Differences between gravitational and electromagnetic radiation

So far we have been emphasizing how, at a fundamental level, the generation and propagation of gravitational and electromagnetic radiation are quite similar. This is a major point in demystifying gravitational waves. But, on a more practical level, gravitational and electromagnetic waves are quite different: we see and use electromagnetic waves every day, while we have only recently (in 2015) detected gravitational waves – which is why they seemed so mysterious in the first place!

There are two principal differences between gravity and electromagnetism, each with its own set of consequences for the nature of their waves and the information they carry.

1. Gravity is a weak force, but has only one sign of charge.
   Electromagnetism is much stronger, but comes in two opposing signs of charge.
   

   This is the most significant difference between gravity and electromagnetism, and is the main reason why we perceive these two phenomena so differently. It has several immediate consequences:

   • Significant gravitational fields are generated by accumulating bulk concentrations of matter. Electromagnetic fields are generated by slight imbalances caused by small (often microscopic) separations of charge.
   • Gravitational waves, similarly, are generated by the bulk motion of large masses, and will have wavelengths much longer than the objects themselves. Electromagnetic waves, on the other hand, are typically generated by small movements of charge pairs within objects, and have wavelengths much smaller than the objects themselves.
   • Gravitational waves are weakly interacting, making them extraordinarily difficult to detect; at the same time, they can travel unhindered through intervening matter of any density or composition. Electromagnetic waves are strongly interacting with normal matter, making them easy to detect; but they are readily absorbed or scattered by intervening matter.
     
     
   • Gravitational waves give holistic, sound-like information about the overall motions and vibrations of objects. Electromagnetic waves give images representing the aggregate properties of microscopic charges at the surfaces of objects.
   
   
2. Gravitational charge is equivalent to inertia.
   Electromagnetic charge is unrelated to inertia.
   

   This is the more fundamental difference between electromagnetism and gravity, and influences the details of gravitational radiation, but in itself is not responsible for the dramatic differences in how we perceive these two types of radiation. Most of the consequences of the principle of equivalence in gravity have already be discussed, such as:

   • The fundamental field of gravity is a force gradient (or tidal) field, and requires an apparatus spread out over some distance in order to detect it. The fundamental field in electromagnetism is a force field, which can be felt by individual charges within an apparatus.
   • The dominant mode of gravitational radiation is quadrupolar: it has a quadratic dependence on the positions of the generating charges, and causes a relative "shearing" of the positions of receiving charges. The dominant mode of electromagnetic radiation is dipolar: it has a linear dependence on the positions of the generating charges, and causes a direct translation of the positions of receiving charges.