Neutron Stars: Billions of Times Stronger Than Steel

Neutron Stars: Billions of Times Stronger Than Steel

New supercomputer simulations of the crusts of neutron stars--the rapidly spinning ashes left over from supernova explosions--reveal that they contain the densest and strongest material in the universe. So dense, in fact, that the gravity of the mountain-sized imperfections on the surfaces of these stars might actually jiggle spacetime itself. If so, neutron stars could offer new insights into a mysterious phenomenon known as gravity waves.

Neutron stars are the remnants of supernovae – basically the corpses of stars that were much more massive than our Sun. After the supernova explosion so much matter remains with no means to support itself (such as radiation pressure from thermonuclear reactions) that it all collapses into a relatively small object having a radius of about 12 km. The density of such an object is extremely high. Because the material is so dense, it is also very strong and rigid. Consequently, it does not collapse to a perfectly smooth sphere, but instead should contain surface imperfections roughly the size of (small) terrestrial mountains, each as massive as Earth.

In neutron stars that spin rapidly, the asymmetrical mass of these imperfections experiencing acceleration due to the periodic spinning motion should generate gravitational waves. The simulations that were performed in this research have shown that the energy in such gravitational waves could be a hundred times more than previously expected.

Tags: neutron stars, gravitational waves

It's all relative

In the previous message we mentioned an interesting article on relativity: Relativity at the centenary, which pointed out that gravitational physics has become an experimental science. There's a lot going on with studies of black holes, the search for gravitational waves, detailed tests of special and general relativity. And on the theoretical side, there's a lot of activity in the (as yet still unsuccessful) quest for a theory of quantum gravity.

Of course, the theory of relativity is intimidating to a lot of people, but it doesn't need to be. Special relativity actually involves little more that basic physics (ideas like mass, velocity, force, energy) and high school algebra with rudimentary calculus. The mathematics needed for general relativity is somewhat more sophisticated. But the basic concept described by the math is pretty simple: concentrations of matter cause space to curve, and the motion of two (or more) massive objects that interact gravitationally can be understood as "straight lines" in the curved space.

Once you've read some overviews, like the Wikipedia articles just referred to, you will have the basic ideas needed to learn more about relativity. Fortunately, there's an excellent online reference with a large number of review articles that explain in more detail many of the most interesting topics in the contemporary theory of relativity. It's called Living Reviews in Relativity -- and it's all free.

The journal, which covers both theory and experiment, is now in its 9^th year. Some of the articles are valuable for understanding basic topics in cosmology. Here are some of the more generally accessible articles:

• Loop Quantum Gravity (1998)
  
• Stationary Black Holes: Uniqueness and Beyond (1998)
  
• The Cosmic Microwave Background (1998)
  
• Gravitational Wave Detection by Interferometry (2000)
  
• The Cosmological Constant (2001)
  
• Computational Cosmology: From the Early Universe to the Large Scale Structure (2001)
  
• The Thermodynamics of Black Holes (2001)
  
• Experimental Searches for Dark Matter (2002)
  
• Gravitational Waves from Gravitational Collapse (2003)
  
• Testing General Relativity with Pulsar Timing (2003)
  
• On the History of Unified Field Theories (2004)
  
• Quantum Gravity in Everyday Life: General Relativity as an Effective Field Theory (2004)
  
• Brane-World Gravity (2004)
  
• Measuring our Universe from Galaxy Redshift Surveys (2004)
  
• Quantum Gravity in 2+1 Dimensions: The Case of a Closed Universe (2005)
  
• Modern Tests of Lorentz Invariance (2005)
  
• Binary and Millisecond Pulsars (2005)
  
• Loop Quantum Cosmology (2005)
  
• The Confrontation between General Relativity and Experiment (2006)
  

Note that there have been updated versions of some articles. In each case, only the most recent update (as of this writing) and its corresponding date are listed.

Tags: relativity, special relativity, general relativity, black holes, quantum gravity, pulsars

Gravitational waves

Nice BBC article:

Science to ride gravitational waves

The real aim is to have a new means of studying the Universe - to trace its exotic phenomena in detail in a way that does not rely on light.

"The analogy I like is this: imagine being able to see the world but you are deaf, and then suddenly someone gives you the ability to hear things as well - you get an extra dimension of perception," explains Professor Bernard Schutz from the Albert Einstein Institute and Cardiff University.

"Up until now we've only been able to see the Universe with our telescopes, but with gravitational waves we will be able to hear it as well; and that's going to convey a different type of information.

"Most of the Universe cannot emit electromagnetic waves - we will never see it with light. But we can see it, or parts of it, with gravitational waves."

Tags: gravitational waves