Einstein’s theory of gravity has been tested in lots of ways, from the slow precession of Mercury’s orbit, to the detection of gravitational waves. So far the theory has passed every test, but that doesn’t necessarily mean it’s completely true. Like any theory, general relativity is based upon certain assumptions about the way the universe works. The biggest assumption in relativity is the principle of general equivalence. 

The equivalence principle was proposed by both Galileo and Newton, and basically states that any two objects will fall at the same rate under gravity. Barring things like air resistance, a bowling ball and a feather should fall at the same rate. Experiments that have tested the principle of equivalence show it’s a good approximation at the very least.

In Newtonian gravity, this just means that the gravitational force on an object is proportional to its mass, so even if the equivalence principle is only an approximation we could still use Newtonian gravity. But Einstein’s theory of relativity, gravity isn’t a force, but simply an effect of the warp and weft of spacetime. In order for this to be true, the equivalence principle can’t be approximately true, it has to be exactly true. If objects “fall” due to the bending of space itself, then everything must fall at the same rate, because they are all in the same spacetime.

But there’s an interesting twist to this principle. One of the things relativity predicts is that mass and energy are related. This is where Einstein’s most famous equation, E = mc², comes into play. Normally the “relativistic mass” of an object is effectively the same as its regular mass, but objects like neutron stars have such strong gravitational and electromagnetic fields that their relativistic mass is a bit larger than the mass of their matter alone. If the gravitational force on an object is proportional to its mass-energy, then a neutron star should fall slightly faster than lighter objects. If Einstein is right, then a neutron star should fall at exactly the same rate as anything else.

A few years ago, astronomers discovered a system of three stars orbiting closely together. Two of them are white dwarf stars, while the third is a neutron star. The neutron star is also a pulsar, which means it emits regular pulses of radio energy. The timing of these pulses are determined by the rotation of the neutron star, which is basically constant. Any variation in the timing of the pulses is therefore due to the motion of the neutron star in its orbit. In other words, we can use the radio pulses to measure the motion of the neutron star very precisely.

Each of the stars in this system is basically “falling” in the gravitational field of the others. Recently a team of astronomers observed this system to see if the neutron star falls at a different rate different from Einstein’s prediction. Their result agreed with Einstein. To within 0.16 thousandths of a percent (the observational limit of their data) the neutron star falls at the same rate as a white dwarf.

Once again, Einstein’s gravitational theory is right.

Paper: A. Archibald et al. Testing general relativity with a millisecond pulsar in a triple system. 231st meeting of the American Astronomical Society, Oxon Hill, Md. (2018)

The post Testing Einstein’s Theory With A Triple Play appeared first on One Universe at a Time.

The life of a star is determined by its mass. Large stars live short lives that end in supernova explosions, while smaller stars live longer, ending their lives as white dwarfs. Knowing the mass of a star helps us understand not only the life of a star, but the evolution of galaxies. But determining the mass of a star can be difficult. 

The best way to weigh a star is to measure how strongly it pulls on another star. If two stars are a binary pair, the speed at which they orbit each other is governed by the gravitational pull between them. By measuring their orbits over time, we can determine the mass of each star. But many stars are solitary. The nearest star to them can be light years away, and its gravitational pull on these stars is too small to measure. So we need another way to determine its mass.

One alternative is to look at the temperature of a star. Larger mass stars burn hotter than smaller ones, so the higher a star’s temperature, the greater its mass. But this has a few downsides. For one, this relation between stellar temperature and mass is only true for main sequence stars. For another, stars get slightly hotter as they age. An old star with the Sun’s mass has a higher temperature than a young solar mass star.

A new way to measure a star’s mass looks at its surface gravity. A ball dropped near the surface of the Earth will fall at a rate of about 9.8 m/s². This is Earth’s surface gravity. Far away from the Earth gravity is weaker. The Moon, for example, “falls” around the Earth at  only about 2.7 mm/s². The surface gravity of a planet or star depends upon its mass and its diameter. By determining the distance to a star, we can use its apparent size to determine its diameter. Determining surface gravity is a bit more tricky.

If you bounce a ball against the ground, it takes a certain amount of time to rise to its maximum height and fall back to the ground. That time depends in part on surface gravity. If you were to bounce a ball in the same way on Mars, the time between bounces would be longer, because Mars has a smaller surface gravity. We can’t bounce balls on a star, but there are surface fluctuations that rise and fall. The surface of a star often churns a bit like boiling water, creating rising and falling pockets known as granules. The rate at which these granules rise and fall is determined by the star’s surface gravity. So by measuring the rate at which a star flickers in tiny ways, we can determine a star’s mass.

A recent paper looked at the observational limits of data from GAIA (currently gathering data) and TESS (planned to launch in March). They found that GAIA could determine a star’s mass give or take 25%, and TESS should be able to determine stellar mass to within 10%. Since these satellites will observe millions of stars, this could become a powerful tool in the study of stars.

Paper: Keivan G. Stassun, et al. Empirical, Accurate Masses and Radii of Single Stars with TESS and Gaia.  arXiv:1710.01460 (2017)

The post How To Weigh A Star appeared first on One Universe at a Time.

Last year astronomers made the first detection of gravitational waves from the merging of two black holes. It gave us an entirely new way to view the cosmos. Now we aren’t limited by the emission and absorption of light by matter. We can explore the universe through ripples in the fabric of spacetime itself. Through recent observations we can study the most mysterious aspect of spacetime, known as dark energy. 

Dark energy is what causes the universe to expand. It makes up about 70% of the universe, but we don’t really know what it is. One reason for this is that we don’t know exactly how much it expands the universe. Cosmic expansion is typically defined in terms of the Hubble parameter H₀. Because the universe expands, more distant galaxies appear to be moving away from us faster than closer galaxies. The velocity of a distant galaxy is related to its distance by v =H₀d. We can measure the speed of a galaxy through the redshift of its light. The greater the galaxy’s speed, the more its light is shifted toward longer (red) wavelengths.

Various methods used in the cosmic distance ladder. Credit: Wikipedia

Knowing the distance of a galaxy and its observed redshift, we can determine the Hubble parameter. When we do this for lots of galaxies, we get a value of about H₀ = 67.6 (km/s)/Mpc. But there is a catch. We can’t measure the distances to the furthest galaxies directly. We use what’s known as the cosmic distance ladder, where we use one type of measurement to determine the distance of nearby stars, use that result with other observations to measure distances to close galaxies, and use that result to measure more distant galaxies. Each step in the ladder has its own advantages and disadvantages, and if one rung in the ladder is off, it throws off all the other ones.

Fortunately, we have other ways to measure the Hubble parameter. One of these is through the cosmic microwave background. This remnant echo of the big bang has small fluctuations in temperature. The size of these fluctuations tells us the rate of cosmic expansion (among other things). Observations by the Planck satellite gave a Hubble parameter value of about H₀ = 67.7 (km/s)/Mpc.

But other methods of measuring the Hubble parameter give slightly different results. For example, one method looked at how light is gravitationally lensed by distant galaxies. Gravitational lensing can create multiple images of distant supernovae, and since each image takes a different path around the galaxy, they arrive at different times. The timing of these images can be used to determine the Hubble parameter, and the result is about H₀ = 71.9 (km/s)/Mpc. A different method using distant supernovae gives a result as high as H₀ = 73 (km/s)/Mpc. So what is the real value of the Hubble parameter?

This is where gravitational waves come in. All of the measurements of the Hubble parameter so far rely upon observations of light. Gravitational waves provide us an entirely new method to measure cosmic distances. As two black holes or neutron stars begin to merge, they spiral ever closer to each other, creating gravitational waves we can detect. The frequency of these waves depends upon their masses, and their masses determine how much energy they produce when they merge. By comparing the energy they produce with the strength of the gravitational waves we observe, we know their distance. This is similar to the way standard candles are used in optical astronomy, where we know the actual brightness of a star or galaxy, and compare it to the observed brightness to determine distance. In fact, this new method has been termed a standard siren.

But distance isn’t enough to determine the Hubble parameter. We also need to determine its speed away from us. We aren’t able to measure the redshift of gravitational waves, so we can’t used them to measure speed. But when two neutron stars merge they produce both gravitational waves and light. For one such merger, we not only observed the light produced, but also its redshift. From that we can find the Hubble parameter. Since the distance is found directly from gravitational waves, it doesn’t rely upon the cosmic distance ladder or an assumed model of cosmic expansion. From this event it found H₀ = 70 (km/s)/Mpc.

While that result points to a larger Hubble parameter, the uncertainty of the result is really large. Based on the data, it could be as large as 82 or as small as 62. But this is only one measurement. As more mergers are observed, we will get more precise results. So gravitational waves will help us pin down the Hubble parameter. It’s only a mater of time, and space.

Paper: The LIGO Scientific Collaboration, et al. A gravitational-wave standard siren measurement of the Hubble constant. Nature 551, 85–88. doi:10.1038/nature24471 (2017)

The post Measuring The Universe With Gravitational Waves appeared first on One Universe at a Time.

Next month a total solar eclipse will be seen across the United States. It is one of the few eclipses to trace a path through several densely populated areas, and that means there’s plenty of opportunity to do some experiments, including one that’s stirred a bit of controversy for the past 60 years.

The most famous eclipse experiment is Arthur Eddington’s 1919 experiment showing that starlight is deflected by the mass of the Sun. It was the first experiment to confirm that Einstein’s theory of relativity was correct. But perhaps the second most famous eclipse experiment was performed in 1954 by Maurice Allais. Allais was an economist, and won the Nobel Prize in Economics in 1988. But he was also interested in alternative theories of gravity and electromagnetism. He thought that gravity could be an effect of a cosmic aether, and that effects of this aether could be observed during a solar eclipse. So in 1954 Allais conducted a simple experiment with a Foucault pendulum.

A basic pendulum is simply a mass connected to a cable or rod. When the pendulum is released, it swings back and forth at a regular rate. But given a bit of time time, the orientation of a pendulum will shift. The direction of its back and forth motion will change. This was first noticed by Léon Foucault in the 1850s. As Foucault demonstrated, the gradual shift of a pendulum is due to Earth’s rotation. Everything on the Earth moves around in a circle once a day. If you are on the equator, you would travel the entire circumference of the Earth in 24 hours. If you are near the north pole, you would travel only a small circle in 24 hours. This means that while everything on Earth moves in a circle once a day, things closer to the equator move faster than things closer to the Earth’s poles. Your speed depends upon your latitude. As a pendulum swings, it will be slightly closer to the equator at one part of its swing, and slightly farther away at another part. As a result, the motion of the Earth causes the orientation of the pendulum to shift slightly with each swing, an effect known as precession. The effect is very small, but it builds up.

Graph showing the precession rate shift during an eclipse. Credit: Allais, original paper.

Because the precession is due to Earth’s rotation, traditional physics says the rate of precession should be the same during an eclipse as any other time. But when Allais did his experiment, he found the rate of precession shifted during the eclipse. This came to be known as the Allais anomaly, or Allais effect (not to be confused with the Allais paradox, which deals with his economics work). This was unexpected, and it generated a lot of criticism. One of the main arguments was that Allais was not an experimental scientist. Although the experiment seems simple, it could be influenced by things such as atmospheric changes of temperature, pressure and humidity which can occur during a total eclipse. Eliminating these factors is challenging, even for an experienced experimentalist. A second criticism was that there is no clear mechanism for such an anomaly. Even Allais didn’t have a claim a mechanism beyond some vague non-traditional effect.

Of course the real proof of the pudding would be to repeat the experiment. Either the effect is real or it isn’t, and further experiments should lead to the truth. But total solar eclipses are not overly common, and they don’t often occur over university research labs. So only a handful of experiments have been done, and they’ve been done with equipment of widely varying precision. The results have been mixed. Allais repeated the effect in 1959, and found the effect again. In 1965 an experiment using a more precise gravimeter device found no effect, while a pendulum experiment in 1970 found some effect, though the cause was unclear. One of the more precise pendulum experiments was performed in 1990 by Tom Kuusela. Kuusela found no effect to within 1 part in 4 million. Another one by Horacio R. Salva in 2010 also failed to see any effect. So it seems pretty clear that the effect isn’t real. But that hasn’t stopped the controversy. There are a few experiments that claim to observe the effect, though they tend to be published in less mainstream journals. Supporters of the effect have cited it as evidence for everything from dark matter to the electric universe and a flat Earth. If you delve deep enough you find accusations that NASA covered up research from several eclipse experiments.

Long story short, the Allais’ effect was likely due to experimental error, but it remains an interesting story.

Paper:  Maurice Allais. Should the Laws of Gravitation be Reconsidered?, Aero/Space Engineering 9, 46–55 (1959)

Paper: Kuusela T. Effect of the solar eclipse on the period of a torsion pendulum, Phys. Rev. D 43, 2041–2043 (1991)

Paper: Horacio R. Salva. Searching the Allais effect during the total sun eclipse of 11 July 2010. Phys. Rev. D 83, 067302 (2011)

The post The Eclipse And The Pendulum appeared first on One Universe at a Time.

At the heart of most galaxies is a supermassive black hole. These powerhouses have a mass of millions or billions of Suns, and can create brilliant quasars when active. So what happens when two galaxies collide?

According to the models, when galaxies collide and merge, their two black holes capture each other in an orbital dance. Over time the black holes would spiral ever closer, eventually merging into a single black hole. We have seen smaller black holes merge through the gravitational waves they produce, but supermassive black holes mergers are rare, so we haven’t seen their gravitational waves. But if our models are correct, many galaxies should contain a binary black hole. The challenge is seeing them.

VLBA image of the central region of the galaxy 0402+379, showing the two cores, labeled C1 and C2, identified as a pair of supermassive black holes in orbit around each other.
Credit: Bansal et al., NRAO/AUI/NSF.

We have seen a few cases of binary supermassive black holes, but these are largely through indirect evidence. Seeing two black holes close together and proving they orbit each other takes careful observation over time. In 2009 a collection of radio telescopes across the world known as the Very Long Baseline Array (VLBA) observed two supermassive black holes that appeared to be close together. They appeared to be located in an elliptical galaxy known as 0402+379, making them an orbiting binary. But it was also possible that the two were a visual binary. That is, from Earth they could appear to be close together, but in reality one of the black holes could be much more distant.

To prove they orbit each other, VLBA made another set of observations in 2015. The positions of the black holes had shifted, which confirmed they were orbiting each other. To determine their orbit, the team also used VLBA data gathered in 2003. From the positions of these black holes over the course of more than a decade, they found they orbit each other with a period of about 30,000 years. They are only about 24 light years apart, and their combined mass is about 15 billion Suns.

Paper: K. Bansal et al. Constraining the Orbit of the Supermassive Black Hole Binary 0402+379. The Astrophysical Journal (2017). DOI: 10.3847/1538-4357/aa74e1

The post Black Hole Dance appeared first on One Universe at a Time.

In 1919 Arthur Eddington traveled to the island of Principe off the coast of West Africa to photograph a total eclipse. Mission was to observe the apparent shift of nearby stars by the Sun’s gravity. His experiment was a success, and it verified Einstein’s gravitational theory of general relativity. Since then, we have observed the gravitational deflection of starlight by the Sun numerous times. We have also seen the deflection of the light from distant galaxies, but we haven’t seen the deflection of distant starlight by another star. The gravitational effect of a single star is extraordinarily small. But recently a team has observed the deflection of starlight by a single white dwarf star.

The white dwarf is known as Stein 2051 B, and it’s been in the middle of a controversy for nearly a century. White dwarfs are formed with a star such as our Sun runs out of the hydrogen necessary to generate heat and pressure through nuclear fusion. The star collapses under its own weight until it reaches a point where the pressure of electrons keeps it from collapsing any further. White dwarfs have the mass of a star, but are about the size of Earth. This small size makes them difficult to study.

In 1930, a 19-year old named Subrahmanyan Chandrasekhar calculated the theoretical structure of white dwarfs. Similar models were developed by Wilhelm Anderson and E. C. Stoner, but Chandrasekhar’s was more accurate, and included the calculation of an upper limit to the mass of white dwarf, now known as the Chandrasekhar limit. His model was highly controversial, and Eddington was one of the biggest opponents of the model. As astronomers found more examples of white dwarfs, it became clear that Chandrasekhar’s model worked. But Stein 2051 B seemed to be an exception. It has a distant companion star that allowed us to get a rough idea of its mass and it seemed that Stein 2051 B had a mass that is much smaller than most white dwarfs. This would imply it has some strange structure such a s a large iron core.

A schematic of the gravitational lensing effect. Credit: Wikipedia

But then in 2014 the white dwarf happened to pass in front of a more distant star, as seen from Earth. This allowed astronomers to observe the effects of gravitational lensing by the white dwarf. Using data from the Hubble Space Telescope, the team analyzed the gravitational deflection of the distant star. Using general relativity, they then calculated the size and mass of the white dwarf. They found it has a mass of 0.675 solar masses, which is larger than previously thought and well within the typical range of a white dwarf. So Stein 2051 B doesn’t have an exotic composition after all.

Arthur Eddington was an extremely prominent astronomer, and his opposition to the young upstart’s model meant it gained little traction early on. But the good thing about science is that in the end the data rules. It is perhaps poetic justice that Chandrasekhar’s model has been vindicated by the very experiment Eddington so famously first used.

Paper: Kailash C. Sahu, et al. Relativistic deflection of background starlight measures the mass of a nearby white dwarf star. Science
DOI: 10.1126/science.aal2879 (2017)

The post The Attraction Of A Star appeared first on One Universe at a Time.

During the Archean Eon of planet Earth, when life was figuring out how to harness energy from the Sun, two black holes in a distant galaxy merged with a ripple of gravitational waves. Over the next 2.9 billion years these ripples traversed a vast and empty space, while on Earth a plucky little species of bipeds learned to use lasers and mirrors to measure gravitational vibrations smaller than the nucleus of an atom. When the gravitational ripples reach Earth, they become humanity’s third detection of merging black holes. 

With this third detection of gravitational waves, named GW170104, gravitational astronomy is coming into its own. Like the previous mergers, the initial black holes were stellar mass black holes (19.4 and 31.2 solar masses, respectively) and they merged to become a 48.7 solar mass black hole, releasing about 2 solar masses worth of energy as gravitational waves. This is similar to the other two mergers we’ve detected, and confirms that stellar mass black holes can be produced with a mass larger than 20 Suns. Observations of x-rays near black holes had previously demonstrated they could have masses between about 5 and 15 solar masses. The size of these mergers also support the existence of medium sized black holes, between the stellar mass size and the supermassive size seen in the centers of galaxies.

The gravitational waves of three confirmed black hole mergers, and one tentative merger. Credit: Credit: LIGO/B. Farr (U. Chicago)

This latest detection is also the most distant black hole merger we’ve observed, happening about 3 billion light years away, which is more than twice as distant as the previous mergers. This greater distance allows us to test Einstein’s theory in new ways, particularly a quantum aspect known as gravitons. In quantum theory, the forces between particles are caused by field quanta. For electromagnetism, these are photons. For the strong nuclear force, they are gluons. For gravity, the field quanta are known as gravitons. Gravitons are the only field quanta we haven’t observed. Gravity is the weakest of the four fundamental forces, so to directly observe a graviton you’d need something like a Jupiter-mass detector orbiting a neutron star. We aren’t likely to do that any time soon. But we do understand the theory of gravitons pretty well. One of the key predictions of relativity is that gravitons should be massless, like photons. As a result, gravitational waves should always propagate at the speed of light. With this new merger we can test this idea through a property known as dispersion.

Dispersion occurs when waves originating from the same source travel at different speeds. You can see this in a prism, where sunlight is spread out into a rainbow of colors. This is caused by the fact that the speed of light through glass varies with wavelength or color. A similar effect occurs in astronomy, when radio waves travel through the ionized plasma of interstellar space. We can actually use this effect to measure the distribution of gas and dust in our galaxy, for example. Light undergoes dispersion when traveling through plasma because the charged particles of the plasma interact strongly with light. When light travels through unionized gas, there isn’t any dispersion.

Since gravitational waves don’t interact strongly with other masses, there shouldn’t be any dispersion as they travel through the vacuum of space. However there is another way that dispersion might occur. If gravitons have mass, then gravitons with different energies would travel at different speeds. Over the course of 3 billion light years this dispersion would be big enough to observe. The latest merger showed no evidence of dispersion, which means gravitons (assuming they exist) appear to be massless. General relativity passes yet another test.

The next step for gravitational astronomy is to bring more detectors online. With the limited data we have, we can’t pin down the exact location of these mergers in the sky, so we can’t connect a merger event to things seen in the optical spectrum. More detectors will also let us gather more information about the rotation of black holes before their mergers, which will let us further test general relativity. We are still at the beginning stages of an entirely new field of astronomy, but already the power of this new field is starting to show.

Paper: B. P. Abbott et al. GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2. Phys. Rev. Lett. 118, 221101 (2017) DOI: 10.1103/PhysRevLett.118.221101

The post Black Holes And Gravitons appeared first on One Universe at a Time.

Dark matter is difficult to study. Since it doesn’t interact with light, it is basically invisible. But it does have mass, and that means it deflects light ever so slightly, an effect known as weak gravitational lensing. By observing the way light from distant galaxies is distorted, we can map the distribution of mass between us and the galaxies. Comparing this to the visible matter of galaxies allows us to map the presence of dark matter. This technique works well when measuring large regions of dark matter, such as the halos around galaxies, but gravitational lensing is such a weak effect it’s difficult to study the detailed structure of dark matter. That’s unfortunate, because the details are what we need to understand the nature of dark matter. 

A computer simulation showing filaments of dark matter between clusters of galaxies. Credit: Michael L. Umbricht

The dominant model for dark matter makes several predictions we can test. For example, it predicts that dark matter will clump together gravitationally, and that means galaxies will also cluster together at a particular scale. This is exactly the clumping scale we observe across the cosmos. But there are also predictions we can’t easily test, such as dark matter filaments. As dark matter clumps together, some of the dark matter should be left behind, forming filaments of dark matter that connect galaxies and clusters of galaxies. These filaments have long been thought to exist, but detecting them is extremely difficult. Their gravitational influence is so small any weak lensing they produce is almost indistinguishable from random noise.

There has been some evidence of dark matter filaments. Comparisons of faint lensing between galaxies agrees with models of dark matter filaments, but with weak data you have to be careful not to presume too much about your model. A new paper in MNRAS tries to overcome this issue by taking a different approach. Rather than trying to observe filaments within a single cluster of galaxies, the team looked at data from thousands of filaments.

Taking data from the Baryon Oscillation Spectroscopic Survey, they focused on about 23,000 pairs of Luminous Red Galaxies (LRGs). These galaxies are particularly bright, and are easy to distinguish from other galaxies. They also have very similar structures, which makes them useful to study statistically. The team then measured the weak lensing between these pairs of galaxies. Individually the lensing between them would be hard to distinguish from random distortions, but they then combined the data from the pairs to create an overall average. In this way any random distortions would tend to wash out, leaving only the effects of dark matter. The result is a statistical image of the dark matter filaments between galaxy pairs.

While the result is statistical, it doesn’t rely upon a dark matter model to infer its presence. It also agrees with the statistical predictions of dark matter filaments. It’s yet another success for the dark matter model.

Paper: Seth D. Epps et al. The weak-lensing masses of filaments between luminous red galaxies. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stx517 (2017)

The post The Dark Web appeared first on One Universe at a Time.

As dark matter continues to vex astronomers, new solutions to the dark matter question are proposed. Most focus on pinning down the form of dark matter, while others propose modifying gravity to account for the effect. But a third proposal is simply to remove gravity from the equation. What if the effects of gravity aren’t due to some fundamental force, but are rather an emergent effect due to other fundamental interactions? A new paper proposes just that, and if correct it could also explain the effects of dark matter.

The idea of emergent gravity isn’t entirely new. The most popular variation was proposed in 2010, where Erik Verlinde argued that gravity is not a fundamental force, but rather an effect that arises from the entropy of the Universe. Entropy is a property of thermodynamics. It’s often described as the unusable part of a system (or the waste heat if you will) and while that’s sometimes a useful description, a better description involves the amount of information contained within a system. An ordered system (say, marbles evenly spaced in a grid) is easy to describe because the objects have simple relations to each other. On the other hand, a disordered system (marbles randomly scattered) take more information to describe, because there isn’t a simple pattern to them. Basically, the more information it takes to describe a system, the more entropy it has.

Verlinde’s model uses this connection between thermodynamics (heat, energy, and forces) and information through a mathematical method known as the holographic principle. Since the information contained within a region of space depends upon the arrangement of objects within that region, moving the objects can change the entropy within the region. Verlinde demonstrated that this produces an entropic force that acts like gravity. From the basic idea of information entropy, one can derive Einstein’s equations of general relativity exactly.

Entropic gravity is an interesting idea, and it would explain why gravity is so difficult to bring into the fold of quantum physics, but it’s not without its problems. For one, since entropic gravity predicts exactly the same gravitational behavior as general relativity, there’s no experimental way to distinguish it as a better theory. There are also theoretical problems with the model. For example, if you try to describe a gravitationally closed system of masses within the model it only matches experiment if you place weird constraints on the entropy of the system.

But despite its problems the idea is at least worth exploring, and this latest work adds a new twist by describing the effects of dark matter. In the original formulation, the model focused on standard gravity. Specifically, it excluded dark energy. This new paper notes that since the dark energy of a region of space requires additional information to describe, including it in the model changes the entropy of a region of space. The paper then goes on to show how this additional information creates an additional entropic force. One that might account for the effects of dark matter similar to other modified gravity models such as Modified Newtonian Dynamics (MoND). Thus gravity, dark matter, and dark energy might all be connected through entropy.

While this seems like an elegant solution to several cosmological problems, there are plenty of reasons to be skeptical. For one, this new variation of emergent gravity still has the same theoretical difficulties of the original. Then there’s the fact that modified gravity models fail to explain large scale effects such as the clustering of galaxies, which regular gravity and dark matter explains very well. This new work is still more of an idea and less of a robust theory.

But even if the model doesn’t work out in the end, it demonstrates how thermodynamics and gravity are deeply connected in ways that aren’t obvious at first glance.

Paper: E. P. Verlinde. Emergent Gravity and the Dark Universe. arXiv:1611.02269 (2016)

Paper: E. P. Verlinde. On the Origin of Gravity and the Laws of Newton.  arXiv:1001.0785 (2010)

The post Emergence Of Gravity appeared first on One Universe at a Time.

Gravitational waves are produced when black holes, stars or planets orbit each other (among other things). As these masses move around each other, they create gravitational disturbances that radiate outward. While we’ve only recently detected gravitational waves directly, we’ve known the exist for decades because of a secondary effect known as inspiraling. As gravitational waves radiate away from two orbiting masses, they carry a bit of energy with them. As a result the two masses lose a bit of energy and move closer to each other. Over time they will spiral ever closer until they collide. 

The direct detection of gravitational waves showed how two black holes entered such a death spiral, merging to become a single black hole. We’ve also observed the inspiraling of a pulsar orbiting a companion star. The two haven’t collided yet, but will eventually collide in about 300 million years. So what about smaller masses? As the Earth orbits the Sun, does it produce gravitational waves? Does the Moon? Yes, but the effect is extremely tiny, so we can’t observe it directly.

Take the Earth, for example. The amount of gravitational energy lost by two masses in a circular orbit is pretty easy to calculate, and for the Earth and Sun it comes out to be about 200 watts. That tiny loss of energy means that the Earth moves closer to the Sun by about the width of a proton each day due to gravitational waves. Of course gravitational waves aren’t the only thing that affects Earth’s orbit. Because the Sun is radiating away about 5 million tons of mass every second, the gravitational attraction of the Sun is decreasing, the Earth actually moves away from the Sun by about 1.6 centimeters per year. Even that tiny effect is far more significant that the inspiraling effect of gravitational waves.

Technically all orbiting masses generate gravitational waves, so everything is inspiraling over time. But unless the masses are really large, such as stars and black holes, the effect is negligible. As far as planets are concerned, we can effectively ignore it.

The post Death Spiral appeared first on One Universe at a Time.

I’ve been getting a flurry of emails and comments recently from folks who don’t believe the Earth is round. It’s pretty straightforward to demonstrate to yourself that the Earth is indeed round, but this time the argument is about gravity and Earth’s (supposed) rotation. Water droplets on a ball will fly off if you rotate the ball due to centrifugal force. If the Earth rotates once a day, then stuff on the equator is moving at over 1,000 mph, while stuff near the poles is barely moving. How can gravity be strong enough to keep things from flying off the equator without simultaneously crushing things at the poles? 

The basic idea of gravity is that masses are mutually attracted to each other. As Newton described it, masses exert a force on other masses depending on how much mass it has, and how far away it is. Near the surface of the Earth, the gravitational force is about 10 times your mass. This number comes from that fact that force is a product of mass and acceleration, and the acceleration of gravity is about 10 meters per square second. That means that if you took a mass and let go of it, its speed would increase by about 10 meters per second (22 mph) each second. If the Earth weren’t rotating, the force of gravity would be basically the same everywhere on the planet, and “down” would always be toward the center of the Earth. But the Earth is rotating, say the scientists, so surely that would have an effect, right?

It turns out centrifugal force is easy to measure in the lab. Just swing a mass and measure how much the mass seems to pull outward. Yes, I know some of you will point out that this actually involves centripetal force, but the end result is the same. A common introductory physics lab involves performing just such an experiment to see how the speed of an object affects the centrifugal force. What you find is that the force depends upon the square of the speed divided by the radius of the circular motion. At the equator an object is moving about 1,000 mph, and it’s moving in a circle with a radius of about 4,000 miles. Plug these into our equation and that gives 250 miles per square hour. That sounds huge, but if you convert it to metric, you get 0.03 meters per square second. So gravity pulls an object at the equator with a force of about 10 times its mass, while the centrifugal force is pulling it away from the Earth at about 0.03 times its mass. Yes, things at the equator are moving fast, but the radius of the Earth is so large that it doesn’t produce much centrifugal force.

Saturn’s fast rotation means it is wider at the equator than the poles.

Since centrifugal force is only about 0.3% of the gravitational force, gravity always dominates, and we don’t notice the centrifugal force in our everyday lives. But modern gravitational measurements are extremely sensitive. We’ve measured the variation of gravity all over the globe, and we find it varies with latitude just as predicted by Newtonian gravity and centrifugal force. Earth’s rotation means you are slightly lighter at the equator, but Earth would have to rotate much, much faster to overcome gravity. Earth isn’t the only place where centrifugal force has an effect. Saturn, for example, has a day that is only 10 hour long, and as a result it’s equator is moving at more than 23,000 mph compared to its poles. That isn’t enough to make things fly off Saturn, but it does mean that the centrifugal force at the equator is about 19% of Saturn’s surface gravity. As a result Saturn bows outward at its equator.

So Earth’s rotation really does mean that you weigh less at the equator. The effect is small, but we can measure it, and it confirms once again that the Earth is round.

The post The Enemy’s Gate Is Down appeared first on One Universe at a Time.

The central idea of Einstein’s theory of gravity is the principle of equivalence. That is, objects will fall at the same rate under gravity regardless of their mass or composition. As long as there isn’t air resistance, a feather and a bowling ball dropped at the same time will strike the ground simultaneously. But what if one object is rotating and another is not? Will they fall at the same rate? 

We know that the rotation of a body can affect how things fall. When a body such as Earth rotates, it creates an effect known as frame dragging, which twists space and time slightly. Computer simulations of merging black holes show that two rotating black holes would merge at a different rate than if they weren’t rotating. So on a large scale gravity is affected by rotation.

This has led some theorists to wonder whether such a rotational effect might be a way to connect Einstein’s gravity with the quantum theory of atoms and molecules. It turns out that atoms and other quantum particles have a property known as spin. If we imagined an atom as a small sphere, we can imagine its spin as the rotation of that spin. The catch is that atoms are not little spheres, and spin is not physical rotation. Spin is an inherent property of a quantum object that behaves similar to the type of rotation we see every day.

So, would an atom with spin fall at a different rate than one without spin? More specifically, does the principle of equivalence hold for quantum objects with spin? A recent experiment tested this question by comparing the free fall of rubidium atoms with different orientations of spin.

The team compared the gravitational acceleration of atoms with spin +1 and -1. To use our rotating sphere analogy, this would be like comparing a sphere rotating clockwise about its north pole vs one rotating counterclockwise. They found that the two orientations of spin fall at the same rate to within 1 part in 10 million, which was the limit of observation for their experiment. In other words, spin has no affect on the rate at which an atom falls.

This result isn’t entirely unexpected. It’s been generally thought that the equivalence principle holds for both classical and quantum objects. The experiment does, however, rule out some of the more radical models trying to unify gravity and quantum theory.

Now we know the equivalence principle holds even for a quantum top.

Paper: Xiao-Chun Duan, et al. Test of the Universality of Free Fall with Atoms in Different Spin Orientations. Phys. Rev. Lett. 117, 023001 (2016).  arXiv:1602.06377 [physics.atom-ph]

The post Einstein’s Top appeared first on One Universe at a Time.