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“In physics today, we have no knowledge of what energy is.” — Richard Feynman1
These words come not from an opponent of science, but from Richard Feynman, one of the greatest physicists of the twentieth century. It does not appear in a philosophical essay: it comes straight from a course intended for first- and second-year students.2
What he actually says
Feynman goes on: we do not have a picture that energy comes in little blobs of a definite amount. It is not that way.3
What we have are mathematical formulas:
- One for the energy of motion (kinetic energy)
- One for the energy of position (potential energy, like a stone at the top of a cliff)
- One for a stretched spring
- One for heat, electricity, chemical reactions
- One for radiation (light and its invisible equivalents)
- One for the nucleus of atoms, and even one for mass itself (E = mc²)4
We calculate each of these forms, we add them up, and the total never changes.5
Feynman offers a striking image. A child has twenty-eight blocks that are absolutely indestructible. Every evening, his mother counts them. One day, she finds only twenty-seven — and discovers the last one under the rug. Another day, she finds thirty: a friend had left two behind.6
Then come the hiding places that cannot be opened. A closed box: the mother cannot look inside, but she knows the weight of one block and that of the box, so she devises a calculation. A bathtub of dirty water in which nothing can be seen: she measures the level, and devises another calculation.7
With each new hiding place, a new term is added to her formula. The formula becomes complicated. But the total always comes to twenty-eight.8
Feynman then adds the decisive detail: there are no blocks.
In the real world, you have to take the first term out of the formula — the one that counts the blocks you can see. All that remains are calculations. We only ever have the terms of the formula, never the object.9
The law in its exact form
There is a universal rule to which science has never found an exception:
There is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes.10
Feynman stresses that this is an abstract idea: the law does not tell us the mechanism or the reasons for the various formulas. It only states that, when we add them all up, we get the same number.11
So it is not the conservation of a substance, but the fact that a number stays the same.
An important caveat, from Feynman himself: to verify this conservation, we must make sure that we have not put anything into the system or taken anything out — exactly as the mother had to take into account the friend who brought two blocks.1213
What this changes in everyday life
If energy is conserved and the total stays the same, two everyday expressions turn out to be scientifically inaccurate:
❌ Energy is not “produced”.
❌ Nor is it “consumed”.14
A power station makes nothing: it transforms one form of energy into another. A moving car destroys no energy: it converts it.
So what do we mean by “consumption”?
What we consume is usable energy. At each conversion, part of the total is degraded into diffuse heat (in the air, in structures, in the surrounding water). This is the phenomenon of energy degradation.15
We never run short of energy: it does not disappear, it spreads out.16
The outside air in winter contains a colossal amount of thermal energy, but it is too diffuse to heat a home directly without an external energy input.
In short: the traps of language
- “Producing energy” suggests that it is created out of nothing.
- “Consuming energy” suggests that it disappears.
These shortcuts are convenient in everyday life, but they mask how the world really works — just like speaking of “zero gravity” for astronauts in orbit or saying that the Sun “burns”.17
Note 1
It is important to realize that in physics today, we have no knowledge of what energy is.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 2
These are the lectures in physics that I gave last year and the year before to the freshman and sophomore classes at Caltech.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, “Feynman’s Preface”, 1963
Note 3
We do not have a picture that energy comes in little blobs of a definite amount. It is not that way.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 4
gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, mass energy
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Energy can be measured and quantified, and it is conserved, yet it does not have the same type of material reality as a building block. What, then, is energy? Coopersmith notes that most physicists are unable to provide a satisfactory answer to that question. […] She then proposes looking to the historical emergence of energy for a deeper understanding of the concept.
Crystal, Lisa, review of Coopersmith, Jennifer, Energy, the Subtle Concept: The Discovery of Feynman’s Blocks from Leibniz to Einstein, Oxford University Press, 2010, Physics Today, vol. 64, no. 4, April 2011, p. 61-62, doi:10.1063/1.3580497
Note 5
there are formulas for calculating some numerical quantity, and when we add it all together it gives "28"—always the same number
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 6
Imagine a child, perhaps “Dennis the Menace,” who has blocks which are absolutely indestructible, and cannot be divided into pieces. […] Let us suppose that he has 28 blocks. […] one day there are only 27 blocks, but a little investigating shows that there is one under the rug […] Another day, careful count indicates that there are 30 blocks! This causes considerable consternation, until it is realized that Bruce came to visit, bringing his blocks with him, and he left a few at Dennis’ house.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 7
Mother is not allowed to open the toy box. Being extremely curious, and somewhat ingenious, she invents a scheme! She knows that a block weighs three ounces, so she weighs the box at a time when she sees 28 blocks, and it weighs 16 ounces. The next time she wishes to check, she weighs the box again, subtracts sixteen ounces and divides by three.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
There then appear to be some new deviations, but careful study indicates that the dirty water in the bathtub is changing its level. The child is throwing blocks into the water, and she cannot see them because it is so dirty, but she can find out how many blocks are in the water by adding another term to her formula.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 8
no matter what he does with the blocks, there are always 28 remaining! […] In the gradual increase in the complexity of her world, she finds a whole series of terms representing ways of calculating how many blocks are in places where she is not allowed to look. As a result, she finds a complex formula, a quantity which has to be computed, which always stays the same in her situation.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 9
The most remarkable aspect that must be abstracted from this picture is that there are no blocks. Take away the first terms in (4.1) and (4.2) and we find ourselves calculating more or less abstract things.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 10
It states that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 11
It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 12
In order to verify the conservation of energy, we must be careful that we have not put any in or taken any out.
Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963
Note 13
In allen Fällen der Bewegung freier materieller Puncte unter dem Einfluss ihrer anziehenden und abstossenden Kräfte, deren Intensitäten nur von der Entfernung abhängig sind, ist der Verlust an Quantität der Spannkraft stets gleich dem Gewinn an lebendiger Kraft, und der Gewinn der ersteren dem Verlust der letzteren. Es ist also stets die Summe der vorhandenen lebendigen und Spannkräfte constant.
In all cases of the motion of free material points under the influence of their attractive and repulsive forces, whose intensities depend only on distance, the loss in quantity of tension force is always equal to the gain in living force, and the gain in the former equal to the loss in the latter. Thus the sum of the living forces and tension forces present is always constant.
Mayer, Julius Robert, “Bemerkungen über die Kräfte der unbelebten Natur”, Annalen der Chemie und Pharmacie, vol. 42, 1842, p. 233-240 ; et Helmholtz, Hermann von, Über die Erhaltung der Kraft, Berlin, 1847
I will therefore conclude by considering it as demonstrated by the experiments contained in this paper, — 1st. That the quantity of heat produced by the friction of bodies, whether solid or liquid, is always proportional to the quantity of force expended.
Joule, James Prescott, “On the Mechanical Equivalent of Heat”, Philosophical Transactions of the Royal Society, vol. 140, 1850, p. 61-82
Note 14
Wrong Track: There is less energy at the end than at the beginning. You can do less. Right Lines: Energy can only be shifted from one store to another.
Institute of Physics, Supporting Physics Teaching, module “Energy”
Note 15
In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy.
Clark, Mary Ann, Douglas, Matthew and Choi, Jung, Biology 2e, OpenStax, 2018, section 6.3 “The Laws of Thermodynamics”
Note 16
If you have chosen what to study carefully, the same quantity of energy is there at any time, it is just shared out differently amongst the stores. […] Perhaps the best approach in such cases is to emphasise, time and again, the spreading out of the energy amongst many small stores. […] Of course the collective name for all of these actions is dissipation.
Institute of Physics, Supporting Physics Teaching, module “Energy”
Note 17
A series of learning difficulties hamper the understanding of energy as a conserved quantity. For example, the aspect of conservation is not covered by the everyday use of the word ‘energy’, and the idea of energy conservation is formed very late in the course of children's development.
Duit, Reinders, “Understanding Energy as a Conserved Quantity—Remarks on the Article by R. U. Sexl”, European Journal of Science Education, vol. 3, no. 3, 1981, p. 291-301
Notes and sources
- ↑
Passage In physics today, we have no knowledge of what energy is.
It is important to realize that in physics today, we have no knowledge of what energy is.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage it comes straight from a course intended for first- and second-year students
These are the lectures in physics that I gave last year and the year before to the freshman and sophomore classes at Caltech.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, “Feynman’s Preface”, 1963, vol. I, “Feynman’s Preface”, first sentence.
Open access open the source · checked on 9 September 2026
- ↑
Passage we do not have a picture that energy comes in little blobs of a definite amount. It is not that way.
We do not have a picture that energy comes in little blobs of a definite amount. It is not that way.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage One for the energy of motion […] One for heat, electricity, chemical reactions […] and even one for mass itself
gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, mass energy
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
Passage the history of the notion and the debate about its abstraction
Energy can be measured and quantified, and it is conserved, yet it does not have the same type of material reality as a building block. What, then, is energy? Coopersmith notes that most physicists are unable to provide a satisfactory answer to that question. […] She then proposes looking to the historical emergence of energy for a deeper understanding of the concept.
Source Crystal, Lisa, review of Coopersmith, Jennifer, Energy, the Subtle Concept: The Discovery of Feynman’s Blocks from Leibniz to Einstein, Oxford University Press, 2010, Physics Today, vol. 64, no. 4, April 2011, p. 61-62, doi:10.1063/1.3580497, 2nd paragraph of the review.
