, part of the object's mechanical energy will be lost due to the friction converting it into thermal energy. But this conversion makes it so the sum between the remaining mechanical energy and the emerging thermal one is always the same, so there's nothing that is being created or destroyed. This also holds true for all processes where you're acquiring a form of energy from another, like electricity from eolic or hydraulic energy. So why should we worry about saving up energy if it cannot be destroyed? Well, read the following law to find out...
  • Second law of thermodynamics: "In all natural processes, energy spontaneously aims towards the least energetic system or body". This law is the reason why heat goes from a hot body to a cold one, not only ensuring the eventual outcome of the zeroth law but also dictating the direction of all processes and phenomena that occur in nature. It is possible to perform a process that opposes the natural trend, but we need to apply energy from our own (or the machine we're using) for that to force it; think of how hard it is to scale a mountain yet how easy it is to slide downward (in the latter case, the kinetic energy is spontaneously taking us back down thanks to gravity, whereas going up requires us to employ potential energy to overcome said gravity). The quantitative measurement of this spontaneity is known as entropy, and as time passes it will gradually aim at a maximum value in the universe. Because of this trend, and due to how imperfect our universe is, energy cannot be totally converted into the form we deem most ideal; it's not because a part of it retains the old form, but because that part will fail to make the cut and instead be spit back as useless thermal energy (think of the food you eat: A part of it fails to nourish your body, so it's later expelled as poop). Save for heating devices like microwave ovens and furnaces (which give useful purposes to thermal energy for daily life), a machine expelling heat is effectively expelling debris, and that debris cannot be recycled unless a huge conversion effort is made (which itself requires using energy as well, so it's hardly worth it). And no heat engine can perform a conversion with 100% efficiency, so expect the aforementioned outcome to happen every time you turn something on in your house (touch it after some time, and you'll notice it's hot). And also expect to pay a steep electrical bill if you're using too much energy — the company that provides it to your home needs to also provide it to all households and energy isn't unlimited!
  • Third law of thermodynamics: "In the limit when the absolute temperature of a system approaches zero, all processes in it cease and its entropy approaches a minimum value". In other words, because temperature indicates the thermal state of a body or system, lower activity begets less heat and thus reducing that activity will also reduce the temperature (this is also a good opportunity to realize that cold, by itself, isn't something a body has, but something it lacks — it is merely the scarcity of heat, similar to how white isn't a color but the absence thereof); as a result, there will be less spontaneity and the variations of entropy will be lower. However, this law is also the reason why reaching absolute zero is impossible to begin with: Even in the coldest regions of the universe, the matter present is still having some internal energy, which means its atoms and molecules are still moving, however slowly (there's also the issue of matter itself being energy in rest form, as seen in Relativity). Many scientists favor using the Kelvin scale of temperature, because its frame of reference is a value of absolute temperature that cannot be reached (absolute zero), but any other value can and thus it's less arbitrary (the Celsius and Fahrenheit scales base their frames of reference on the values of temperature when water freezes and when it boils respectively).

Building upon these laws, it's possible to understand why some energetic processes happen in certain ways and why others cannot happen on their own unless we intervene. When describing the laws, we used the behavior of thermal energy as a recurring example, but it applies to all other forms. Electricity, just to name one, always goes from a negatively charged body to a positively charged one, never the other way around; electrons have negative charge, so if a body that has a surplus of electrons approaches or touches one with a deficit or is at least electrically neutral, the surplus electrons will be transferred to the latter body (the elementary particles that are charged positively, the protons, always stay in the atomic nuclei unless these are unstable and thus the body is radioactive). A moving body can clash against a resting one and make it move by transferring some of its energy into it (how the resting body would instead give energy to the moving one is anyone's guess). In chemistry, the most electronegative elements are guaranteed to react with the most electropositive ones to form salts, which are neutral as a result. And so on.

And in all those cases, the processes that happen will always try to reach a common state that applies to all involved systems, thus reaching equilibrium. With very few exceptions (which, even then, have to be assisted by a voluntary external intervention), all processes are irreversible and obey to what we know as the "arrow of time". All actions, due to their spontaneity, will aim at a state of disorder or randomness, leading to an increase of entropy. Even the things we do to put something into a state of higher order (cleaning a bedroom and putting everything where it has to be, for example) require an input or investment of energy on our part, and our actions imply an increase of entropy that adds up to the latter's total amount in the universe.

Because energy degrades when it's used, making a rational use of it allows companies to optimize its availability to satisfy society's needs and reduce the costs whenever posible. In highly industrialized parts of the world, consumption of energy is high and has accelerated over the decades; because a large percentage of applied energy originates from limited natural resources like natural gas, oil and nuclear matter, it has been a topic of concern between politicians, economists and ecologists, and thus many countries have aimed to at least partially balance things out by acquiring electricity by conversion from light (solar panels), wind (eolic turbines) and water (hydroeletric plants), on the heels of the realization that the traditional sources will deplete completely one day.


