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The Power of Theory in Science

The views expressed are those of the author and are not necessarily those of Scientific American.

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"He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast."—Leonardo da Vinci

It’s often lonely, these days, as a theorist. As soon as most people hear the word theory, in fact, they start thinking about something like this:

 (Image credit: F. Steiger.)

But if you’re scientifically minded, you know just how powerful your theory is. Because your theory – if it’s any good – allows you to not only explain what you’ve already seen, but allows you to predict something new, which you can then go look for.

By the early 1800s, there were two theories about the nature of light. One of them, going back to Newton, is that light is a ray.

(Image credit: University of Iowa.)

And, of course, it is a ray. But there was another idea – going back even farther (to Christiaan Huygens) – that light might also be a wave. And this gained a lot of support in 1799, when Thomas Young first passed light through two thin, nearby slits.

 (Image credit: Matthew Parry-Hill and Michael Davidson.)

The pattern that comes out of this – the famed double-slit pattern, below – can only be explained if light were, in fact, a wave.

 (Image credit: Benjamin Crowell.)

So this was the leading theory in the 1800s: that light is a wave. So if you’re a good theorist, and you’re interested in studying light, what do you do?

Well, if you’re famed French mathematician and physicist Simeon Poisson, you would think of the most ridiculous configuration you could imagine in the hopes of disproving the light-is-a-wave theory. And that’s exactly what he did in 1818.

(Image credit: Auburn University.)

He imagined that you took a wave source of light, and had it shine on and around a completely black, spherical obstacle, setting up a screen behind it. Obviously, he reasoned, you would see some light on the screen indicating the outside of the sphere, and darkness, or a shadow, on the inside.

But, he calculated, if the wave theory of light were correct, you would get something completely absurd!

(Image credit: Robert Vanderbei.)

Sure, you’d get light on the outside, and shadow on the inside, but what’s that at the very center? Poisson predicted, using the wave theory of light, that you’d actually get a bright spot of light at the very center of this shadow! How absurd was that! And therefore, he reasoned, the wave theory of light was absolutely crazy, and had to be wrong.

Hard to argue with that, isn’t it?


(Image credit: Alexandre Sixdeniers, from a painting by Henry Scheffer.)

Meet Francois Arago, former Prime Minister of France (among many other things). Shortly after Poisson’s prediction, Arago decided to put the theory to the test, and actually performed the experiment to look for the "theoretically absurd" spot.

What happens if, in fact, you perform this experiment yourself?

(Image credit: Thomas Bauer at Wellesley.)

Amazingly, the spot is real! If your theory is any good, scientifically, this is exactly what it will do. It will not only explain what’s already been observed, it will allow you to apply it to new situations, and make testable predictions about what you can expect to find. The crazier the prediction, and the more successful the experiment, the more compelling the theory becomes.

And examples abound. In 1927, Georges Lemaitre predicted that the Universe would be expanding, based on his application of Einstein’s theory of General Relativity to the Universe. Einstein’s initial response was, "Your math is correct, but your physics is abominable."

(Image source unavailable.)

And yet, two years later, Edwin Hubble discovered, in fact, that the farther away a galaxy is from us, the faster it expands away from us. The only thing that was abominable was Einstein’s inability to recognize just how powerful his theory actually was.

In the 1960s, the Big Bang Theory was very much in doubt. But it had a prediction: that there would be a very low-temperature, uniform background of radiation in the Universe, permeating all of space and appearing in every direction.

(Image credit: U.S. National Park Service.)

But in 1964, exactly that was discovered, by Arno Penzias and Bob Wilson. The Big Bang has – quite justifiably – gone largely unchallenged ever since.

And, although they’re outside of my expertise, the same goes for evolution and natural selection in biology, as well as global warming in climate science. They’re theories, to be sure, just as sound and compelling as the ones I’ve described above.

So the next time someone tells you that the Big Bang, Evolution or Global Warming is just a theory, you’ll know what to do.

(Image source unavailable.)

You’ll tell them, yes, it is the best theory.

In science, that’s as close as we ever get to certainty and the truth. The better we understand and test it, the more compelling, valid, and powerful it gets. And when that happens, we can learn from it, find and identify if there’s a problem, and try to deal with it.

But only if you listen to the right theory.

