# Are White Holes Real?

Sailors have their krakens and their sea serpents. Physicists have white holes: cosmic creatures that straddle the line between tall tale and reality. Yet to be seen in the wild, white holes may be only mathematical monsters. But new research suggests that, if a speculative theory called loop quantum gravity is right, white holes could be real—and we might have already observed them.

A white whole is, roughly speaking, the opposite of a black hole. “A black hole is a place where you can go in but you can never escape; a white hole is a place where you can leave but you can never go back,” says Caltech physicist Sean Carroll. “Otherwise, [both share] exactly the same mathematics, exactly the same geometry.” That boils down to a few essential features: a singularity, where mass is squeezed into a point of infinite density, and an event horizon, the invisible “point of no return” first described mathematically by the German physicist Karl Schwarzschild in 1916. For a black hole, the event horizon represents a one-way entrance; for a white hole, it’s exit-only.

Theorists apply loop quantum gravity theory to black hole

Physicists have published a study applying Loop Quantum Gravity to an individual black hole, showing that singularities – or the infinite strengthening of the gravitational field that occurs deep within a black hole, insuring the annihilation of anything entering – may not be encountered. Instead, their model shows that gravity would eventually change, suggesting that the “other end” of a black hole might take one to another location within our own universe.

The Center of a Black Hole: Infinitely Massive Singularity or Portal into another Universe?

Black holes are one of the most naggingly peculiar objects in the universe. Beyond the event horizon of a black hole, our equations are turned upside down; they also get turned inside out when we attempt to fathom the singularity at its center when using the equations given to us by Einstein. To make life simpler, what if we removed the singularity all together? There is some math for that.

Coming full circle, it suggests that black holes do not hold singularities in their interior, but portals to another universe.

Image Credit: mondolithic.com

As a quantum theory of gravity, loop quantum gravity could potentially solve one of the biggest problems in physics: reconciling general relativity and quantum mechanics. But like all tentative theories of quantum gravity, loop quantum gravity has never been experimentally tested. Now in a new study, scientists have found that, when black holes evaporate, the radiation they emit could potentially reveal “footprints” of loop quantum gravity, distinct from the usual Hawking radiation that black holes are expected to emit.

In this way, evaporating black holes could enable the first ever experimental test for any theory of quantum gravity. However, the proposed test would not be easy, since scientists have not yet been able to detect any kind of radiation from an evaporating black hole.

The scientists, from institutions in France and the US, have published their study called “Probing Loop Quantum Gravity with Evaporating Black Holes” in a recent issue of Physical Review Letters.

“For decades, Planck-scale physics has been thought to be untestable,” coauthor Aurélien Barrau of the French National Institute of Nuclear and Particle Physics (IN2P3) told PhysOrg.com. “Nowadays, it seems that it might enter the realm of experimental physics! This is very exciting, especially in the appealing framework of loop quantum gravity.”

In their study, the scientists have used algorithms to show that primordial black holes are expected to reveal two distinct loop quantum gravity signatures, while larger black holes are expected to reveal one distinct signature. These signatures refer to features in the black hole’s energy spectrum, such as broad peaks at certain energy levels.

Using Monte Carlo simulations, the scientists estimated the circumstances under which they could discriminate the predicted signatures of loop quantum gravity and those of the Hawking radiation that black holes are expected to emit with or without loop quantum gravity. They found that a discrimination is possible as long as there are enough black holes or a relatively small error on the energy reconstruction.

While the scientists have shown that an analysis of black hole evaporation could possibly serve as a probe for loop quantum gravity, they note that one of the biggest challenges will be simply detecting evaporating black holes.

“We should be honest: this detection will be difficult,” Barrau said. “But it is far from being impossible.”

We present a pedagogical introduction to the notions underlying the connection formulation of General Relativity - Loop Quantum Gravity (LQG) - with an emphasis on the physical aspects of the framework. We begin by reviewing General Relativity and Quantum Field Theory, to emphasise the similarities between them which establish a foundation upon which to build a theory of quantum gravity. We then explain, in a concise and clear manner, the steps leading from the Einstein-Hilbert action for gravity to the construction of the quantum states of geometry, known as spin-networks, which provide the basis for the kinematical Hilbert space of quantum general relativity. Along the way we introduce the various associated concepts of tetrads, spin-connection and holonomies which are a pre-requisite for understanding the LQG formalism. Having provided a minimal introduction to the LQG framework, we discuss its applications to the problems of black hole entropy and of quantum cosmology. A list of the most common criticisms of LQG is presented, which are then tackled one by one in order to convince the reader of the physical viability of the theory.

An extensive set of appendices provide accessible introductions to several key notions such as the Peter-Weyl theorem, duality of differential forms and Regge calculus, among others.  The presentation is aimed at graduate students and researchers who have some familiarity with the tools of quantum mechanics and field theory and/or General Relativity, but are intimidated by the seeming technical prowess required to browse through the existing LQG literature. Our hope is to make the formalism appear a little less bewildering to the un-initiated and to help lower the barrier for entry into the field.

# Space-Time Is Not the Same for Everyone

July 9, 2013 — Before the Big Bang, space-time as we know it did not exist. So how was it born? The process of creating normal space-time from an earlier state dominated by quantum gravity has been studied for years by theorists at the Faculty of Physics, University of Warsaw. Recent analyses suggest a surprising conclusion: not all elementary particles are subject to the same space-time.

New Quantum Gravity-Based Hypothesis Suggests Black Holes Become White Holes Soon After They Form

Black holes, as fascinating as they are, are still sort of a mystery to science. Well, perhaps not the black holes themselves as much as the things that happen inside of them. Namely, what happens to information once it’s consumed? Physics says that it can’t be destroyed entirely, yet the nature of black holes themselves seem to indicate otherwise.

Now, a team of researchers have proposed a new hypothesis suggesting that black holes indeed do not destroy information. Instead, all of it is sent barreling back out into space following their transition into white holes (the polar opposites of black holes). Learn all about it here: http://bit.ly/1rU5edV

Image Credit: Victor Habrick VISIONS/SPL/Getty

The Tenants of Semi-classical Gravity

There are a number of hypotheses running around trying to reconcile general relativity and quantum mechanics. Some ideas have more credence than others, but across the board, there isn’t enough falsifiable testing one can perform on these hypotheses to separate fact from clever ideas. One such theory is approaching testability, the theory of semi-classical gravity. This, of course, is very exciting for theoretical physicists across the board.

First, what is it? Find out: http://goo.gl/9us7Ek

Image A Credit: Warner Brothers; Image B (http://ow.ly/tFKOd)

This is what I did this semester -- Loop Quantum Gravity

Let’s see how this works. Click the zoom in button once, and the text actually becomes legible. Or you can click open in a new window (or click here). I don’t even think you need a gmail account! This is hosted on google drive.

I wrote all of this document this semester. If anyone is interested in some of the background, but little of the technical detail, of loop quantum gravity, check it out. I can’t vouch for the accuracy nor readability, but I did the best I could. And LaTeX makes everything look nice.

Tumblr doesn’t host documents, so I thought I’d give this a shot.

Screw it, it wasn’t worth it. Whenever it loaded, the page was forced to jump to the document. The document is still up: click the link given so see my semester’s notes on quantum gravity.