Energy and matter:

There is one final thing we need to discuss before applying our knowledge on energy to the purview of TV Tropes (fiction and popular culture), for it will be useful too: How energy relates to matter. As explained in Relativity, the two entities are closely linked to each other, in the same way space and time are. During many centuries, it was known that energy served as the metaphorical driving force for matter (from a literal standpoint, applying mechanical energy onto an object already implies applying force to it to move it through a distance, so you can calculate work by multiplying those two quantities), but it was also believed that both were still fully separate concepts. This was proven false with the mass-energy equivalence: Because the mechanical properties of a body are affected by how fast it's moving, the value of its momentum is affected considerably by how time and length vary when the body is moving at a speed that is comparable to light's. As a fun fact, the famous formula E=mc2 is actually a simplified form of the broader equation E=p2c4+mc2, in which p (the body's momentum) equals zero when it's resting in its referential position. Because moving at a speed close to that of the light's speed requires a huge input of energy (thus applying a prohibitively high amount of work), the relativistic values for momentum and energy are unreachable to most macroscopic objects, and the speed at which things and living being move is so mundane that the first term of the broader equation is easily negligible, leaving only the simplified form we're more familiar with. Thus, the frame of internal energy for a particle is largely defined by its mass, and the constant that ties both quantities is the squared form of the speed of light.

For atoms and elementary particles, approaching the speed of light is less difficult, because their masses are so extremely small that the momentum required to make them move at relativistic speeds is much smaller, and thus can be applied when we expose those small particles into powerful electromagnetic fields.note  The momentum gained by the particle is strong enough to overcome the nuclear force of atoms, allowing it to break the bond between protons and neutrons inside the nuclei. Einstein's mass-energy relation predicts that part of the mass in the system is freed as radiant energy.note  While this appears to contradict the first law of thermodynamics (since one would then claim that energy can be created from matter), Einstein's formula ensures that the overarching total amount of mass-energy in the universe remains constant, so you can think of that equivalence as the unified version of the laws of conservation of mass and energy. If anything, the first law of thermodynamics gains an even greater validity thanks to this relation.

The reason why it's the speed of light, and no other constant or quantity, which marks the connection between mass and energy is because of Einstein's postulates in special relativity. If the speed of light is intended to remain the same for all referential frames (including both moving and resting bodies), then it's everything else that needs to change so the relationship between the pairs of variables (space and time on one hand, energy and matter on the other) remained consistent, which led to them varying. This realization wasn't noted before the dawn of the 20th century because most physicists believed that there was a hidden, indiscernible medium that made possible the travel of light across vacuum and media called aether (analogous to sound and mechanical waves needing a medium to be transmitted), meaning that light's speed would somehow change even between two referential frames in the vacuum (the speed does lower when light travels across a physical medium like air, but that's because matter is obstructing its motion, hence the use of the index of refraction to measure the ratio between the speed in the vacuum and the speed in the medium). This belief led to the failure of detecting aether through the famous Michelson-Morley experiment, and motivated physicist Hendrik Lorentz to devise a mathematical model that suggested that electromagnetic waves like light moved independently from their referetial frames. Einstein, seeing the failure of that experiment as a Necessary Fail to solve the mystery, made his aforementioned postulates because they complied with Lorentz's transformation model and allowed him to extrapolate them to all physical phenomena.

Another reason why mass and energy are tied to the speed of light is because, as revealed in Quantum Physics, light itself is a quantified form of energy. Whereas in the macroscopic scale it behaves like a wave, and that behavior can easily explain properties like reflection, refraction, diffraction and (most importantly) polarization, in the scale of atoms and molecules we see that it travels as a torrent of small particles of concentrated energy called photons. And being particles, they move with momentum like material particles do. Classically, momentum would require a value of mass to be calculated, but photons don't have mass because they aren't matter, so their momentum necessarily has to be based on the pure energy they're made of. Notice how resting photons don't exist because they always move, so the surviving term in the generalized form of the mass-energy equivalence is the one governed by momentum, which is always nonzero (meanwhile, the mc2 term is zero due to no mass). It was, once again, Einstein who realized this (the model of quantified energy came beforehand from fellow German physicist Max Planck, but he only assumed it was valid for thermal radiation when he was explaining how heat was absorbed by black bodies), and it was how he successfully explained the mechanism of the photoelectric effect, yielding him the Nobel Prize in Physics (relativity wasn't embraced by the scientific community until much later).


Fact versus fiction:

Now that we have reviewed the subject of energy and how it works in Real Life, let's examine some of the most common portrayals of it in popular culture so we can clarify any myths or misconceptions surrounding them.


