QUESTION #658

# I have read that gravity waves travel at the speed of light; does this mean that gravity waves are a part of the electromagnetic spectrum?

Asked by: John Oreopoulos

Put simply, the answer to your question is no, although the electromagnetic force has some similarities to the gravitational force. Modern Physics has now accepted something known as the Standard Model as a description of all fundamental particles and three of the four fundamental forces that act between these particles (electromagnetism, weak nuclear force, strong nuclear force). The model regards all forces as being 'transmitted' by a type of particle known as bosons, which are exchanged between particles in order to transmit a force. The Standard Model regards all electromagnetic radiation, which comprises the 'electromagnetic spectrum' and is responsible for the electromagnetic force, as consisting of discrete quanta (particles) known as photons. These photons have a number of distinguishing properties. Equally, although the Standard Model does not formally include gravity, it is currently accepted that the gravitational force must be transmitted in the same way as the other forces. The boson responsible has become known as the graviton, and again has a number of distinguishing properties. The reason for any uncertainty and assumption here is because the graviton is yet to be observed (this would be expected because gravity is by far the weakest force, making the graviton most difficult to observe). By comparing the fundamental properties of these bosons, it is clear that photons and gravitons are different, although they do share some of the same properties. e.g. Transmission velocity - c (speed of light) for both
Rest mass - zero for both
Charge - zero for both
Range - infinity for both
Spin - 1 for photon, 2 for graviton
Charge coupling - 2.31x10-28 Jm for photon, 1.87x10-64 Jm for graviton String Theory - open string for photon, closed string for graviton As you can see, many of the more commonly known, superficial properties of the two bosons are identical, but this fails to hold for a number of other properties that cause the two bosons, and so the two forces, to be very different. This is easily seen by the extreme difference in strength between the two forces (a small fridge magnet can hold a mass against the entire gravitational force of the planet). Hence it is the case that 'gravity waves', which can be viewed as simply consisting of coherent states of many gravitons, are not that the same as electromagnetic waves, can be viewed as simply consisting of coherent states of many photons, because photons and gravitons are fundamentally different. As a final note, it should be noted that some Physicists believe that the four forces will, in fact, be shown to be different low energy variations of the same phenomenon when viewed at very high energies, meaning all four forces are actually the same. This has already be shown to be true for the electromagnetic and weak nuclear force, which are now often referred to as the electroweak force. Whether the very weak gravitational force will follow this trend is unknown and a quantum theory of gravity is first required. Nevertheless, it should be remembered that we can only observe the Universe at relatively low energies, and we do not know how the forces behave at very high energies.
Answered by: Sam Cohen, Physics Student, LGS, Leeds, England

The speed of light should not be thought of as a characteristic of light specifically but should rather be thought of as a sort of "speed limit" of the universe. Information simply cannot move from one region in space to another region in space without some time delay. This should make some intuitive sense. Picture a line of marbles that are interconnected by springs, forming a line alternating between marble and spring over and over. If a marble on one side of the line is pushed in, before the rest move, it's logical to assume that time would have to be taken to compress the spring to a sufficient amount before the spring would exert enough force on the marble next to it, and this process would repeat there. Events that occur on one side of the line would take time to reach the other. Of course, you could stiffen the springs more and more until they were solid. However, inside the solids this same game would be played with molecules being held together by coulombic forces. If a sufficiently precise instrument would be used to measure the disturbances felt at one end of the marble-link chain, it would be clear that there still was some time delay between stimulation at one end and observation at the other. Scientists believe that the fabric of space has encoded into it a speed limit on information transfer. Depending on how "rigid" the fabric of space is, this speed limit could be very high; however, eventually at some level it will be there. It is not possible to make a change somewhere in the universe and have it be "felt" IMMEDIATELY at ALL other places in the universe -- it simply wouldn't make sense. At a very small scale, light can be thought of as made up of small quantized units called photons which have characteristics that follow probabilistic wave functions that give rise to behavior which appears wave-like when many of them are taken together. Because these particles have no mass, they have the easiest time moving through the fabric of space. Massless particles are like "marbles" with the stiffest springs between them. If there is any delay between the communication (i.e., the propagation of one of these particles through space) of information from one spot to another by way of these massless particles, that delay must be the delay of space itself. It must be a delay that can never be eliminated. That delay is what gives light its "speed." This speed is not only the speed of light but it is really the maximum speed of information propagation in the universe. Because light was studied intensely before the actual fabric of the universe was, this speed originally was tacked onto light specifically. This actually occurred when Maxwell modeled the time delay of light transmission and reception as the combination of reactive properties of space called permittivity and permeability. Maxwell didn't realize he was actually predicting the speed limit of all observable things; he was only concerned with light and other areas of physics concerned with other things were not yet invented. If gravity is modeled is a similar fashion, then its quantized units can be called gravitons, and those gravitons would behave in the same probabilistic way. When considering the simultaneous effect of many of them, they give rise to gravity waves. Because these gravitons also might be massless, they too would be governed by the same speed limit as light. If the existence of these gravitons can be confirmed, then we might call the "speed of light" the "speed of gravity" too. So you see it's inappropriate to attach properties of light to gravity just because it propagates at the same speed. They do share some characteristics, of course, or else they would not have the same top speed. However, the characteristics that they share exist at such a low level that they are the characteristics all observable things share. So the answer to your question is no.
Answered by: Ted Pavlic, B.S., Electrical Engineering Grad Student, Ohio State

