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New Impact Hypothesis Might Explain Our Moon’s Uniqueness

“This also helps explain some nice oddities all in one go. “What’s beautiful about this work is that we can end up with the current state of the moon — its orbit, its chemistry — with just one step, without invoking any other event,” says Sarah Stewart, a coauthor on the study. If the Earth rotated on its axis before the impact, and if the protoplanet that collided with Earth were in the Earth-Sun plane, none of this would be possible. But with this, we not only get a rapidly spinning young Earth and a 24-hour, 23.5º tilted Earth today, but a Moon that orbits out of the plane, nearly locked to the Sun rotationally, and locked to the Earth revolution-wise. The pieces all fall together beautifully.”

Why is our Moon so unlike every other moon in the Solar System? No other moon is such a large percent of its parent planet’s mass or size; no other moon rotates so far afield of its planet’s rotational axis; no other moon orbits so far out of the planet-Sun plane. Yet our Moon does it all. The giant impact hypothesis might explain why the Moon is made of the same material as Earth, but wouldn’t explain these features. Unless, that is, the giant impact occurred with a very large velocity out of the plane of proto-Earth’s orbit. Unless, again, the Earth weren’t rotating at 23.5º prior to the impact. This new tweak on the impact hypothesis, put forth by a team of authors earlier this week in the journal Nature, might explain the unique history of the Earth-Moon system, including some features we don’t normally think about as being puzzling.

It isn’t just Earth that’s unique in our Solar System, but the Moon as well. Combined, the great cosmic detective story might have a new lead suspect!

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First signs of weird quantum property of empty space?

A team led by Roberto Mignani from INAF Milan (Italy) and from the University of Zielona Gora (Poland), used ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile to observe the neutron star RX J1856.5-3754, about 400 light-years from Earth [1].

Despite being amongst the closest neutron stars, its extreme dimness meant the astronomers could only observe the star with visible light using the FORS2 instrument on the VLT, at the limits of current telescope technology.

Neutron stars are the very dense remnant cores of massive stars – at least 10 times more massive than our Sun – that have exploded as supernovae at the ends of their lives. They also have extreme magnetic fields, billions of times stronger than that of the Sun, that permeate their outer surface and surroundings.

These fields are so strong that they even affect the properties of the empty space around the star. Normally a vacuum is thought of as completely empty, and light can travel through it without being changed. But in quantum electrodynamics (QED), the quantum theory describing the interaction between photons and charged particles such as electrons, space is full of virtual particles that appear and vanish all the time. Very strong magnetic fields can modify this space so that it affects the polarisation of light passing through it.

Mignani explains: “According to QED, a highly magnetised vacuum behaves as a prism for the propagation of light, an effect known as vacuum birefringence.”

Among the many predictions of QED, however, vacuum birefringence so far lacked a direct experimental demonstration. Attempts to detect it in the laboratory have not yet succeeded in the 80 years since it was predicted in a paper by Werner Heisenberg (of uncertainty principle fame) and Hans Heinrich Euler.

“This effect can be detected only in the presence of enormously strong magnetic fields, such as those around neutron stars. This shows, once more, that neutron stars are invaluable laboratories in which to study the fundamental laws of nature.” says Roberto Turolla (University of Padua, Italy).

After careful analysis of the VLT data, Mignani and his team detected linear polarisation – at a significant degree of around 16% – that they say is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding RX J1856.5-3754 [2].

Vincenzo Testa (INAF, Rome, Italy) comments: “This is the faintest object for which polarisation has ever been measured. It required one of the largest and most efficient telescopes in the world, the VLT, and accurate data analysis techniques to enhance the signal from such a faint star.”

“The high linear polarisation that we measured with the VLT can’t be easily explained by our models unless the vacuum birefringence effects predicted by QED are included,” adds Mignani.

“This VLT study is the very first observational support for predictions of these kinds of QED effects arising in extremely strong magnetic fields,” remarks Silvia Zane (UCL/MSSL, UK).

Mignani is excited about further improvements to this area of study that could come about with more advanced telescopes: “Polarisation measurements with the next generation of telescopes, such as ESO’s European Extremely Large Telescope, could play a crucial role in testing QED predictions of vacuum birefringence effects around many more neutron stars.”

“This measurement, made for the first time now in visible light, also paves the way to similar measurements to be carried out at X-ray wavelengths,” adds Kinwah Wu (UCL/MSSL, UK).

This research was presented in the paper entitled “Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX J1856.5?3754”, by R. Mignani et al., to appear in Monthly Notices of the Royal Astronomical Society.

European Southern Observatory

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PEOPLE OF THE ANCIENT WORLD: Lucius Verus (Marcus Aurelius’ Co-Emperor) 

LUCIUS Verus (161-169 CE) was Marcus Aurelius’ adopted brother and co-emperor, a man whose time on the throne is overshadowed by the reign of the last of the Five Good Emperors.

 In the final years of the Pax Romana, a period of almost two centuries of relative peace and stability following the rule of Augustus, a young Stoic philosopher, Marcus Aurelius, came to the throne in 161 CE, and for the first time in the empire’s short history he chose to have someone rule at his side; his adopted brother Lucius Verus.

The future emperor Lucius Ceionius Commodus was born in 130 CE and raised in Rome, the eldest son of the one-time consul Lucius Aelius Caesar.

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Article by Donald L. Wasson on AHE

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