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The Question

(Submitted February 16, 1997)

My questions relate to two matters that have been troubling me and for which I have not seen any comments by any astrophysicist or astronomer. The first question concerns the possibility for light emitted by a body that is 5 billion light years away from the earth, to survive for 5 billion years without being reduced to nothing during such long period of time. In other words, once light leaves its source it is no longer being fed with energy and thus it only dissipates energy through space and time. That being the case, how is it possible for such light to survive not only the distance but also the time. The only explanation that makes any sense to me would be that which would hold that space is curved and that the distances we think we observe are nor real in a physical sense. Rather, they are relativistic.

My second question relates to the theory of the big bang. If the theory is correct, then all of the observable Universe and beyond must be surrounded in a sphere of light that was created at the moment of the big bang. But because such sphere is expanding at the speed of light, we will never be able to observe it unless some of that light is reflected inwards towards the center of the sphere. Hence, if the Universe will eventually contract, the sphere of light will collapse back towards the point of origin and on its way there at the speed of light, it will illuminate (or burn up) everything in its path including our earth. I do not really understand any of this but I would welcome the comments and views of an appropriate qualified individual.

The Answer

As for your first question, No, light does not dissipate its energy as it travels through space. It can only dissipate its energy if it interacts with matter. Light is a form of energy, and does not need to be "replenished" once it is it emitted. This is because light is actually made up of an electric field and a magnetic field which produce and support each other as the light beam travels through space. If you've ever seen an electric generator/motor you know that the coils of wires being spun inside the magnets can produce electricity (i.e. an electric field). Also near power lines or motors compasses will become deflected because of the magnetic field produced by the electricity flowing through the power line or by the motor. It was in the 19th century that James Clerk Maxwell discovered that a changing electric field produces a magnetic field, and that likewise a changing magnetic field produces an electric field. He also discovered that light was comprised of these changing electric and magnetic fields. Hence, a light beam is "self-sustaining".

The only thing that stops this from going on forever is when a light wave interacts with some form of matter (ie. a planet, dust, gas etc...). Its energy is then absorbed by the matter. Sometimes the material may re-emit the light, but usually at a lower energy. Since space is mostly empty the chances of these waves encountering some matter is relatively small. Hence, light can propagate outward for long lengths of time.

Of course, as the waves spread out the intensity (or brightness) does gets weaker. This is because there are fixed number of waves spreading out into a larger area. This is known as "the inverse square law", because at any given point in space the intensity of the light decreases as the inverse square of the distance from the light emitting source. Nonetheless, its energy remains unchanged.

As for your second question, there is a residual effect from the Big Bang, and we can and have observed it. It's the Cosmic Background Radiation, which is observable in infrared wavelengths. Right after the first instant of the Big Bang, the energy was so great and dense that matter was constantly being created and destroyed (as predicted by Einstein's E = mc^2). The Universe was an expanding and cooling "soup" of energetic particles and photons. Around a year after the Big Bang, the "soup" had expanded and cooled enough that the photons in the soup no longer interacted with matter. This left a "gas" of photons that has since expanded and cooled to 3 degrees Kelvin. This radiation permeates all of the Universe. The Cosmic Background Explorer (COBE) measured this radiation to an unparalleled precision. For example, it has found that all but one part in 3000 of this "photon gas" contains energy from the Big Bang (in other words, the photons have essentially not interacted at all with the rest of the Universe since the Big Bang). You can learn more about COBE from http://www.gsfc.nasa.gov/astro/cobe/cobe_home.html. COBE has shown that much of the Big Bang is a good representation of how our Universe began and has ruled out some competing theories.

We hope this helps you understand better these things that have been puzzling you.

Jim Lochner, Andy Ptak, and Mike Arida
for Imagine the Universe!

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