Presentation on theme: "You’re So Predictable! Scientific Theories vs. Hypotheses."— Presentation transcript:
You’re So Predictable! Scientific Theories vs. Hypotheses
E = mc 2 What features distinguish those on the left from those on the right? Split the 10 items on this page into TWO categories, according to whatever distinguishing features you deem relevant. Be ready to defend the criteria you used to partition the items. Real World“Models” of Reality
E = mc 2 The bottom row shows photographs of real things in the real world (although technically even a photo is a “representation” (a re-presentation) of reality …and remember, ultimately EVERYTHING is a “representation of reality” in our minds/brains) A “Model” is: …a deliberately SIMPLIFIED “representation of reality” …created by humans …to depict, describe, interpret, and/or EXPLAIN some aspect of the real world vs.
Scientific Theories A scientific theory is an explanatory model. It seeks not only to depict or describe reality, but also to EXPLAIN it. Specifically, a scientific theory seeks to explain reality in terms of cause-and-effect relationships. If a particular scientific theory is a good model of reality – that is, if it is good at explaining things in terms of cause-and-effect – then it will be able to generate many successful predictions. We call these predictions hypotheses. The best way to understand this is to witness some vivid examples of scientific theories generating successful predictions…
Example #1: Does light “feel” gravity? Isaac Newton’s model of gravity – the “law of universal gravitation” – depicts gravity as an attractive “force” that any two objects exert on each other. This force (F G ) depends on the mass (m) of BOTH objects, and thus exists ONLY between objects that have mass. Since light has no mass – that is, since photons are packets of pure energy, not matter – Newton’s model argued that light does not “feel” gravity. When photons pass through a gravitational field, they will zip straight through, unaffected. F G = G m 1 m 2 d 2 (In terms of the mathematical model, gravitational attraction is directly proportional to the product of the two masses. Since a photon has zero mass, and the product of zero and any other number is still zero, no force of gravity exists between light and objects.)
The Theory of General Relativity Einstein’s theory of general, however, described gravity instead as a local curvature in the fabric of space itself (or technically, in “space-time”). Anything passing through such a curvature would therefore turn – or deflect – much the way a golf ball turns when it rolls across an indentation in the putting green. A satellite orbits the earth NOT because of a force that holds it in orbit, but because it’s caught in a curvature of space-time.
Newton’s Explanatory Model: Gravity is CAUSED by a force between massive objects. This theory PREDICTS that we’ll only see the EFFECTS of this force on objects that have mass. Einstein’s Explanatory Model: Gravity is CAUSED by a curvature in space-time. This theory PREDICTS that we’ll see the EFFECTS of this curvature on everything, with or without mass. Again, a scientific theory is an explanatory model that makes predictions (testable hypotheses) based upon cause-and-effect relationships. So here we have two competing theories:
Orion – with his famous 3-star belt – is a winter constellation. We see him during winter because that’s the time of year that the dark side of the earth faces toward Orion’s part of the sky. Orion IS in the sky during summer, but only during daytime, such that the brightness of the sun blinds our view of him. WinterSummer A Prediction and a Test…
But there IS one very rare occasion when you can see the winter constellations during summer: during a solar eclipse! Now, suppose you were to look at Orion’s belt during a solar eclipse, and suppose it was fairly close to the occluded sun, and suppose you were to see something very odd: That the 3 stars are NOT in their usual positions!* IF this were your observation, what would you conclude??? *This is illustrative only: NOT to realistic scale, and with GREATLY exaggerated distortion. Observed positions Expected positions
When Einstein published his theory of relativity in 1916, physicists were quick to develop a prediction – a hypothesis – to test the new theory: If Einstein’s model is valid, then when starlight from distant galaxies passes by the sun during an eclipse, it will bend from its straight-line path. This will create the illusion that the stars are displaced from where we know them to be – much like the distortions created by magnifying glasses and fish bowls. Crucially, Einstein’s model also enabled scientists to mathematically predict the precise degree of distortion. They then scrambled to the site of the next solar eclipse to see if observations matched predictions. Sure enough, when the midday sky darkened and the stars came into view, they were exactly where they were predicted to appear. The researchers found the stars right where they were looking for them!
Example #2: Quantum theory and neutrinos There are four basic interactions or “forces” that bind the universe together: 1)Gravity (holds things together at the largest scales and prevents them from flying apart: solar system, stars, galaxies, superclusters, etc.) 2)Electromagnetic force (holds things together at the atomic and molecular level, and prevents them from flying apart) 3)Strong Force (holds things together at the subatomic level, especially the nucleus, and keeps it from flying apart) 4)Weak Force (also holds things together inside the nucleus) But not all particles of matter “feel” all four forces. For example, electromagnetic attraction exists between electrons and protons – but not neutrons – and this keeps the electrons in orbit around the nucleus. Otherwise, the electrons would fly off and away. Similarly, the strong force exists between neutrons and protons – but not electrons – and this binds the nucleus together. Otherwise, all those positively charged protons would fly apart from one another.
The Theory of Quantum Mechanics mathematically predicted – purely on theoretical grounds – the existence of neutrinos, extremely tiny but not quite mass-less particles that fly through space at close to the speed of light. It predicted these would be created by the nuclear reactions inside the heart of the sun and other stars. Because they would be electrically neutral, they wouldn’t interact with “ordinary” matter by way of the electromagnetic force. This would allow them – predicts the theory – to pass straight through most things, including Earth itself. (Billions of them are zooming through your body right now!) This would also make them nearly impossible to detect. They would, however, have a smidgen of mass, and they would also participate in the “weak” force, and so – predicts the theory – every once in a long, long, LONG while, one will interact with an ordinary atom. That is, it’ll collide into a nucleus. And when it does – predicts the theory – the collision would emit a pair of gamma rays travelling in precisely OPPOSITE directions. And unlike neutrinos, gamma rays are readily detectable!
The original solar neutrino detector inside the Homestake Mine in South Dakota. To test the prediction, scientists in the late 60’s made some statistical calculations, and based thereon, built a huge tank filled with a chlorine solution – enough chlorine atoms to capture the occasional neutrino. They lined the tank with gamma ray detectors and buried it deep underground …where cosmic gamma rays and other radiation cannot tread, but neutrinos (by the zillions) could! Sure enough, they were able to detect – every day or so – a gamma signature consistent with a neutrino collision: gamma rays always in synchronized PAIRS striking OPPOSITE sides of the tank, and thus travelling on opposite trajectories. So they found the neutrinos right where they were looking for them!
Example #3: “Big Bang” theory and CMBR Contrary to popular (mis)conception, big bang theory does NOT portray the birth of the universe as a giant explosion of matter into pre-existing space. Rather, the “big bang” was a rapid EXPANSION of the fabric of space itself (or technically, of space-time itself …relativity theory again). Here’s a helpful way to understand it: Suppose we reduce 3-D space to a 2-D surface. Now imagine that this surface curves back on itself into the shape of a round, black balloon. Again, space itself is the surface (NOT the interior!) of the balloon. Now, the surface of this balloon is speckled with clusters of matter – like islands – that we call galaxies. Finally, imagine the balloon rapidly inflating, as in the following animation…
The disks here represent galaxies. They exist on the SURFACE of the black balloon (= space). Watch what happens as the balloon expands… (click) Notice that the islands of matter – the galaxies – do NOT come apart, nor do they expand or grow in size. That’s because they’re held tightly together by those four fundamental forces: Atoms and molecules are held together by the electromagnetic, strong, and weak forces, while solar systems and galaxies are held together by gravity. (So no, you yourself are NOT expanding, even though you’re part of an expanding universe.) However, the empty space between galaxies – space itself – STRETCHES, such that the distance between galaxies DOES grow. Watch it again…
According to the Big Bang model, the cosmos began as an incredibly rapid inflation, expanding within seconds from a pinpoint into a vast universe. (Much astronomical evidence shows that it continues to expand today, though not nearly as fast.) Based on cause-and-effect reasoning, Big Bang theory predicted the existence of “cosmic microwave background radiation,” a sort of afterglow of the big bang that permeates the entire universe. This radiation is the residue of ancient, high frequency light waves whose wavelengths have been gradually stretched and cooled into microwaves by the steady expansion of space itself. The theory predicted that the radiation should be detected within a certain very cold temperature range (about -270°C) and within a specific range of wavelengths (very long, close to radio waves), and in a certain pattern in the sky relative to Earth. In other words, once again, the theory told astronomers where to look…
So in the 1960’s, astronomers turned their new radio telescopes skyward, and when they did, they found the exact radiation fingerprint that they had already predicted must exist: the predicted temperature, the predicted wavelengths, and the predicted pattern in the sky. There it was – the smoking gun of the big bang – right where they were looking for it!
Example #4: Why Isn’t the Sea Getting Saltier? Most of the ions that make up the salt in seawater were delivered to the oceans by rivers. Separated from the parent rock, they dissolved in rain runoff, a process known as “rock weathering.” Thus the oceans should slowly be getting saltier, right?! Robert C. WhisonantVA Div of Mineral Resources
No, the oceans are NOT getting saltier, because ions are being removed from seawater, too. Some ions bind to sediments on the seafloor, while living organisms also incorporate ions into their bodies and later sink to the bottom. Once an ion finds its way into the ocean, it typically stays in the water column for millions of years, but all ions eventually end up on the seafloor. Tom Garrison’s Oceanography
…and remember, according to plate tectonics theory – and since we live on a round planet – old seafloor is swallowed back into the Earth at roughly the SAME RATE that new rock is made. Rock is recycled, so over the long haul of geologic time, the oceans lose old ions and gain new ions in equal amounts. New rock is made here (seafloor spreading center) …and here (volcanoes) …but gets consumed here (subduction zone /oceanic trench)
Principle of Constant Proportions Although salinity varies from place to place (due to local rainfall or evaporation), on average it’s 3.5% by weight. But no matter what the overall salinity of a sample of seawater, the ratios of the ions that compose the salt remain constant. Because the input and removal of ions are equal, and because the oceans are well mixed over geologic time, we get the “principle of constant proportions.”
Oceans Rivers (on average) BUT WAIT! Interpret these pie charts. Is there a problem/puzzle here? How so? What gives???
Relative to seawater, river water is enriched with certain ions (like calcium) and impoverished in others (like chlorine). Thus there must be OTHER “sources” and “sinks” for ions. Shell-building critters extract most of the excess calcium and bicarbonate to make their CaCO 3 shells. Volcanoes and undersea lava flows also add ions, especially chlorine and sulfate. BUT THIS STILL DOESN’T BALANCE THE EQUATION. There still wasn’t enough potassium, and there was too much magnesium and sulfate. Chambered nautilus Shedd Aquarium Scallop and barnacle Randy Shuman Brain coral UCBMP
Geochemists realized that the missing chemicals could – in theory – be supplied or extracted by superheated seawater spewing up from deep sea hydrothermal vents (rather like the geysers of Yellowstone) …if they exist. Plate tectonics theory did indeed PREDICT the existence of HTV’s in the vicinity of divergent plate boundaries, all a mile or more below the surface of the sea. Finding them, however, would be the proverbial needle in a very, very big haystack. Old Faithful in Yellowstone Park
So the theory of plate tectonics told oceanographers WHERE to look - namely on the outskirts of the mid-ocean ridges where new seafloor is ever being created. Using sensors lowered from ships, they detected seawater signatures consistent with the predictions, and finally in the 70’s were able to visit the sites in deep sea submersibles. And there they found – right where they were looking for them – hydrothermal vent fields, most no bigger than a football field and so short-lived that they go extinct in a decade or two …a bulls eye hit in both space and time! Woods Hole A “black smoker,” spewing the missing chemicals into the sea Woods HoleNOAA
What they did NOT expect find were whole oases of life around these vents – crabs, worms, fish, etc. – sustained not by the sun’s energy but by energy harvested by bacteria from the vent’s chemical emissions …the first ecosystems ever discovered whose food chains were not based upon plants and photosynthesis! Serendipity! Smithsonian NSF NOAA
Example #5: Theory of Evolution and Tiktaalik The Theory of Evolution explains that natural selection normally can only favor very small adaptive changes. Sudden, giant leaps in body form are not possible. Thus big, long-term changes (“macroevolution”) in animal body forms are caused by the gradual accumulation of many, many tiny changes (“microevolution”). For this reason, the theory of evolution predicts the existence of “transitional forms” between major body forms. With luck, some of these transitional animals might have been fossilized. U T Austin UMD wikipedia Eusthenopteron, an ancient lobe-finned fish, 390 million years old. Note the conical skull fused to the pectoral fins. Acanthostega, an early amphibian, 365 million years old. Note the flattened skull, wrist bones, and finger-like digits.
Acanthostega was almost entirely aquatic, poorly adapted to come onto land. Its front limbs weren’t flexible enough to bear weight in a high- gravity environment, and although it had lungs, the rib cage wasn’t robust enough to prevent lung collapse on land. Its fingered limbs were probably an adaptation for clambering around in the shallows, not walking on land. It had both gills and lungs, and the latter probably just helped it supplement its O 2 intake in oxygen-poor waters. UMD wikipedia Nevertheless, with its limbs, wrists, and “hands” instead of fins, it represents a transitional form between lobe-finned fish and terrestrial tetrapods (reptiles and their 4-limbed descendants like birds and mammals).
But might there be a fossilized transitional form out there somewhere between lobe fish and Acanthostega itself??? UMD U Chicago THINK! Where would you look for such a fossil? What kind of rock? What sort of geological setting or landscape? WHY?
UMD Paleontologist Neil Shubin and colleagues set out in search of this “missing link” …and evolutionary / geologic THEORY told them where to look: 1)Sedimentary rock (not igneous, because even if an unlucky fish got caught in a lava flow, its tissue would be destroyed; not metamorphic, because that rock – and everything in it – has been radically reconfigured by extreme pressure and heat) 2)Shale, mudstone, or siltstone (because that forms in calm bodies of water, ideal for fossil formation; sandstone, by contrast, forms from wave-beaten shoals and beaches, or else dry, fish-free deserts) 3)Rock that is about 375 years old (because Acanthostega shows up 365 ma, and before 380 ma there are only fish) 4)Rock that intersects the surface of the Earth, such that “rock weathering” by wind and water has had time to expose fossils 5)Landscape free of vegetation, so one can visually scan for fossils
UMD One place on Earth fit the bill: The Arctic Badlands of Canada’s Ellsmere island
UMD After 5 long, cold, tough summers of scrambling over rock, camping out in tents, and dodging polar bears, Shubin and his crew found it …right where theory predicted it would be: An ancient fish – NOT an amphibian – with a flattened head, a flexible neck (skull no longer fused to the pectoral girdle), and fins – NOT “hands” – containing wrist and finger-like bones. They named the species Tiktaalik, the local Inuit (“Eskimo”) word for “large freshwater fish” wikipedia U Chicago wikipediaU Chicago
Hey kids, have a go at the following brainteasers! Jot your answers down on paper.
Part of your new secretarial job at the local high school is to make sure that student documents have been processed correctly. Your job is to make sure the documents conform to the following alphanumeric rule: “If a person has a ‘D’ rating, then his documents must be marked code ‘3’.” You suspect that the previous secretary has accidentally misclassified some of the students’ documents. The cards below bear information about the documents of four students. Each card represents one student. One side gives a student’s letter rating and the other side gives the number code currently assigned to that document. Flip over only those card(s) that you definitely need to turn over in order to see if the documents violate the classification rule. DF37 Adapted from Tooby & Cosmides 1992
In a crackdown against drunk drivers and underage drinking, local law enforcement officials are revoking liquor licenses left and right. You’re the owner of a local pub, and your liquor license requires that you enforce the following rule: “If a person is drinking beer, then he must be at least 21 years old.” These cards bear information about four people sitting at one of your tables. Each card represents one person. One side tells what a person is drinking and the other side tells that person’s age. Flip over only those card(s) that you definitely need to turn over in order to see if anyone is breaking the law. Drinking Beer Drinking Coke 24 Years Old 18 Years Old Adapted from Tooby & Cosmides 1992
You hate baseball, but your spouse is an avid fan, and he/she has dragged you to the local pro team’s opening game. All around you, people are eating everything from peanuts to cotton candy to hot buffalo wings, and they are drinking everything from cold beer to coke to bottled water. Bored with the game, you start looking for patterns in people’s food and drink choices. You predict that the following rule will always hold: “If a person eats hot buffalo wings, then he will drink a cold beer.” These cards bear information about four people who are sitting in front of you. One side tells what a person is eating and the other side tells what that person is drinking. Flip over only those card(s) that you definitely need to turn over in order to see if anyone is going against your predicted rule. Eating Hot Wings Eating Peanuts Drinking Beer Drinking Coke Adapted from Tooby & Cosmides 1992
Each rule is of the form: “If P, then Q” In all 3 scenarios, the four cards correspond to the SAME four possibilities: PNot-PQNot-Q
D7 Drinking Beer 18 Years Old Eating Hot Wings Drinking Coke P P P Not-Q F3 Drinking Coke 24 Years Old Eating Peanuts Drinking Beer
How’d you do? If you made some mistakes, don’t fret …you’re in the majority. This is a Wason Selection Task, named for Peter Wason, who found that formal logic doesn’t always come “naturally” to us. When faced with a Wason task, most people correctly choose the P card, but far fewer – often less than 25% – will correctly choose the not-Q card …even among college students at Stanford and Harvard! BUT… some Wason tasks prove easier to solve than others. Which tasks did you find easier than others? Which were especially challenging? What do you think made them easy or hard?
Example #6: Social Exchange Theory and Cheater Detection Most folks think that FAMILIARITY and ABSTRACTNESS determine the difficulty of a Wason task. Thus, familiar scenarios (like the bar and baseball scenarios) should be easier to solve than unfamiliar or abstract scenarios (like the secretary task). Psychologist Leda Cosmides put forth a different proposal. The theory of evolution – combined with several sub-theories: game theory, kin selection theory, and reciprocal altruism theory – predicts that cooperative behavior among non-kin cannot evolve in the animal kingdom UNLESS the animal is also able to detect potential cheaters (“defectors” who refuse to return past “favors”). Since humans DO readily cooperate with non-kin, Cosmides predicted that our brains/minds must contain specialized psychological mechanisms for detecting cheaters within contexts of “social exchange.” Consider the next scenario: It’s quite UNFAMILIAR, yet it involves a SOCIAL rule. Is it hard to solve?? (Warning: the cards have been move around this time.)
You are a local chief among the Halé-àkà people who inhabit the small tropical island of Tontiki in the South Pacific. In the mornings, the people wander into the local rainforest to gather fruits, nuts, and other plant foods. Molo nuts are abundant but bitter. Much rarer is the cassava root, highly prized as an aphrodisiac (sex stimulant). All married persons have a tattoo on their face, and because extramarital sex is strictly frowned upon among the Halé-àkà, it is part of your role as chief to enforce the following custom: “If a man eats cassava root, then he must have a tattoo on his face.” These cards bear information about four men who you see foraging in the forest one morning. Each card represents one person. One side tells what food a person is eating and the other side tells whether that person has a marital tattoo on his face. Flip over only those card(s) that you definitely need to turn over in order to see if anyone is violating the local custom. Eating Cassava Root Eating Molo Nuts Tattoo On Face No Tattoo
Familiarity Theory: Success on Wason tasks will mainly depend on the familiarity and abstractness of the scenario. Social Exchange Theory: Success on Wason tasks will mainly depend on whether the scenario involves the potential for people to violate a social contract. Such situations – whether familiar or not – will “switch on” the cheater detection mechanism in our brains and help us solve the task. Once again we have two competing theories:
Results of one experiment, using a variety of scenarios. Interpret, please! (like the cassava root scenario) (like the baseball scenario) (like the secretary scenario)
Another experiment gave participants a chance to select cards that were NOT logically correct, but WERE “correct” if one’s goal were to catch cheaters. Interpret!
Now consider this rule: “If an employee gets 3 weeks annual vacation, then she must have worked for the firm for at least 10 years.” These cards bear information about the employment records of four employees. Each card represents one person. One side tells how many weeks paid vacation the person is currently receiving and the other side tells how long that person has worked for the firm. Worked < 10 years Getting 3 weeks vacation Not getting 3 weeks vacation Worked 10+ years In one experiment Cosmides asked half the participants to imagine themselves as the vice president in charge of personnel for a commercial oil drilling company in Texas, and the other half to imagine themselves as the secretary of the local trade union. Either way, the LOGICALLY correct answers are the SAME, so the role shouldn’t affect their choices…
Results. What happened? (logically correct)(logically incorrect)
In these and other ingenious experiments, Cosmides ruled out competing hypotheses and the theories that gave birth to them. She argued that she had revealed the existence of an innate “cheater detection” mechanism built into the human mind. She found it right where she was looking for it …right where THEORY told her to look!
Take-Home Messages 1)Scientific theories – or explanatory models – are usually too big to test all at once, but they do generate predictions – or hypotheses – that ARE testable. By culling out the good hypotheses from the bad ones, science slowly sculpts and re-sculpts its theories and models …over and over and over. This makes our models ever more accurate representations of reality “out there.” 2)Science can make testable “predictions” about the PAST as well as the future. Specifically, science can predict future observations about past events. (Examples: cosmic background radiation, Tiktaalik, and cheater detection mechanism)
Take-Home Messages 3)Science enables us to detect real things in the real world that would otherwise remain invisible to us. From deep space to deep inside atomic nuclei, to the deep seafloor, to deep in ancient rocks, to deep inside the human mind, science enables us to PREDICT the existence of things that we otherwise would never even imagine. Einstein’s distorted stars, solar neutrinos, cosmic background radiation, hydrothermal vents, fossilized fingered fish on Arctic islands, cheater detectors in the brain …we might NEVER have discovered ANY of these unless preexisting THEORY – principled, explanatory, cause-and-effect models of reality – had not raised the right questions to begin with, and then pointed researchers in the right directions to find them.