Physics below the Planck scale

A month ago (I know! So far back! How do I dare blog about something so old!), I read this article on sciencedaily.com, a website I visit regularly to get my daily survey of what is being published in scientific journals. Before summarizing the story, let me give you the necessary but fascinating background.

If you have an interest in the nature of the world in which you live in, you might know that at the most fundamental level, the mighty science of physics has boiled down reality to two inconsistent theories: quantum field theory and general relativity. The former describes with unbelievable precision everything that happens at the smallest scales we have ever been able to probe (although that may change in the very near future), while the latter describes just as well everything we see when we look at the largest scales we can probe a.k.a. the entire visible universe. Let me stress this in case you missed it: these two theories are inconsistent!

You might say, well, if one describes the very small and the other describes the very large, don’t they deal with non-overlapping domains? In our day-to-day life, that is certainly true. However, there are two instances for which both descriptions of reality must be summoned: the beginning of the Universe and black holes. A physical theory that would accurately describe these would be called a theory of quantum gravity, of which we have several candidates like string theory and quantum loop gravity.

Let me be a little more precise in one of my previous statements. I said that general relativity was used to describe systems at large scales, but in reality, it is used to describe systems for which gravity is the most influential force. Generally speaking, that becomes the case at large scales, but at the early stages of the Big Bang or in the core of a black hole, gravity becomes influential on very small scales because matter is so dense. The other types of interactions described by quantum field theory are also present, and you have a complex interchange between gravity and the other fundamental forces. These interchanges are exactly what a quantum theory of gravity is supposed to shed light on, since it should describe adequately all types of interactions all at once.

Another way of looking at the problem between quantum field theory and general relativity comes from basic knowledge of both theories. If you know something about general relativity, it is that matter and energy bend space-time. If you know something about quantum mechanics, it is that you can’t know velocity and position of a system as accurately as you want simultaneously. This fact about quantum mechanics implies that in quantum field theory, you will have arbitrary amounts of matter and energy fluctuating in and out of existence, and the smaller the scales you probe, the larger these fluctuations will become. I know this is far from a straightforward conclusion, but I’d rather be incomplete than inaccurate in my explanation. For now bear with me. Putting these two facts from both theories together, you get that at small enough scales, there will be enough matter-energy fluctuating to distort space-time, transforming space-time into some kind of fractal foam.

From a back of the enveloppe calculation, this should happen at the Planck scale, which is around 1.6−35 meters. This is the so-called Planck scale. This is extremely small, and the most powerful particle accelerator we have right now, which is the LHC, can only probe down to 2.8−20 meters. This is 15 orders of magnitude away. To give you a slight idea, this is about the same as going from the length of your leg to the size of a single proton.

A typical back-of-the-enveloppe calculation...

We do have experiments out there which can probe the Planck scale. The idea is to take advantage of these very, very, very, oh-so-very dramatically energetic cosmic rays we observe every once in a while. I believe we may not yet be clear on the origin of these highly energetic cosmic rays, although conventional wisdom tells us that some of these come from supernova explosions. Some of these cosmic rays take the form of gamma ray bursts, which are a bunch of highly energetic photons arriving together in the very sensitive IBIS detector of the Integral satellite.

Whatever is the origin of these gamma ray bursts, they are orders of magnitude more energetic that what we produce at the LHC, and that they traverse many light-years of space. If there is any unexpected structure to space-time at the Planck scale, careful observation of these gamma ray bursts should tell us about it. This is exactly what is being reported on the Science Daily story I started with.

According to the Science Daily story, observation of the gamma ray burst designated GRB 041219A led to ruling out the possibility of unexpected structure to space-time down to… wait for it… 10-48 m or smaller!

This is a BIG DEAL! This would mean that our current ways of thinking about quantum gravity are almost all wrong!

Or so I thought. I couldn’t resist looking into the actual publication on which the story is based, and here is the all-too-important detail that the Science Daily story left out:

Unfortunately, the dimension-5 operator of (3) is not compatible with supersymmetry. We therefore have to resort in this case to dimension-6 operators.

Uh, OK. That doesn’t really help. However, I just noticed how late it is. And that this article is already well over 900 words. And I have a pretty good idea of how much I still have to write to make this intelligible… You have been a courageous reader to bear with me so far, you shall be rewarded soon, in the epic upcoming second part of this post!

Secret Hint: limits are model-dependent…

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One Response to Physics below the Planck scale

  1. Jean-Gab says:

    Tu vulgarises quand même bien je trouve… ça serait mentir que j,ai tout compris, mais je sais qu’en personne tu pourrais combler les troues d’ignorances de ma part!

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