Timeline: the history of gravity

Geraint Lewis, University of Sydney

Our understanding of gravity has gone through a few permutations, from Newton’s equations through to Einstein’s general relativity. With today’s discovery of gravitational waves, we look back on how our grasp of gravity has evolved over the centuries.

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1687: Newtonian gravity

Isaac Newton publishes Philosophiae Naturalis Principia Mathematica, giving a comprehensive account of gravity. This gave astronomers an accurate toolbox for predicting the motions of planets. But it was not without its problems, such as calculating the precise orbit of the planet Mercury.

All planets’ orbits precess – with the closest point of their orbit moving slightly with each revolution – due to the gravitational tugs from other planets.

Wes Mountain/The Conversation, CC BY-ND

The issue with Mercury’s orbit was that the amount of precession did not match what Newton’s theory predicted. It was only a small discrepancy, but big enough for astronomers to know it was there!

Wes Mountain/The Conversation, CC BY-ND

1859: Planet Vulcan

To explain Mercury’s odd behaviour, Urbain Le Verrier proposed the existence of an unseen planet called Vulcan, which orbited closer to the sun. He suggested that the gravity from Vulcan was influencing Mercury’s orbit. But repeated observations revealed no signs of Vulcan.

Wes Mountain/The Conversation, CC BY-ND

1905: Special relativity

Albert Einstein shakes up physics with his special theory of relativity. He then started incorporating gravity into his equations, which led to his next breakthrough.

1907: Einstein predicts gravitational redshift

What we now call gravitational redshift was first proposed by Einstein from his thoughts in the development of general relativity.

Wes Mountain/The Conversation, CC BY-ND

Einstein predicted that the wavelength of light coming from atoms in a strong gravitational field will lengthen as it escapes the gravitational force. The longer wavelength shifts the photon to the red end of the electromagnetic spectrum.

1915: General relativity

Albert Einstein publishes general theory of relativity. The first great success was its accurate prediction of Mercury’s orbit, including its previously inscrutable precession.

The theory also predicts the existence of black holes and gravitational waves, although Einstein himself often struggled to understand them.

Wes Mountain/The Conversation, CC BY-ND

1917: Einstein theorises stimulated emission

In 1917, Einstein publishes a paper on the quantum theory of radiation indicating stimulated emission was possible.

Einstein proposed that an excited atom could return to a lower energy state by releasing energy in the form of photons in a process called spontaneous emission.

In stimulated emission, an incoming photon interacts with the excited atom, causing it to move to a lower energy state, releasing photons that are in phase and have the same frequency and direction of travel as the incoming photon. This process allowed for the development of the laser (light amplification by stimulated emission of radiation).

1918: Prediction of frame dragging

Josef Lense and Hans Thirring theorise that the rotation of a massive object in space would “drag” spacetime around with it.

1919: First observation of gravitational lensing

Gravitational lensing is the bending of light around massive objects, such as a black hole, allowing us to view objects that lie behind it. During a total solar eclipse in May 1919, stars near the sun were observed slightly out of position. This indicated that light was bending due to the sun’s mass.

Wes Mountain/The Conversation, CC BY-ND

1925: First measurement of gravitational redshift

Walter Sydney Adams examined light emitted from the surface of massive stars and detected a redshift, as Einstein predicted.

1937: Prediction of a galactic gravitational lensing

Swiss astronomer Fritz Zwicky proposed that an entire galaxy could act as a gravitational lens.

1959: Gravitational redshift verified

The theory was conclusively tested by Robert Pound and Glen Rebka by measuring the relative redshift of two sources at the top and bottom of Harvard University’s Jefferson Laboratory tower. The experiment accurately measured the tiny change in energies as photons travelled between the top and the bottom.

Wes Mountain/The Conversation, CC BY-ND

1960: Laser invented using stimulated emission

Theodore H. Maiman, a physicist at Hughes Research Laboratories in California, builds the first laser.

