Wednesday, March 02, 2005 ... Deutsch/Español/Related posts from blogosphere

Gia Dvali solves 1/2 of the C.C. problem?

My friend Oldřich Klimánek who is currently translating "The Fabric of the Cosmos" by Brian Greene to Czech has pointed out the following popular article to me:

What does he say about the cosmological constant? Well, it does not exist. The accelerating expansion of the Universe is, Dvali argues, due to a "leak of gravitational force" into extra dimensions that only appears at very long distances.

So far I don't exactly understand how Gia's picture works. But definitely, it's one of the proposals to modify gravity in the extreme infrared - he talks about the Yukawa-Newton potential (arising from massive gravity). Dvali predicts new modifications of the motion of the Moon and the planets. In the case of the Moon, it should be testable because of the mirrors on the lunar surface that have been installed by Apollo 11 (and terrestrial lasers).




I am now looking at several papers such as
and although the general words sounds appealing, the details have not convinced me yet.

Find the first gravitational wave

Incidentally, did you consider the possibility that you would be the first person in the world to discover the gravitational waves, in co-operation with the LIGO experiment? It's easy as long as you have

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reader Anonymous said...

Would you expect anything different? If he cannot respond to a person's arguments intelligently, he'll just sling mud because it works everytime.


reader Anonymous said...

dear Dr. Motl,

What would happen to papers on Dark Energy then?There are quite many in the arXiv...


reader Quantoken said...

As I promised, I just posted a message on my BLOG, discussing why both Bohr and Einstein made fatal logical blunders in their great debate of the light box gedanken experiment. I quote from my message:

"But what Einstein never realized was that Bohr's logic was frauded. Intentionally or un-intentionally, he confused two completely different logical concept. One is the uncertainty of time measurement, Delta T, which should be the time internal during which the shuttle is open. Another concept, also expressed using the same symbol Delta T, is the time difference caused by time dilation due to general relativity."

See my blog for more details of the discussion:

http://quantoken.blogspot.com/

Quantoken


reader Matti Pitkanen said...

Dear All,

for a few days ago Lubos's response to my comment explaining the role of Mersenne primes and Gaussian Mersennes in TGD was following:

"Matti - well, let me admit that I don't believe any of the things about "TGD" you wrote."

I tried to write my own comment but it was administered automatically out. I was not able to see even preview! Characteristic for Lubos's views about democratic discussion.

Amusingly, I heard only a day later that Sir Roger Penrose seems to disagree with Lubos Motl about TGD in his book Road to Reality: at least here refers to my work on p-adic numbers.

The concept of fractal many-sheeted space-time made quantitative by the p-adic length scale hypothesis is also very relevant for dark matter and energy and the acceleration of the Universe. As one might guess, dark matter is at larger space-time sheets rather than in higher dimensions.

Matti Pitkanen


reader Quantoken said...

Let me make on On-Topic comment. I had a look at the Einstein@Home web site.

My conclusion is the whole LILO project is a billion dollar piece of bullshit. How are they going to measure a displacement of one-hundred-millionth the diameter of a hydrogen atom as they claimed? The laser beam itself has a wavelength of several thousand Angstrom. How could a light of several thousand Angstrom wavelength be used to detect a length scale of 10^-11 times its wavelength? What happens to thermal vibrations of atoms and molecules, not to meantion environmental noises etc.

Now, assuming their instrument is absolutely frawless, and there is no uncertainty principle, no zero point vibration, no thermal fluctuation, everything as perfect as a gedanken experiment can be. How are they going to detect gravitational waves from remote supernovas?

Let see, assuming there is one ant crawing just one meter away, and there is a remote star one million light years away.

Let's not worry about the effect of tiny mechanical vibrations due to the movement of the ant's legs. Although that's more than enough to throw the mirror out of balance many times more than the tiny 10^-11 angstrom sensitivity.

Who is going to exert more gravity pull on the mirror? The ant or the star? The answer will surprise you. The star is one million light years away, that's 10^22 meter. So the star's gravity per kilogram is 10^44 times weaker. A typical star like the Sun has a mass of 2x10^30 kg. An ant would weight 2 miligram. So there's 10^36 difference in mass.

Overall, the tiny ant still exert a gravity 10^8 times stronger than the star one million light years away!!!

Let's give them some benefit of doubt, assume we are talking about a star closer, 1000 light years away. The ant still wins over by 100 times in gravity.

So are they detecting gravity wave of ants near by, or of stars far away? And we haven't even talked about bigger nearby objects yet. A bird shit on the ground and that could generates a "huge" gravity shock wave a million times stronger than that of a remote supernova :-)

And we haven't even talk about gravity wave being a quadropole wave, and hence decays to the fourth power of distance, not the second power law.

The whole thing is a piece of crap.

Quantoken


reader Anonymous said...

Dear Lubos, as a professional physicist I find your posts interesting and enlightening -- especially the one on Wick rotation. Please don't be discouraged by all the nuts posting on your blog, there are a lot of people like me who don't post but *do* read. It would be helpful though if you could be a lot more aggressive about cancelling crackpot posts. They just clog things up, especially when there is a long discussion. They were funny at first but the joke has gone stale.


reader Lumo said...

Dear participants,
thanks for your comments.

"What would happen to the papers on dark energy?"

In the unlikely scenario that a model like Gia's is valid, the other papers would indeed become mostly irrelevant, even though many aspects of them would still be usable.

Many people have tried to explain the accelerating Universe - or explain the value of the cosmological constant - in many different ways, and no solution is convincing enough even to define the "mainstream opinion".

It's always hard to decide what to do with a strong inflow of posts such as "LIGO is a b*l*s*it" etc. I am apparently not allowed to erase them, and I am not allowed to label the author; moreover, he or she has a legitimate account here. But nevertheless, I encourage you to be skeptical about these statements.

I've also deleted a couple of comments which were unconstructive because they were intended to initiate off-topic flamewar here.

This blog is not intended as a cheap forum to discuss naive and unjustified conjectures about the "lack of innate aptitude" of one of two major U.S. parties, and whoever wants to discuss these questions at this level, or just attack me for my political opinions, may consider to do so on a different blog.

This article and thread is not political.

All the best
Lubos


reader Lumo said...

Dear Matti,

your comment is at least remotely about science, so it was kept.

Roger Penrose is a great guy, but some of the examples why not even Penrose may be treated as an infallible God were here.

All the best
Lubos


reader Anonymous said...

Hi Lubos

First let me agree with the comment a few lines up. Many physicists read and enjoy your blog for your physics commentary. Now to the topic at hand, Gia's idea is very interesting and honestly no more crackpot than some of the other propositions out there for Dark Energy and Dark Matter. There is a wealth of interesting literature out there but I wonder if you have seen the following papers. astro-ph/0309300 and astro-ph/0408415. They seem to both predict some very different local physics and dark energy within the same model.


reader Anonymous said...

But Quantoken is right, you know, the Bohr-Einstein debates are all flawed. The crucial key is quantum entanglement, not the uncertainty principle, but that's understandable since quantum entanglement would not be "discovered" until years later.


reader Plato said...

Missing Energy EventsI am glad you brought this article forward, but it is to bad it is buried already behind 2 more new posts.

Perhaps gravity bleeds into this fifth dimension, Arkani-Hamed theorized, or even more dimensions. But, given our four-dimensional reality, we're able to experience only the gravity left over. In other words, gravity is much stronger than we realizeIs there anything you could add to enlighten these issues more?


reader Anonymous said...

Dear Dr. Motl,

Thanks for answering my question on dark energy.Bytheway, I was not the anonymous who posted that insuanating comments, I've read them before I posted my question yesterday though I was hesistant to use the anonymous ID because of those comments posted by the other anonymous I still went through to post my question here with an anonymous ID also.