Energy cannot be created or destroyed in any ordinary physical process. This principle, known as the conservation of energy, is one of the most rigorously tested ideas in all of science. But whether energy is truly “eternal” depends on what you mean by the word, and the answer gets surprisingly complicated once you zoom out from a chemistry lab to the scale of the entire universe.
Why Physics Says Energy Is Conserved
The first law of thermodynamics states that the total energy of a closed system stays constant. Energy changes form constantly (chemical energy becomes heat, motion becomes electricity, mass becomes radiation), but the total amount never increases or decreases. Every experiment ever conducted in a laboratory confirms this.
There’s a deep mathematical reason behind this. In 1918, mathematician Emmy Noether proved that whenever the laws of physics don’t change over time, energy must be conserved. It’s not just an observation. It’s a logical consequence of the universe’s symmetry. Energy is, by definition, the quantity that stays the same when the laws of physics remain constant from one moment to the next.
Mass and Energy Are the Same Thing
Einstein’s famous equation, E=mc², revealed that mass itself is a form of energy. When an object emits radiation, its mass decreases by a tiny but real amount. When it absorbs radiation, its mass increases. In a closed system where one object radiates energy and another absorbs it, the total mass-energy remains exactly the same. Nothing is gained or lost.
This means “energy” is broader than most people realize. The mass of every atom in your body is a form of stored energy. Nuclear reactions in the sun convert a small fraction of that mass into light and heat, but the total quantity of mass-energy in the system doesn’t change. It just shifts form.
How Living Things Use Energy
Your body is a constant energy-conversion machine. You eat food containing chemical energy stored in the bonds of sugars, fats, and amino acids. Your cells break those molecules apart and use the released energy to build ATP, the molecule that powers nearly every cellular process. Carbohydrate and fat metabolism alone account for over 90% of your body’s energy needs.
The process works like a relay. Nutrients get broken down into a molecule called acetyl-CoA, which enters a cycle of chemical reactions in your mitochondria. Electrons get passed along a chain of proteins, and the energy released at each step is captured to produce ATP. Oxygen is the final acceptor of those electrons, combining with hydrogen to form water. This is why you breathe.
When an organism dies, this conversion stops, but the energy doesn’t vanish. It dissipates as heat, gets consumed by decomposers, and disperses into the environment. The atoms and energy that made up a living body return to the broader system. Nothing disappears.
Where Conservation Gets Complicated
On the scale of the cosmos, the clean rule of energy conservation starts to blur. General relativity, Einstein’s theory of gravity, describes a universe where space itself can stretch and curve. In flat, unchanging spacetime, energy conservation holds perfectly. But in the curved, expanding spacetime of our actual universe, the picture is less straightforward.
The most striking example is cosmological redshift. Light traveling through expanding space gets stretched to longer wavelengths, losing energy in the process. The cosmic background radiation left over from the early universe has been redshifting for over 13 billion years. Each photon carries less energy now than it once did. Where does that energy go? Physicists disagree. Some argue it converts into gravitational energy. Others say it’s simply lost, a consequence of the fact that energy conservation in its global form doesn’t strictly apply in an expanding universe.
The key distinction is this: at any tiny, local patch of spacetime, energy is always conserved. The differential (infinitesimal) version of the conservation law works perfectly in general relativity. But when you try to add up all the energy across a large, curved region of space, the math breaks down. There’s no single, agreed-upon way to define “total energy of the universe.”
Dark Energy Makes It Stranger
About 70% of the universe’s energy content comes from dark energy, a mysterious force driving the accelerating expansion of space. If dark energy behaves as a cosmological constant (the simplest model), its density stays the same per unit volume even as space expands. That means as the universe grows, the total amount of dark energy increases, seemingly from nowhere.
Recent results from the Dark Energy Spectroscopic Instrument (DESI), based on the largest 3D map of the universe ever created, spanning nearly 15 million galaxies and quasars, hint that dark energy’s influence may actually be weakening over time. This hasn’t reached the threshold physicists require to call it a discovery, but multiple combinations of data show statistical significance between 2.8 and 4.2 sigma. If dark energy evolves rather than staying constant, it would reshape our understanding of how the universe’s energy budget works.
The Vacuum Is Never Truly Empty
Even perfectly empty space contains energy. Quantum mechanics requires that every field has a minimum energy level, called zero-point energy. This isn’t speculative. It’s been measured experimentally since the 1920s and affects everything from molecular bonds to the behavior of materials at very low temperatures.
Some popular claims suggest this vacuum energy represents an infinite reservoir humanity could tap into. The reality is far more modest. While zero-point energy is real and can be manipulated in laboratory settings, the total vacuum energy of the observable universe appears to be extremely small. Many physicists suspect it may be exactly zero. It’s a permanent feature of quantum fields, but not a free lunch.
Energy Lasts Forever, but Becomes Useless
Even if energy is conserved in the everyday sense, the second law of thermodynamics imposes a separate limit. Energy always tends to spread out and become less concentrated. Every time energy changes form, some of it becomes low-grade heat that’s harder to use for anything productive. This is entropy, and it only increases over time in a closed system.
Taken to its logical endpoint, the second law predicts a “heat death” of the universe: a state trillions of years in the future where everything reaches the same temperature. At that point, energy still exists, but it’s spread so uniformly that no work can be done. No stars can form, no machines can run, no life can survive. All the energy in the universe becomes nothing more than the random jiggling of particles.
So in the most practical sense, energy is eternal in quantity but not in quality. It persists, transforms, and disperses, but it never vanishes. What fades is not the energy itself, but our ability to do anything with it.