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Igor Khavkine
Oct12-06, 04:09 AM
P: n/a
On 2005-09-01, BJ <> wrote:
> Everything loses energy as it travels -- the planet relies upon the
> work of gravity; the airplane consumes fuel. How come light can travel
> without losing energy?
> Granted the electric and magnetic components of light oscillate at
> right angles to enable it to travel in a very efficient way, but the
> oscillation should consume energy like everything else.
> Where does this energy come from?

That's a common misconception that goes back to Aristotle. He thought
that an object needs to be pushed to remain in motion. Galileo put that
misconception to rest with his experiments on inertia.

The simple answer is that energy is not lost in motion, it can only be
transfered from one place to another. So light can travel indefinitely
unless it comes into contact with which it can interact and to which it
can transfer energy.

You may wonder how the apparently observed energy loss, that you refer
to, can be reconciled with the above principle of conservation of
energy. This very question, actually, is what lead people to the
understanding of heat as a form of energy. Whenever a motion is executed
against some resistence, air friction or friction of break pads agains
the wheels of a car, some of the energy due to large scale mechanical
motion, flight of a plane or the turning of a car's wheels, gets
transfered to the atoms and molecules of the material providing the
resistence, the air or the break pads and the metal of the wheels. These
molecules start moving around faster but on a much smaller scale,
essentially imperceptible. The only way we can tell this energy transfer
took place is through changes in temperature, which indicates heat
exchange. By the second law of theormodynamics, once energy is turned
into heat, it's kind of hard to get it back into mechanical motion.
However, when things cool down, again, the heat energy is not lost, only
transevered to the much larger surrounding medium (the athmosphere,
usually) which is so large that the entailed change in temperature is

BTW, the kinetic energy of a planet in motion around the Sun is not
constant. It goes up when the planet is closer to the Sun and goes down
when the planet is farther away. So the Sun's gravity is giving kinetic
energy to the planet for part of the orbit, and takes it away for
another part of the orbit. However, if the gravitational potential
energy is added to the kinetic energy, their sum (the total mechanical
energy) is conserved throughout the orbit.