# Q & A: Gravity and photons

Q:
1.If gravity is considered to be a "dimple" in space time, why didn’t the materials which now make up the planets simply "fall" to the center of the solar system?. What causes the planets to orbit the sun? 2.What is the mass of a Photon? Why/how is light bent by gravity? 3.Wave-particle duality of light says that light behaves as both a wave and a particle. Is it unreasonable therefore to assume that it is actually niether? 4.Can light create a current as it passes through a liquid? 5.Why do the planets spin? Do they all spin, and if so, is the spin in the same direction?
- Gerry Puente De La Vega (age 17)
Heaton Manor, Newcastle, England
A:
Lots of questions! Here are some answers, one by one:

1) All objects in the solar system are in fact falling towards the center all the time, being pulled in by the gravity of the sun (these are words associated with the Newtonian view of gravity). The planets don't get any closer to the sun on average because they are in elliptical orbits. Their veolicites are perpendicular (on average) to the gravitational force of the sun, and so while they are constantly accelerating towards the sun, the effect is to constantly veer away from a straight line and be pulled towards the sun, and to travel in ellipses. If you attach a ball to a string and whirl it around your head, the tension in the string constantly points inwards, towards your hand. But the ball doesn't get any closer, it just goes in circles -- it needs to accelerate constantly in order to change direction, but if the acceleration is perpendicular to the velocity, it won't pick up speed.
(For a more General Relativistic answer, see below. The travel of the planets is also 'straght' in a curved space-time./mbw)
2) Photons are massless. Please see our other answers about massless objects which must travel at the speed of light. This answer requires some General-Relativity-speak, which is different from the Newtonian-speak of answer #1. Light always travels in straight lines, but what a "straight line" is depends on the local geometry of space. Space is warped by gravitational fields, and you cannot have a uniform coordinate system everywhere in space in which all the laws of physics are obeyed, if there's gravity. Instead, little patches of space each act like little uniform coordinate systems, but to patch them all together requres bending and warping near a gravitational field. Light follows the bent space in each little section, but faraway observers, trying to picture the paths in flat coordinate systems, will claim that the light has been "bent".

3) Light is as light does. The ideas of waves and particles are our ways of describing what light does, and these descriptions work extremely well within their domains of applicability (often within the same experiment, the two different descriptions must be used). Maybe I'm too hard-nosed about it, but I would say that our best idea of what light "is" is our description of it -- maybe we'll discover something new about it someday, but this will always have something to do with how light interacts with other things in detectable ways. (Some people think photons, and all other particles, are made up of tiny bits of string. I say that this is a fine idea, but we need a way to test to see if it is true or false).

4) Sure. Light helps initiate electrical currents in some solids I know of offhand, like solar cells and photodiodes. Light also can knock electrons off of a surface, via the photoelectric effect. I suppose this surface can be liquid, say, mercury, and the photoelectric effect should work fine even with a liquid. The solar cells and photodiodes rely on specific material properties which depend on them being solids, but I wouldn't rule out liquid versions someday.
5) Planets spin because the stuff that fell together gravitationally to make them up had angular momentum relative to the planet's axis. If a meteor comes in and hits the Earth, the chances are nearly zero that it will hit head on. A glancing blow will contribute to the Earth's angular momentum. If a planet or star falls together gravitationally from a cloud of matter, even if the cloud has very little rotational motion, the angular speed of spin is increased as the planet or star becomes more compact. This has to do with the conservation of angular momentum, and a typical example is that of a spinning ice skater pulling in his arms to rotate faster. Another example is the water draining in a sink. It almost always swirls around as it goes down the drain, even if there was very little apparent motion to begin with in the water in the sink.

Tom

(published on 10/22/2007)