Kick-off for the Dark Energy Survey

Last 31st of August, the Dark Energy Survey (DES) kicked off. Actually it had already started before through the development of the camera, the different calibration measures and the necessary tests to make that what was being developed was useful for science and not another camera to take beautiful pictures of the sky.

DES aims at scanning the observable universe with the Dark Energy Camera (DECam). This camera is installed in the Blanco telescope in Chile (Chile is a fantastic place to do Astronomy and Astrophysics) and for about 100 nights every year during the next five years will carry out observations of millions of galaxies and thousands of supernovae to find out if the light, that started its travel towards us thousands of millions of years ago, gives us information about why the universe is expanding at an accelerated rate. The final objective is to determine, from the measure of the light gathered by the DECam, if the accelerated expansion of the universe is because of the mysterious dark energy, what would help us to understand it, or if it is necessary to change our concepts about gravity.

The DECam is designed to make measures of the observable universe, which, in few words, is the universe we can ‘see’ from the earth. When I use the verb ‘to see’, I don’t mean everything we can see when we look at the night sky, but everything we can observe when the light from a stellar object reaches the cameras and detectors the human beings are able to develop. The fact that a detector is very sensitive doesn’t mean that it can observe everything that exists in the universe because the light requires time to reach us from a remote point in the universe, and, as everybody knows, light travels at a constant and finite speed (300.000 km/s)

The DECam is a photographic camera that, in concept, is similar to the ones that we all have. However it is prepared for the detection of tiny and/or distant objects whose light has been travelling for several years in an expanding universe and, because of it, it is redshifted. When the light is redshifted, the light is not in the visible range of the electromagnetic spectrum, which is the one that we can see with our cameras, and it is shifted towards longer wavelengths in the red or infrared part of the spectrum. The DECam is made of about 62 detectors, which in total make about 500 Megapixels (let’s see when Apple or Samsung install one of this cameras in their mobile phones) that are sensitive to these redshifted wavelengths.

The objective of DES is to find out whether the Universe is expanding at an accelerated rate, or in other words, to determine if the galaxies that are moving away from us are doing it at accelerated rate instead of at a constant speed. In 1998, observations of type Ia supernovae indicated that the universe is expanding in an accelerated manner, what led to the scientists who conducted these observations to win the Nobel Price in Physics in 2011.

One of the widest accepted models explaining the accelerated expansion of the Universe is the one that postulates the existence of a dark energy which is a form of energy that permeates all the space and that it is thought that make up about the 68% of the universe (the remaining 32% is completed by the dark matter with a 27% and ordinary matter we are made of with a 5%). Currently it is thought that dark matter could be of two forms, a constant energy that fills the space homogeneously or scalar field whose density varies in space and time.

DES aims at finding out whether the accelerated expansion of the universe is because of dark matter. It DES’s results were negative, we should think about our concepts of gravity and the universe. Luckily, although Newton and Einstein are not longer between us, there are lots of Young physicists that can perfectly replace the absence of these two geniuses.

Good luck to the Dark Energy Survey team!


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Radiation and Radioactivity (I)

Often, when we talk about the danger of nuclear energy we talk about the danger of radiation. Also, when we talk about the danger, not yet demonstrated by the way, of mobile telephones we talk about radiation too. Is it correct to use the word radiation in both cases? Is it always the same radiation?

Historically, radiation is a combination of oscillating electric and magnetic fields propagating in a medium. In other words, it is a moving wave. Opposite to sound waves which need a physical medium to propagate, electromagnetic waves, radiation, can also propagate in the vacuum such as the outer space.

As a side note, due to the vacuum is empty, if we ignore quantum fluctuations that continuously create particle and antiparticle pairs of course, and thus there is nothing that helps the propagation of sound waves, the big explosions heard when the starship Enterprise shoots a photon torpedo against a Klingon’s bird of prey should not be possible to be heard, but Star Trek is Star Trek and this fact can be forgiven.

These electromagnetic waves are characterised by an energy that depends on its wavelength (distance between two points with the same phase, such as crests) or its frequency (number of waves per second).

A wave

Wavelength and frequency are inversely related, so if the wavelength is high or there distance between two crests is large its frequency is low or there are few waves per second and vice versa. It we take the frequency as the reference magnitude, we have that the higher the frequency the greater the energy.

c=λν and E=hν

Mathematical relations between wavelength λ and frequency ν, and between the energy E and the frequency ν. c is the speed of light which is constant with an approximate value of 300.000 km/s and h is the Plank constant with a value of  6,63 x10-34 joules per second and that, as it names indicates, is constant.

Up to now, this does not say much about radiation, thus to go deeper we have to know that every body with a temperature T above absolute zero, which is equivalent to -273.15 Celsius (pure water freezes at 0 Celsius), emits radiation. If we take into account that the third principle of thermodynamics tells us that it is impossible to reach the absolute zero in a finite number of steps, we come to the conclusion that everything emits radiation! And if everything emits radiation, we have a problem, a big one, if every radiation is of the same kind! Because if it is of the same kind as the mobile telephone radiation and of the same kind of the one that escapes from nuclear power plants, since everything emits radiation we should have disappeared from the earth a long time ago.

Keep calm because it is not as bad. As I mentioned earlier, radiation is a wave that propagates with a frequency and thus it has an energy. Depending on the frequency we have a type of radiation or another and this is known as the electromagnetic spectrum.

Now is when things become more interesting. Electromagnetic radiation is classified as non-ionizing radiation and ionizing radiation. Non ionizing radiation is the one that doesn’t carry enough energy (its frequency is not high enough) so as to liberate one or more electrons from the atoms or molecules it collides with, while the ionizing radiation has enough energy to liberate such electrons. I guess we can now deduce what radiation is the bad on, cannot we?

The limit between what it is known as ionizing and non-ionizing radiation is not, however, very clear. If we have in mind that the ionization energy of a hydrogen atom is 13.6 eV (1 EV or electronvolt is the energy that an electron requires so overcome an electric potential barrier of 1 volt), the radiation that makes the hydrogen to lose an electron will be ionizing. Notwithstanding, matter, including ourselves, is made of different chemical elements which make that there is a wide range of ionization energies. In any case if we consider the hydrogen ionization energy as the lowest limit, by looking at the electromagnetic spectrum we have that for radiation with energies in the ultraviolet range and above towards gamma rays, it will be considered as ionizing radiation. Anyway, there is not an agreement on this limit yet, because as I said everything depends on the material the radiation is colliding with considering the ultraviolet frequency range as a lower limit.

We have seen that the electromagnetic radiation is everywhere and cannot avoid it; in fact, we are a large radiation source. Because we are visible to others, it means that we are emitting radiation, in this case in the visible frequency range. Additionally, we usually have a body temperature above 36 Celsius and thus we are also emitting radiation in the infrared frequency range. Therefore, is radiation dangerous? The answer is, in general terms, no. What we need to do is to use the correct terms when we speak and say that what it is dangerous is the ionizing radiation, especially at high energies.

A classic: E = mc2. Is it everything? Its history

Why, when someone sees an equation, the first thing he/she thinks about is to run away? And why, when someone sees a t-shirt with a picture of an old man with white messy hair sticking out his tongue with an equation below the picture, the first thing he thinks is, cool! I want one of those too!? The old man is Albert Einstein and the equation is E = mc2 which is the most famous equation in history, probably even more famous than the Pythagorean theorem (hypotenuse)2 = (cathetus 1)2 + (cathetus 2)2 that, by the way, is related to the Einstein equation in way that I will explain later. The problem is that many of those who have, or want to have, such a t-shirt don’t know exactly what the meaning of the equation is and even less what its origin is. What we all probably know is that, in the equation E is the energy, m the mass and c the speed of light.

Another thing that very few people know is where the famous equation comes from and what Einstein wanted to explain when he worked it out, so lets do a bit of history about how Einstein came to it without using mathematical formulae (this is difficult, so if you are curious I recommend you to have a look at the original paper even if you don’t understand anything, it is so beautiful and simple that it is worthy to have a look at it)

The original paper where Einstein formulated the problem had as title ‘Ist die Trägheit eines Körpers von seinem Energiegehalt abhängig?’[1], what means, does the inertia of a body depend upon its energy-content? Einstein’s objective was to explain an aspect derived from his previous studies about electrodynamics where he used Maxwell’s equations, which explain why we see what we see and that are for me one of the most beautiful set of equations in history.

He also used the relativity principle as follows:

‘The laws by which the states of physical systems alter are independent of the alternative, to which of two systems of coordinates, in uniform motion of parallel translation relatively to each other, these alterations of states are referred’

As you can see it does not say that everything is relative what it is used by many people when they want their opinion being in a better position than a different one.

From this point Einstein asked himself what happens when a stationary body, with an energy E1 in a system of coordinates, let us call it C, and E2 in another system of coordinates C moving with a uniform parallel translation, emits energy in two directions. To understand it let us suppose that we are the body with energy E1, that we got it after a good breakfast and that we are stopped in train station’s platform. The train station is the system of coordinates C. The system of coordinates C would be a train moving in a railroad track parallel to the platform where we are. The emission of energy E1 by the body means that we emit energy of a certain type. To be original, let us suppose that we have a pair of eyes capable of throwing X-rays then, when we throw X-rays, we emit energy in two directions (one per eye). If we take into account the energy we emitted and measure it with regard to the system of coordinates that is moving (the train) and to the stationary one, as well as we consider the principle of relativity and the energy conservation law (energy cannot be created or destroyed, but can change form, or in other words, the initial energy, the one gained after the breakfast, has to be the same that the one at the end, the energy emitted in the form of X-rays together with the one from the breakfast that we didn’t use), after subtracting the energies in both systems of coordinates we find the result that obtained Einstein which explains that if it is emitted energy in the form of radiation its mass diminishes, what is the same as ‘the mass of a body is a measure of its energy-content (I must admit that this is much more easier to explain with formulae). The c2 appears because of measuring the energy of the body in the system of coordinates that is moving.

In summary, the energy and mass are equivalent through a constant which is the speed of light, because although the OPERA experiment says the opposite, wait… they don’t say it anymore because they forgot to correctly plug a wire, nothing can travel faster that the speed of light (in vacuum) which is constant with a numerical value of 300,000 km/s approximately. Or what is the same E = mc2.

Although it may seem at useless result, it is not because the mass is continuously transforming into energy and the energy into mass. The former is easier to understand. In the interior of the sun there are permanent nuclear reactions transforming where two hydrogen atoms (this is quite more complex but enough to understand it) are fusing or ‘colliding’ together to produce a Helium atom plus an amount of energy that escapes from the reaction, which reaches us to heat and tan us. It is something similar to what happen to us when we are ‘big’ and run and make exercise to burn fat, the excess of mass disappear, doesn’t it? And, where does it go? It is the energy needed to run and make exercise!

That the energy transforms into mass is more difficult to understand, but it is, for example the fundamental principle that particle accelerators are based on. When in the CERN’s Large Hadron Collider (LHC) two protons travelling at almost the speed of light collide, protons (in fact the quarks and gluons that make up protons) transform into energy for a tiny period of time. Until now, mass transformation into energy again. If this was the only thing that happens it would be useless to spend so much money in a particle accelerator, but what happens next is the important thing, energy transforms into mass again! However instead of producing protons again, the energy transforms in a particle of the immense particle zoo that has been discovered and in particles that are waiting to be discovered. This is how, for example, the famous Higgs boson was found.

This is interesting, but… is E = mc2 everything? The answer is a big NO. If you remember, in the example of the protons, I said that they are travelling at the speed of light, but in addition they have mass on their own, known as energy at rest. This mass is the one that appears in the Einstein equation. Where is then the speed that protons have? Einstein equation does not say anything about the bodies that, apart from the mass they have, are moving. In addition, it does not say anything either about the bodies that does not have mass, as it is the case of photons (light) because if there no mass, then energy would be zero, what is not possible because, for example the photons (light in the range of infrared frequency, which is made of photons too) heat us, which is the same that saying that they transmit energy to us. This problem is fixed by including the momentum of the particle in the equation. Momentum is, broadly speaking, a measure of the speed of the particle. In the case of photons, they don’t have mass but momentum, in other words they are moving. If we introduce the momentum in the Einstein equation, we have the extended form of the equation, which is as follows:

E2 = (mc2)2 + (pc)2

Where p is the momentum previously mentioned. Now we have a complete formula for the energy because if the particle does not have mass it can have energy (E = pc) and if it has not momentum (speed), it is at rest, it also has energy given by the famous Einstein equation. The fact that E is squared means that we have to take the square root to find the solution. But wait a moment, maths tells us that when we take the square root of a number, we always have two solutions, a positive and a negative one. For example  has two solutions, 2 and -2, this is because 22 = 4 and (-2)2 = 4, too! Does it mean that we can have negative energies? Well, the answer is not so easy. History attributes Paul Dirac in 1931 [2] the interpretation of negative energies as antiparticles. In this way, all (charged) particles have associated an antiparticle, the proton has its antiproton, the electron its antielectron or positron, etc.

Coming back to the extended form of the Einstein equation, if we have a close look at it, it has a similar form to the one of the Pythagorean theorem as I said before. How do we interpret this from a Physics point of view? Lets draw each element of the equation as part of a right-angled triangle to represent it as in the Pythagorean theorem [3]

Extended form of Einstein equation represented as the hypotenuse and cathetus of a right-angled triangle

Extended form of Einstein equation represented as the hypotenuse and cathetus of a right-angled triangle

This means that if a particle has mass, we could only give it energy to a certain limit; we could never give it an infinite energy. The reason can be seen in the figure. If we raise energy, then the hypotenuse E will be longer. When we raise energy we make that the particle increases its momentum (it increases its speed to keep it simple) and therefore the cathetus pc would be longer. To keep the form of a right-angled triangle the cathetus mc2 should be longer which is the same as saying that its mass should increase. Thus, if we keep on giving energy the mass increases continuously giving as result that we would need more and more energy to make the particle keep moving. This is not efficient or worthwhile. It is not even useful! The limit on the speed is imposed by the speed of light; therefore if we increase the energy until the speed of the particle reaches the speed of light, we would need more and more energy to keep it moving!

I admit that this is difficult to ‘visualize’ it with the bunch of text I wrote, thus I leave you with a video of MinutePhysics (@minutephysics) where he explains it, in a wonderful way, in a bit more than two minutes.


[1] Einstein, A. Ist die Trägheit eines Körpers von seinem Energiegehalt abhängig?, Annalen der Physik. 18:639, 1905 (versión en inglés: Does the Inertia of a Body Depend upon its Energy-Content)

[2] (in Spanish)


Why do we do science?

First I’d like to say that I’m not a native English speaker and that I may make many mistakes writing in English, both spelling and grammar, thus I’d like to apologise in advance for it. However as I think that science communication should reach to the largest possible number of people, I’ve decided to write first in Spanish (my mother tongue) and then in (at least understandable) English. I hope you excuse my mistakes and enjoy the reading.

I have always liked reading and learning about the history of science, more concretely about the history of Physics, in fact I’m physicist; at least that is what it is said in the certificate I received from my university after paying the appropriate taxes. After finishing the degree I didn’t have the chance to continue a research career but that didn’t stop me from keeping my interest in Physics (with capital letters!).

Among the aspects that I most like from the history of science, and about the history of Physics in particular, is to look for the original papers that gave rise to the biggest revolutions in the understanding of nature and the impact these discoveries had in our quality of life.

At the same time, I found interesting the lack of interest about science that the general public has. I have heard too many times things like, is that thing useful for me? Behind this question there is a large unawareness of the reality we live in. What would have happened if, by the end of the 1850 decade, Robert Bunsen had not became interested in the possibility of analysing salts according to the colours they have when burnt and Kirchhoff had not became interested in spectral analysis [1]? The answer is that probably the discoveries that happened after it and that gave rise to the birth of Quantum Physics, what was followed by the study of the subatomic levels of matter, what gave rise to the discovery and study of semiconductors that lead to thousands of technology developments like the computer I’m using to write these lines (a Mac Book Pro, for your information) would not have happened. I also wonder what would have happened if after the discovery of the particles that matter is made of like hadrons and leptons, which is basically another way of naming the protons, neutrons and electrons that we all know, it hadn’t existed the curiosity to ‘see’ it there was something else. The answer is that, among other things, the Conseil Européen pour la Recherche Nucléaire (also known as CERN. Yes, the one of the Higgs boson and the black holes that will destroy our world, hahaha) had never been established and it had never existed the need to develop advance control systems for the Super Proton Synchrotron (SPS) where in May 1973 Beck and Stumpe [2], from the control group, proposed to use tactile technology for it, which is the same technology that almost all of us have in our smartphones and tablets to send whatsapps and play to solitaire game (at least this is what I mostly do with with my iPhone and iPad. Is it too obvious that I’m addict to Apple things? Another example, and this is the last one for now, is that what would have happened if due to the need of exchanging the data generated in the CERN accelerators, mainly the LEP at that time (Large Electron Positron Collider), between European and US researchers, Berners Lee in 1983 [3] had not made the first proposal for the World Wide Web and that he decided that it was developed for the benefit of all mankind. The answer is that none of you would be reading this, neither this nor anything that you get after duly paying your internet service provider every month (I wonder if this is what Berners-Lee was thinking about when he decided to yield the property rights to the benefit of mankind). In summary, either directly or indirectly science, an in particular research in pure science, has always been done with two purposes in mind, a main and a secondary purpose.

The main purpose: to widen the human knowledge. Human beings are curious by nature and all of us (even those who say the opposite) need to know. I don’t know anybody yet who has questioned himself something like, why…? Or that has never experimented, even unconsciously, in the same manner that scientists do. If you don’t believe me, try the next experiment: set up a fire and put your hand in it. When you are back from the hospital, set a fire again and repeat the experiment. Why don’t you do it? Because you have done the same that any scientist does, you have experimented and drawn conclusions from the experiment. The only difference between you and the poor fellows from CSIC (Spanish science research institution) is that they perform experiments at another level because they have already gone through the phase where they knew that the fire burns and need to go beyond it. They ask themselves about what the fire is made of and what is the mechanism that makes it burn. Furthermore, they ask themselves why we feel that the fire burn and cold water refreshes us. Another difference is that probably you earn more money than the poor CSIC fellows, but that is another story.

The secondary purpose, which actually is more important to all of us, even for scientists but it is not why scientists research, is to enhance the quality of our lives. Either in the fields of Physics, Chemistry, Mathematics, Biology or Geology, etcetera, pure research has a strong component of applied research as we’ve seen in the examples of the tactile screens or the web. If not, ask Faraday when, according to the legend, in 1831 after a conference about the dynamo he had invented, the then Minister of Finance asked him ‘what is the practical value of electricity’. Faraday’s answer was: ‘One day sir, you may tax it’. How right he was! [4].

As the great Richard P. Feynman said, ‘Physics is like sex, sure it may give some practical results, but that’s not why we do it’. Replace Physics by Science and the result is the same. Each one may draw his / her own conclusions.

Finally, I recommend you that because Bunsen and Kirchhoff worried about researching something that finally gave rise to computers, that because Beck and Stumpe worried about developing tactile screes and that because Berners-Lee worried about developing the web to make science advance, you worry about everything they gave us and search the web for a while and learn something about science. There hundreds of thousands of websites and blogs where science communication (by the way, science communication was started by Faraday with his conferences and that is why he is considered as the first science communicator) is very well explained and I’m sure you find out things that you never had thought you may be interested and that maybe will inspire you.

By the moment, I think I will stop writing and start looking for a website where maybe I find something interesting to write about next time.


[1] Historia de la Física Cuántica. José Manuel Sánchez Ron. 2ª Edición. 2005