RE: The Beauty of Theoretical Physics
December 17, 2011 at 11:11 pm
(This post was last modified: December 17, 2011 at 11:13 pm by Perhaps.)
Introduction
Krauss begins his lecture by discussing Einstein's Equations and his theory of relativity in regards to a static universe - one that neither expands or contracts. He asserts that curvature equates to momentum in these equations and he introduces Einstein's cosmological constant as a factor which neutralizes the equation to support the idea of a static universe.
He moves onto Edwin Hubble and his observations of fuzzy blobs (nebuli) which have led to the conclusion of an expanding universe - in direct contradiction of Einstein's findings. He takes time to jokingly assert that we are not the center of the universe while elaborating on the fact that the universe is constantly expanding. In order to illustrate this expansion he uses transposition of 2d graphs.
By observing expansion we can introduce the idea of origin - the big bang. He then traces himself back to Einstein's equation and the cosmological constant. He uses simple math to relocate the constant on the other side of the equation completely changing the predictions. This now allows us to say that empty space weighs something, but that means that nothing weighs something which isn't possible by definition.
This leads to the determining of the curvature of the universe by measuring the weight held within the universe. Are we flat, closed, or open? If we are closed we would be able to look out really far and eventually see the back of our head. If we are an open universe then space is infinite. The flat universe has just enough matter to make space flat.
If the universe is dominated by matter (closed) then the expansion will eventually stop and collapse upon itself. In an open universe it will expand forever, and in a flat universe the expansion will eventually level out while never reaching 0. If we determine which universe we live in then we can determine our cosmological future.
Dark Matter
We weigh the universe by using clusters of galaxies (largest structures in the universe). This is done by observation of light curving around masses, which is what Einstein stated in his early work on the theory of relativity. Matter acts as a lens in this case from which we are able to use mass distribution. We are able to calculate how much mass is needed to avert the light around the matter.
The interesting observation from this calculation is that the dominate mass is not in the galaxies but between the galaxies. About 50 times of the mass in the system is in the stuff that doesn't shine. This is called dark matter, which in fact is the dominate stuff of the galaxies. We assume, with reasonably certainty, that this dark matter is made up of particles which are new and elementary. This means that we can do experiments to look for it and hopefully find out the identity of this dark matter.
From this we are able to weigh the universe by measuring how much dark matter there is. We now have great certainty that the universe is open. We, however,
know that we live in a flat universe. We know this because the total energy in a flat universe is 0. This is interesting because if you want a universe that came from nothing then you want a universe that has a total energy of 0. But, if you want to measure geometry, wouldn't it make more sense to measure it directly as opposed to using weight? It is now possible to do so, using triangular lasers. According to non-euclidean geometry a triangle may have three right angles which would prove that the thing being measured is curved. If the angles are 180 degrees when added then we prove that the universe is flat.
CMB
This leads to the analysis of the Cosmic Microwave Background which is the afterglow of the big bang (the radiation left). One observation made is that we can only see so far out into the universe because at one point the radiation from the big bang (believed to be about 100,000 years after the bang) was above 3,000 Kelvin - above absolute zero. At this temperature hydrogen atoms separate their charges and there exists a plasma. This plasma has an opaque feature which doesn't allow light to penetrate it, thus we can't see past this point because light was not able to penetrate it. We are only able to see up to the point when matter became neutral.
Krauss then observes that once the universe reaches a certain point in expansion light is not able to cross and gravity is not able to take effect. Both light and gravity have an effective radius in which they may be used, but after 100,000 years of expansion the universe escaped these effects, thus when we look at one section of the cosmic background it should not mirror another section on the opposite side, as they can have no effect on each other due to distance.
We now are able to use our triangle. We apply it to the cosmic microwave background and we observe a one degree difference. (I can't do this section proper credit so if you wish to hear it from him start at the 25 minute mark in the video). It is concluded that we are in a flat universe by comparing what we observed from our triangulation with a predicted flat universe.
Dark Energy
This leads to a realization that we are missing 70% of the material we need to create a flat universe (which is why we concluded on an open universe earlier). In 1998 a group measured the expansion rate of the universe which once again showed that we need 70% of some material to create a flat universe. This 70% is accredited to dark energy. The question then becomes - what the hell is dark matter? - the answer is - we don't know.
This essentially states that empty space is what accounts for the majority of the mass within the universe. We, however, have no idea why this is - it is one of the biggest mysteries in physics today.
The future
Objects greater than some distance, d, from us are now escaping faster from us than the speed of light. Nothing can travel through space faster than the speed of light, but space can do whatever the hell it wants. We are at rest in respect to ourselves, but the space between us and everything else is moving away faster than the speed of light. This means that in the far future we will be ignorant to the existence of anything other than our own galaxy. We won't be able to see anything in the distance, merely ourselves.
We live at a special time where the density of matter in the universe is about equal to the density of energy in the universe. Why should we happen to be around when this is the only time in the universe that this will occur? This leads to the essential question posed to Einstein - Did God have any choice in the creation of the universe? - This essentially asks if nature is unique or if there is only one physics. (I would highly advise watching this section of the lecture starting at 42:30) Krauss briefly addresses string theory and its relation to the dimensions of the universe and how each could potentially have different laws of nature or physics.
Once again, I highly advise you to watch this section for yourself as it would explain much better than I can in my own words. from 47:00 onward I will not explain, for it must be heard to have the full effect.
(December 17, 2011 at 9:04 am)Welsh cake Wrote: Or those who have busy chaotic lifestyles.
I do not have time to watch an hour-long video. Perhaps, if you want to talk about some of the key points or arguments raised in the lecture, please be more specific and address them.
Brevity is the soul of wit.
![[Image: you_are_here_galaxy.jpg?w=300&h=206]](https://images.weserv.nl/?url=stagesource.files.wordpress.com%2F2011%2F09%2Fyou_are_here_galaxy.jpg%3Fw%3D300%26amp%3Bh%3D206)
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Yeah RRIIIIIIGGGHHHHHHHHT.
