Is Energy Created or Destroyed? The Law of Conservation

Energy is neither created nor destroyed. It changes form, moves from one place to another, and sometimes hides in ways that seem strange, but the total amount stays the same. This idea, known as the law of conservation of energy, is one of the most thoroughly tested principles in all of physics. It holds across scales from subatomic particles to galaxy clusters, and it has survived every serious experimental challenge for more than 170 years. Yet its implications are stranger and more far-reaching than the simple statement suggests.

How the Law Was Nailed Down

Before the mid-1800s, scientists had good reasons to suspect that energy was conserved, but no one had produced clean measurements to prove it. Heat, motion, electricity, and chemical change were treated as mostly separate phenomena. The decisive step came from James Prescott Joule, a brewer from Manchester, England, who spent years devising increasingly precise experiments to show that mechanical work and heat were interchangeable at a fixed rate. In his most famous setup, falling weights turned a paddle wheel inside an insulated container of water. The friction heated the water by an amount Joule could measure with a sensitive thermometer. He concluded that the quantity of heat produced by friction is always proportional to the mechanical force expended, and he pinned down a specific conversion factor: the energy from a 772-pound weight falling one foot would raise the temperature of one pound of water by one degree Fahrenheit.¹

That number, the mechanical equivalent of heat, was revolutionary not because of its precision but because it existed at all. It meant that energy wasn’t being created when a paddle wheel warmed water, and it wasn’t being destroyed when a falling weight stopped moving. It was being converted. Other researchers were circling the same conclusion independently, and within a couple of decades, conservation of energy was promoted from a useful hypothesis to a bedrock law of thermodynamics.

Why Energy Conservation Isn’t Just an Observation

You might wonder whether the law of conservation of energy is simply an empirical pattern, something we happen to see every time we check, or whether there is a deeper reason it holds. In 1918, mathematician Emmy Noether proved a set of theorems that revealed a profound connection between symmetries and conservation laws.² The relevant symmetry for energy conservation is time-translation symmetry: the laws of physics work the same way today as they did yesterday, and as they will tomorrow. If you set up an identical experiment next Tuesday, the same rules apply. Noether showed mathematically that this time symmetry necessarily implies a conserved quantity, and that quantity is energy.

This means conservation of energy is not just something we keep observing and hoping continues. It is a logical consequence of the uniformity of physical law across time. As long as the rules of the game do not change from moment to moment, energy must be conserved. This is a much stronger footing than simply saying “we’ve never caught it failing.” It also reveals the conditions under which the law could, in principle, break down: if the fundamental laws of physics changed over time, energy conservation would not hold. In the expanding universe, this actually gets complicated on cosmological scales, which we will return to later.

What About E = mc²?

Einstein’s famous equation is often misunderstood as saying that energy and mass are different things that can be swapped for each other, as though mass vanishes and energy appears from nowhere. That framing is misleading. Mass and energy are two aspects of a single conserved quantity. A better way to think about it: mass is a form of energy, specifically a very concentrated one. When a nuclear reactor splits uranium atoms, the fragments weigh slightly less than the original uranium. That tiny “mass defect” shows up as a large amount of kinetic energy and radiation. Nothing has been created or destroyed. Energy that was locked up in the nuclear binding structure has been released into other forms.

The mass changes in nuclear reactions are large enough to measure directly. In chemical reactions like burning wood or digesting food, the mass equivalent of the energy released is vanishingly small. One analysis notes that while mass-energy equivalence underpins the immense energy release in nuclear fission and fusion, where a measurable mass defect is directly converted into energy, the principle is frequently misapplied to common chemical reactions.³ The mass change from burning a log of firewood is real in theory but far too small to weigh. You would need extraordinary precision to detect it. For practical purposes, the mass stays the same, and the conservation law operates in its pre-Einstein form just fine.

The takeaway is that E = mc² expanded the conservation law rather than breaking it. The bookkeeping now includes mass as a line item in the energy ledger, and the total still balances.

How Your Body Obeys the Law

Living things can seem to defy energy conservation. Plants pull carbon dioxide and water from the environment and assemble complex molecules; animals move, grow, and think. Where is all that energy coming from, and where does it go? The answer is that biological systems are open systems: they take in energy from their surroundings (sunlight for plants, food for animals) and release it as heat and waste products. No energy is created. Every calorie of work your muscles do, every degree of body heat you radiate, traces back to chemical energy in the food you ate, which traces back to sunlight captured by a plant.