  1. Traveling at light speed in outer space: Impossible, unfortunately. As explained before, moving an object requires applying work onto it, and moving it faster will require applying a higher amount of work; in theory, making an object move at light speed would require an infinite amount of work, which for a rocket cannot be achieved even with all the fuel in the universe. This is because you'd need the rocket to achieve an unlimited amount of momentum, and remember that a physical object can only have momentum if it has mass. A vehicle expels the mass of its fuel while it moves, and once a rocket depletes all its fuel it will only have its own mass left. It is more realistic, but still technologically unreachable in our current era, to build a rocket or spacecraft that travels at a speed that is reasonably close to that of light, like 50% to 80%.
  2. Having your own electricity at home by installing an eolic turbine, a waterwheel next to a river, or a solar device: This is probably the most common portrayal in works with a Down on the Farm or Tomorrowland setting. Now, make no mistake; converting natural sources of energy into electrical is a healthy contribution to helping the environment, but it's not a magical solution to the energy crisis (otherwise, we'd have discarded nuclear energy, gas and oil a long time ago!). Remember the second law of thermodynamics: There's no process, whether natural or man-made, that is capable of performing an energy conversion with 100% efficiency. Even if a river is flowing rapidly, if the skies are clear enough for sunlight to land onto the solar devices at all times, or if wind is blowing so rapidly that the eolic turbine will spin like crazy, only a part of that energy will be successfully converted into electrical. For a solar device, the photoelectric effect will at best have a conversion rate of 15% to 20%; eolic turbines fare a bit better with a 20-40% range, while a waterwheel can go as high as 85% but it's not saying much if it's working with a normal river rather than Roaring Rapids. The rest of the energy will either pass by the machines or be converted into heat. This is why cities and countries that rely on water-based sources to provide energy to households use enormous hydroelectric power plants and why they're built onto lakes: They're powerful enough to perform a conversion rate of 90%, which is indeed a lot but is also much more costly and requires an obsessive care of the operating setups.
  3. Having a Bulletproof Vest for a guaranteed protection against bullets: Not fully guaranteed, and in fact the subject on how bulletproof vests work is a lot more complicated, as noted in the description of that very page as well as its real life section. A bullet, despite being small in size and having a low mass, is shot with such a tremendous kinetic energy that the vest it lands onto will receive a lot of material damage by trying to absorb the impact to nullify the bullet's momentum. And at least part of that impact will make it into the person's flesh. This is why police officers are still taught not to act recklessly when facing armed criminals: Being well-equiped with a vest only reduces the probability of suffering a fatal wound.
  4. Not being burnt by simply staying away from lava, a large source of fire or a very hot object: Ahhh, the Convection, Schmonvection trope, long time no see. You may already know that being close to a hot place or object will make you sweat or even get blisters. What few know, however, is that being moderately far from that hot place or object is still dangerous, especially if we're talking about lava or a house on fire. To understand why, let's remember the zeroth law of thermodynamics: The average temperature of lava is 2200 Fahrenheit degrees, or 1204.4 Celsius, whereas air is usually as warm as 68 Fahrenheit or 20 degrees. The two systems (lava and air) will attempt to reach thermal equilibrium by the former sharing its deadly heat with the latter, and the transfer will occur via convection. Therefore, even if you are 10 kilometers (six miles) away from an erupting volcano, you'll be vaporized. In the case of a house on fire, it's not so extreme, but for a human body it can still be lethal. Depending on how big the house is, the temperature can range between 600 and 1100 Fahrenheit degrees, so the surroundings will be still very hot. If your house is in front of another that is burning, make sure to soak the outer walls with as much water as possible, as doing this will mitigate the Heat Wave that comes from the burning house. and you may still be ordered to evacuate by firemen for the sake of safety.
  5. Nuclear energy being the most common source of radiation disease: The answer is no, surprising as it may seem. Due to the catastrophic consequences of World War II, nuclear energy got a very bad reputation in the minds of modern-day society, and this has been regrettably reflected in fiction countless times. While it's true that nuclear energy can be dangerous even when it's not weaponized, it has also contributed to the development of powerful methods in medicine to save lives, and the majority of those methods are painless. Furthermore, even if you're feeling cynical towards nuclear technology, know that it's not the only source of radiation that can kill you. Remember that radiation is any emission of energy (we used thermal as an example when describing the zeroth law, but it also refers to any radiation from the electromagnetic spectrum) that is independent from any transfer-assisting medium. You may be less likely to die from gamma radiation if you wear proper equipment when approaching a radioactive source (or, well, if you simply stay away from it), but you still have to be mindful about the ultraviolet radiation transported by everyday sunlight. It's not merely about avoiding sunburn due to the heat, kids: The less obvious UV radiation, while present in small quantities thanks to the bigger proportion being damped by the Earth's ozone layer, is still capable of endangering a person's life over the years, as it is accumulated over time. Up to 60000 people worldwide die every year because of this, whereas a safely-maintained nuclear power plant is much less likely to kill that many people.

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