Seeing what we want to see

Seeing what we want to see

The M33 Galaxy • Image by ESO (CC By 4.0)

Originally published 13 August 2006

A Newsweek mag­a­zine poll some years ago found that 87% of Amer­i­cans believe in a God who answers prayers. More than three-quar­ters of Amer­i­cans believe God some­times inter­venes to heal some­one with an incur­able dis­ease, and near­ly as many believe prayers are answered for such mun­dane requests as find­ing a job. What is true for Amer­i­cans is undoubt­ed­ly true for believ­ers of every faith right around the world. If so many are con­vinced of the effi­ca­cy of peti­tionary prayer, how can they pos­si­bly be wrong?

Four cen­turies ago, Fran­cis Bacon said that what a per­son would like to be true, he pref­er­en­tial­ly believes. Even the most fair-mind­ed observ­er can be led into error by uncon­scious or unex­am­ined prej­u­dices, which is why sci­en­tists place so much empha­sis on con­trolled exper­i­ments, the sta­tis­ti­cal analy­sis of data, peer review, math­e­mat­ics, dia­grams, pho­tographs, spe­cial­ized lan­guages, and the strict exclu­sion of per­son­al, reli­gious, and polit­i­cal affil­i­a­tions from sci­en­tif­ic com­mu­ni­ca­tion. The point of all these strate­gies is to min­i­mize the effect of “see­ing what we want to see.”

But no knowl­edge sys­tem, not even sci­ence, can be entire­ly free of per­son­al and cul­tur­al pre­dis­po­si­tions. The his­to­ry of sci­ence is full of wrong turns tak­en on the basis of per­son­al or cul­tur­al prej­u­dices. A sub­tle exam­ple of how easy it is to mis­judge evi­dence is the work of the astronomer Adri­aan van Maa­nen on the rota­tion of galaxies.

In the sec­ond decade of the last cen­tu­ry, one of the biggest unan­swered ques­tions in astron­o­my was the dis­tance to the so-called “spi­ral neb­u­la”, pin­wheel-shaped whirls of stars and gas that could be observed with tele­scopes. Were the spi­rals among the stars of the Milky Way Galaxy, and there­fore rel­a­tive­ly near­by and small, or were they far beyond the Milky Way and per­haps as large as our own galaxy? One way to answer this ques­tion was to see if rota­tion­al motion could be detect­ed in the spi­rals. If the spi­rals are small and near­by, they might appear to rotate more quick­ly than if they are larg­er and far away.

Van Maa­nen looked for rota­tion by com­par­ing pho­tographs of face-on spi­rals tak­en some years apart. It was a sim­ple and appar­ent­ly fool­proof task he under­took: com­pare the posi­tions of stars in two pho­tographs of the same object. In spi­ral after spi­ral he found con­sis­tent evi­dence of rota­tion, as if the spi­ral neb­u­las were unwind­ing. The star dis­place­ments he observed were tiny but appar­ent­ly real. Since van Maa­nen was one of the most respect­ed astronomers in the world for this sort of work, his results were wide­ly accept­ed. His mea­sure­ments sug­gest­ed that the spi­ral neb­u­la were inside the Milky Way.

It took more than a decade for astronomers to real­ize that van Maa­nen’s con­sis­tent and con­vinc­ing mea­sure­ments were in error. Oth­ers who attempt­ed to repro­duce his work were not able to do so. And oth­er lines of evi­dence sug­gest­ed that the spi­rals were out­side of the Milky Way. In fact, we now know that the spi­rals are so far away that it would have been impos­si­ble to detect a change in the posi­tions of stars dur­ing the inter­vals used by van Maanen.

So what was the source of his error? His­to­ri­ans Richard Berendzen, Richard Hart, and Daniel See­ley have exam­ined the van Maa­nen’s work and rule out sys­tem­at­ic instru­men­tal and com­pu­ta­tion­al errors. It seems that van Maa­nen had a slight per­son­al bias toward believ­ing that the spi­rals were in rota­tion, and his results reflect­ed this bias. In oth­er words, while striv­ing for com­plete objec­tiv­i­ty, his mea­sure­ments were nonethe­less affect­ed by his expectations.

The machin­ery of sci­en­tif­ic know­ing is designed to pre­vent or detect exact­ly this kind of error, and even­tu­al­ly did so in the case of van Maa­nen’s galax­ies. The arbiter of sci­en­tif­ic truth is repeat­able, repro­ducible exper­i­ment. Nature, not our hopes or desires, must have the last word.

One should always be skep­ti­cal about exper­i­men­tal results that lie close to the mar­gin of sys­tem­at­ic instru­men­tal, sta­tis­ti­cal, or per­son­al errors. All of the so-called evi­dence for the effi­ca­cy of peti­tionary prayer falls into this cat­e­go­ry, and that is why every claim for answered prayers should be con­sid­ered skep­ti­cal­ly. His­to­ry has taught us how easy it is to see what we want to see.

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