About the Author: The views and opinions expressed here are the opinions of theoretical astrophysicist, physics professor at Lewis and Clark College, and writer of the 2010 Physics Blog of the year, Ethan Siegel. And yes, the beard is real. (Photo credit: Chad Shahan.)

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Metaxy 9:00 am 06/15/2011

    All I can say is that we need more of the same. The lack of understanding of science in general, and "theory" in particular, among a significant segment of the American people but, more critically, among an incredible number of American politicians is appalling and scary. To politicize science is not to understand science at all; however, not to recognize that scientists have political positions is equally shortsighted.

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  2. 2. brerlou 11:13 am 06/15/2011

    How can you write an article on the wave theory of light and NOT mention the wave-particle duality paradox and theory of light? I must assume that this is a severely truncated article, edited by a non-physicist. The one point to be gained from the wave-particle duality problem is that human perception and description is severely limited in trying to describe natural phenomena.

    This duality is actually a feature of the human mind and not a real feature of nature. Science theory is like the 6 blind men of Indostan trying to describe an elephant. "All were partly in the right, and all of them were wrong."

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  3. 3. tharriss 1:56 pm 06/15/2011

    Yes brerlou, but those blind men are probably closer to the true description of the elephant than the blind man that doesn’t touch the elephant, but just makes stuff up randomly and BELIEVES it is true.

    And given enough time and effort, and repetition of the experiment, the blind men could bit by bit build up a good description of the beast… by your judging their first impressions from the single experiement where each only got to touch part of the elephant, you are mis-representing the way science develops better and better information over time.

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  4. 4. aatish 2:58 pm 06/15/2011

    What an incredible whirlwind of spectacular predictions!

    I love your no-nonsense quirky writing style, seems to capture quite a bit of your personality. Your blog kicks ass.

    Link to this
  5. 5. Mong H Tan, PhD 3:07 pm 06/15/2011

    RE: No reductionist theories in biology — or in psychology — please!

    This is a most reductionist philosophical essay that I have had ever read in my intellectual life! — Brief for the Big Bang theory; but misleading for Life sciences! — And it is coming out of from the brain of a physics professor at Lewis and Clark College!?

    As the writer has duly and misleadingly concluded above that: "And, although they’re outside of my expertise, the same goes for evolution and natural selection in biology, as well as global warming in climate science. They’re theories, to be sure, just as sound and compelling as the ones I’ve described above.

    "So the next time someone tells you that the Big Bang, Evolution, or Global Warming is just a theory, you’ll know what to do.

    "You’ll tell them, yes, it is the best theory."

    I thought the writer should have had known better: Reductionism in physics and other physical sciences (including chemistry or even biochemistry) is fine, powerful, and predictable! Whereas reductionist theories of any materialism may not be directly applied to biology nor psychology or spiritualism, at all; as I strongly argued before here: "Too Hard for Science? The Genetic Foundations of Intelligence — RE: Absolutely not! — The scientific answer may lie in the "neocortical foundation" of our brain: whereby our "intelligence" is primarily accumulated and expressed by learning and communications — and not inherited or encoded in our "genes" as neo-Darwinists would have had persistently and reductively believed!?" (ScientificAmericanUSA; June 3).

    Best wishes, Mong 6/15/11usct2:07p; practical science-philosophy critic; author "Decoding Scientism" and "Consciousness & the Subconscious" (works in progress since July 2007), "Gods, Genes, Conscience" (iUniverse; 2006 — ) and "Gods, Genes, Conscience: Global Dialogues Now" (blogging avidly since 2006 — ).

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  6. 6. fortress 8:21 pm 06/15/2011

    Dear Mr. Mong H. Tan, PhD:

    Having been the producer and publisher of three best-selling books, as well as an author of two of my own, I think that I am qualified – as a writer and author – to weigh-in and critique the silver from the dross.

    Take a huge hint and learn how to write before you, kind sir, go on and give people your version of what a writer’s responsibilities are and are not. Certainly from my perspective Mr. Ethan Siegel’s word choice, meter, tone, rhythm, and above all his ability to know and understand his audience are impeccable.

    I take this position inasmuch as I do know how difficult the process is when attempting to explain science particulars with those who are not science-minded. Furthermore, as an essay or any type of composition, Mr. Siegel’s knowledge of language; moreover, his ability to make easy transitions – from topic area to additional and sequential topic areas – are in fact quite difficult.

    And finally…ease up! One should not throw stones when one lives in a glass house.

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  7. 7. scientific earthling 1:39 am 06/16/2011

    Language Please: Watch your words, the same word has several meanings plus look how it is derived.

    You will realise Theory in science is not the same as theory in loose English language.

    Read here for further info:

    you can also look up a dictionary.

    Link to this
  8. 8. zstansfi 2:51 am 06/16/2011

    I’m not a climate scientist so I really don’t know the answer to this, but is global warming actually considered a Theory? I mean isn’t this term rather vague? My understanding was that the correct term is "climate change" and that most of the public debate (i.e. in the wider society outside the climate science community) is over whether or not there is sufficient evidence that this is caused by human activity. It seems to me that just throwing down the term "global warming" and calling it a Theory without any qualification is a bit of a stretch. Am I wrong here?

    Link to this
  9. 9. morp 9:32 am 06/16/2011

    Professor Siegel tells exactly the opposite of what he writes.
    Science consists in the observation of the kosmos,including the earth,and in trying to find the physical laws governing the observed phenomena.
    A physical theory explains phenomena by logical conclusions based on observed phenomena and on already found phyical laws.
    Siegel’s conclusions are based on nothing firm.He is swimming in the air.As long as he has no wings he must keep his feet on the firm soil of logical thinking.

    Link to this
  10. 10. brerlou 12:53 pm 06/16/2011

    What you say is obviously true. (Why are you trying to rehash the science versus religion argument here. It is not relevant to this discussion.) I am simply making the argument upon which gestalt psychology is founded, namely that the human intellect tends towards the error of perceiving incomplete knowledge as a complete whole. It is the same argument that Immanuel Kant made when he explained the difference between phenomena and noumena. It is explanatory, not contentious.

    I related the foregoing to this article because I thought that this particular article failed as a piece of expository writing because it presented an observed phenomenon as though it was the whole truth. In this case I thought that this particular article FAILED to reconcile the particle theory of light (as photons) with the wave theory. I REALLY wanted to know if I was seeing the article in its entirety, that’s all.

    Link to this
  11. 11. gmperkins 12:49 am 06/18/2011

    I was really hoping for an article discussing how the word <theory> gets abused alot. Alot of "theories" are actually hypothesis. To be a theory a large percentage of scientists in the field must agree that the hypothesis is correct, and thus becomes a theory.

    For instance, within the article two scientists contend on the nature of light. Both ideas are hypothesis, not theories. People call hypothesis theories to make them sound correct when, in fact, it is just a good idea waiting to be verified or disproven.

    Now lets consider more complex ideas like Evolution, Big Bang and Global Warming. Currently, the Evolutionary Theory is the only one I consider to be a theory.

    I do agree that both the big bang and global warming are the best front runners for what they propose to explain. And they have a decent number of factual observations, like glaciers are definitely melting.

    The Big Bang Hypothesis lacks a general understanding of our universe and on a number of key points in phsyics (like how can light be both a particle and a wave? and other such ‘nitpicky’ points ;) ).

    Global Warming lacks a sound model of a very complex system (which is why it is easy to attack, unfortunately). More CO2 in the atmosphere means warming, that idea is a theory and the backbone of Global Warming. Unfortuantely, the Earth may compensate and what not because it is a complex system. In general, I am against pollution and I happen to like more Oxygen, being a oxygen breather and all that ;) . I also understand that its better to play it safe, their is a lot of history showing how poisons have affect our health and the health of our environment. Still, the idea is a hypothesis.

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  12. 12. rwstutler 1:04 am 06/18/2011

    Mong H Tan writes: "Reductionism in physics and other physical sciences (including chemistry or even biochemistry) is fine, powerful, and predictable! Whereas reductionist theories of any materialism may not be directly applied to biology nor psychology or spiritualism, at all …"

    reductionism is a philosophers term, not a word that has any real meaning to a scientist. Though
    I agree that reductionism is unlikely to be applicable to spiritualism …

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  13. 13. iWind 7:37 am 06/18/2011

    He can write an article "on the wave theory of light and not mention the wave-particle duality," because the article is not actually about the history of theories of light and neither is it about the nature of light.

    Personally I found the example used to be far more interesting and refreshing (and it also emphasized both the development and impermanence of theories). It is rare to hear about scientific debates predating the most recent ones leading to the current major theories.

    The wave-particle duality, having been one of the great issues for over a century now, is pretty worn as an example, and should be well known to all readers except perhaps the very youngest, but furthermore would not in itself do much for the subject at hand, particularly if compared with the complexities of explaining the necessary details. (Whether that duality is something "real" or the result of scientific terminology or human perception, is certainly completely outside the scope of the article.)

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  14. 14. Wilhelmus de Wilde 10:26 am 06/18/2011

    The year is 2111,

    We are aware of the fact that the LHC from a century ago was a great loss of time and effort just to prove a Theory that could never be tested, we now are aware of the "fact" that the Big bang was not at all occurring , This also was a theory that explained a solipsistic universe .

    Now that we are aware of the the "NON CAUSAL NON DETERMINISTIC DIMENSION" around us, where all the possible times and places of what we one hundred years ago called paralel Universes and multiverses are simultaneously PRESENT, this is now the most promising theory we are aware of it explains that the collapses of wave functions by our observations have created our universe, so that in fact we (our consciousness) are our own creators.

    We are now trying to achieve a higher state of intelligence and consciousness, however it still remains a theory because we might change each moment of reality, so we hope that the Theory of Every Reality the TER will be in reach for the next 1000 years to come.

    keep on thinking


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  15. 15. rwstutler 12:38 am 06/20/2011

    I do not think the words you are using mean the same to me as they appear to mean to you.

    The concensus of scientific thought is that the big bang did in fact happen, and that the details of the event and subsequent evolution of ‘existence’ which came from the bang are still areas worth further study and research. At least, that is the way it appears to me, here on planet earth.

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  16. 16. Wilhelmus de Wilde 11:42 am 06/27/2011

    My dear RW,(sorry for the late reaction)

    I do not like to follow any consenses; the freedom of thought is the most important thing in our human existance (and even this we have to question), we have to be very attentive when such consenseuses are posed to us, surely when we are not (yet) able to test them.
    What I am thinking of at this very moment can and will be old fashioned two seconds later when we found new facts that prove the opposite of what is the consensus of today.

    keep on thinking (free)


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  17. 17. Torbjörn Larsson, OM 11:33 pm 07/21/2011

    I would add to the post description of naive (i.e. in-theory) testing that a) predicting something old is already that fails erroneous theory b) <a href="">eventually the process converges observably</a>.

    Late, but FWIW:

    @ brerlou:

    "mention the wave-particle duality paradox" is a misnomer. It went away with quantum mechanics, where the wavefunction is the entity. (In quantum field theory it is more complex, as the field is relativity description of local particle interaction.)

    @ Mong H Tan:

    "No reductionist theories in biology … please"

    Sorry, but evolution theory is not subject to philosophy.

    But FWIW, "reductionism" is among other things how you put up and test phylogenetic trees, using bayesian parsimony in exactly the same way the parameters of standard cosmology is derived.

    @ ztansfi:

    "is global warming actually considered a Theory?"


    The underlying theory of atmospheres is greenhouse theory. The residual forcing that predicts our observably increasing global temperature is, now uniquely, anthropic. So GT has been supplemented with the new main theory AGW theory (Anthropic Global Warming theory). Check with IPCC reviews.

    @ morp:

    "explains phenomena"

    Your theory on science as explanation is called inductionism, and fails test. (Ironically precisely since testing fails erroneous theory, so testing works.) The reason it is popular among "folk science" is because it supports theology.

    @ gmperkins:

    "the hypothesis is correct, and thus becomes a theory."

    There is no "hypothesis-theory-law" hierarchy, it is folk science (see inductionism above). The main difference is that a theory is a set of connected hypotheses, whether it is tested or not.

    "The Big Bang Hypothesis"

    It is supplanted by standard cosmology, with an inflation era before big bang expansion. (Ethan loves to describe this, check his blog!)

    Standard cosmology is a tested theory (more than 3 sigma certainty).

    "Global Warming lacks a sound model"

    See above on GT and AGW both being theories. AGW is currently tested to 2 sigma (see WIREs papers), which is better than a medical diagnosis would give you and satisfies climate scientists.

    @ Wilhelmus de Wilde:

    "when we found new facts that prove the opposite of what is the consensus of today."

    That is the very point of testing, it verifies that theories are correct to certainty. So what you speak of has so low likelihood that it is rejected.

    Link to this

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