Is Space Digital? asks the cover story of this past February's Scientific American (by Michael Moyer). The subject of that article, the Fermilab's Holometer experiment (officially called Experiment E990), is a remarkable project. It is designed not to study some exotic particle or the properties of matter or energy, but to examine the fabric of space itself. The device at the heart of it is called an "interferometer". It is a 40-meter-long device that projects lasers down its length and then reflects them back. The goal is to be able to detect and analyze tiny fluctuations in the light beams as they travel along the devices arms and interfere with themselves.
The project's director is Professor Craig Hogan
of the University of Chicago's Department of Astronomy and Astrophysics. Hogan has long been involved in studying a theory of reality called "the Holographic Principle
". Simply put, the Holographic Principle describes th... okay
, there's nothing really "simple" about this.
Allow me to start again.
The Holographic Principle describes the Universe as a finite set of data stored on the two-dimensional edge of the cosmos. From that data, all the information that describes everything in existence is "projected", in the same way a three-dimensional hologram is projected from a two-dimensional surface.
That projection is us and everything around us
Don't get the wrong idea. When people hear hologram or projection, they think "illusion". As the Fermilab's FAQ
page puts it succinctly, all three spatial dimensions and everything in them are real, it's just that the Universe only needs two dimensions to store the data required to describe it all. The spatial dimensions with which we are familiar, are considered "emergent". It emerges from the two-dimensional information.
Professor Hogan and his team are looking for something called "holographic noise
". They are trying to determine if, at the finest level--"the Plank length
"--the Universe is composed of discreet, individual values, like the 1's and 0's in the binary system of a computer code.
In this interview, he explains why this is critical to testing the Holographic Principle.
The first interferometer was built over a hundred years ago. How does this experiment--and the device itself-- differ from previous ones?
Modern interferometers are much more precise because they use laser light and electronic detectors. Michelson's interferometers
used a very simple detector-- his eyes!
Our machine differs from other modern ones mainly in two respects--- it gathers data very quickly, so we can track variations on light travel time, and it correlates the outputs of two nearly-coincident but separate interferometers.
If I understand correctly, a Planck length is a trillion-trillion times smaller than a hydrogen atom. How is it possible to study anything at that scale?
We cannot see it directly, but detect its indirect effects. If a system moves randomly by a Planck length every Planck time
, then in the course of a microsecond it will move more than an attometer
, a detectable amount.
Is each Planck length just a unit of measure, like an inch, or are they actual discreet things, like a page in a book?
Prof. Hogan: A Planck length is a unit of measure. We don't yet know the detailed physics that goes with that.
The article in Scientific America is entitled "Is Space Digital?" How is that question related to the Holographic Principle?
Prof. Hogan: The holographic principle refers to a finite amount of information, which is to say, the state of things can be described by a finite string of zeros and ones.
So does that mean that if the results of the experiment have the potential to rule out an infinite Universe?
Prof. Hogan: They have the potential to rule out an infinite density of information in space. The universe itself could still go on forever, but would be a countable infinity of information.
What do we know already about how the universe uses information?
Prof. Hogan: I would say, that's physics. It's pretty rich.
Are you saying that information is "the stuff" of physics? Or "the stuff" of the Universe itself?
Prof. Hogan: It's kind of hard to tell the difference. Physics describes transformations, relationships and behaviors of the world; if you like, you can call those things transformations in the form or representation of information.
If the holometer indicates that space is digital, how might that change the way we see the world at large?
Prof. Hogan: It would be a clue that would help lead us to the underlying theory, but by no means the final answer. A lot of ideas could be ruled out if they could not account for the effect. The alternative is to say that it really is continuous, or has much more information than the holographic principle allows.
When should we expect the results and what should we look for in them?
Prof. Hogan: In a year or two, we hope to have the machine working at Planckian sensitivity. We will either detect the Planckian noise or get an upper limit that will eliminate a class of ideas for unifying space-time with the quantum.