## relativity

• ### Demonstration Highlight: Inertial Reference Frame

Welcome back! Today we’re taking a look at a popular demonstration related to the concept of relativity.

We’re accustomed to thinking about the motion of a projectile from a perspective outside of its motion, the generally safer option in real life! The PhET collection of physics simulations has a lovely one for seeing how different parameters like mass, gravity, and air resistance affect the motion of a projectile; try it out here: https://phet.colorado.edu/en/simulations/projectile-motion

When we observe and measure motion, we are inevitably making the measurement against some frame of reference. An inertial reference frame is the technical term for a frame of reference in which an object is observed to have no outside forces acting on it, so that it is moving freely in space. Sometimes we have to go to great lengths to determine what such a frame of reference might be – and in the case of this demonstration, it is literally a metal frame!

In demonstration P1-02 in our collection, two spring-powered cannon have been pointed so that if a projectile came out of either of them and moved in a straight line, the projectile would pass through a hole in a transparent barrier and then land in a sophisticated projectile catchment mechanism, also known as a sock. But of course, if we just launch a ball out of the cannon, that doesn’t happen! As soon as the ball leaves the cannon, it starts to fall due to the acceleration of gravity, following a parabolic path, so it slams into the transparent barrier far below the hole.

But, if we raise up the whole aluminum frame that holds the cannon, barrier, and catchment, and then drop it, we can fire the cannon while the aluminum frame is falling. Now, from the perspective of the frame, there’s no separate acceleration pulling the ball down, because the frame is falling at the same rate that the ball is! So the projectile moves “straight” across the frame, through the hole, and lands in the catchment. Meanwhile, from our own perspective outside the experiment, we see the ball following a parabolic path just like always, while the whole experiment falls down.

• ### STEM News Tip: 2020 Nobel Prize in Physics Announced

The recipients of the 2020 Nobel Prize in Physics were announced this morning. This year, the prize has been awarded for discoveries related to black holes. The recipients: Reinhard Genzel, Andrea Ghez, and Roger Penrose.

Black holes are a phenomenon predicted by the theory of relativity, that a sufficiently massive object could have enough gravity to warp spacetime such that even light could not escape from it. Roger Penrose carried out some of the important theoretical calculations establishing the reality and properties of black holes; Andrea Ghez and Reinhard Genzel led experiments that established that a supermassive black hole exists at the center of our own Milky Way galaxy.

Reinhard Genzel made a virtual visit to UMD last month as the speaker for the Astronomy Colloquium; he is director of Germany’s Max Planck Institute for Extraterrestrial Physics and is also a professor at the University of California, Berkeley.

Andrea Ghez is a professor of physics and astronomy at the University of California, Los Angeles. She and her research were featured in a NOVA television special on black holes in 2018. She won the APS Maria Goeppert Mayer Award in 1999.

Roger Penrose is a professor at the University of Oxford. In 1988 he shared the Wolf Prize in Mathematics with Stephen Hawking for their mathematical research regarding spacetime singularities.

We’ll share more information on black hole physics in an upcoming post; meanwhile, check out these links for more about the physics, the people, and the prize.

• ### STEM News Tip: Measuring the gravitational force of tiny masses

A recent publication announced some remarkably fine new measurement of the gravitational attraction between extremely small objects. Also reported in Scientific American, the Aspelmeyer group at the University of Vienna has measured gravitational interactions between millimeter scale objects. You can read all the details in the papers below.

B. Brubaker, Physicists Measure the Gravitational Force between the Smallest Masses Yet, Scientific American, 2021 March 10

Westfal, Hepach, Pfaff, & Aspelmeyer, Measurement of gravitational coupling between millimetre-sized masses, Nature, 20201 March 10

Aspelmeyer Group at the University of Vienna

If you’re discussing this kind of measurement in class, check out our model of the classic Cavendish Experiment, E1-01 in the demonstration collection.

• ### STEM News Tip: New Gravitational Waves Discoveries!

UMD Physicists are heavily involved with the LIGO collaboration, the Laser Interferometer Gravitational-Wave Observatory that detects and analyzes gravitational waves to study distant celestial phenomena. Several recent papers have announced important new findings. One highlight is the observation of merging black holes including the largest one yet observed in such a merger.

The merger of these massive objects distorts spacetime around it, creating a ripple that we can detect here on earth through the use of extremely precise interferometery. Some of you may recall presentations we hosted a few years ago when LIGO announced their first detections. New research from this team is coming in all the time!

More places to visit:

• ### STEM News Tip: Prof. Buonanno elected to NAS

UMD Physics Professor Alessandra Buonanno was elected to the US National Academy of Sciences last week. In addition to being a professor here at UMD, she is a department director at the Max Planck Institute for Gravitational Physics in Germany. She is well known internationally for her work with the LIGO Collaboration and their detection of gravitational waves.

Earlier this spring, Prof. Buonanno also was awarded the Galileo Galilei Medal from the Italian National Institute for Nuclear Physics for her gravitational wave work.