1960s: First evidence for black holes

The 1960s was the beginning of the renaissance of general relativity, and saw the discovery of galaxies that were powered by the immense pull of black holes in their centres.

There is now evidence of massive black holes in the hearts of all large galaxies, as well as there being smaller black holes roaming between the stars.

1966: First observation of gravitational time delays

American astrophysicist Irwin Shapiro proposed that if general relativity is valid, then radio waves will be slowed down by the sun’s gravity as they bounce around the solar system.

Wes Mountain/The Conversation, CC BY-ND

The effect was observed between 1966-7 by bouncing radar beams off the surface of Venus and measuring the time taken for the signals to return to Earth. The delay measured agreed with Einstein’s theory.

We now use time-delays on cosmological scales, looking at the time differences in flashes and flares between gravitationally lensed images to measure the expansion of the universe.

1969: False detection of gravitational waves

American physicist Joseph Weber (a bit of a rebel) claimed the first experimental detection of gravitational waves. His experimental results were never reproduced.

Wes Mountain/The Conversation, CC BY-ND

1974: Indirect evidence for gravitational waves

Joseph Taylor and Russell Hulse discover a new type of pulsar: a binary pulsar. Measurements of the orbital decay of the pulsars showed they lost energy matching the amounts predicted by general relativity. They receive the 1993 Nobel Prize for Physics for this discovery.

Wes Mountain/The Conversation, CC BY-ND

1979: First observation of a galactic gravitational lens

The first extragalactic gravitational lens was discovered, when observers Dennis Walsh, Bob Carswell and Ray Weymann saw two identical quasi-stellar objects, or “quasars”. It turned out to be one quasar that appears as two separate images.

Since the 1980s, gravitational lensing has become a powerful probe of the distribution of mass in the universe.

1979: LIGO receives funding

US National Science Foundation funds construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

1987: Another false alarm for gravitational waves

A false alarm on direct detection from Joseph Weber (again) with claimed signal from the supernova SN 1987A using his torsion bar experiments, which consisted of large aluminium bars designed to vibrate when a large gravitational wave passed through it.

1994: LIGO construction begins

It took a long time, but the construction of LIGO finally began in Hanford, Washington, and Livingston, Louisiana.

 

2002: LIGO starts first search

In August 2002, LIGO starts searching for evidence of gravitational waves.

2004: Frame dragging probe

NASA launches Gravity Probe B to measure the spacetime curvature near the Earth. The probe contained gyroscopes that rotated slightly over time due to the underlying spacetime. The effect is stronger around a rotating object which “drags” spacetime around with it.

Wes Mountain/The Conversation, CC BY-ND

The gyroscopes in Gravity Probe B rotated by an amount consistent with Einstein’s theory of general relativity.

Wes Mountain/The Conversation, CC BY-ND

2005: LIGO hunt ends

After five searches, the first phase of LIGO ends with no detection of gravitational waves. The sensors then undergo an interim refit to improve sensitivity, called Enhanced LIGO.

2009: Enhanced LIGO

An upgraded version called Enhanced LIGO starts new hunt for gravitational waves.

2010: Enhanced LIGO hunt ends

Enhanced LIGO fails to detect and gravitational waves. A major upgrade, called Advanced LIGO begins.

2014: Advanced LIGO upgrade completed

The new Advanced LIGO has finished installation and testing and is nearly ready to begin a new search.

2015: False alarm #3 for gravitational waves

The indirect signature of gravitational waves in the early universe was claimed by the BICEP2 experiment, looking at the cosmic microwave background. But it looks like this was dust in our own galaxy spoofing the signal.

2015: LIGO upgraded again

Advanced LIGO starts a new hunt for gravitational waves with four times the sensitivity of the original LIGO. In September, it detects a signal that looks likely to be from the collision between two black holes.

2016: Gravitational wave detection confirmed

After rigorous checks, the Advanced LIGO team announce the detection of gravitational waves.

