General Relativity Archives

March 12, 2021March 12, 2021 by Brian Koberlein

Alcubierre Gives us an Update on his Ideas About Warp Drives

The Enterprise using warp drive, as seen in Star Trek Beyond. Credit: Paramount Pictures

If you want a galaxy-spanning science fiction epic, you’re going to need faster than light travel. The alternative is taking decades or centuries to reach an alien star system, which isn’t nearly as much fun. So, you start with some wild scientific idea, add a bit of technobabble, and poof! Quam Celerrime ad Astra. Everything from wormholes to hyperspace has been used in sci-fi, but perhaps the best known FTL trope is warp drive.

December 13, 2020December 14, 2020 by Matt Williams

If We Used the Sun as a Gravitational Lens Telescope, This is What a Planet at Proxima Centauri Would Look Like

mage of a simulated Earth, at 1024×1024 pixel resolution, at the distance of Proxima Centauri,at 1.3 pc, as projectedby the SGL to an image plane at 650 AU from the Sun. Credit: Toth H. & Turyshev, S.G.

As Einstein originally predicted with his General Theory of Relativity, gravity alters the curvature of spacetime. As a consequence, the passage of light changes as it encounters a gravitational field, which is how General Relativity was confirmed! For decades, astronomers have taken advantage of this to conduct Gravitational Lensing (GL) – where a distant source is focused and amplified by a massive object in the foreground.

In a recent study, two theoretical physicists argue that the Sun could be used in the same way to create a Solar Gravitational Lens (SGL). This powerful telescope, they argue, would provide enough light amplification to allow for Direct Imaging studies of nearby exoplanets. This could allow astronomers to determine if planets like Proxima b are potentially-habitable long before we send missions to study them.

December 10, 2020December 10, 2020 by Brian Koberlein

Black Holes Gain new Powers When They Spin Fast Enough

Computer simulation of plasma near a black hole. Credit: Hotaka Shiokawa / EHT

General relativity is a profoundly complex mathematical theory, but its description of black holes is amazingly simple. A stable black hole can be described by just three properties: its mass, its electric charge, and its rotation or spin. Since black holes aren’t likely to have much charge, it really takes just two properties. If you know a black hole’s mass and spin, you know all there is to know about the black hole.

October 17, 2020October 17, 2020 by Brian Koberlein

We actually don’t know how fast the Milky Way’s supermassive black hole is spinning but there might be a way to find out

This illustration depicts a gas halo surrounding a quasar in the early Universe. The quasar, in orange, has two powerful jets and a supermassive black hole at its centre, which is surrounded by a dusty disc. The gas halo of glowing hydrogen gas is represented in blue. A team of astronomers surveyed 31 distant quasars, seeing them as they were more than 12.5 billion years ago, at a time when the Universe was still an infant, only about 870 million years old. They found that 12 quasars were surrounded by enormous gas reservoirs: halos of cool, dense hydrogen gas extending 100 000 light years from the central black holes and with billions of times the mass of the Sun. These gas stashes provide the perfect food source to sustain the growth of supermassive black holes in the early Universe.

Unless Einstein is wrong, a black hole is defined by three properties: mass, spin, and electric charge. The charge of a black hole should be nearly zero since the matter captured by a black hole is electrically neutral. The mass of a black hole determines the size of its event horizon, and can be measured in several ways, from the brightness of the material around it to the orbital motion of nearby stars. The spin of a black hole is much more difficult to study.

October 12, 2020October 12, 2020 by Brian Koberlein

Black Holes Make Complex Gravitational-Wave Chirps as They Merge

Simulated merger of two black holes. Credit: NASA's Goddard Space Flight Center

Gravitational waves are produced by all moving masses, from the Earth’s wobble around the Sun to your motion as you go about your daily life. But at the moment, those gravitational waves are too small to be observed. Gravitational observatories such as LIGO and VIRGO can only see the strong gravitational waves produced by merging stellar-mass black holes.

The chirp of a gravitational merger is clear. Credit: LIGO/Caltech/MIT/University of Chicago (Ben Farr)

October 8, 2020October 8, 2020 by Brian Koberlein

Gravitational-Wave Lensing is Possible, but it’s Going to be Incredibly Difficult to Detect

This diagram shows how Hubble is able to observe a quasar, a glowing disc of matter around a distant black hole, even though the black hole would ordinarily be too far away to see clearly. Credit: NASA and ESA

Gravity is a strange thing. In our everyday lives, we think of it as a force. It pulls us to the Earth and holds planets in orbits around their stars. But gravity isn’t a force. It is a warping of space and time that bends the trajectory of objects. Throw a ball in deep space, and it moves in a straight line following Newton’s First Law of Motion. Throw the same ball near the Earth’s surface, and it follows a parabolic trajectory caused by Earth’s warping of spacetime around it.

October 3, 2020October 3, 2020 by Brian Koberlein

Einstein. Right again

Simulation of M87 black hole showing the motion of plasma as it swirls around the black hole. Credit: L. Medeiros; C. Chan; D. Psaltis; F. Özel; University of Arizona; Institute for Advanced Study

Most of what we know about black holes is based upon indirect evidence. General relativity predicts the structure of a black hole and how matter moves around it, and computer simulations based on relativity are compared with what we observe, from the accretion disks that swirl around a black hole to the immense jets of material they cast off at relativistic speeds. Then in 2019, radio astronomers captured the first direct image of the supermassive black hole in M87. This allows us to test the limits of relativity in a new and exciting way.

September 25, 2020September 25, 2020 by Brian Koberlein

The Shadow from M87’s Supermassive Black Hole has Been Observed Wobbling Around the Galaxy for Years

The history of the EHT and the images they captured. Credit: M. Wielgus, D. Pesce & the EHT Collaboration

In April 2019, the Event Horizon Telescope (EHT) released the first direct image of a black hole. It was a radio image of the supermassive black hole in the galaxy M87. Much of the image resulted from radio light gravitationally focused toward us, but there was also some light emitted by gas and dust near the black hole. By itself, the image is a somewhat unimpressive blurry ring, but the data behind the image tells a more detailed story.

August 26, 2020August 26, 2020 by Matt Williams

One Theory Beyond the Standard Model Could Allow Wormholes that You Could Actually Fly Through

Molecular clouds scattered by an intermediate black hole show very wide velocity dispersion in this artist’s impression. This scenario well explains the observational features of a peculiar molecular cloud CO-0.40-0.22. Credit: Keio University

Wormholes are a popular feature in science fiction, the means through which spacecraft can achieve faster-than-light (FTL) travel and instantaneously move from one point in spacetime to another. And while the General Theory of Relativity forbids the existence of “traversable wormholes”, recent research has shown that they are actually possible within the domain of quantum physics.

The only downsides are that they would actually take longer to traverse than normal space and/or likely be microscopic. In a new study performed by a pair of Ivy League scientists, the existence of physics beyond the Standard Model could mean that there are wormholes out there that are not only large enough to be traversable, but entirely safe for human travelers looking to get from point A to point B.

August 13, 2020August 14, 2020 by Brian Koberlein

Fastest Star Ever Seen is Moving at 8% the Speed of Light

This artist's impression shows part of the orbit of one of the stars very close to the supermassive black hole at the centre of the Milky Way. Analysis of data from ESO’s Very Large Telescope and other telescopes suggests that the orbits of these stars may show the subtle effects predicted by Einstein’s general theory of relativity. There are hints that the orbit of this star, called S2, is deviating slightly from the path calculated using classical physics. This close-up of the orbit of star S2 shows how the path of the star is slightly different when it passed the same part of its orbit for the second time, 15 years later, due to the effects of general relativity.

In the center of our galaxy, hundreds of stars closely orbit a supermassive black hole. Most of these stars have large enough orbits that their motion is described by Newtonian gravity and Kepler’s laws of motion. But a few orbits so closely that their orbits can only be accurately described by Einstein’s general theory of relativity. The star with the smallest orbit is known as S62. Its closest approach to the black hole has it moving more than 8% of light speed.