physics

Shepard elephant

Shepard elephant is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Shepard elephant rather than just read about it. In short: The Shepard elephant, also known as l'egs-istential quandary or the impossible elephant, is a perceptual illusion, of the type impossible object, based on figure-ground confusion. As its creator Roger Shepard explains: The elephant…belongs to a class of objects that are truly impossible in that the object itself cannot be globally segregated from the nonobject or background.

Key takeaways

Shepard elephant belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Shepard elephant to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Shepard elephant from memory before moving on to harder problems.

Reference excerpt

The Shepard elephant, also known as l'egs-istential quandary or the impossible elephant, is a perceptual illusion, of the type impossible object, based on figure-ground confusion. As its creator Roger Shepard explains: The elephant…belongs to a class of objects that are truly impossible in that the object itself cannot be globally segregated from the nonobject or background. Parts of the object (in this case the elephant’s legs) become the background, and vice versa.

History

Shepard first published this optical paradox in his 1990 book Mind Sights (page 79) giving it the name "l'egs-istential quandary". It is the first entry in his chapter on "Figure-ground impossibilities". The pen-and-ink drawing is based on a dream Shepard had in 1974, and on the pencil sketch he made when he woke up.

Interpretation

The image is widely reproduced and discussed. Brad Honeycutt, author of Exceptional Eye Tricks, calls the Shepard elephant "one of the most famous and classic optical illusions." The MIT Encyclopedia of the Cognitive Sciences classifies it under "Illusions of Interpretation" as an example of "Impossible Pictures", saying, "Impossible figures embody conflicting 3D clues...Shepard's elephant (figure 2) confuses its legs with the spaces in between. Local votes about depth are not properly integrated." The Oxford Companion to Consciousness suggests as a way to understand "Shepard’s many-legged elephant": "try slowly uncovering the elephant from the top, or from the bottom." (If you cover the bottom of the drawing, you see the top of an elephant with four legs. If you cover the drawing's top, you see four elephant feet, plus trunk and tail.) Al Seckel, who devotes Chapter 18 of his book Masters of Deception to Roger Shepard, draws a contrast between Shepard's elephant and the impossible trident (aka the "blivet" or "Devil's tuning fork"). Although an impossible trident has a closed edge, Seckel says the "conspicuous line discontinuity" of the elephant's tail is necessary to avoid a "counting paradox": a blivet is an imaginary object, but everyone knows an elephant has exactly four legs and exactly four feet.

Influence The Shepard elephant has attracted interest outside scholarly sources. Author Clive Gifford included it in his 2013 book for children Eye Benders, and told The Guardian it is one of his favorites. Sometime before 2012, someone created a modified version of the Shepard elephant. The modified elephant has an extra hind leg, with foot attached, made by extending the curved line that Shepard left ambiguous (to look like either the top of a leg or the end of a tail.) This modified image, which now has four legs but five feet, was circulated as "How many legs does this elephant have?" The Shepard elephant has also inspired some other derivative works. The OpticalSpy website also cited the Shepard elephant as inspiration for manipulating a photograph of an Indian elephant to give it six legs.

Illustrations

Worked examples

Example 1 — a first encounter with Shepard elephant

Start with the simplest possible case. Write down what Shepard elephant claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Shepard elephant before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Shepard elephant ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Shepard elephant

In research: Shepard elephant appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Shepard elephant in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Shepard elephant is common in secondary-school and first-year university syllabi. It links to neighbouring topics Elephants in art, Elephants in popular culture, Impossible objects, so understanding it makes those chapters shorter.
In everyday life: Look for Shepard elephant outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Shepard elephant in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Shepard elephant means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Shepard elephant out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Shepard elephant in simple terms?

Why does Shepard elephant matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Shepard elephant?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Shepard elephant.