Recent research has even quantified how tightly biology is constrained by thermodynamics. A study of unicellular organisms across different metabolic types and domains of life found that the amount of energy dissipated per unit of biomass grown is largely conserved, regardless of whether the organism is a bacterium, an archaeon, or a eukaryote.⁴ In other words, the universe charges a remarkably consistent energy tax for building living matter. Aerobic respiration, the process animals use, shows a trade-off between how efficiently it uses energy and how fast it can drive growth. But the total budget always balances. Life is creative in how it shuffles energy around, but it never gets a free lunch.

The biochemistry inside cells follows the same thermodynamic balance equations that govern any chemical reaction network. The entropy, heat content, and free energy of an open biochemical system change over time in ways that can be tracked with standard thermodynamic bookkeeping.⁵ Cells are impressive chemical factories, but they operate firmly within the rules.

Does Empty Space Have Energy?

One of the stranger frontiers for energy conservation is the quantum vacuum. Even the emptiest patch of space is not truly empty. Quantum fields permeate all of space, and they fluctuate constantly, producing fleeting disturbances that physicists sometimes describe as “virtual particles.” These fluctuations are not just theoretical bookkeeping; they produce real, measurable effects. The most famous is the Casimir effect, in which two uncharged metal plates placed very close together in a vacuum experience a small but detectable force pushing them toward each other. This force arises from zero-point fluctuations in quantum fields.⁶

Does this mean the vacuum is creating energy from nothing? Not in a way that violates conservation. The energy associated with vacuum fluctuations is already part of the quantum fields’ ground state. The Casimir force does not generate energy; it reflects a difference in vacuum energy between the region inside the plates and the region outside. When the plates move closer, the system releases energy, but only at the cost of doing work against whatever was holding them apart. The books still balance.

There is an ongoing debate in physics about the nature and significance of these fluctuations. Some researchers argue that quantum fluctuations are the most fundamental feature of reality, not caused by the uncertainty principle but rather the other way around: the uncertainty principle arises as a consequence of the fluctuations themselves.⁷ This is an active area of investigation, not a settled question. But whatever the resolution, no experiment has shown vacuum energy violating conservation.

Perpetual Motion and Why It Keeps Failing

If energy is conserved, you might think you could build a machine that simply recycles its own energy forever, running without any fuel or external input. People have been trying for centuries. Patent offices around the world receive a steady stream of applications for perpetual motion machines, despite the fact that such devices are scientifically impossible.⁸ Most patent offices have explicit policies rejecting these applications outright.

The fundamental problem is not just conservation of energy. A machine that perfectly recycled its energy with zero losses would technically not violate the first law of thermodynamics: it would not create energy, it would just keep using the same energy forever. But the second law of thermodynamics rules this out too. Every real process converts some usable energy into waste heat that disperses into the environment. Friction in bearings, air resistance, electrical resistance in wires, the list of unavoidable losses goes on. You can minimize these losses, and modern engineers are spectacularly good at that, but you can never reach zero. A machine without any external energy input will always run down.

The designs that people submit are often ingenious. Overbalanced wheels, magnetic loops, buoyancy engines, and elaborate arrangements of levers have all been proposed. In every case, a careful analysis reveals hidden energy losses or a misunderstanding of the forces involved. A magnetic motor, for example, might seem to spin indefinitely on first glance, but the magnets do not provide an energy source. They produce forces, but the arrangement of those forces in a loop always results in a net zero energy gain per cycle. If a machine appears to run indefinitely, it has a hidden energy source, often a temperature gradient in the room, a battery, or the person demonstrating it giving the wheel a periodic nudge.

The Energy Cost of Erasing Information

One of the more surprising extensions of energy conservation is its connection to information. In 1961, physicist Rolf Landauer proposed that erasing a bit of information, flipping a memory register to a blank state, requires a minimum expenditure of energy, released as heat. This minimum is tiny: about 0.0000000000000000000003 joules per bit at room temperature. But it is not zero, and it is fundamental. The Landauer principle states that an irreversible change in stored information, such as merging two computational paths, dissipates a minimum amount of heat per bit to the surroundings.⁹

This matters because it links information processing to thermodynamics. Your computer, your phone, every digital device generates heat not just because of engineering imperfections but because computation itself has a thermodynamic floor. Research has confirmed that there is a fundamental lower bound on the energy cost of both measurement and information erasure.¹⁰ Current computers operate far above this floor, wasting vastly more energy per operation than the Landauer limit. But as computing technology advances and engineers push for ever-greater efficiency, this theoretical limit becomes more relevant. It represents a wall that no amount of clever engineering can break through without violating thermodynamics.

