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Physics LibreTexts

1.2: Evidence

There is no science without evidence. This means that when a scientist makes an assertion, he or she must back up the assertion with some observational or empirical data. These observations can be examined and checked by other scientists (or anyone else who is interested). This method differs from pre-scientific styles of learning about nature, in which an assertion might be made on the strength of mysticism, moral authority, political ideology, or the old standby, "everybody knows that...." For example, for centuries "everybody knew" that stones could not fall out of the sky. Rumors that stones had fallen out of the sky were written off as superstition. Then, in 1803, a shower of meteorites fell on a town in France. French physicist Jean Baptiste Biot visited the town, heard numerous consistent eyewitness accounts, and collected actual specimens of the stones, which proved to be unlike any known terrestrial rocks. The convincing body of evidence helped Biot prove to doubters that stones do indeed fall from space. We call them meteorites.

156 bolides were detected on a single (pointed) photographic plate of the all sky fish-eye photographic camera during the Leonid meteor shower in 1998 at Modra observatory. The exposure time was 4 hours. Click here for original source URL.

What is the story with UFOs? The idea that some unidentified flying objects are extraterrestrial visitors permeates the popular culture. Do alien spaceships visit Earth? That is a legitimate hypothesis, but how can we test it? It is difficult to investigate UFOs scientifically because there is so little physical evidence. No one has discovered fragments of alien material that can be tested in labs. Many "UFO photos" are fakes; others show identifiable natural phenomena. It is true that absence of evidence is not the same as evidence of absence. However, no real science can be done on this subject without tangible evidence. People are free to propose that many UFO sightings actually involve alien spaceships, but the reports are not believable without better evidence to support the hypothesis. Scientists do not ignore the UFO issue because they believe that alien spaceships are impossible (although many think that they are extremely unlikely), but because there is little or no physical evidence to work with.

Evidence must be available for strutiny and corroboration. Science is a social and public enterprise, so open access to data is a critical aspect of how we learn about the world (contrast this with some aspects of the world of commerce, where some information is tightly held or kept secret). That means that evidence must be published so that other scientists can evaluate it. Federal funding agencies in the United States and Europe now insist that scientific data be made publicly available. Bad data or poor evidence will not stand the test of time, because if it relates to an important advance, other scientists will try and replicate the experiment. Repeating your own experiment is good; having another scientist repeat your experiment is even better. This is the way rare instances of fraud or fabrication of data are discovered. Progress in science is not a smooth and steady march. There are false starts or dead ends or experiments that can't be replicated.


Mars Meteorite - NWA 7034 Designated Northwest Africa (NWA) 7034, and nicknamed "Black Beauty," the Martian meteorite weighs approximately 11 ounces (320 grams). Click here for original source URL.