May 2005 Sky from the Keeble Observatory
Albert Einstein is probably the most recognizable scientist of the 20th century.
Indeed, he stands for most as the ultimate icon of science itself. Just 100 years
ago he wrote four fundamental papers while working as a patent clerk, upending much
of classical physics and laying the foundations for quantum mechanics. Perhaps most
famous for his formulations of special and general relativity, in that year he also
proved the existence of atoms and explained how light interacts with matter.
To commemorate that “miracle year,” we’ll spend this and a few following columns
exploring some of those revolutionary ideas. First, let’s look at relativity.
Imagine riding through Ashland on a northbound train, and playing a game of catch
with the person just ahead of you on the train. When you toss the ball forward,
an observer in the train station will see the ball traveling north just a bit faster
than the train. When your partner tosses it back, the same observer sees it traveling
just a bit slower. From the train’s point of view, the ball is simply moving back
and forth, and the station appears to be moving backward. We say that the velocities
add, and that the measured motions are relative to which frame of reference is doing
the measuring. Intuition suggests that the same addition of velocities should apply
to light, which travels much faster than the train or ball. (The speed of light
is almost exactly 300,000 kilometers per second - that’s 186,282 mi/hour – equivalent
to more than seven trips around Earth’s equator each second.) So we expect a northbound
flash of light emitted on the train to travel at the speed of light plus that of
the train, and a southbound flash at the speed of light minus that of the train.
Experiments in the 1860s and since have shown that not to be the case. The speed
of light is a constant, irrespective of the relative motion of source and observer.
Einstein describes a “thought experiment” in which he imagines running along with
a beam of light, matching its speed. What would that look like? He concluded that
it would look like nothing in nature, and therefore must be impossible.
Speed is a measure of distance traveled divided by the elapsed time. If that ratio
is fixed for light, then relative motion must distort the measurements of space
and time. Einstein’s Special Theory of Relativity describes precisely those distortions.
We don’t notice them in everyday life because we don’t travel at speeds anywhere
close to that of light. The size of effect depends on the square of the ratio of
speed to the speed of light, which is normally a very small number – other than
saying that, I’ll spare you the equations!
What are the key results? First, a clock in motion relative to you as the observer
will record time passing at a slower rate. For example, you would hear your clock
go “tick-tock-tick” while a moving, identical clock would go “tick-tock.” An even
faster clock might only go “tick!” But, the moving clock watcher would notice nothing
unusual about her clock, and would see your clock running slow! That’s the “relativity”
part. The effect is formally called time dilation.
Second, objects in motion relative to you will be shortened in the direction of
their motion by exactly the same factor that the clocks are dilated. This effect
is called length contraction, and it also is relative to which observer is doing
the measurement. You will see her meter sticks contract, she will see yours contract!
Counter-intuitive though these may seem, the predictions of special relativity are
well-confirmed by precise experiments. We’ll say more next month.
Lunar phases for May: Last Quarter on the 1st, at 2:24 am, EST; New Moon at 4:45
am, on the 8th, EDT; First Quarter on the 16th, at 4:57 am; Full Moon on the 23rd,
at 4:18 pm; and a second Last Quarter on the 30th at 7:47 am.
Jupiter is already up as the Sun sets this month. Look for it about 30 degrees above
the southeast horizon early in the month, climbing to about 40 degrees by month’s
end. Saturn is still lingering near Castor and Pollux in Gemini. You’ll find it
at about 60 degrees altitude to the southwest, but getting lower and closer to the
Sun. It’s about 40 degrees above the western horizon by the time the calendar changes
to June. Venus is about 10 degrees off the western horizon early in the month at
twilight. On a clear evening you’ll see it as the first “star” to emerge from twilight.
It will be a few degrees higher later in the month, and still quite bright.
Mars rises about 3 hours before the Sun, and is about 30 degrees above the southeast
horizon at dawn. It’s moving away from the Sun’s position on the sky, so look for
it to rise earlier and appear a little higher each day.
At mid-month, an overhead view about 2 hours after sunset still shows a mostly empty
patch of sky! Nothing dramatic has changed since last month, but all the constellations
are about 30 degrees further west. At this time the Milky Way rings the horizon,
so the vast majority of stars in our Galaxy are too low to see. Castor and Pollux
are keeping company with Saturn, about 30 degrees above the western horizon. Turning
to the southwest we see Leo, marked by its brightest star Regulus about 60 degrees
above the horizon. To the south we see the constellation Virgo, with Jupiter a bit
brighter than Spica, that constellation’s brightest star. Turning to the north,
we see the familiar inverted shape of the “Big Dipper” high above the horizon. Following
the curve of the handle towards the east will bring us to Arcturus, the brightest
star in Bootes.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.