Open access open the source · checked on 27 October 2026
- ↑
Passage We calculate each of these forms, we add them up, and the total never changes.
there are formulas for calculating some numerical quantity, and when we add it all together it gives "28"—always the same number
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage A child has twenty-eight blocks that are absolutely indestructible […] One day, she finds only twenty-seven — and discovers the last one under the rug. Another day, she finds thirty: a friend had left two behind.
Imagine a child, perhaps “Dennis the Menace,” who has blocks which are absolutely indestructible, and cannot be divided into pieces. […] Let us suppose that he has 28 blocks. […] one day there are only 27 blocks, but a little investigating shows that there is one under the rug […] Another day, careful count indicates that there are 30 blocks! This causes considerable consternation, until it is realized that Bruce came to visit, bringing his blocks with him, and he left a few at Dennis’ house.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage A closed box: the mother cannot look inside, but she knows the weight of one block and that of the box, so she devises a calculation.
Mother is not allowed to open the toy box. Being extremely curious, and somewhat ingenious, she invents a scheme! She knows that a block weighs three ounces, so she weighs the box at a time when she sees 28 blocks, and it weighs 16 ounces. The next time she wishes to check, she weighs the box again, subtracts sixteen ounces and divides by three.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
Passage A bathtub of dirty water in which nothing can be seen: she measures the level, and devises another calculation.
There then appear to be some new deviations, but careful study indicates that the dirty water in the bathtub is changing its level. The child is throwing blocks into the water, and she cannot see them because it is so dirty, but she can find out how many blocks are in the water by adding another term to her formula.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage With each new hiding place, a new term is added to her formula. The formula becomes complicated. But the total always comes to twenty-eight.
no matter what he does with the blocks, there are always 28 remaining! […] In the gradual increase in the complexity of her world, she finds a whole series of terms representing ways of calculating how many blocks are in places where she is not allowed to look. As a result, she finds a complex formula, a quantity which has to be computed, which always stays the same in her situation.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage there are no blocks […] take the first term out of the formula — the one that counts the blocks you can see. All that remains are calculations.
The most remarkable aspect that must be abstracted from this picture is that there are no blocks. Take away the first terms in (4.1) and (4.2) and we find ourselves calculating more or less abstract things.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage There is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes.
It states that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage the law does not tell us the mechanism or the reasons for the various formulas
It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
Passage to verify this conservation, we must make sure that we have not put anything into the system or taken anything out
In order to verify the conservation of energy, we must be careful that we have not put any in or taken any out.
Source Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, section 4-1.
Open access open the source · checked on 9 September 2026
- ↑
For this whole section
Passage two statements of conservation, in 1842 and 1847
In allen Fällen der Bewegung freier materieller Puncte unter dem Einfluss ihrer anziehenden und abstossenden Kräfte, deren Intensitäten nur von der Entfernung abhängig sind, ist der Verlust an Quantität der Spannkraft stets gleich dem Gewinn an lebendiger Kraft, und der Gewinn der ersteren dem Verlust der letzteren. Es ist also stets die Summe der vorhandenen lebendigen und Spannkräfte constant.
Editors’ translation In all cases of the motion of free material points under the influence of their attractive and repulsive forces, whose intensities depend only on distance, the loss in quantity of tension force is always equal to the gain in living force, and the gain in the former equal to the loss in the latter. Thus the sum of the living forces and tension forces present is always constant.
Source Mayer, Julius Robert, “Bemerkungen über die Kräfte der unbelebten Natur”, Annalen der Chemie und Pharmacie, vol. 42, 1842, p. 233-240 ; et Helmholtz, Hermann von, Über die Erhaltung der Kraft, Berlin, 1847, Helmholtz 1847, p. 17.
Public domain · checked on 9 September 2026
Passage the constant ratio between work and heat
I will therefore conclude by considering it as demonstrated by the experiments contained in this paper, — 1st. That the quantity of heat produced by the friction of bodies, whether solid or liquid, is always proportional to the quantity of force expended.
Source Joule, James Prescott, “On the Mechanical Equivalent of Heat”, Philosophical Transactions of the Royal Society, vol. 140, 1850, p. 61-82, final conclusion, p. 82.
Public domain · checked on 24 October 2026
- ↑
Passage Energy is not “produced”. […] Nor is it “consumed”.
Wrong Track: There is less energy at the end than at the beginning. You can do less. Right Lines: Energy can only be shifted from one store to another.
Source Institute of Physics, Supporting Physics Teaching, module “Energy”, IOPSpark (Supporting Physics Teaching), page “Energy can run out” (https://spark.iop.org/energy-can-run-out), subheading “Energy running out”.