Wes Mountain/The Conversation, CC BY-ND

Geraint Lewis, Professor of Astrophysics, University of Sydney

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Why is Einstein’s general relativity such a popular target for cranks?

Michael J. I. Brown, Monash University

Scientists maybe celebrating the 100th anniversary of Albert Einstein’s general theory of relativity, but there was also a death in 1915. It was one of the many deaths of simple and intuitive physics that has happened over the past four centuries.

Today the concepts and mathematics of physics are often removed from everyday experience. Consequently, cutting edge physics is largely the domain of professional physicists, with years of university education.

But there are people who hanker for a simpler physics, toiling away on their own cosmologies. Rightly or wrongly, these people are often labelled cranks, but their endeavours tell us much misconceptions about science, its history and what it should be.

I regularly browse open access website arxiv.org to look for the latest astrophysics research. Real astrophysics, that is. But if I want to take a look at what pseudoscientists are up to, I can browse vixra.org. That’s right, “arxiv” backwards. The vixra.org website was founded by “scientists who find they are unable to submit their articles to arXiv.org” because that website’s owners filter material they “consider inappropriate”.

There are more than 1,800 articles on vixra.org discussing relativity and cosmology, and many don’t like relativity at all. Perhaps one reason why cranks particularly dislike relativity is because it is so unlike our everyday experiences.

Einstein predicted that the passage of time is not absolute, and can slow for speeding objects and near very massive bodies such as planets, stars and black holes. Over the past century, this bizarre predication has been measured with planes, satellites, and speeding muons.

But the varying passage of time is nothing like our everyday experience, which isn’t surprising as we don’t swing by black holes on our way to the shops. Everyday experience is often central to cranky ideas, with the most extreme example being flat earthers.

Thus many crank theories postulate that time is absolute, because that matches everyday experience. Of course, these crank theories are overlooking experimental data, or at least most of it.

History and linearity

One of the most curious aspects of pseudoscience is an oddly linear approach to science. To be fair, this can result from an overly literal approach to popular histories of science, which emphasise pioneering work over replication.

A pivotal moment in relativity’s history is Albert Michelson and Edward Morley’s demonstration that the speed of light didn’t depend on its direction of travel nor the motion of the Earth.

Of course, since 1887 the Michelson-Morley experiment has been confirmed many times. Modern measurements have a precision orders of magnitude better than the original 1887 Michelson-Morley experiment, but these don’t feature prominently in popular histories of science.

Interestingly many pseudoscientists are fixated on the original Michelson-Morley experiment, and how it could be in error. This fixation assumes science is so linear that the downfall a 19th century experiment will rewrite 21st century physics. This overlooks how key theories are tested (and retested) with a myriad of experiments with greater precision and different methodologies.

Another consequence of the pseudoscientific approach to history is that debunked results from decades past are often used by buttress pseudoscientific ideas. For example, many pseudoscientists claim Dayton Miller detected “aether drift” in the 1930s. But Miller probably underestimated his errors, as far more precise studies in subsequent decades did not confirm his findings.

Unfortunately this linear and selective approach to science isn’t limited to relativity. It turns up in cranky theories ranging from evolution to climate.

Climate scientist Michael E Mann is still dealing with cranky accusations about his seminal 1998 paper on the Earth’s temperature history, despite the fact it has been superseded by more recent studies that achieve comparable results. Indeed, it devoured so much of Mann’s time he has literally written a book about his experience.

What about the maths?

During the birth of physics, one could gain insights with relatively simple (and beautiful) mathematics. My favourite example is Johannes Kepler’s charting of the orbit of Mars via triangulation.

In the 17th century, Johannes Kepler used elegantly simple mathematics to chart the motion of Mars. Johannes Kepler / University of Sydney

Over subsequent centuries, the mathematics required for new physical insights has become more complex, as illustrated by Newton’s use of calculus and Einstein’s use of tensors. This level of mathematics is rarely in the domain of the enthusiastic but untrained amateur? So what do they do?

