## Testing Quantum Spacetime

Posted by on Friday, December 4, 2015

Scientists at Fermilab have released some interesting new experimental results, and although it is a relatively minor result in the grand scheme of scientific research (especially since they found nothing new), the exotic nature of the this experiment makes it interesting. Especially for theorists, who in the last few decades have moved further away from the forefront of experimental knowledge.

The Holometer experiment has now disproven one popular theory of a quantized spacetime, and added more evidence for spacetime being continuous.

Since the dawn of science it has been assumed that space and time are continuous. If you pick any two points in space, or any two moments in time, you can always find a third point that lies halfway between them. And then a fourth and fifth point that lie between that new point and the original points. And so on ad infinitum.

And yet in quantum mechanics, which is arguably the most successful theory in history in terms of experimental confirmation, argues that this cannot be true. The electrons in an atom must exist at very specific energies and orbits. Light exists in discrete units called photons, which cannot be divided. And as the position of a particle is measured more precisely, knowledge of its momentum must always decrease due to the uncertainty principle.

And so it follows that space and time should not be continuous. For a variety of reasons - most of which are more technical than I can cover here - spacetime must be discrete as well. A continuous spacetime means that gravity is not quantized, which in turn would affect the quantization of everything else in nature. And we know that quantum mechanics works extremely well. Maybe nature has a solution for this conflict, but as yet humanity hasn't found it.

If spacetime is quantized, then it would also mean that there is a limit to the amount of information that a region of spacetime can hold. Each unit of spacetime would only be able to hold one bit of information, and so at some point spacetime would become saturated with information and nothing more could be pushed into it. And if the holographic principle is true (which I covered in an earlier article) then even less information can be held. Part of the purpose of the holometer is to try to saturate a small region of space using high energy one kilowatt lasers, which would then provide a maximum information density for spacetime and possibly prove one of these theories.

In this current result though, they are passing the lasers through an interferometer and looking for signs of quantum fluctuations in the positions of the mirrors in the experiment. The laser beams follow two different paths, each of which gets reflected from a mirror, and then the beams are combined at a detector. If the interference pattern is constant, spacetime could be continous. If it varies, then it could indicate that the mirrors are being moved by quantum fluctuations in their positions, which could in turn indicate spacetime is quantized.

And today Fermilab has released data that shows no indication of such fluctuations. Their data is already precise enough to rule out some of the simplest models of quantum spacetimes, and they are expected to continue increasing the precision in future. This means that either spacetime is continuous and the theorists need to find other solutions to the quantum mechanics conflicts, or that the scale of the spacetime quantization is smaller than previously thought. (As a theorist I tend to favour the latter solution, as the conflicts with quantum mechanics are much more difficult to resolve)

For now though it is an interesting result, and it has proven that our theories of a discrete spacetime need more development. It is also interesting because we finally have an experiment precise enough to study subatomic spacetime scales. And that can only lead to more interesting physics in the future!

The Holometer experiment has now disproven one popular theory of a quantized spacetime, and added more evidence for spacetime being continuous.

Since the dawn of science it has been assumed that space and time are continuous. If you pick any two points in space, or any two moments in time, you can always find a third point that lies halfway between them. And then a fourth and fifth point that lie between that new point and the original points. And so on ad infinitum.

And yet in quantum mechanics, which is arguably the most successful theory in history in terms of experimental confirmation, argues that this cannot be true. The electrons in an atom must exist at very specific energies and orbits. Light exists in discrete units called photons, which cannot be divided. And as the position of a particle is measured more precisely, knowledge of its momentum must always decrease due to the uncertainty principle.

And so it follows that space and time should not be continuous. For a variety of reasons - most of which are more technical than I can cover here - spacetime must be discrete as well. A continuous spacetime means that gravity is not quantized, which in turn would affect the quantization of everything else in nature. And we know that quantum mechanics works extremely well. Maybe nature has a solution for this conflict, but as yet humanity hasn't found it.

If spacetime is quantized, then it would also mean that there is a limit to the amount of information that a region of spacetime can hold. Each unit of spacetime would only be able to hold one bit of information, and so at some point spacetime would become saturated with information and nothing more could be pushed into it. And if the holographic principle is true (which I covered in an earlier article) then even less information can be held. Part of the purpose of the holometer is to try to saturate a small region of space using high energy one kilowatt lasers, which would then provide a maximum information density for spacetime and possibly prove one of these theories.

In this current result though, they are passing the lasers through an interferometer and looking for signs of quantum fluctuations in the positions of the mirrors in the experiment. The laser beams follow two different paths, each of which gets reflected from a mirror, and then the beams are combined at a detector. If the interference pattern is constant, spacetime could be continous. If it varies, then it could indicate that the mirrors are being moved by quantum fluctuations in their positions, which could in turn indicate spacetime is quantized.

And today Fermilab has released data that shows no indication of such fluctuations. Their data is already precise enough to rule out some of the simplest models of quantum spacetimes, and they are expected to continue increasing the precision in future. This means that either spacetime is continuous and the theorists need to find other solutions to the quantum mechanics conflicts, or that the scale of the spacetime quantization is smaller than previously thought. (As a theorist I tend to favour the latter solution, as the conflicts with quantum mechanics are much more difficult to resolve)

For now though it is an interesting result, and it has proven that our theories of a discrete spacetime need more development. It is also interesting because we finally have an experiment precise enough to study subatomic spacetime scales. And that can only lead to more interesting physics in the future!