Open access open the source · checked on 24 October 2026
- ↑
Passage At each conversion, part of the total is degraded into diffuse heat
In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy.
Source Clark, Mary Ann, Douglas, Matthew and Choi, Jung, Biology 2e, OpenStax, 2018, section 6.3 “The Laws of Thermodynamics”, section 6.3, subsection “The Second Law of Thermodynamics”, 1st paragraph.
Open access open the source · checked on 27 October 2026
- ↑
Passage We never run short of energy: it does not disappear, it spreads out.
If you have chosen what to study carefully, the same quantity of energy is there at any time, it is just shared out differently amongst the stores. […] Perhaps the best approach in such cases is to emphasise, time and again, the spreading out of the energy amongst many small stores. […] Of course the collective name for all of these actions is dissipation.
Source Institute of Physics, Supporting Physics Teaching, module “Energy”, IOPSpark (Supporting Physics Teaching), page “Energy can run out” (https://spark.iop.org/energy-can-run-out), subheadings “Energy running out” (“Right Lines”) then “Conservation of energy”.
Open access open the source · checked on 24 October 2026
- ↑
Passage These shortcuts are convenient in everyday life, but they mask how the world really works
A series of learning difficulties hamper the understanding of energy as a conserved quantity. For example, the aspect of conservation is not covered by the everyday use of the word ‘energy’, and the idea of energy conservation is formed very late in the course of children's development.
Source Duit, Reinders, “Understanding Energy as a Conserved Quantity—Remarks on the Article by R. U. Sexl”, European Journal of Science Education, vol. 3, no. 3, 1981, p. 291-301, abstract (“Summaries English”).
Paywalled · checked on 25 October 2026
Bibliography
11
- Atkins, Peter, The Laws of Thermodynamics: A Very Short Introduction, Oxford University Press, 2010, in libraries (energy degradation and the second law; see Entry 4.9).
- Clark, Mary Ann, Douglas, Matthew and Choi, Jung, Biology 2e, OpenStax, 2018, section 6.3 “The Laws of Thermodynamics”, open access (in every energy transfer, a share lost in an unusable form, most often heat). https://openstax.org/books/biology-2e/pages/6-3-the-laws-of-thermodynamics
- Coelho, Ricardo Lopes, “On the Concept of Energy: History and Philosophy for Science Teaching”, Procedia – Social and Behavioral Sciences, vol. 1, 2009, p. 2648-2652, open access.
- Coopersmith, Jennifer, Energy, the Subtle Concept, Oxford University Press, 2010, in libraries (the history of the notion and the debate about its abstraction).
- Crystal, Lisa, review of Coopersmith, Jennifer, Energy, the Subtle Concept: The Discovery of Feynman’s Blocks from Leibniz to Einstein, Oxford University Press, 2010, Physics Today, vol. 64, no. 4, April 2011, p. 61-62, doi:10.1063/1.3580497, open access (a book that looks to the history of the notion of energy for an answer to the question of its nature). https://physicstoday.aip.org/reviews/energy-the-subtle-concept-the-discovery-of-feynmans-blocks-from-leibniz-to-einstein
- Duit, Reinders, “Understanding Energy as a Conserved Quantity—Remarks on the Article by R. U. Sexl”, European Journal of Science Education, vol. 3, no. 3, 1981, p. 291-301, paywalled (the teaching of conservation and degradation).
- Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, chap. 4 “Conservation of Energy”, 1963, open access (our ignorance of what energy is; the twenty-eight blocks, the closed box, the dirty water, the terms that are added; “there are no blocks”; the list of forms of energy; the need to check that nothing goes in or out). https://www.feynmanlectures.caltech.edu/I_04.html
- Feynman, Richard P., Leighton, Robert B. and Sands, Matthew, The Feynman Lectures on Physics, vol. I, “Feynman’s Preface”, 1963, open access (the audience of the course: Caltech’s first- and second-year students). https://www.feynmanlectures.caltech.edu/I_91.html
- Institute of Physics, Supporting Physics Teaching, module “Energy”, open access (“produce” and “consume” as traps of language; usable energy). https://spark.iop.org/
- Joule, James Prescott, “On the Mechanical Equivalent of Heat”, Philosophical Transactions of the Royal Society, vol. 140, 1850, p. 61-82, public domain (the constant ratio between work and heat).
- Mayer, Julius Robert, “Bemerkungen über die Kräfte der unbelebten Natur”, Annalen der Chemie und Pharmacie, vol. 42, 1842, p. 233-240 ; et Helmholtz, Hermann von, Über die Erhaltung der Kraft, Berlin, 1847, public domain (two statements of conservation, in 1842 and 1847).
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