One option is to hark back to an earlier era. For example, trying to disprove general relativity by using the assumptions of special relativity or even Newtonian physics (again, despite the experiments to the contrary). Occasionally even numerology makes an appearance.

Another option is arguments by analogy. Analogies are useful when explaining science to a broad audience, but they aren’t the be-all and end-all of science.

In pseudoscience, the analogy is taken to the point of absurdity, with sprawling articles (or blog posts) weighed down with laboured analogies rather than meaningful analyses.

Desiring simplicity but getting complexity

Perhaps the most fascinating aspect of pseudoscientific theories is they hark for simplicity, but really just displace complexity.

A desire for naively simple science can produce bizarrely complex conclusions, like the moon landing hoax conspiracy theories. NASA/flickr

Ardents of the most simplistic pseudoscientific theories often project complexity onto the motives of professional scientists. How else can one explain scientists ignoring their brilliant theories? Claims of hoaxes and scams are commonplace. Although, to be honest, even I laughed out loud the first time I saw someone describe dark matter was a “modelling scam”.

Again, this isn’t limited to those who don’t believe in relativity. Simple misunderstandings about photography, lighting and perspective are the launch pad for moon landing conspiracy theories. Naively simple approaches to science can lead to complex conspiracy theories.

Changing intuition

Some have suggested that pseudoscience is becoming more popular and the internet certainly aids the transmission of nonsense. But when I look at history I wonder if pseudoscience will decay.

In the 19th century, Samuel Rowbotham promoted Flat Earthism to large audiences via lectures that combined wit and fierce debating skills. Perhaps in the 19th century a spherical world orbiting a sun millions of kilometres away didn’t seem intuitive.

But today we can fly around the globe, navigate with GPS and Skype friends in different timezones. Today, a spherical Earth is far more intuitive than it once was, and Flat Earthism is the exemplar of absurd beliefs.

Could history repeat with relativity? Already GPS utilises general relativity to achieve its amazing precision. A key plot device in the movie Interstellar was relativistic time dilation.

Perhaps with time, a greater exposure to general relativity will make it more intuitive. And if this happens, a key motivation of crank theories will be diminished.

Will general relativity become more widely understood via popular media, such as the movie Interstellar?

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Michael will be on hand for an Author Q&A between 4 and 5pm AEDT on Tuesday, November 24, 2015. Post your questions in the comments section below.

Michael J. I. Brown, Associate professor, Monash University

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

 

How we plan to bring dark matter to light

Alan Duffy, Swinburne University of Technology and Elisabetta Barberio, University of Melbourne

Long before we had the atomic theory of matter, scientists knew the air was real, even though it was invisible. This was because we could see its action as the wind caressed the leaves in trees.

Likewise we see the influence of another invisible force in the wider cosmos in the movement of stars within galaxies. But we don’t yet know what this mysterious “dark matter” is made of.

Now a new generation of detectors – including one we’re building in a gold mine in Victoria – is giving us hope that we might finally shed some light on dark matter.

Glow in the dark

Some models predict that whatever particle makes up dark matter is also its own antiparticle. This leads to the fascinating prediction that if two dark matter particles interact they annihilate into a shower of either exotic particles or radiation.

If it annihilates into particles, then space-based detectors, such as the Alpha Magnetic Spectrometer (AMS) on the International Space Station, might detect unusual numbers of, say, positrons. If it annihiliates into radiation (or if the positrons themselves annihilate), then the radiation will be in the form of highly energetic gamma-rays, which could be detected by NASA’s Fermi Gamma-ray Space Telescope orbiting above the Earth.

The Alpha Magnetic Spectrometer mounted on the International Space Station could help detect the signs of dark matter. NASA

If so, the signal will be strongest where the density of dark matter is highest. This could be near the centre of our galaxy, where it is pulled close by the enormous gravity of the densely-packed stars and supermassive black hole.

Unfortunately, black holes and nearby exploding stars can all produce similar signals to annihilating dark matter. This makes it hard to discriminate any dark matter signal from black hole or supernovae noise.

