Evidence

 Scientific evidence consists of quantitative observations or experimental results that can be confirmed by other investigators.

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.

 

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 scrutiny 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.

What type of evidence do we consider in astronomy? There is very little direct evidence. Astronauts have returned a few hundred pounds of Moon rocks to Earth, and every now and then a chunk of the outer Solar System falls to Earth as a meteorite. We have even been lucky enough to find a few dozen "free" pieces of Mars rock! Spacecraft have viewed all the major planets of the Solar System. However, our knowledge about most of the universe has been learned by pointing telescopes at the skies and interpreting the radiation that is gathered. While many of these telescopes collect and detect visible light, in the last few decades astronomers have devised technologies to record radiation that the eye cannot see.

 

Science thus depends on the idea that we can extend our senses with instruments. For example, physicists deal daily with electric and magnetic fields and subatomic particles, none of which register with our five senses. Astronomers routinely detect many types of radiation from celestial objects, including radio waves, X-rays and gamma rays. These invisible types of radiation can be converted into an intensity map, so we can "see" the object. Telescopes are used to gather light from sources that are billions of times too faint for the eye to see. All of this information is the evidence of astronomy.

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