PDA

View Full Version : Questions on EM and asteroid hunting.



Albion
2008-Oct-30, 03:02 PM
Firstly, is there an upper and lower limit to the wavelengths of the electromagnet spectrum? Is it possible to have something with energy levels above Gamma rays or below Radio waves? Do all photons shift toward the red end of the spectrum at the same rate over time? Does the particle itself lose structure or decay at all during the shift to the red? And lastly, can we tell at what level of energy the CMB was when it was first emitted by checking the time vs. rate of red shift?

On asteroid hunting... Instead of using the light from the sun to detect near earth asteroids in telescopes, why don't we create our own light using lasers. Why couldn't we create a cone of 10 or 20 independently moving lasers to scan the sky for reflections? I'm sure we have computers fast and powerful enough to detect any slight reflections even if the asteroid is coming at us from the sun.

-Craig

Jeff Root
2008-Oct-31, 08:50 AM
Hello, Craig!

When asking unrelated questions such as these two, it may be best
to put them in separate threads because they could each generate
significant discussion.



Firstly, is there an upper and lower limit to the wavelengths of the
electromagnet spectrum? Is it possible to have something with energy
levels above Gamma rays or below Radio waves?
There are no really important limits. The high energy end is limited
only by how much energy nature happens to put into any individual
particle or photon. The low energy end is limited essentially only by
the limits of our radio receivers. There may be a limit on how much
energy can be packed into a particle. I think it has been mentioned
here on BAUT previously, but is considerably higher than anything
that has been observed and is not really of any practical significance
even if it exists. There may be a limit on how long a wavelength of
radio energy can pass through the interstellar medium without being
absorbed. That, too, is beyond our detection range.

If higher or lower energies are generated or observed, there is no
particular reson to call them as anything other than gamma or radio.



Do all photons shift toward the red end of the spectrum at the same
rate over time?
If you are refering to cosmological redshift, or any form of Doppler
shift, yes, all photons shift by the same amount.



Does the particle itself lose structure or decay at all during the
shift to the red?
No, the photon does not change in any way. The same photon
would be redshifted relative to an observer moving away from the
light source and blueshifted relative to an observer moving toward
the source. So the change in wavelength is completely reversible.



And lastly, can we tell at what level of energy the CMB was when
it was first emitted by checking the time vs. rate of red shift?
If I understand the question correctly, we do more like the reverse:
We see a blackbody spectrum all over the sky, having a peak
wavelength of roughly one centimetre. A plausible explanation of
its source would be hydrogen gas which was at a temperature of
about 3000 kelvins when it was emitted, and redshifted by a factor
of about 1000. The temperature of 3000 kelvins is the temperature
at which hydrogen transitions between opaque (above 3000 K) and
transparent (below 3000 K). The transition occurs when electrons
and protons combine to form hydrogen atoms, giving off light.

The amount of redshift is the difference between the wavelength
of the observed cosmic microwaves and the wavelength of 3000 K
hydrogen in the laboratory. From that, the speed of recession is
calculated. Making the naive assumption that the recession speed
has always been constant, the time since the light was emitted is
then deduced. Variations in the recession speed imply a different
time since the light was emitted. Such variations can be discovered
by measuring redshifts of light from galaxies at various distances,
emitted more recently than the cosmic background radiation.



On asteroid hunting... Instead of using the light from the sun to
detect near earth asteroids in telescopes, why don't we create our
own light using lasers. Why couldn't we create a cone of 10 or 20
independently moving lasers to scan the sky for reflections? I'm sure
we have computers fast and powerful enough to detect any slight
reflections even if the asteroid is coming at us from the sun.
There would not be enough reflected light to detect. Even aiming
a laser at the retroflectors on the relatively nearby Moon, typically
only one photon is returned in each pulse. In order to distinguish
single photons from the background, I believe the observations
need to be done at night, when Earth's sky is dark, and when the
retroreflector on the Moon is also in the darkness of lunar night
(constraining the time to a rather narrow window); the detector
needs to look at a very small spot of carefully optimized size on the
Moon's surface; the light needs to be carefully filtered for color and
for the time of arrival at the detector; and many pulses need to be
observed and averaged together.

Over much shorter distances, spacecraft orbiting Earth and Mars
use laser ranging to measure topographic features.

-- Jeff, in Minneapolis

Albion
2008-Oct-31, 08:46 PM
Hello, Craig!
If you are refering to cosmological redshift, or any form of Doppler
shift, yes, all photons shift by the same amount.


No, the photon does not change in any way. The same photon
would be redshifted relative to an observer moving away from the
light source and blueshifted relative to an observer moving toward
the source. So the change in wavelength is completely reversible.


-- Jeff, in Minneapolis

The wavelength of a photon cannot change over distance because only expansion, velocity, and gravity can change that wavelength, correct? Thus two stationary objects in a non-expanding universe would emit and detect the same wavelength photon from each other no matter their distance. It's only when one of the objects starts moving in relation to the other object that the wavelength starts to change.

For example

x = wavelength of a photon when it arrives at a detector.
y = wavelength of a photon when it's emitted.
c = speed of light
d = The difference in velosity between the emission source and the detection source.

x = (((c - d)/c) - 1) + y

The way I am seeing it is like this. You have two objects in space both traveling at 60mph. The wavelength of a photon is analogous to the energy released when those two objects collide. If the objects are traveling toward each other the time before the interaction would be very short and the energy released would be tremendous (blue shift). But if the two objects were traveling on an almost parallel path, the time necessary to interact would be great but the energy release would be much much smaller (red shift). The kinetic energy in each of the objects doesn't change as they travel it's the time and force of impact that change. Thus the wavelength of a photon never changes, it's just the way we see it based on the difference in velocity between the emission source and the detection source. Is that correct?

So technically, if two objects were traveling toward each other at .6 the speed of light. Given enough distance between the two objects, could a visible light photon emitted by one of the objects be detected in the gamma wavelength by the other?