Gravity is definitely not part of the electromagnetic spectrum. Properties of the gravitational force are very different to the electromagnetic force, (apart from the 1/r^2 law). 1) Gravity is only ever attractive, electromagnetism can be both attractive and repulsive (Unless we have exotic matter in which case gravity also can be repulsive) 2) All particles feel the gravitational force in the same way, electromagnetism is only felt by particles carrying charge. Gravitational "charge" is mass/energy. 3) The electromagnetic field does not carry charge and hence photons do not interact with each other. Gravitons have energy and hence gravitate. The most striking similarity is that both are gauge theories and have a very similar description in terms of differential geometry despite their respective Lagrangians looking very different. Electromagnetism has a local U(1) gauge invariance on an "internal" space. Gravity has a local Poincare invariance on physical space-time. Uniting these two different symmetries is highly nontrivial and leads us directly to supersymmetry and superstrings. Early attempts such as Kaluza-Klein theories tried to describe electromagnetism and gravity as a 5-dimensional generalisation of general relativity failed. It was much later that "no-go" theorems were discovered that blocked the way forward until supersymmetry was discovered. There is however, a very interesting relation between gravity and Yang-Mills theory (generalisation of Electromagnetism) that arose from string theory. The Kawai-Lewellen-Tye (KLT) relations relate pertubativly the closed string vertex operators to two open string vertex operators. This heuristically allows us to write Gravity = (Gauge theory) x (Gauge theory). The KLT relations were then used to calculate things in quantum gravity from gauge theory, more recently quantum gravity has been used to answer some questions about QCD. About the speed of gravity. General Relativity, just like Special Relativity there is a maximum speed limit that anything cannot beat. This speed is c, which "happens" to be the speed of light in vaco. This already means that gravity could at best propagate at the speed of light. [I should point out that quantum effects such as vacuum polarisation in QED can allow photons to propagate faster than c on a curved space-time background!] Gravity (quantum) is only consistent with observation if we have a massless gravitational field of spin-2 (spin-0 is not completely ruled out) . This was proved by Feynman and others. Since the quanta of gravity are massless, then via the usual arguments they must propagate at the speed of light c. Now, gravitational waves are slightly different. These are essentially localised (classical) gravitational energy that propagates through some background space-time. As solutions of the field equations of general relativity they formally travel at the speed of light. Again, this is the fastest we could hope for. Gravitational waves are like "classical gravitons", but I wouldn't take the analogy too the extreme. So far, all experimental tests (as far as I know) put bounds on the propagation speed near c. So to recap, gravity is not part of the electromagnetic spectrum, but does propagate at the speed of light. Both theories have differences, but it is believed that the KLT relations point to a deeper fundamental relation between Yang-Mills theory and General Relativity.
Answered by: Andrew James Bruce, Grad Student UK