Gravitational waves are a theoretical consequence of a propagating energy disturbance through space-time. They are predicted by Einstein’s general relativity equations, and astrophysicists are going to great pains to try to detect the faint signature from the passage of these waves through local space. Unfortunately, even though millions of dollars have been spent on international experiments, the gravitational wave remains in equation form; there is little (direct) evidence to support their existence.
However, this doesn’t stop the US military from worrying about them and commissioned a 40-page report into whether high frequency gravitational waves could be used by an enemy. Excuse me? Gravitational waves… as a weapon?
Gravitational waves are more commonly associated with black hole collisions or spinning neutron star binaries, but it would appear the US Defense Intelligence Agency (DIA) has been losing some sleep about the perceived risk of gravitational wave generation falling into the wrong hands.
So what’s all the fuss about? Gravitational waves are predicted to permeate throughout the Universe, travelling at the speed of light; anything with a large mass is expected to generate them. The most frequently used example is a neutron star binary, where both neutron stars orbit each other. As neutron stars have such a high mass, they will distort space-time greatly, and as they orbit, swirls in space-time will be generated. This is analogous to swirling your finger in a pond; ripples will propagate away from the disturbance. In the case of the neutron stars, ripples in space-time will propagate away from the binary pair. These ripples are gravitational waves, and they carry energy away from the system.
As energy is constantly being removed from the orbiting binary, orbital energy will be lost, causing the binary to spiral closer and closer together. It is for this reason that gravitational waves are often referred to as carriers of gravitational radiation. Gravitational waves are not only generated by neutron stars. Colliding black holes, spinning neutron stars and supernovae are all thought to generate detectable gravitational waves.
A Null Result to Nowhere
The famous Hulse-Taylor binary is indirect proof that gravitational waves exist, as since the pulsar binary PSR B1913+16 was discovered in 1974, the measured orbital period has decreased at a rate of 0.0000765 seconds per year. This is a sure sign that energy is being lost from the system via gravitational waves.
So, gravitational waves are predicted, and neutron star binaries suggest gravitational waves do exist. But there has been no direct measurements of gravitational waves via numerous sensitive laser interferometers (such the US-based LIGO). The lack of detection means one of three things, either: a) gravitational waves do not exist, and general relativity has been misinterpreted, b) although sensitive, gravitational wave observatories are not sensitive enough (or they need a longer baseline), or c) detectors require more “exposure time” to collect data (in other words, “wait and see). As general relativity has proven itself to be pretty robust, a) is (hopefully) not the case. It is more likely a combination of b) and c).
Results from the Laser Interferometer Gravitational Wave Observatory (LIGO) have been more than a little frustrating in recent years, as nothing has been discovered. The problem is not that LIGO doesn’t work, the problem is that we are having a hard job in trying to characterize what a gravitational wave signal looks like. This ambiguity has lead to some bizarre claims from the LIGO team, including the assertation that the observatory was able to “probe” deep into the Crab Nebula to “see” the rapidly spinning pulsar in the middle. However, they didn’t do this by detecting gravitational waves… they did it by not detecting gravitational waves. Huh? The logic is that the Crab pulsar must be a very smooth neutron star (as, in some cases, the magnetic structure of neutron stars are thought to cause “lumps” of matter on the surface), and therefore cannot generate gravitational waves.
No gravitational waves = smooth neutron star = proof that gravitational waves exist!
I’m sure the situation is a little more detailed than that (the null result presents a lower bound to the detectable gravitational wave frequency), but it isn’t looking great for poor old LIGO…
Not So Evil
So, gravitational waves are simply ripples in space-time. As we live in space-time, how do we feel these waves? What’s more, how can we use these waves?
As for feeling gravitational waves, the only way we think we can observe them is by looking out for microscopic changes in distance between objects. Generally, even for the biggest interferometers (with baselines of kilometres), a traversing gravitational wave is predicted to disrupt the distance between two objects at either end of the detector by a fraction of the width of a proton. This might sound tiny, but the advanced optics used in LIGO will be able to detect this change.
If the effect of a gravitational wave is so small, and only the largest cosmic events are likely to generate detectable gravitational waves, why did the DIA commission a study into the uses of them?
The DIA wanted to know whether gravitational waves can be used for:
- Enemy communications.
- Object detection or imaging (i.e. gravitational wave “radar”).
- Vehicle propulsion.
Although this might sound a little silly, I can see where the DIA’s sci-fi imagination may have been stimulated. One of the main advantages to building a gravitational wave observatory would be to see deep into the hearts of stars, neutron stars and even black holes. Objects normally opaque to electromagnetic observation techniques would suddenly become transparent as gravitational waves will not be absorbed or easily deflected. The structure of stellar objects, as suggested by the “smooth” neutron star example, could be probed.
Say if an enemy of the USA was able to generate gravitational waves, or detect them with any degree of accuracy, the possibility of an ultra-penetrative radar could be used to peer deep into Ally territory. The thickest walls or underground bunkers could be vulnerable to such a device. Even better, the Earth wouldn’t be a problem either; gravitational waves travel straight through matter, regardless of size. An enemy on the opposite side of the planet could receive gravitational waves that had travelled through the US! It would be the ultimate form of privacy invasion.
Alas, it is a physical impossibility to do such things with gravitational waves. We are having a hard enough job discovering the things, let alone generate them. There is no known physical process that could harness this theoretical space-time ripple to be used in vehicle transportation. As a recent New Scientist article points out, the theories for using gravitational waves for “evil” or strategic advantage is pseudo-science.
“The proposal is utter nonsense,” said Karsten Danzmann, a gravitational wave physicist from the Max Planck Institute for Gravitational Physics in Hanover, Germany. “I’m a bit surprised the agency bothered to commission an investigation – it would probably have been enough to just ask an in-house science advisor.”
I’m actually very glad the agency commissioned an investigation. Although it took 40 pages of convincing, at least they didn’t jump straight in to develop the world’s first gravitational wave drive, wasting millions in the process.
Two tips for the DIA for the future:
1) Ask a physicist before spending money on an expensive study.
2) Give the money you would have spent on the expensive study to the guys at LIGO. They may be having a hard time at the moment, but they could sure use more funding…
Source: New Scientist