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Posts Tagged ‘Quantum Gravity’

Turtles all the way down?

Tuesday, March 20th, 2012

I recently got an interesting e-mail about the Big Bang. The writer said she didn’t see how you could make something out of nothing. She collects creation myths and thought that, no matter how you sliced it, it’s always “turtles all the way down.” This is a reference to creation myths where the world is poised on top of a turtle, which is itself poised on top of something else, but raises the issue: Is there any firm ground?

This is worth addressing because it illustrates the gulf between the understandings in people’s minds about the Big Bang on one hand, and how physicists deal with it on the other. To be clear – we have a wealth of observations that support the Big Bang, but you have to be careful. We can only look back into the universe to a moment 300,000 years after the ‘start,’ as best we can discern it. At this early moment, the universe went from being opaque to transparent. Before this moment, ionized gas kept light from traveling any distance, but once protons and electrons cooled enough to form neutral hydrogen, light (photons) could travel long distances. The remnant photons from this time are seen as the so-called cosmic microwave radiation. These photons were first observed by Arno Penzias and Robert Wilson in the 1960’s and continues to be a rich source of information about the early universe.

What do we see? We see galaxies moving away from each other. The further away we look, the faster they appear to recede. Einstein’s gravity has a number of solutions for possible universe structures. One of these solutions describes the expanding universe very well, and, if taken at face value, would extrapolate back in time to an initial state when all matter in the universe existed as a single point of infinite density. But, does a point of infinite density make sense? The author of the e-mail question thinks not, that it’s like pulling a rabbit out of the hat. You can’t make something from nothing, and this apparent absurdity invalidates the Big Bang model.

The main issue is that, although our observations are very consistent with this model of a Big Bang universe, we cannot actually see the initial moment. It’s hidden from view. We strongly suspect that the laws of physics might change dramatically when distances scales and energy densities approach the conditions very close to initial moment. We know that when the classical laws of physics are combined with quantum mechanics, new phenomena emerge. This was the case of our theory of electromagnetism. When we incorporated quantum mechanics with electromagnetism, the phenomenon of anti-matter became apparent. We have yet to find a satisfactory theory of gravity that combines quantum mechanics. The manifestations of quantum mechanics in gravity will only emerge at extremely high energy densities, such as those in the very early universe, near the time of this infinite density, and will likely modify our current models. For all we know, space-time might resemble some Escher print, eluding the concept of an infinite density starting point through a twisted configuration that folds in on itself.

Rather than dealing with a concept that seems almost theological in nature, physicists try to reconcile models against data. We fully realize that our models will extrapolate to conditions that raise difficult issues, like infinite densities. More often than not, the difficult conditions are something we avoid talking about, because, largely, we cannot really test or measure these. If it is inaccessible, it is inaccessible. The work can be perhaps more likened to the work of explorers. Our job is to map new territories, and, if anything, we can only report on territories we’ve explored. What lies beyond the horizon is a matter of speculation.
Responses? Questions? Contact me on Twitter @JohnHuth1


Weighing Antimatter

Thursday, July 28th, 2011

How much does antimatter weigh?

It is a great question and to be honest physicists don’t know. In fact, it is a great question precisely because we don’t know. To clarify: I am talking about “weight,” not “mass.” I wrote a few words at the bottom of this post about the difference between the two. For now I will just say that mass is what makes pushing or pulling something in a new direction harder; weight is that pull, by a planet’s gravity, on things that have mass. In the Universe, there are some kinds of matter that do not have mass, like photons (packets of light), and thus are also weightless. Other kinds of matter, like protons & electrons, do have mass and consequentially weigh something.

Figure 1: CERN’s Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) Experiment. (Photo: CERN)

Okay, so here is where things get interesting. Back in the 1920’s a guy named Paul Dirac discovered the theory of antimatter.  The theory not only predicted that each piece of matter has an “antimatter partner” but also that the two partners have the same mass. This morning, the ASACUSA Experiment (Fig. 1) at CERN announced that the anti-proton has the same mass as its partner, the proton. Well, at least up to experiment’s capabilities of resolving the two. Anyone keeping track of CERN’s anti-matter physics program, or has watched the first 15 minutes of “Demons & Angles,” might know that the lab has been making significant progress trapping and collecting anti-hydrogen. While the amount being produced at CERN may not be enough to make a small city-state disappear, it is close to the amount needed to determine the weight of anti-hydrogen. This is good news for physicists at Fermilab who are working on the Antimatter Gravity Experiment (AGE), the goal of which is to measure anti-hydrogen’s weight. Interesting, no?

Figure 2: A hydrogen atom consists of an electron and a proton orbiting around one another, and are kept together because of their mutual electric attraction. Similarly, an anti-hydrogen atom consists of a positron (anti-electron) and an anti-proton. (Image: Wikipedia)

Now for the exciting part. Our theories, e.g. the time-tested Standard Model of Physics, only say that matter-antimatter partners should have the same mass. There is NO reason whatsoever, other than helping one sleep at night, that the partners should have the same weight. Since weight is innately related to gravity, any measurement of an individual anti-particle’s weight is inherently a measurement of gravity at the quantum scale. Additionally, any description of the behavior of antimatter acting under gravity is at the very least a stepping stone to Quantum Gravity. Quantum Gravity, by the way, is the theory of gravity at the microscopic scale; it does not really exist, yet; and is preventing physicists from constructing a full description (theory) of our universe. Determining that the proton and anti-proton have the same mass makes it easier to spot any differences in their weight. On top of that, if there is a difference in the weight of hydrogen & anti-hydrogen, then it might also explain why there is so much more matter in the universe than antimatter.

If you are not excited by now, I give up. 🙂 Note: A big thanks to @symmetrymag for bringing this news to my attention.

A Few Words on Mass vs. Weight


Physically, “inertia” is the natural resistance to a change in movement; a measurement of inertia is called “mass.” One way to think about mass is if you & I were running down a football pitch, side-by-side, and you tried pushing me over. Mass is that bit of resistance you feel when you try pushing me over. If I were twice as tall, it would be harder to push me over. If I were half as tall, it would be easier to push me over. Next time you are playing football, like right after you read this Quantum Diaries post, try it out. “Weight” is that specific, attractive pull (force) a planet has on an object. The big difference is that mass is universal property of an object whereas weight can vary. A single electron will always have the same mass but a human will weigh less and less the further away he/she is from the Earth. Since this rock I like to call home is approximately a sphere, the gravitational pull it has at its surface is approximately constant. Consequentially, the difference between 1 lb (a unit of force) and 1 kg (a unit of mass) is a numerical constant. I hope this helped.


Happy Colliding.

– richard (@bravelittlemuon)