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Going nearly light speed — can you see your reflection? Lasers can’t be white!
Q: If you are traveling at the speed of light, and you hold
up a mirror that is also traveling that fast will you be able to see yourself?
Please give an explanation. (Riveesh, Glasgow, Scotland)
 A:
Nothing that has mass can move at the speed of light, but we can imagine an
astronaut moving at nearly that fast.
Albert Einstein, ca 1905 at the patent office in Berne,
Switzerland.
The astronaut, cruising through space at near light speeds,
picks up her mirror, and takes a look. Yes. She sees herself.
The reason why is so simple that it took a genius to state it.
In 1905, Albert Einstein formulated his Special Theory of Relativity, which
results from just two postulates:
The speed of light in a vacuum is a constant.
The rules of physics have to work the same in all frames of
references — moving or at rest — as long as the moving frames are not
accelerating. Things have to work as usual.
Thus, our astronaut traveling at a relativistic speed can see
herself in a mirror just as easily as an Earthling sitting at a make-up mirror.
The rules of nature are the same for both situations. Light from the astronaut’s
face bounces off her mirror and reflects back at light speed, even though she’s
traveling at nearly the same speed. She can see herself.
This, of course, disturbs our "common sense" and everyday
experiences. We might expect that light moving ahead from the face of the
astronaut, who is moving at near light speed, might move at nearly double light
speed. But, Einstein says, No. Light always moves at the same constant
speed in a vacuum.
Admittedly, Einstein merely postulated that this be true. He,
however, built his entire theory of relativity based on the two postulates. We
have tested the resulting theory over the past century. Never have we found a
discrepancy. It always works. So, probably his postulates are true, no matter
what our "common sense" thinks.
Further Reading:
Conceptual framework of special relativity by Rod Nave, HyperPhysics
Theory: special relativity by Helen Quinn, Stanford Linear Accelerator
Center
Special
relativity by John Zavisa, How stuff works
Q: White light contains all
wavelengths. But, I understand a laser is created from just one wavelength. So,
is it possible to create a White Laser? (Ray, Memphis, Tennessee)
 A:
Nope, for just the reason you mentioned. A laser must be coherent light whose
waveforms march together in the same direction, in phase and all at the same
frequency, which means having the same exact color. When two waveforms are in
phase, that means that all their peaks and troughs line up. By definition,
however, white light consists of all frequencies of visible light from infrared
to ultraviolet. Furthermore, the waveforms of white-light’s constituent
frequencies jumble together without a common direction or phase.
Laser light reflecting off mirrors. Courtesy of the US Air
Force and Wikipedia.
By the way, white Light Emitting Diode (LED) light exists. A
light emitting diode, however, emits light whose frequencies are clustered about
a given color. But it’s not a laser. The emitted light is not a single
frequency, nor coherent.
My LED flashlight that produces white light does so using a
blue (gallium nitride) LED, covered with a yellow phosphor. The chip emits the
blue light; the yellow phosphor absorbs the narrow-spectrum blue light and
re-emits a broad-spectrum
 light
centered about yellow. Then, cleverly enough, the yellow light stimulates the
red and green receptors in my eyes. So, with the residual blue, my brain
receives a mix of blue, red and green — which gives white. I see a bluish white.
But it isn’t a white laser, merely a white LED.
The LED (magnified) used in a Pelican flashlight. The
yellow rectangle in the center is the semiconductor material. No electricity is
being applied to the LED, and so it emits no light now. Courtesy of Keith
Swenson, copyright, Pelican Products.
Further Reading:
Lasers by Rod Nave, HyperPhysics
Pelican LED flashlights — white light
Lasers by April Holladay, Encarta
Spectral colors by Rod Nave, HyperPhysics
Lasers, Wikipedia
(Answered Feb. 7, 2006)
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