Of Einstein's genius

This man oozes brainpower. Maybe not in this picture, though.

The real genius in Einstein's work is in the fact that he discovered a fundamental property of the universe from only a few assumptions.
He set the speed of light as constant in different frames of reference and developed the mathematics of Lorentz transforms further to come up with one of the most beautiful theories in physics.

While usually in physics direct evidence and data is studied to deduce a phenomenon, he inductively derived his theory having no evidence at all. He made his own axioms and derived everything again from them. And the best part is that it worked, at least in theory. Despite a few physicist immediately recognized his genius, his theories were not completely recognized by the scientific community at the beginning, especially when the paper for his theory of Special Relativity presented no references. Einstein's reasoning was outside the lines, but sharp, nonetheless, and it proved no mistakes.

Evidences for his special theory of relativity started to come up only much later, around 1932, with the Kennedy-Thorndike experiment testing the dependence of the speed of light on the velocity of the measuring device.
Direct evidences of the general theory of relativity are still feeble, even though it is now a firmly accepted theory.

No, it is not photoshopped

The difficulty in testing this theory (linked to the difficulty to discover it) lies in the fact that we need astronomically strong gravitational fields in order to detect the space-time fabric and its tiny ripples: the gravitational waves. I did not use the word astronomical by chance, as the best empirical evidences come from deep space. Usually quasars (active galactic nuclei), which are very far, but bright x-ray sources, are used as indirect test for the theory of general relativity, as their ginormous gravitational field can bend light. Pictures of galaxies behind quasars could be worked out from their light being bent right into our eyes (or telescopes), forming "rings" of light around the quasars.

A more direct evidence of general relativity would be detecting its evident result: gravitational waves. Unfortunately, these waves are far less energetic than anything we can imagine, making it easy to let them disappear into the much higher and chaotic background of radiation. Experiments exist trying to achieve the impossible through interferometers, but a much more concrete evidence, at least for now, is presented by binary pulsars.

Not the most scientifically
accurate gif I could find,
but surely the coolest.

Binary pulsars move into space and their gravitational field emits gravitational waves, which is a leak of energy from the system. Everything, in fact, emits gravitational waves, but for small (less heavy, to be precise) objects their emission is undetectable. As energy is leaking from the binary system of pulsars, their period of revolution will get slightly slower than usual and with enough time passed, this accumulated time will get significantly big to be measured. This is not a direct evidence, as the energetic leak that slows the period down is not necessary given by gravitational waves, but that is the only phenomenon we know that could cause it, for now, and Einstein's model works really well when applied to these bodies, so it is still evidence, even if indirect.

When I started writing this post, it was meant to be an introduction to one big recent news in science in one of the FSND series. I was so excited writing about how marvelous Einstein's theories are that it got too long and I decided that it would easily be a blog post on its own.

The news regards the recent first visible-light evidence of gravitational waves from a pair of dwarf stars, and I'll now leave you in a pointless cliff-hanger (pointless as the news is already out elsewhere) as I will talk about it in the next Fortnightly Science News Digest on 15 September.

5 "must-see at least once in a lifetime" Celestial events

Most of the travellers choose their destinations based on the wonders of the world. Some of them, choose them for the wonders of the universe.

There are some beautiful celestial events that are visible only on certain locations on Earth at certain times. Their rarity and poor availability, combined with their beauty, drives people to travel to be able to see them.

Here is a list of 5 events worth seeing at least once in a lifetime. For each one of them I wrote a blog post on their nature and how and when to find them:

1. Aurorae
2. Solar Eclipse
3. Midnight Sunset
4. Milky Way
5. Meteor Shower

Enjoy.

1. Aurora

Aurorae are one of the most impressive views of a night sky and they are very famous to be an event that not all the skies can host.
They happen around the polar regions, both in the northern and southern hemisphere taking respectively the more common name of northern and southern lights.
The curvy movements and the lightness with which they fly across the sky is a really hypnotizing and fascinating sight that cannot leave even the most apathetic man without a "wow" out of his lips.

[This is a post which is part of the series: 5 unmissable celestial events]

2. Solar Eclipse

Solar eclipses are natural phenomena that occur when the Moon passes between the Earth and the Sun, obscuring the latter.
This event is so unnatural that it astonishes and amazes not just humans, but many others living creatures. Studies have shown that animals react strangely to solar eclipses. Their behaviour is driven by the absence of sunlight where there should be and, in fact, depending on the animal, they usually prepare to sleep.

 [This is a post which is part of the series: 5 unmissable celestial events]

3. Midnight Sunset

The midnight sun is a quite surreal phenomenon happening in very northern or southern latitudes, nearby the polar regions. It is nothing more than having the sun out in the sky, only, at midnight!
What is stunning, though, is that (it depends on the latitude and the season) the sun does not set, but remains still on the horizon before rising again and giving sleepless "nights" to visitors.

 [This is a post which is part of the series: 5 unmissable celestial events]

4. Milky Way

A starry sky is always a very relaxing and touching view, and often, if we spend some time to contemplate it, when our eyes are well adapted to darkness, we could spot some steady "white clouds" between the stars.
Fortunately that is no premonition of rain, because those clouds are well beyond the Earth's atmosphere. That is the Milky Way, no less than the very galaxy we live in!

[This is a post which is part of the series: 5 unmissable celestial events]

5. Meteor Shower

Seeing a falling star is a peculiar event. Its rarity permitted the birth of the popular conception of expressing a desire when seeing one. But maybe this habit founds more solid roots in the fact that people nowadays spend a very little time looking at a night sky, than on the rarity of the event itself.
Millions of meteoroids are in orbital collision with the Earth every day and we should thank our atmosphere that only a tiny percentage reach the ground with much smaller sizes than the original object. On certain seasons the frequency of falling meteors is so high that the event is called meteor shower. But why the Earth is tormented by these intruders?

 [This is a post which is part of the series: 5 unmissable celestial events]

Moonset and Jupiter again

Same telescope as in the previous post, different situation: waxing moon.
After some time wasted (the temptation to point the telescope to the ground and spy innocent people from the top of my house is unimaginable) I convinced myself to watch the orange setting moon a little nearer (20mm eyepiece):

 

And after I proved that Jupiter's moons (obviously) move and then (indirectly) that gravitational laws are true. As you can see in the pictures below, with the same telescope (20mm eyepiece) the moons have a different configurations and it agrees with Stellarium. (mouse rollover to circle the moons):

Stellarium (mouse rollover to name the moons):

I've seen what Galileo saw

Clear sky, Tramontana (a Northern wind known to be very dry in Southern Italy), new moon, neighbourhood lights off: perfect occasion to dust off my brother's telescope and do some night-sky observations.

A refracting telescope, 10cm objective lens and two 34mm and 20mm eyepieces; not so bad for the Moon and planets.

I tried to spot some stars but they were too faint because of that damn light pollution, so I decided to see one of the most luminous astral bodies in the Northern Sky: Jupiter.
After some focusing, I spotted Jupiter with its prominent lighter-hued zones.
But I also noticed what at first sight I thought were some refractions/reflections. There were smaller dots aligned near Jupiter, with different brightness. They were too strange to be some optical effect, so the second assumption was: the Jupiter's moons!
That dots were quite far from Jupiter, in my opinion, to be its moons so I wasn't so sure about that, but they were four (as the Galilean moons), aligned and with different sizes.
I took a picture with my phone (one of the most difficult things in my life, but I was determined to take it) and the result is a very fuzzy and dirty image. Unfortunately the view through the telescope was much clearer and defined, but it can get the idea across (click on the image to enlarge):

After reversing and some photoshopping (or better "gimping"):

Then I immediately checked with Stellarium what kind of bodies they could be, whether Jovian moons or stars. This is the screenshot:

Fascinating.

Stellarium: night sky simulation software

Exploring the educational section of softwares for Gnome, I stumbled upon this incredible program: Stellarium.

You just enter your location and it simulates the sky over you at that moment. Very useful for amateur astronomers or night sky passionate.

There are also a lot of cool features to make the sky similar to the real sky: you can regulate the magnitude and the light pollution, or you can accelerate or choose the time, you can label costellations, stars or nebulae (so you can learn star's names or costellations), make zooms and a lot more.

Practical, easy to use and interesting.
It is also available for Windows.

Why aren't there any green stars?

Sun, green wavelengths filtered

I've wondered for long before studying physics why there are no green star. There are many beautiful picture of stars around, but why green is always missing? Has Nature decided to discriminate green?

The simple answer is: no, but we don't see it because of how we perceive colours and because there are no stars that emit only green wavelengths.
Not satisfied?

Good, because we need to use some physics to understand why we don't percept green in stars.

Firstly, we see the stars because they emit light.
Light is an electromagnetic wave, also know as radiation and the different colours of light emitted depends on its wavelength. The visible range of wavelengths can be seen here.
Stars, for an empirical reason (but there is also an explanation for that), emit a spectrum, that is a range of wavelengths. Then, for example, a red star does not emit only "red wavelengths", but a specific range of wavelengths that include the red.
The star colour depends on its temperature: higher temperatures correspond to shorter wavelengths, that is "bluer" colours, and lower temperatures correspond to longer wavelengths, that is "redder" colours. Then, even if red is a warm colour and blue is a cold one, a blue star is actually much hotter than a red one.
Is this range of colours emitted with the same intensity? No, the intensity of each colour emitted follow a specific path, discovered with quantum physics, called blackbody radiation curve and you can find a cool toy (applet) to play with it here. For each temperature, there is a different path that includes a different range of colors.

Now let's see a bit how our eyes interpret spectra.
When we see a color of an object, we are actually seeing the composite color, that is the mix of the colors emitted (or reflected). Black and white are not really "colors" since black is just an absence of color, while white is formed by all the colors. A prune has that nice purple color because it basically reflects red and blue wavelengths and our eyes mix them up.
The same applies for stars, but you have to mix up a very large range of colours with different intensities.

Known that, let's use those information to answer the question.
The applet used before helps a lot: it mixes the colours under the curve and shows the composite result. You can change the temperature in Kelvin of the curve (i.e. of the star) on the bottom (our Sun has a surface temperature of 6000 Kelvin, very roughly).
When the temperature is low, the star is evidently red because there is almost no blue at all in the spectrum (or it has a very low intensity). It is clear also why very hot stars have a bluish colour, since in the spectrum the red colour has low intensity in respect to blue.
It is interesting to see what happens when we choose the peak near the green wavelength. The sum of the colours is white, not green!

You can't see green stars because of the nature of the star emission, in which the green color is included, but is not perceived by our eyes because it is mixed with all the other colors to form white.