However, if we were to find a clump of dark matter that was glowing brightly in gamma-rays, and there were barely any stars within, then we could be far more confident that we were seeing signs of dark matter.

Fortunately, there are such objects orbiting the Milky Way, known as an ultra-faint dwarf spheroidal galaxies. But, unfortunately, there appears to be no confirmed detection of gamma-rays from these objects, although there are hints there might be something interesting going on within.

To confirm the nature of dark matter there is no substitute for direct detection in the lab. It might be possible to produce dark matter during collisions in the Large Hadron Collider at CERN, in which case it would fly through the detectors without ever setting them off.

Its presence would be revealed in the same way as a dodgy accountant: we measure all the energy that goes into a collision, and we measure all the energy that comes out. If it doesn’t add up, we know that some energy has escaped in the form of dark matter.

The Large Hadron Collider might be able to create dark matter particles. CERN

Digging for dark gold

There is another option, and that is to try to detect the naturally occurring dark matter of our galaxy that the Earth ploughs through each year. This relies on the ghost-like dark matter colliding with the nucleus of an atom in a head-on collision.

Indeed, in the time it’s taken you to read this article, it’s likely that you’ve had an atom knocked away by a dark matter particle. It’s unlikely that you felt it, though, as humans make for bad detectors. But we’re building a better one.

With an international consortium of universities, research agencies and industry we are constructing the Stawell Underground Physics Laboratory (SUPL) a kilometre underground at a gold mine in Stawell, Victoria. This will house the world’s first dark matter detector in the southern hemisphere, known as SABRE.

We use the layers of rock above to block radiation from space that would otherwise overwhelm our sensitive detector. This ensures that only the ghostly dark matter is able to pass through the solid rock, and will occasionally collide with the detector.

Some of the lead scientists of the SABRE experiment in the Stallwell gold mine. In the background is the radiation testing facility. Carl Knox (Swinburne University), Author provided

The SABRE experiment consists of an ultra-pure sodium iodide crystal doped with thallium that has extraordinarily low levels of radiation (we don’t want to see our own radioactive “glow” after all). This unique crystal, created by Princeton’s Professor Frank Calaprice, will occasionally be struck by a dark matter particle, causing the nucleus of an atom to recoil away like a game of billiards. The atom will be energetically excited during the collision and eventually release this energy as a high energy gamma-ray.

The sodium iodide crystal itself is a natural scintillator, taking this gamma-ray and producing a flash of optical light that the sensitive cameras around the crystal can detect. So, in hunting for ghosts, we look for faint flashes of light in the dark.

We hope that, in time, we might finally shed some light on dark matter, and gain an insight into this mysterious substance that makes up five times more of the mass of the universe than what we can see.

Alan Duffy, Research Fellow, Swinburne University of Technology and Elisabetta Barberio, Professor of High Energy Physics, University of Melbourne

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Isaac Newton on seeing further

Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1726/7) was an English physicist and mathematician (described in his own day as a “natural philosopher“) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), first published in 1687, laid the foundations for classical mechanics. Newton made seminal contributions to optics, and he shares credit with Gottfried Leibniz for the development of calculus.

Feynman on scientific method

Physicist Prof. Richard Feynman explains the scientific and unscientific methods of understanding nature.

Is Philosophy Dead?

Philosophy Professors Daniel Kaufman & Massimo Pigliucci discuss the value of philosophy in light of recent attacks from a few well-known scientists. They argue that such attacks on philosophy are expressions of sheer ignorance, and result in a certain kind of anti-intellectualism.

Centrifugal force fiction

A claim of the existence of a ‘centrifugal force‘  is not strictly a logical fallacy, except when it is argued to be a real force. It is actually a fictional force that appears to draw a rotating body away from the center of rotation. It is caused by the inertia of the body, which would otherwise continue in  the direction of a straight line, if it were not for the constraining force causing  the body to rotate about the centre. Professor Julius Sumner Miller entertainingly demonstrates these forces here: