Free-falling dead stars show that a cornerstone of physics holds up

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Triple star system involving a pulsar suggests Einstein was right.
Kevin Gill/Flickr, CC BY-ND

James Geach, University of Hertfordshire

It may not be intuitive, but drop a hammer and a feather and – in the absence of air resistance – they will hit the ground at exactly the same time. This is a key principle of physics known as “universal free fall”, stating that all objects accelerate identically in the same gravitational field. In fact, it’s an important theme in Albert Einstein’s immensely successful theory of general relativity, which describes how gravity works.

But although we know it holds true for hammers and feathers, it’s been unclear whether the principle extends to extreme objects such as stars. Now a new study, published in Nature, has tested the principle using a remarkably extreme astrophysical environment: a triple star system containing two white dwarfs and a pulsar (a rotating neutron star that beams radio waves). These objects are the extremely dense remnants of dead stars.

Spoiler alert: It turns out Einstein is still right, and it is getting even harder to prove him wrong.

But let’s start with the basics. Hold an object in your hand. It doesn’t matter what it is – the object will have some mass. We can think of that mass in two ways. Isaac Newton taught us that if we apply a force to a body it will undergo an acceleration, and the size of that acceleration is directly proportional to the amount of force applied – and inversely proportional to the mass itself. Give a broken-down car a push and it won’t accelerate very quickly, but apply that same push to a shopping trolley and you’ll send it careering down the aisle. When thinking about the acceleration of an object due to a force exerted on it, we think about the “inertial mass” of the body.

Any two objects with mass are attracted to each other through the gravitational force. So the object you are holding in your hand is attracted to the Earth, and the size of the force pulling it down is dependent on the mass of the object. In this case, we think about the “gravitational mass”.

If you dropped it, the object you are holding would “free fall” – the force of gravity would accelerate it towards the ground. The size of the force pulling the object down depends on the gravitational mass, but the amount of acceleration depends on the inertial mass. But is there any difference between the two types of mass? To find out, we can write down an equation of motion linking the two types of mass: inertial mass on one side of the equation and gravitational mass on the other.

The equation predicts something we can test using an experiment: if inertial mass is equivalent to gravitational mass, then all objects should fall towards the Earth with an identical acceleration regardless of their mass. That often surprises people. This is called the “Equivalence Principle”.

Galileo first noticed that plummeting objects fall at the same rate, and you can do this experiment yourself by simultaneously dropping two objects of different mass. However one problem doing this on Earth is the presence of another force acting on the falling bodies, called air resistance. If you drop a hammer and a feather, the feather will tend to gently drift down to the ground, lagging behind – the objects aren’t strictly in free fall. But go to the moon and do that experiment, as astronaut David Scott did during Apollo 15, where there is no air resistance, and the Equivalence Principle is clear to see.

Now, it has been unclear whether the theory does a good job of describing gravity in all situations. There is a lot at stake – if general relativity breaks down for certain situations then we would need a revised or modified theory of gravity. In particular, scientists have been wondering whether the universality of free fall applies to objects that have strong “self gravity” – a significant gravitational field of their own. Indeed some modified theories of gravity predict that the Equivalence Principle might be violated for strongly self-gravitating bodies in free fall, whereas general relativity says it should be universal.

Dance of stars

Thanks to an extreme laboratory in space – a triple stellar system 4,200 light years away – the new study managed to test this. That name doesn’t do it justice: we’re talking about two white dwarfs and a more massive “millisecond” pulsar (a neutron star rotating about 366 times a second, and beaming radio waves like a lighthouse). One white dwarf and the pulsar are orbiting each other every 1.6 days. In turn they also orbit around the other white dwarf every 327 days.

A triple stellar system involving normal stars, similar to the sun.

The pulsar-white dwarf pair can be considered to be in free fall towards the other white dwarf, because an orbit is just the case of free fall without ever reaching the ground, like satellites around the Earth. Of course, the pulsar and white dwarf are very massive objects themselves, with strong self gravity. General relativity predicts that the accelerations of the white dwarf and pulsar, due to being in free fall towards the outer white dwarf, should be identical – despite differences in mass and self-gravity.

Combining observations that span six years of monitoring, the astrophysicists carefully modelled the orbits of the pair. They measured a parameter called Delta, which describes the fractional difference between the acceleration of the white dwarf and the more massive pulsar. If general relativity holds, then Delta should be equal to zero. The results indicate that, within the uncertainties of the measurements, the difference in accelerations is indeed statistically consistent with zero – we can say with 95% confidence that Delta is less than 0.0000026.

This new constraint is far better than anything previously measured. It provides valuable new empirical evidence that general relativity remains our best model of how gravity works, so we are unlikely to need any new or modified theories at this point. This come just weeks after general relativity was proven right on a galactic scale for the first time.

The ConversationWill we ever find a situation where general relativity breaks down? In a way I do hope so, because it would reveal new physics. But the continuing success of general relativity, first written down a century ago, must surely be celebrated as one of the most incredible intellectual achievements of our species.

James Geach, Royal Society University Research Fellow, University of Hertfordshire

This article was originally published on The Conversation. Read the original article.

Discovering dopamine’s role in the brain: Arvid Carlsson’s important legacy

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Swedish pharmacologist and Nobel laureate, Arvid Carlsson.
Vogler/Wikimedia Commons, CC BY-SA

Patrick Lewis, University of Reading

Arvid Carlsson, the Swedish neuroscientist and Nobel laureate, died on June 29, 2018 at the age of 95. He had devoted his life to understanding how the brain works and was awarded the Nobel for his research into dopamine – an important chemical found in the brain.

So what is dopamine, and why did finding out about it merit the Nobel Prize?

Dopamine is a simple chemical, made in the body from an amino acid called tryptophan. Despite its simplicity, it plays an important role as a neurotransmitter – chemicals that brain cells use to communicate with one another.

What Carlsson did was to reveal exactly how significant dopamine is to the function of the brain. Before his research, most people thought that dopamine was just a precursor of a brain hormone called noradrenaline. By decreasing dopamine levels in the brains of rabbits in his lab in Gothenburg, Carlsson was able to show that if you don’t have the right level of dopamine in your brain, the circuits that determine how the brain controls movement don’t work properly.

Although Carlsson was investigating basic neuroscience, it wasn’t long before scientists and doctors realised that there were similarities between the problems with movement that Carlsson had observed in rabbits and the symptoms of Parkinson’s disease.

Parkinson’s disease is a neurodegenerative disorder, a type of disease where increasing numbers of brain cells die over time, causing patients to develop problems with their movement, including uncontrollable shaking, slowed movement and sudden freezing.

Following on from Carlsson’s research, doctors soon realised that if they examined the brains of people with Parkinson’s there was much less dopamine than you would find in a healthy brain. This is because the cells that make and use dopamine in the brain, dopaminergic neurons, are the cells that die in Parkinson’s disease.

This led researchers to propose a simple solution. If the symptoms of Parkinson’s are caused by too little dopamine, why not boost these levels, with a dopamine pill or injection, to help the brain work again?


Unfortunately, this approach didn’t work, as dopamine isn’t able to cross from the bloodstream into the brain. But providing people with Parkinson’s a precursor to dopamine, a chemical called levodopa that can get into the brain and is converted into dopamine, did work and provided relief from many of the symptoms of Parkinson’s. This was immortalised in the book Awakenings, written by neurologist Oliver Sacks and later made into a film starring Robin Williams and Robert De Niro.

In Awakenings, patients with post-encephalitic Parkinsonism (a viral disease similar to Parkinson’s disease) who were treated with levodopa, had almost miraculous improvements in their symptoms.

The ConversationUnfortunately, levodopa, and other drugs that target dopamine levels in the brain, only treat the symptoms of Parkinson’s, they don’t slow down the loss of brain cells that underlie the disease. Despite this, and some serious side effects, they remain the frontline drug in our fight against the disease. But we wouldn’t have these frontline drugs if it wasn’t for the important work that Carlsson conducted in the 1950s, and for which he shared the Nobel Prize for Physiology or Medicine in 2000.

Patrick Lewis, Associate Professor in Cellular and Molecular Neuroscience, University of Reading

This article was originally published on The Conversation. Read the original article.

Whale sharks gather at a few specific locations around the world – now we know why

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A whale shark basking in the Maldivian shallows.
Melody Sky, Author provided

Joshua Copping, University of Salford and Bryce Stewart, University of York

The whale shark is the largest fish in the world, but much of its lifecycle remains shrouded in mystery. These gentle giants gather in just a handful of places around the globe – something which has long baffled scientists – but our new research has started to explain why. Better understanding of whale shark movements could help prevent further population loss in a species that has already experienced a 63% population decline over the past 75 years.

When swimming solo, the whale shark, which can grow up to 18.8 metres in length and 34 tons in weight, travels all over the world. Recently, a group of scientists tracked the remarkable journey of one whale shark across the Pacific from Panama to the Philippines. At more than 12,000 miles it proved to be one of the longest migrations ever recorded.

Yet whale sharks are known to come together at just a few specific locations around the world. Anything from ten to 500 whale sharks may gather at any one time in areas off the coasts of Australia, Belize, the Maldives, Mexico and more.

Face to face with the world’s largest fish.
Jil Kühne, Author provided

Approximately 20 hotspots have been identified – mere pinpricks in the vastness of the world’s oceans – but we don’t know what exactly attracts the whale sharks to them. In some cases the sites are linked to a specific biological phenomenon – such as the spawning of land crabs at Christmas Island in the Indian Ocean, which provides whale sharks with the seasonal equivalent of a Christmas feast. Our new research aimed to discover whether there was something else that united the places where these giants of the ocean hang out.

It’s all about bathymetry

The physical features of these spots – known as their bathymetry – have been shown to influence gathering points in other marine species. So in collaboration with the Maldives Whale Shark Research Programme, we decided to investigate whether it drives whale shark gatherings in the same way.

Our new global study shows that whale sharks congregate in specific areas of shallow water, next to steep slopes that quickly give way to areas much deeper water (usually between 200 metres and 1,000 metres).

We identified three main reasons. First, the deep water is used by whale sharks for feeding. Studies have shown the sharks diving to depths of almost 2 kilometres (1,928 metres to be precise) to feed on zooplankton and squid.


Second, the steep slopes are known to bring nutrients up to the surface from the deep, which in turn increases the abundance of plankton and attracts large numbers of filter feeding species. And finally, in shallow water, as well as feeding on coral and fish spawn, the sharks are able to thermoregulate, warming themselves back up after their dives into deep water which gets as cold as 4℃.

Valuable but vulnerable

If you’ve ever seen or swum with a whale shark, it was most likely in one of these relatively shallow aggregation areas. Knowing where these hotspots are has provided local communities with a windfall from ecotourism. In the Maldives alone, economic benefits from whale shark-related activities were estimated at US$9.4m per year. Whale sharks are worth a lot more alive than dead – and with many of these meeting points in developing countries, the income is invaluable.

But with the increasing pressures of tourism comes new dangers for the sharks. Crowds of snorkelers and tourist vessels are increasingly disturbing the whale shark’s waters, and – more worryingly – risk potentially fatal strikes by boats. To protect these beautiful creatures and continue to reap the rewards of ecotourism, we recommend that marine protected areas should be set up around whale shark gatherings and codes of practice be followed when interacting with them.

Whale sharks are imposing, but feed on krill and other plankton.
MWSRP, Author provided

Deep mysteries remain

These discoveries have narrowed down some of the key reasons why whale sharks congregate where they do, but many mysteries remain. Do individuals travel between these hotspots? Coastal gatherings are predominately made up of immature male sharks, usually still just four or five metres long. So where are all the girls? And where do whale sharks mate and give birth? Mating and pupping have never been seen in the wild – but, intriguingly, up to 90% of the whale sharks passing through the Galapagos marine reserve are female and thought to be pregnant.

The ConversationCould this be a key labour ward for the world’s whale sharks? Last year a BBC film crew at the Galapagos attempted to follow a pregnant female in a submersible to watch it give birth, but to no avail. That’s one secret that the depths are keeping for now.

Joshua Copping, PhD Candidate in Environmental Sciences, University of Salford and Bryce Stewart, Lecturer in Marine Ecosystem Management, University of York

This article was originally published on The Conversation. Read the original article.