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Entries in relativity (40)


What is Dark Energy?

70% of the mass of the universe is made of a mysterious stuff we call Dark Energy. It cannot be seen directly, but its existence is inferred through the way it affects the motion of galaxies. We have a limited understanding of what this dark stuff is supposed to be. The best explanation comes from quantum mechanics. If you take a box of empty space, vacuum, and if you were able to weigh it with great accuracy, you would find out that it ain't really empty... Quantum mechanics allows for a level of controlled fuzz at the microscopic level: vacuum is really not empty, but looks more like a soup of particles being constantly created and destroyed out of nothingness. The more massive a particle that is created from nothingness, the shorter it lives. The end result is a picture of the vacuum which is a breathing living animal of random particles... hence, the empty box will have some weight. The most amazing thing about this picture is that it implies that the vacuum of space "anti-gravitates": regular matter collapses under the pull of gravity; vacuum mass - let's call it dark energy - does the opposite! It creates a repulsive gravitational effect. The end result is that empty space will tend to expand violently because of dark energy. So, there's a competition in the universe between a pull from the regular matter in it, and the repulsion from the dark energy in the vacuum. Currently, dark energy is winning out and we see the universe is undergoing an expansion: the fabric of space is being stretched to larger and larger sizes! Now, here's the depressing part. The story I just outlined is only a partial explanation. There are other layers to understanding dark energy as a condensate of stuff in the vacuum. However, if we were to take this most simple explanation seriously, we can try to estimate the amount of dark energy expected and compare it with the amount we actually measure... we find that a cube of empty space - with a size of 1 centimeter for its sides - weighs 10 to the power of minus 32 kilograms... That's tiny, by measurable on astrophysical scales! The best sensible prediction we have from quantum mechanics gives a value slightly bigger: 10 to the power 112 times bigger... it is then safe to say experiment doesn't agree with theory... And we are still to understand what 70% of the universe if made of!


Gravity As A Lens

Gravity as a lens... General Relativity predicts that all things that carry energy must gravitate, including light! Much like the curving trajectory of a tennis ball in flight, a light-ray also bends because of the pull of gravity. This effect is undetectable when gravity is too weak, like near the surface of the Earth. But when light travelling towards us from stars and galaxies far away encounters the pull of entire galaxies during its trek, it can bend significantely. And we can see this effect dramatically in distorted images of the universe behind the curtain of massive galaxies that interrupt the light. Through such observations, we can infer how much energy and mass there is between us and some far away stars whose images are getting distorted. The conclusion is rather shocking: 26% of the stuff in the universe is some unknown substance that we cannot see! We call it Dark Matter, and we know it's there from the way it bends light that travels through it. Only 4% of the stuff in the universe is luminuous and can be seen! The remaining 70% is something even more mysterious; we call it Dark Energy. Gravitational lensing - the phenomenon where gravity's pull bends light - has become a crucial tool in measuring the stuff the universe is made of; and hence is helping us understand the cosmological history of our universe. It was also the phenomenon originally used (in the early 1900's) to confirm General Relativity as the correct description of gravity - at the expense of the centuries-old Newtonian gravity. Check out the accompanying video for a brief overview of this tentalizing twist that Nature has apparently provided us; as if trying to help us see what we otherwise cannot...


LHC: an emotional take

And for those of you in a particularly romantic mood about physics and science, check out this more sensionalist version of the video in my previous post.


The Big Bang in a bottle

Creating the Big Bang in a bottle... give a child a toy he or she is curious about and you will soon see a dramatic manifestation of human curiosity in action: the child will throw the toy to the ground to break it up into pieces... he or she wants to see what it is made of and smashing it is a very efficient way to achieve this. As physicists, we never outgrow this instinct. When we want to find out what stuff is made of, be they atoms, or even smaller consitutents like protons and neutrons, we just smash them together! The harder we throw subatomic particles towards each other, more of the inner microscopic workings we uncover. This is the basic principle behind multi-billion dollor particle accelerators. They are basically giants microscopes that can "see" inside subatomic particles by throwing them at each other at immense speeds; and watching what comes out... These are nothing less than the most sophisticated machines that humans are able to build to date. The most recent one is the Large Hadron Collider (LHC) which started its operation just months ago and will go in full operational swing within a year or so. It will try to duplicate - within a small area - the conditions that our universe experienced moments after the Big Bang. It does this by smashing protons onto each other at high speeds in an underground tunnel envelopped by superconducting magnets. What it can potentially discover can change the world as we know it, much like the particle accelerators did 50 years ago. Before I write about the physics that one can expect from all this, check out this great video overview of this amazing instrument...


The Three Pillars


Back to basics - Quantum Mechanics, Relativity, and Gravity... many physicists, including many graduate students, may not realize this very fundamental aspect of how physics perceives the world. Yet this is so easy and important to understand and is immensely helpful to get a gut feeling of how the natural laws tick. Here it is. 

We all have, from birth, a perception of three different measures of the world around us: one for length, one for time, and one for mass. We see 3 realizations of the length measure, i.e. we live in 3D; we have a partial grasp of the concept of time since it seems to thrust us uncontrollably into our miserable futures; and mass has to do with a quantification of the amount of "stuff" in an object.

A priori, these 3 measures have no scales associated with them: 1 meter is equivalent to 1000 meters, etcetera. Philosophically, there is no reason to have a special scale for any of the three. Interestingly however, Nature has provided us with exactly three natural scales to make the world interesting: one is speed of light, the other is Planck's constant for Quantum Mechanics, and the third is Newton's constant for the gravitational force! These three scales add all the flavor to the world around us, and they delineate the three pillars of modern physics: Relativity, Quantum Mechanics, and Gravity. All three frameworks are tested ad nauseum individually in the lab. They also all fit together perfectly, except for Gravity and Quantum Mechanics... these two are inconsistent with each other! and hence arises one of the greatest puzzles of modern physics still desperately awaiting a resolution...


Distortion of spacetime

Distortion of space and time by gravity, and the Global Positioning System... For more than 200 years, the Newtonian theory for gravity reigned supreme as the correct description of the most familiar force of Nature, gravity. At the beginning of the 20th century, the story changed dramatically: the theory of General Relativity, developed by Einstein, dethroned Newtonian gravity as the correct framework for the gravitational force. One of the most interesting and beautiful aspects of this new theory purports that the fabric of space and time is malleable... mass/energy bends space and time around it! For example, a massive star would change the measure of distance between two nearby fixed points as it pulls on and distorts the space in its neighborhood. The effect is larger for more massive stars, but is also present around our own Earth. Our planet's gravity distorts time around it in such a way that time runs faster at higher altitudes... Astronauts who live about a year in the space station - in orbit about 300 km above sea level - come back a few microseconds older than otherwise... While this is a small effect, it is very important for the functioning of the Global Positioning System (GPS). GPS is all about measuring time accurately: you locate yourself with a GPS device by measuring the time it takes for a signal to travel from your handheld device to four other satellites above your head. This measurement needs to be highly accurate given that the signals travel at the speed of light. If the distortion of time by Earth's gravity is not taken into account, GPS's accuracy will go down from a few meters to many kilometers! Simply put, GPS is the first technological application that relies on the theory of General Relativity! Check out this video to satisfy your curiosity more:


Black holes

Black holes are the most extreme and violent objects of the universe... They push our understanding of Nature to its limits. A black hole is basically a star that has exhausted all its nuclear fuel and has collapsed under its own weight. If our Sun were to form a black hole (which it won't because it is not heavy enough), all its stuff would collapse into a ball of radius about 3 km (that's around 2 miles) - its current size is 200,000 times more... The surface of a black hole is known as the black hole horizon. If you were falling into a black hole, as you approach its horizon, the rate at which you age will slow down as seen by your worried relatives far away: time gets severly warped near a black hole. You may survive crossing the horizon, but, once you do, you are doomed forever... you will never ever be able to escape - at least not in your preferred form... This is what happens to entire planets and stars when they come too close to a black hole: they get sucked in! A black hole is basically an engine that devours and consumes large amounts of mass as it roams around the universe. It is a drain hole in space that gets larger as it sucks more stuff. In recent years, we have discovered that there are billions of black holes in the universe! more dramatically, some are supermassive: they have devoured billions and billions of stars - that's billion with a 'b'... they typically sit at the center of galaxies as the stars in the galaxy spiral into it - sort of like water flushing down the toilet... Our own galaxy has a supermassive black hole at its center and we are on our way down the drain... I'll write more about this subject in time, specially from the perspective of string theory. For now, check out this amazing video about black holes... a bit long but worth every minute: