Quantum Gravity

Quantum gravity is an overall term for theories that attempt to unify gravity with the other fundamental forces of physics (which are already unified together). It generally posits a theoretical entity, a graviton, which is a virtual particle that mediates the gravitational force. This is what distinguishes quantum gravity from certain other unified field theories ... although, in fairness, some theories that are typically classified as quantum gravity don't necessary require a graviton.

The standard model of quantum mechanics (developed between 1970 & 1973) postulates that the other three fundamental forces of physics are mediated by virtual bosons. Photons mediate the electromagnetic force, W & Z bosons mediate the weak nuclear force, and gluons (such as quarks) mediate the strong nuclear force. The graviton, therefore, would mediate the gravitational force. If found, the graviton is expected to be massless (because it acts instantaneously at long distances) and have spin 2 (because gravity is a second-rank tensor field).

The major problem in experimentally testing any theory of quantum gravity is that the energy levels required to observe the conjectures are unattainable in current laboratory experiments. Even theoretically, quantum gravity runs into serious problems. Gravitation is currently explained through the theory of general relativity, which makes very different assumptions about the universe at the macroscopic scale than those made by quantum mechanics at the microscopic scale.

Attempts to combine them generally run into the "renormalization problem," in which the sum of all of the forces do not cancel out and result in an infinite value. In quantum electrodynamics, this happened occasionally, but one could renormalize the mathematics to remove these issues. Such renormalization does not work in a quantum interpretation of gravity.

The assumptions of quantum gravity are generally that such a theory will prove to be both simple and elegant, so many physicists attempt to work backward, predicting a theory that they feel might account for the symmetries observed in current physics and then seeing if those theories work.

Time Travel Theory from LOST

By my count, we've seen three types of time travel recently on ABC's Lost. Desmond's mind-trips, the island itself vanishing, and people jumping around the island's past. Will one theory ever explain all three?

Personally, I love time travel stories, even when they don't quite make sense - as is frequently the case. At least Lost is scoring points for me in one area: it has already stated definitively that a time-traveler can't change his/her own past, which is an important question to resolve. In other words, when you travel through time, you don't create a new timeline, you just visit other points on the same, immutable timeline. (Of course, we've already got an escape clause, because the rules don't apply to Desmond. We'll have to see whom else they don't apply to...

Most TV shows involving time travel try to have it both ways at various times - I'm looking at you, Doctor Who and Star Trek. They'll show how you can go back in time and alter your own timeline, but then try to play the "predestination paradox" game. (Data traveled back in time because he saw his head in the ground, and his head was in the ground because he traveled back in time. But if traveling back in time creates a new timeline, as Trek has shown on numerous other occasions, Data's head shouldn't have been in the ground until he'd already traveled backwards in time.) So far, Lost hasn't tried anything so silly.

Besides Doctor Who, a few other current shows seem to be toying with time travel on a regular basis. Lost's companion show Life on Mars has a cop whose mind has been thrown back in time to 1973, but that show seems months (or years) away from explaining what's actually happening. On Heroes, I honestly can't tell what the rules are supposed to be - like, I thought the goodies erased the future timeline where Ando killed Hiro, but last night they were talking about it as if it was a fixed point in the future. So far, The Sarah Conor Chronicles has kept a pretty consistent view of time travel: every time you travel back, you create a new timeline. So the future that Kyle Reese came back from in the original movie is long gone, and even the future that his brother Derek Reese came from is kaput.

Lost's episode "The Constant" basically made sense to me - Desmond was unstuck in his own timeline, skipping between his past and his present. His body wasn't going anywhere, and the time travel was strictly neurological. It seemed just about plausible that that could happen. (And even though Desmond is "special," we've seen that happen to other people, including the radio operator and that lady in Oxford.) But now we've seen two other things: the island vanishing, and the castaways coming physically unstuck. They're treated as if they're sort of the same thing, but they're obviously not. (If the island itself was skipping through time, shouldn't its inhabitants be staying put, as the waters around the island change?)

"Think of the island as a record, spinning on a turntable," says Daniel Faraday. "Only now, that record is skipping."

It's interesting that - as far as I can remember - we've never seen what happens when Locke, Sawyer and friends "time travel" from the point of view of someone else. What does Ethan or Richard Alpert see, when Locke vanishes? Does Locke actually physically disappear, or does something else happen? I have a feeling there may be a surprise waiting for us there.

Commenter Putch pointed out that temporal wrangling comes into play a couple of other ways: Daniel proved that time is moving at a different speed on the island than off. And when Ben left the island, he apparently traveled forward in time to 2005.

What's the actual science here?

In the season premiere "Because You Left," Dr. Marvin Candle says the Orchid Station is on top of a store of "exotic matter." Physicist Michio Kaku told Popular Mechanics that nobody really knows much about exotic matter:

Such matter would have "formed when the Earth was young, and then floated into outer space," Kaku says, "and therefore there's none left on Earth." However, it may have been possible for a pocket of the matter to become accidentally trapped underground. Physicists theorize that exotic matter could have antigravitational properties (so it would fall up) or it would have negative energy (absorbing energy around it, possibly making it implosive). And if it were to have antigravitational properties, it wouldn't want to stay on Earth either; instead, it would rocket into space-violently. "It would be quite dangerous to people who encounter it," says Kaku.

Kaku also has suggested that a black hole or wormhole could be involved in the island's time travel, but it would require fantastic amounts of energy. That would explain Mrs. Hawking's equations: they're probability equations, because she's calculating a radiation effect. (Radiation has to do with the stability of the wormhole.)

Note that in one episode, we see one of the Others reading Stephen Hawking's A Brief History Of Time, and it's open to a page on black holes.

Does Alpert have two compasses?

Lost seems to be trying to avoid time paradoxes with its rule against changing the timeline. So when Locke meets Alpert in 1954, from Alpert's point of view this meeting "always happened." But what about Alpert's compass?

FutureAlpert gives Locke his compass, so Locke can give it back to him in 1954. Does this mean that the compass is stuck in a time loop? Is there only one compass, which Alpert gives to Locke in the future, and then Locke gives back to him in the past? Or does the 1954 version of Alpert have two compasses now? Also, does Alpert only know to give his compass to Locke because he remembers Locke giving it to him in 1954?

Also Daniel sends the brain of his rat, Eloise, back in time, and now she already knows how to run his maze - because FutureDaniel taught her. But why will Daniel teach Eloise to run the maze, when she now already knows how?

Where/when did the island itself go?

As I mentioned, I don't think we've ever seen Locke or Sawyer "disappear" when a flash happens and they travel through time. We don't know what happens to them from the standpoint of an outside observer. But we have seen the island itself vanish. So where/when is it?

The fact that the people who were on the island at the time are "skipping through time" wouldn't explain the disappearance of the island itself. The island can't be visiting its own past, because then you'd have two near-identical islands occupying the same space. (Which I'm guessing would be bad.)

We probably won't learn more about the island's actual, physical disappearance until we see the Oceanic Six try to return. At least, I'm hoping that returning to the island will clarify where/when it's got to.

So will Lost's time travel ever entirely make sense? Well, it'll probably make more sense than time travel on Heroes or Star Trek, but I'm guessing there will be some hand-waving involved. At the same time, we're probably 27 Earth-shattering revelations and twists between now and the final, absolute explanation of Lost's clock-buggery.

Jupiter's stripes

Today's morning I heard a very interesting discussion about the Jupiter at National Public Radio. Namely, One of Jupiter's stripes has gone missing; it just faded out. What is more, scientists aren't entirely sure why. Enjoy the link below!

Why Objects Float?

As the story goes, Archimedes was bathing in a public bathhouse several thousand years ago when he noticed the water level rise around him in the tub. Truly thrilled by his apparently miraculous realization, he jumped out of the tub and ran through the streets naked screaming "Eureka!!! Eureka!!!" (I've found it!!! I've found it). What Archimedes had discovered is now referred to as Fluid Displacement. He had noticed that when a body is submerged in a fluid, that body displaces a volume of the fluid equal to its own. That is, to say, that if I placed a cubic meter block of stainless steel into a pool (don't ask me how), the block would displace exactly one cubic meter of water. Simple, right?

Anyway, back to our naked friend. Archimedes also noticed that the objects felt lighter when they were submerged. This phenomena made him want to quantify this observation. After a while, Archimedes made the wonderful discovery that the objects were buoyed up by a force equal to the weight of the fluid that they displace.

So let's take a look at our block in the pool. That cube of stainless steel (which is more dense than water) would have a mass of approximately 8020 kg or would weigh 78596 N. As we learned before, the cube would displace one cubic meter of water, which has a mass of (quite conveniently) 1000 kg or a weight of 9800 N. So the cube would be pushed up, in effect, by 9800 N of water. 78596 (its real weight) - 9800 (what's pushing up on it) = 68796 N (its weight while submerged). Great, now you know this thing weighs 9800 N less while I try to get it out of my pool.

Now let's try something we know is less dense than water. We'll take, oh I dunno, cedar wood, which has a density of .49 g/cu cm. Keep in mind that water's density is 1 g/cu cm. So if I had one cubic meter of wood (if you haven't noticed by now,I go to Price Club), which has have a mass of 490 kg, it would, if forced underwater, displace, again, one cubic meter of water. This displaced water has a mass that is greater than the object being submerged, so in other words, the water is pushing up harder than the wood is pushing down. The wood will float at a position such that the submerged portion would displace a volume of water whose mass is equal to the mass of the entire block. Or the wood would displace its weight in water.

Humans are only slightly less dense than water, which is why we must submerge a large portion of our bodies to stay afloat (more displaced fluid means greater force pushing upwards). Women are less dense than men (due to a higher fat percentage), which is why it is easier for women to float than men. It's not our fault, it's science!

How Will Physics End?

What of the future of this adventure? What will happen ultimately? We are going along guessing the laws; how many laws are we going to have to guess? I do not know. Some of my colleagues say that this fundamental aspect of our science will go on; but I think there will certainly not be perpetual novelty, say for a thousand years. This thing cannot keep on going so that we are always going to discover more and more new laws. If we do, it will become boring that there are so many levels one underneath the other. It seems to me that what can happen in the future is either that all the laws become known - this is, if you had enough laws you could compute consequences and they would always agree with experiment, which would be the end of the line - or it may happen that the experiments get harder and harder to make, more and more expensive, so you get 99.9 percent of the phenomena, but there is always some phenomenon which has just been discovered, which is very hard to measure, and which disagrees; and as soon as you have the explanation of that one there is always another one, and it gets slower and slower and more and more uninteresting. That is another way it may end. But I think it has to end in one way or another.

How Does That Satellite Stay in Orbit?

In order to understand how satellites stay in orbit, we need to first look at gravity. Everything that mass has a gravitational field. Right now, your computer's gravitational field is pulling on you, and your gravitational field is pulling on the computer. However, since gravity is a relatively weak force, unless you're an extremely large object like a planet, you can't feel this pull. We do however feel the pull of the Earth on us.

Everytime you jump into the air, the Earth's gravitational field pull's you back down. If you use more force you can jump higher, but you always return to the Earth. The force that the Earth's gravity pulls a specific object with is expressed in the formula F=mg, where m is the mass of the object and g is the acceleration due to gravity (approximately 9.8 m/s at the Earth's surface). However, that is a specific case at the surface of the planet. The full formula is:
F=GmM/R^2

where F is force, G is the universal gravitational constant (6.67 x 10-11 Nm2/kg2), m is the mass of an the small object, M is the mass of the planet, and R is the distance from the center of the planet to the object.

At the Earth's surface, neglecting all friction, an object could be put into orbit. However the speed would have to be 8 km/s (5 mi/s or 18,000 mi/h). We discover this by combining two formulae. The object in orbit would have to have a centripetal force (F=mv2/r) that counter-balances the gravitational force (F = mg). The two have a common factor of "F" so we combine them as mg = mv2/r. The m's can then be divided out, giving us g = v2/r. Then we multiply both sides by "r", giving us gr = v2. Finally, by taking the squareroot of both sides, we get :
v = (gr)1/2

Interpretation of this formula shows us that the velocity required for orbit is equal to the squareroot of the distance from the object to the center of the Earth times the acceleration due to gravity at that distance. At the surface this would be (6.4 x 106 m * 9.8 m/s2)1/2, or (6.272 x 107)1/2, or approximately 7,920 m/s. This agrees with our previous number of 8 km/s. (Isn't great when somebodyelse does the math for you?)

However, satellites don't orbit at the surface (they could though). Going back to our other formula for the force of gravity on an object from any height, we can go through the process of rewriting the formula again. This time I think I'll just tell you it:
v=(GM/R)1/2

So if we wanted to put a satellite in a circular orbit at 500 km above the surface, it would need a speed of ((6.67 x 10-11 * 6.0 x 1024)/(6900000))1/2, or (58 x 106)1/2, or 581/2 x 103, or:
7615.77 m/s

As you can see, this number is considerably smaller. If you use a larger speed, you can get an elliptical orbit, but don't use too much. If you do, you just may escape from the Earth's gravitational pull.

Dark Matter & Dark Energy

Across astronomical distances, the only significant fundamental force of physics is gravity. However astronomers usually find that their calculations don't quite match up. An undetected form of matter (dark matter) was theorized to fix this.

Recently I have read an article about that the universe consist of 70% dark energy, 25% dark matter, and only 5% of visible matter or energy. Now, some of you may be confused by words 'dark matter' and 'dark energy'. Let me explain it.

'Dark matter' is a matter which doesn't reflect or emit electromagnetic radiation. The existence of dark matter is inferred from gravitational effects on visible matter (like stars or galaxies). 'Dark energy' on the other hand, is an energy which permeates space and exerts a negative pressure. This could have gravitational effects on visible matter. Dark energy cannot be observed directly. It can be rather inferred from observations of gravitational interactions between astronomical objects. This was all very well explained by Michael Turner so you can google it.

The interesting fact is that only 5% of universe is a visible matter/energy. We cannot be sure what is in 95%. However we do know for sure that calculations don't match when we assume that 100% of universe consist of visible matter/energy. That's why scientist came up with theory of dark matter/energy. Yet we still don't know what stands behind the phrase 'dark matter'.