Discuss with students how recent findings concerning the unique gravitational interaction of three dead stars have the potential to poke holes in Einstein’s famous theory.
Grade Level: 9-12
Student Learning Objectives:
Students will gain an understanding of (1) Einstein’s physics principles, (2) basic astronomy (star systems) and (3) how scientific knowledge evolves.
As you review the following information with students, follow the links to articles and YouTube videos, where you’ll find detailed explanations of key facts and concepts from Helen B. Warner Prize winner Scott Ransom.
Three recently discovered dead stars are presenting a possible challenge to Einstein’s equivalence principle and theory of relativity.
(The equivalence principle states that the effect of gravity on a body of mass doesn’t depend on the nature or internal structure of that body of mass. In other words, objects of different sizes and weights fall at the same speed. The principle was famously proven by Galileo’s experiment at the Leaning Tower of Pisa, but because the principle doesn’t jibe with quantum mechanics [quantum gravity], physicists have suspected that equivalence would not hold true under extreme conditions. Any evidence contrary to the equivalence principle would, in turn, cast doubt on Einstein’s theory of general relativity.)
The discovered three-star system includes a millisecond pulsar, which is located 4,200 light-years from Earth, and is closely orbited by a hot white dwarf and a cooler, more distant white dwarf. The stars formed as the result of younger stars' deaths and are comprised of those stars' remains.
The extreme conditions produced by the system led to very pure gravitational interactions among the stars' orbits. This means that all sorts of interesting information can be calculated. More specifically, millisecond pulsars spin rapidly and give off radio waves that race through space as the stars rotate. They are used as a measurement device in astronomy because after the amount of times they spin per second is determined, millisecond pulsars can help scientists understand the effects of gravity. Scientists measure the arrival time of the stars’ radio waves in order to calculate the gravitational properties of the system and its stars’ masses.
The equivalence principle assumes that the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the pulsar. If, however, the equivalence principle is invalid under the conditions in this system, the outer star's gravitational effect on the inner white dwarf and the pulsar would be slightly different, and the high-precision pulsar timing observations could easily show that. Finding a deviation from the equivalence principle would indicate a breakdown of general relativity and would point us toward a new, revised theory of gravity.
The stars in this specific system are unusual, to say the least, originating from a rare set of circumstances. Previously, similar systems haven’t survived when tested, because when they form, pulsars typically blow away anything that could orbit them.
Introducing the discussion to students:
According to Einstein’s equivalence principle, the effect of gravity on a body of mass doesn’t depend on the nature or internal structure of that body of mass. But because the principle doesn’t jibe with more modern quantum mechanics (quantum gravity), physicists have suspected that equivalence would not hold true under extreme conditions.
Those extreme conditions haven’t truly presented themselves—until now. Let’s explore how the unique gravitational interaction of three recently discovered dead stars has the potential to poke holes in not only Einstein’s equivalence principle, but also his theory of general relativity.
Options for student discussion questions: