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Entries in gravity (41)

Monday
Oct112010

Higher Dimensions w/o Alcohol

Do we really live in three dimensions? The recent hype of 3D movies in Hollywood seems to emphasize that fact, but Hollywood does not always get things right... There are two main reasons why you would want to consider this possibility. First, if you allow for more than three dimensions of space, interesting things happen in physics: a unification of certain physics laws that you may otherwise think are very different. The second reason comes from String Theory: this new framework of theoretical physics is the only consistent way we currently know for marrying quantum mechanics and gravity, and it predicts the existence of higher dimensions...

What does it mean to be living in higher dimensions; say four dimensions for fun. Imagine first a simpler case. An ant lives its miserable and monotonic life on a cylinder. It can move in merely two dimensions. But one of the directions it can move in is very different from the other: if it goes around the cylinder, it comes back to the same point; but if it walks along the cylinder, in principle it can walk forever (as always, cylinders in physics are infinite...). You, on the other hand, are a larger creature. You look at this picture from very far away. You just see a wire; you cannot resolve the thickness of the cylinder. To you, the ant lives in one dimension! In this manner, we could be living in four dimensions, three of which are very large or even infinite, but the fourth is curled up in a circle so tiny that we cannot resolve it. That is all nice and well, and would make for a great conversation topic in a bar, but does it buy you anything new in terms of understanding Nature?

The answer is a big yes. Imagine we are tracking an electron from our pitiful three dimensional perspective. From the real four dimensional perspective, the electron may move around in the fourth dimension that we cannot resolve - just like the ant circling the cylinder. An electron has an attribute called electric charge. It is the reason it feels an electric force. But this quantity called charge is rather mysterious: what is it? where does it come from? From the viewpoint of the four dimensional  world, electric charge is simply a measure of the speed of the electron in the fourth dimension! And the electric force is a part of the four dimensional gravitation force! When you cannot resolve the tiny size of the fourth dimension, you instead see two forces: gravity and electric force... but they may be the same thing in reality. So, despite not being able to see the fourth dimension, we can still see its effects dramatically from our three dimensional perspective. Hence, knowing about a fourth dimension can give us deep insight about how Nature ticks.

In String Theory, higher dimensions are a must. In achieving the long anticipated marriage of quantum mechanics and gravitation, String Theory needs a series of logical consistency checks. One of these requires that the world is higher dimensional,  10 space dimensions… That means 7 of the additional dimensions must be curled up into small sizes and we are thus unable to resolve them. But their existence provides a myriad of indirect effects on our world, effects that unify the natural laws and the spectrum of subatomic particles in elegant ways. There are currently several experiments that are trying to "see" or "resolve" extra dimensions. Depending on how small these extra dimensions are, and how intoxicated the experimental physicists conducting the experiments are, we may sooner or later find out we need a new pair of dorky eyeglasses to go to the next blockbuster flick. 

Sunday
Oct102010

Back To The Future

Relativity tells us that Nature has a speed limit: no information can travel faster than the speed of light. That's quite fast, but not fast enough to travel the universe quickly enough. When you look at the Sun (please don't!), you see it as it was 8 minutes ago: it takes 8 minutes for light from the Sun to reach us. Stare at Saturn through a telescope: you are seeing it as it was 1 hour 30 minutes ago. The deeper you look into space, the further back in the past you are imaging... Our closest star, Proxima Centauri, is about 4 years in the past as we look at it today. We say it is 4 light-years away. Our nearest galaxy, Andromeda, is 2.5 million freaking years in the past! And the edge of the visible universe is 14 Billion years in the past... This means that when we look at the Cosmic Microwave Background radiation (see previous post), we are actually looking at the universe as it was starting out, in its infancy...

One of the strangest effects resulting from this time lag comes about when singular violent events occur far away: Supernovae or exploding stars... When a star exhausts its fuel and undergoes total (and I mean total) gravitational collapse, it typically undergoes a series of violent (and I mean violent) explosions... these are so nasty that near-by stars can be torn apart by the explosion... And we can see these events all the way from the safety of our Earth. They appear as a sudden brightening of an otherwise unspectacular star; and a subsequent dimming as the star starts to settle into a neutron star or, better yet, a black hole. The accompanying picture is that of Supernove SN2006X, before and after - much like pictures taken in hair transplant commercials. The star is 54 million years in our past. So, while the explosion was seen in 2006, it actually happened 54 million years ago... the star is there no more...

Saturday
Oct092010

The Information Paradox

What would happen to you if you were to jump into a black hole... we have three possible answers to this, two from General Relativity, the other from String Theory. In all cases, your fate would not be something to look forward to. But for many physicists, this would actually be a great way to go!

A black hole is a hole in the fabric of space and time, a collapsed star that sucks in everything around it through its immense gravitational pull. We now know there are billions of these extreme objects scattered throughout our universe... so, what would happen to you if you were sucked into a large black hole? Einstein's theory of General Relativity, which predicts the existence of black holes, suggests the following outcome: as you approach the surface of the black hole - called the horizon - you wouldn't necessarily notice anything special. But as soon as you cross the horizon, you suddenly will realize that you cannot communicate with your relatives outside the black hole, even if you wanted to… Ironically, this is perhaps the only time you would want to talk to your relatives! As soon as you cross the horizon, you cannot escape from the black hole ever again; nothing can, not even messages in a bottle. You will continue to fall towards its center as the forces of gravity become stronger and stronger, and pulls your internal organs apart... no one outside will hear your screams... 

Now add to this Quantum Mechanics. Steven Hawking showed in the 1970's that a black hole actually evaporates! When quantum effects are taken into account, one finds that a black hole has a faint glow that slowly dumps the stuff it's made of into the outside world as random radiation! And there arises the Information Paradox... So, you jumped into the black hole and disappeared forever; does then the black hole evaporate into random radiation? if so, where have you gone? the information that is you, where is it? is a black hole a sink of information? This is deeply troubling, for physics and for psychology. We believe information cannot be lost, only change its form! The Information Paradox preoccupied theoretical physicists for decades... We now think we have a good answer to this puzzle from String Theory... stay tuned for another post that will help save your sanity... 

The accompanying lengthy video (5 parts) gives you a history of Hawking and the Information Paradox he proposed.

Friday
Oct082010

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!

Thursday
Oct072010

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...

Wednesday
Oct062010

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...

Thursday
Sep302010

Distortion of spacetime

 

 http://www.youtube.com/watch?v=AwjSCQnx5lg

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: