Science bits of Hawking's documentary

[twofish-quant]

Since the religious bits closed down the other thread, I wanted to mention something about the science bits. Since I didn't see the documentary myself, I can't say whether Hawking got it wrong or was misquoted. But both of these things are dodgy...

"It is a zero sum game, positive energy in matter is balanced by negative energy in space itself".

This is known to be wrong. One of the ideas that was popular in the 1990's was the idea that the total energy of the universe was zero. If that were the case, you could have the universe appear as a quantum fluctuation.

This won't work, because the latest observations indicate the that total energy of the universe isn't zero. Which poses a big problem. If it's zero, it will stay zero. If not-zero, it will move away from zero over time. Getting to the point were it's close to zero but non-zero requires fine-tuning.

It's also known that the energy of space is *positive*.

This is why science is cool. We had a very simple, very elegant model of how the universe could have started, and observations shows that it won't work, and someone gets a free trip to Stockholm for showing this...

"And time slows down to zero and actually begins at the big bang singularity".

This is also terrible from a science standpoint. The trouble is that by selecting coordinates, you can make *any* problem go away. I create a set of coordinates x^\pime = x/(distance to river thames). Because distances shrink as I go to the Thames, I can't cross it.

In order to make this work, you have to do something more clever.

Again, I haven't seen the documentary myself, and it may be that Hawking is being misquoted, but even being misquoted in a popular documentary can cause issues.

 

[Mark M]

This is known to be wrong. One of the ideas that was popular in the 1990's was the idea that the total energy of the universe was zero. If that were the case, you could have the universe appear as a quantum fluctuation.

This won't work, because the latest observations indicate the that total energy of the universe isn't zero. Which poses a big problem. If it's zero, it will stay zero. If not-zero, it will move away from zero over time. Getting to the point were it's close to zero but non-zero requires fine-tuning.

It's also known that the energy of space is *positive*.

Which observations are these?

 

[Tanelorn]

I just saw the new thread, twofish thanks for replies. I am out of time right now.

 

[twofish-quant]

Which observations are these?

http://en.wikipedia.org/wiki/Lambda-CDM_model

There are links to WMAP and associated observations.

The current parameters exclude a "zero-energy" universe.

 

[twofish-quant]

I'm going to be a good boy and stick to physics, but it appears that Dawkins is getting his GR from Lawerence Krauss and Krauss gets his GR wrong

blog.vixra.org/2010/09/08/krauss-missunderstands-energyhttp://en.wikipedia.org/wiki/Lambda-CDM_model-in-general-relativity-to-argue-against-god/

http://online.wsj.com/article/SB10001424052748703946504575469653720549936.html

Leaving aside the religious aspect. As a matter of *science* Krauss is plain wrong.

The total energy of the universe is positive and Krauss is using the wrong equation.

 

[Mark M]

http://en.wikipedia.org/wiki/Lambda-CDM_model

There are links to WMAP and associated observations.

The current parameters exclude a "zero-energy" universe.

Thanks, I never knew WMAP ruled out a zero-energy universe. For others, here is the WMAP cosmological parameters site.

 

[Tanelorn]

It seems there isn't even agreement on these two simple statements. It is why I read with great interest, but don't necessarily jump in with both feet when it comes to fully believing something which I cannot measure myself or understand much of the maths of either argument.

 

[twofish-quant]

This is not scientific disagreement. It appears to me that Krauss is making statements that are flat out *wrong*. There are people on this board with more cosmology/GR experience than me, and they can chime in.

Also the fact that the appears to be "flat" is hardly interesting. You take a curved surface and stretch it out to some enormous size. Whatever you started out with, by stretching, you end up with something "flat" and so flatness falls naturally out of the inflationary models.

The problem is that you can't go the other way. The universe looks "flat" now, but that's because of massive inflation that flattened out the universe. The flatness of the universe today can't be extrapolated back pre-inflationary.

 

[Tanelorn]

No science in the thread? I reject that, it is all science, everything is:

 

[Tanelorn]

twofish, unfortunately I am unable to make a judgement call on what you and Hawking are saying. I suspect that there are many others the same.

Take the CMBR and BB theory, I cannot measure the CMBR accurately myself so I have to trust other people's measurements and then I have to trust other people's theoretical analysis which may or may not be complete. We are all human and we all make mistakes even with simple everyday tasks. So to believe what I am told I need to place great faith and trust in the abilities of others.

 

[twofish-quant]

The fact that layman can't tell when a scientist is talking nonsense, is why I think scientists have a special responsibility to get the facts right, and to make it clear when they are talking about something that is the scientific consensus, or if they are just guessing. Even when they are talking about the scientific consensus, there needs to be a caveat that the scientific consensus can change, and change wildly.

If Krauss was talking about random stuff on his blog or on a forum, then it's OK for him to be sloppy. If he starts writing NYT best sellers, then he'd better get his facts right, and here I think he has gotten them very, very wrong.

Here is a semi-technical explanation of the problem

http://ned.ipac.caltech.edu/level5/March01/Carroll/Carroll1.html

One you add a cosmological constant, you have a *constant* positive energy term. As the universe expands, the matter energy goes down, the negative energy due to gravitation attraction goes down, so you are left with constant positive energy.

The other problem with the universe being created from "nothing" is that it doesn't explain why it doesn't happen now. If "quantum fluctuations" can create gravitationally bound particles with a net energy of zero when why don't we see new matter being created today?

 

[Mark M]

"And time slows down to zero and actually begins at the big bang singularity"

This is also terrible from a science standpoint. The trouble is that by selecting coordinates, you can make *any* problem go away. I create a set of coordinates x^\pime = x/(distance to river thames). Because distances shrink as I go to the Thames, I can't cross it.

This is also my biggest problem with Hawking's cosmology, that it is unjustified to declare that time truly didn't exist before the big bang. The Hawking-Penrose singularity theorems relied on classical general relativity to prove that the universe at the big bang was geodesically incomplete. But there's the problem - considering general relativity predicts its own breakdown at singularities, and quantum mechanics holds that a particle cannot be fit into a space shorter than its wavelength, it would seem that an 'initial' singularity is a result of using a classical theory to describe something it cannot.

On top of that, theories of quantum gravity have ways of avoiding initial singularities - take, for example, string theory. Using AdS/CFT correspondence, singularities can be avoided. If you let a region of space undergo gravitational collapse in the AdS₅ x S⁵ bulk, it appears that a singularity develops as expected. However, in the N = 4 super Yang-Mills theory on the boundary, it doesn't seem that time 'comes to a halt' as it appears by simply applying classical GR.

I don't know enough about LQG to comment on it, but as far as I know, it provides a 'bounce' . So, the singularity would just be the universe going from a contracting phase to an expanding one.

Either way, I think it is misguided to declare that time 'comes to a halt' at the 'initial' singularity.

In terms of Krauss, I don't know how he hasn't seem the error yet. It's strange, because he spends an awfully long time speaking about dark energy in his book.

 

[Tanelorn]

I am not a complete layman, I know enough Physics to be dangerous, but Cosmology is a strong interest not a speciality. My speciality is electron Physics.

Regarding time slowing down I think Hawking used the fact that it happens near and in black holes as a similar example.

 

[twofish-quant]

That's the standard reductionist philosophy chart, and it's a good example of how some physicists (especially particle physicists) think the world works. I happen to reject that philosophy so to me that chart is total non-sense.

Also that chart is extremely old...

http://en.wikipedia.org/wiki/Tree_of_life_(Kabbalah)

One reason that I think it's non-sense is that you can make statements about collections of objects that are philosophically independent of the properties of the objects themselves. You aren't going to understand traffic jams by taking apart a care. This happens a lot in thermodynamics.

The other science issue from Krauss is that he doesn't seem to address this paper

http://arxiv.org/abs/hep-th/0208013

Dyson, Kleban, Susskind argue that if the cosmological constant exists, and if you make something called the AdS/CdT correspondence, then the universe is destined to repeat, and we will have nearly the same conversation N years from now, where N is a huge number.

 

[twofish-quant]

Regarding time slowing down I think Hawking used the fact that it happens near and in black holes as a similar example.

Are we talking about the same Stephen Hawking? Because the Hawking that you are referring to is making silly statements. On the other hand, it could be that the issue is that Hawking is a human being that either says wrong stuff or says stuff that is correct but easily misinterpreted.

The idea that "time slows down" near a black hole is a very common mistake that people make with GR. Time *doesn't* slow down near a black hole. (My area of speciality are supernova simulations. I don't know a huge amount of GR, but just enough so that i can do collapse calculations.)

A good analogy is what happens with latitudes and longitudes near the north pole. If you try to do math using latitudes and longitudes near the north pole, then you get all sorts of bad stuff happen. However, that's because you've chosen coordinates that just don't work. You can choose another set of coordinates to get rid of the problem.

If you use the most obvious set of coordinates to model a black hole then you get all sorts of badness near the event horizon, but that's like trying to model longitude near the north pole. This isn't a "real" issue. Now once you get to the singularity of the BH, then you have a "real" weird thing.

This happens in my simulations. I care about supernova. I'm not researching black holes, so that I choose a "stupid" coordinate system so that time slows down and stops as you approach something that is turning into a black hole so that I don't have to simulate it. This doesn't change the result of my simulations because what happens inside of a black hole cannot effect what happens outside of it (hence the name "black hole"), so I choose a "stupid coordinate" system so that the computer doesn't have to calculate stuff that's irrelevant to what I'm interested in.

But this is just a computational trick. It's not "real".

 

[twofish-quant]

One other thing...

I tend to find the early, early universe to be non-interesting, because you can make up anything you want about it. The late early universe is much more interesting. One problem with *gee-whiz* presentations of the big bang is that it doesn't mention how excitingly boring things are at later times.

By "excitingly boring". At t=0 you can make up anything you want. At t=300,000 years, you can't. When t=300,000 years, the temperature of the universe is about 3000 kelvin and is a mixture of hydrogen and helium gas. I can create a mixture of hydrogen-helium gas in my kitchen and heat it to 3000 K. At t=0, I can invoke all sorts of weird physics. At t=300,000 years I can't, and the fact that I can't make up anything makes it a much more interesting puzzle. The most exciting part of cosmology to me today is not t=0, but galaxy formation. If you ask how the universe started, there is so little data that any explanation that you come up with is pretty much equivalent to "God did it." If you ask how the galaxies form, we'll we can sort of see it, and the temperatures and conditions are things that we can do experiments on. If you have to invoke a new particle or something weird, it's really a mystery because you have to explain why that new particle doesn't show up in your kitchen.

The focus toward the odd and esoteric is a bad thing, because it misses how much cosmology is an *experimental* science. We know that something big went bang 13.9 billion years ago, because if you look at the sky in microwave, you can see something big going bang.

 

[Tanelorn]

twofish thanks for your replies, and the time you give. I think I agree with most if not all the points you made, my mind has shut down for the evening so I can't add anything right now.

I want to add that if I have misquoted Stephen Hawking then I sincerely apologise to him, my memory isn't what it used to be.
I think he is an inspiration to us all with the fortitude he has shown in overcoming such a difficult illness and still able to give so much, I truly wish him all the very best.