The Landauer principle also resolved a century-old thought experiment known as Maxwell’s demon. In this scenario, an imaginary creature sorts fast and slow molecules to create a temperature difference, seemingly violating the second law of thermodynamics. The resolution is that the demon must store information about each molecule and eventually erase that information, which costs energy. The demon’s bookkeeping generates enough heat to preserve the overall energy balance.

Black Holes and Extracting Energy from Gravity

Black holes might seem like objects that destroy energy by swallowing matter and light into a region from which nothing can escape. In reality, the situation is more nuanced. A spinning black hole stores an enormous amount of rotational energy, and physicists have shown, at least theoretically, that this energy can be extracted. Roger Penrose proposed a mechanism in which matter falling into the region just outside a spinning black hole’s event horizon can be split, with one piece falling in and the other escaping with more energy than it originally had. The black hole slows down in the process, losing rotational energy. Research into this mechanism notes that the rotational energy of a black hole should, in general, be comparable with its total mass-energy.¹¹

This is not free energy. The black hole pays for the extracted energy by losing angular momentum and, in a real sense, by losing mass. The energy budget stays balanced. Astrophysicists believe that variants of this mechanism power some of the most energetic phenomena in the universe, including certain types of relativistic jets that shoot out from the poles of supermassive black holes in the centers of galaxies. These jets can extend for thousands of light-years and carry staggering amounts of energy, all ultimately drawn from the black hole’s spin.

There is also the question of Hawking radiation, the theoretical prediction that black holes slowly emit particles and shrink over time. This process converts the black hole’s mass-energy into radiation. Over incomprehensibly long timescales, a black hole could evaporate entirely, leaving only radiation behind. The total energy in the radiation would equal the mass-energy the black hole started with. Even in this extreme scenario, conservation holds.

Where the Law Gets Tricky on Cosmic Scales

The one arena where energy conservation becomes genuinely murky is cosmology. The universe is expanding, and that expansion stretches the wavelength of light traveling through space. A photon emitted by a distant galaxy arrives at your telescope with less energy than it had when it left, because the space it traveled through expanded while it was in transit. Where did that energy go? There is no widely accepted accounting for it. Some physicists argue that the lost energy goes into the gravitational field of the expanding universe, but general relativity does not define a global, unambiguous total energy for the universe as a whole. The math that works so cleanly in a laboratory becomes slippery when applied to the cosmos.

This does not mean the law of conservation is wrong. It means that defining “total energy” for the entire universe is harder than it sounds. In any local region of space, energy conservation holds perfectly well. You can track every joule entering and leaving a laboratory, a star, or even a galaxy cluster, and the books balance. The difficulty arises only when you try to sum up the energy of the whole expanding universe at once. Some cosmologists take this as a sign that energy conservation is a local law, spectacularly reliable in every measurable context, but not straightforwardly applicable to the universe as a whole.

Dark energy adds another layer of confusion. The accelerating expansion of the universe appears to be driven by a constant energy density filling all of space. As space expands, there is more of it, so the total amount of dark energy increases. Yet the energy density per cubic meter stays the same. Whether this counts as energy being “created” depends on how you define the total, and there is no consensus among physicists on the right way to do the accounting at this scale. For any experiment you could actually run, in any lab or observatory, conservation of energy remains ironclad.

Common Misconceptions Worth Clearing Up

Several popular misunderstandings persist around energy conservation. One is that “energy can be created from nothing in quantum mechanics.” Virtual particles in vacuum fluctuations are sometimes described this way, but as discussed earlier, the energy involved is already part of the quantum vacuum’s ground state. No net energy appears. Another misconception is that nuclear reactions “turn mass into energy” as though mass disappears. Mass is a form of energy. When a nucleus splits, the total energy, including the mass-energy of the products plus the kinetic energy and radiation released, equals the original total. Nothing has been conjured or annihilated in the grand ledger.

A subtler confusion involves heat. People sometimes say that waste heat represents “destroyed” energy, energy that has become useless. It is true that dispersed heat is hard to harness for further work, and this is the domain of the second law of thermodynamics, not the first. The energy still exists. It has simply spread out into the environment in a form that is thermodynamically inconvenient. A cup of hot coffee cooling on your desk does not lose energy to nothingness; it warms the air, the table, and everything nearby by an imperceptibly tiny amount. The first law says the energy is all still there. The second law says you will have a very hard time gathering it back into your coffee.

The persistence of perpetual-motion claims and “free energy” devices in popular culture is perhaps the most practical consequence of these misconceptions. Devices marketed as producing more energy than they consume rely on measurement errors, hidden power sources, or outright fraud. If you encounter one, the most reliable filter is the question: where is the energy coming from? If the answer is vague or appeals to mysterious “quantum” or “zero-point” sources, the device is almost certainly not doing what its promoters say.