University_

SAM readers have questions, and University people have answers. This time we investigate black holes in space, Schrodinger's equation, and why planes suddenly drop mid air.

#### If nothing can escape from a black hole, how can the mass of a black hole be known and therefore have an effect past the event horizon?

Nothing has to escape a black hole for us to be able to calculate its mass. In Einstein’s general theory of relativity, gravity is produced when massive objects (like planets) bend space-time, which is a state that envelops everything. (Editor: to learn more, see the star collision story on page 5). The more massive (or dense) an object, the more it bends space-time so, in effect, the stronger its gravity. Black holes are among the most massive objects in the universe. When an object gets caught in the gravity of a black hole, the shape of the object’s orbit tells us how much the black hole is bending space-time. From that, we can calculate the black hole’s mass.

Answered by Geraint Lewis (CertEdStud ’04), Professor of Astrophysics at the Sydney Institute for Astronomy in the School of Physics.

#### I’ve been wrestling with Schrödinger’s equation for the helium atom. Has it been solved? If it has, I’d be grateful to know.

Erwin Schrödinger was an Austrian theoretical physicist who was awarded the 1933 Nobel Prize in physics. His work suggested the very quantum-physics idea that atoms could have the properties of particles and waves at the same time. He came up with equations to prove this and predict the behaviour of various atoms. A particularly tricky equation was for helium, one of the most important elements in the Universe, being the fuel for our sun and most other stars. What made this equation complicated is that helium has a heavy nucleus orbited by two light electrons. But there is indeed a solution now. We can solve the equations by using an approximation in which the two electrons are considered to be so light they don’t really move the nucleus around. It’s like our model of the solar system, where the sun sits at the centre and is not moved (much) by the small planets around it. You may have to find another equation to wrestle with.

Answered by Stephen Bartlett, Professor of Physics at the Centre for Engineered Quantum Systems in the School of Physics.

#### Why do planes suddenly drop sometimes? Is it air pockets, like people say?

The effect is called windshear (or wind gradient), a difference in wind speed and/or direction over a relatively short distance in the atmosphere. The aeroplane drops due to a very concentrated ‘downwelling’ column of air that pushes it downwards very rapidly, in the same way an ‘upwelling’ column of air pushes it up very rapidly. Severe turbulence is the result of an aeroplane flying through a series of alternating downwelling and upwelling air currents.

Answered by Warwick Holmes, Executive Director of Space Engineering in the School of Aerospace, Mechanical and Mechatronic Engineering.

#### Any burning questions?

If you have a question for our researchers that you'd like answered, we'd love to hear from you. Nothing is too obscure. Please email your questions to sam@sydney.edu.au