Black Holes: Seeking proof for the no-hair theorem

According to general relativity, a black hole has three measurable properties: mass, rotation (angular momentum), and electric charge.

That's it. If you know those three things, you know all there is to know about the black hole.

If the black hole is interacting with other objects, then the interactions can be much more complicated, but an isolated black hole is just mass, rotation (angular momentum) and electric charge.

In general relativity this is known as the no-hair theorem.

The basic idea of the no-hair theorem is that the material properties of any object (referred to as "hair" because a physicist named John Wheeler once coined the phrase "a black hole has no hair") become unmeasurable (hence unknowable) as the object collapses into a black hole.

On the surface this seems fairly reasonable. If a neutron star collapses into a black hole, for example, all the neutrons and their interactions become trapped inside the black hole's event horizon when the black hole forms.

The same would be true for an object that was lopsided (say with a mountain range on one side). As it collapses into a black hole, any irregularities would be squashed flat as it approaches the black hole limit.

But there are also difficulties with the no-hair theorem. For one, even though it's referred to as a theorem, it has never been proved in general relativity. So it really should be called the no-hair hypothesis.

There have been lots of demonstrations that the theorem is reasonable, and computer simulations tend to agree that black holes stabilise to a structure defined by mass, rotation (angular momentum) and electric charge, but none of these reach the level of proof.

Then there is the problem that if a black hole really is just defined by mass, rotation (angular momentum) and electric charge, then it has no temperature, and that means that its entropy is zero.

This violates the principles of thermodynamics. Of course when we try to include quantum theory into our black hole description we know that black holes do have a temperature.

In Stephen Hawking's theory, the temperature of a black hole depends upon its mass, so even a Hawking black hole would be definable by mass, rotation (angular momentum) and electric charge.

It's possible that the no-hair theorem is valid even for a quantum black hole.

But there is a more subtle mystery that hides within the no-hair theorem, because it would seem that a black hole is much simpler than other massive objects such as planets, stars and the like.

If you think about an object like the Sun, it has a certain chemical composition, and it's giving off light with different wavelengths having varying intensities.

There are sunspots, solar flares, convection flows that create granules, and the list goes on.

The Sun is a deeply complex object that we have yet to fully understand, and yet, if our Sun were compressed into a black hole, all that complexity would be reduced to mass, rotation (angular momentum) and electric charge.

So what happens when a complex object like a star collapses into a black hole? Where does all that complexity go?

In physics we refer to that complexity as the physical information of a system. According to quantum theory, physical information is never lost, but according to general relativity and the no-hair theorem, physical information that enters a black hole is lost forever.

This contradiction is known as the black hole information paradox, or sometimes the 'firewall paradox.'

Now you might think that the easy answer is just to presume the no-hair theorem is wrong but it's not that simple, and if we started exploring that paradox, things would get a bit hairy.

Physicist suggests speed of light might be slower than thought

This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the centre of the remnant. 

Hubble (in green) and Chandra (in blue) data show the expanding shock wave. 

Credit: ALMA/NASA

Physicist James Franson of the University of Maryland has captured the attention of the physics community by posting an article to the peer-reviewed New Journal of Physics in which he claims to have found evidence that suggests the speed of light as described by the theory of general relativity, is actually slower than has been thought.

The theory of general relativity suggests that light travels at a constant speed of 299,792,458 meters per second in a vacuum.

It's the c in Einstein's famous equation after all, and virtually everything measured in the cosmos is based on it, in short, it's pretty important. But, what if it's wrong?

Franson's arguments are based on observations made of the supernova SN 1987A, it exploded in February 1987.

Measurements here on Earth picked up the arrival of both photons and neutrinos from the blast but there was a problem, the arrival of the photons was later than expected, by 4.7 hours.

Scientists at the time attributed it to a likelihood that the photons were actually from another source.

But what if that wasn't what it was, Franson wonders, what if light slows down as it travels due to a property of photons known as vacuum polarization, where a photon splits into a positron and an electron, for a very short time before recombining back into a photon.

That should create a gravitational differential, he notes, between the pair of particles, which, he theorizes, would have a tiny energy impact when they recombine, enough to cause a slight bit of a slowdown during travel.

If such splitting and rejoining occurred many times with many photons on a journey of 168,000 light years, the distance between us and SN 1987A, it could easily add up to the 4.7 hour delay, he suggests.

If Franson's ideas turn out to be correct, virtually every measurement taken and used as a basis for cosmological theory, will be wrong.

Light from the sun for example, would take longer to reach us than thought, and light coming from much more distant objects, such as from the Messier 81 galaxy, a distance of 12 million light years, would arrive noticeably later than has been calculated, about two weeks later.

The implications are staggering, distances for celestial bodies would have to be recalculated and theories that were created to describe what has been observed would be thrown out. In some cases, astrophysicists would have to start all over from scratch.

More information: Apparent correction to the speed of light in a gravitational potential, J D Franson 2014 New J. Phys. 16 065008 DOI: 10.1088/1367-2630/16/6/065008 

Clumped galaxies give General Relativity its toughest test

More than 600 000 galaxies from the BOSS survey were utilized to measure the strength of gravitational interactions of galaxies extremely far away from each other. 

This is a visual representation of that measurement; the amount that the circles are distorted, or squashed from perfect concentric rings, indicates the velocity that galaxies are falling towards each other and hence the strength of the gravitational interactions. 

Credit: BOSS/U. Portsmouth

Nearly 100 years since Albert Einstein developed general relativity, the theory has passed its toughest test yet in explaining the properties of observable Universe.

The most precise measurements to date of the strength of gravitational interactions between distant galaxies show perfect consistency with general relativity's predictions.

The results will be presented by Dr Lado Samushia at the National Astronomy Meeting 2014 in Portsmouth on Wednesday 25 June.

Using the observed distortions in galaxy positions, the team were able to measure the strength of gravity with a precision of 6 per cent, the strongest constraint of its kind as yet.

The measurements turned out to be perfectly consistent with the predictions of Einstein's general relativity theory.

"Gravity is the main driving force behind the growth of structure in the Universe. According to general relativity, gravity is a manifestation of the space-time curvature, massive objects curve the space-time around them, which affects the movement of other objects around them."

"It's a very elegant theory that has been successful in explaining the outcomes of many experiments, however it is not the only theory of gravity," explained Samushia.

"Theoretical physicists have proposed many alternative theories and modifications of general relativity and the challenge for observational physicists is to test the alternative theories with ever increasing precision."

Each point on the left panel represents a galaxy, while a right panel is an actually image of one of the patches of the sky observed by SDSS. 

Credit: SDSS.

More information: The research has been published in Samushia et al, "The Clustering of Galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS): measuring growth rate and geometry", Monthly Notices of the Royal Astronomical Society vol. 439, p. 3504, 2014. 

A preprint of the paper is available.