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Warning: Brain Blowing Stuff...


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From the February edition of Astronomy Now - "Astronomy Probes Space-time" (Alan Longstaff)

The article starts off:

"The Universe started out in an unimaginably small, dense state. This Planck scale, when the Universe was just 10^(−43) seconds old and 10^(−35) metres across, cannot yet be described by quantum field theory, ..."

But if the Universe is compressed, who is "out there" to measure it? And if the Big Bang was the event that created space and time (which is curved according to the models and predictions of GR), wouldn't all these dimensions have been "squashed" in some way too? I'm thinking of the expanding Universe on the surface of a balloon simple analogy. In other words, could it be that our rulers are stretching in front of our eyes as the Universe expands (but so are we, so we don't notice it ... OR.... the ratios involved now in terms of how much bigger the Universe is becoming over millions of years is so small, it would be imperceptible anyway?

Serves me right for trying to read this stuff at 11pm before going to bed. Should be cuddling up with my wife and watching a romantic movie instead........

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Maybe, space and time are like two positive charges repelling each other and giving rise to expansion and the relatively weak energy field we call gravity. And maybe, there was another universe before this one that for some reason or another gave up on expanding and started to contract. There arrived a point where the older universe contracted so much that time and space repelled each other in a mighty bounce back that we call the Big Bang. In this sense, we could say universes 'breath', its 'inhalation' is what we call expansion and its exhalation is what we call contraction :icon_razz:

How's that as a guess before lunch time :smiley:

I can begin to appreciate why Nietzsche regretted the death of gods...it used to be so much simpler :lol:

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The idea that leads you astray is that you can scale the universe up or down (space-wise) and nobody would notice. That is not true and the reason is that there exist fundamental particles that are, in effect, this big and no bigger and they have this mass and not any other. Particles with mass can always be used to fix the scale of things. In a universe without massive particles we would have trouble finding out if the universe scales up or down, we would have a scale invariant universe. That's my understanding, anyway.

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The idea that leads you astray is that you can scale the universe up or down (space-wise) and nobody would notice. That is not true and the reason is that there exist fundamental particles that are, in effect, this big and no bigger and they have this mass and not any other. Particles with mass can always be used to fix the scale of things. In a universe without massive particles we would have trouble finding out if the universe scales up or down, we would have a scale invariant universe.

Thought-provoking.

But even in different inertial frames lengths are different. So a rod moving away from me in an inertial frame appears shorter than the rod to the observer moving with it. Do fundamental particles building the atoms and molecules that create the rod have dimensions in themselves, and, if so, are they contracted with that moving rod, or just the space in between them?

I'm still trying to get my head around a Universe which is only 10^(−35) metres across. Just that comment assumes one can gaze upon it from beyond. But there is nothing outside it. Not even space. Even empty space with no particles at all is something.

And when we get to the end of the Universe, even now, what do we find? A brick wall? Or do we "push the Universe out" as we go? Or do we involuntarily curve around (thinking we are going straight) and ultimately approach where we left from behind, following some kind of geodesic?

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But even in different inertial frames lengths are different. So a rod moving away from me in an inertial frame appears shorter than the rod to the observer moving with it. Do fundamental particles building the atoms and molecules that create the rod have dimensions in themselves, and, if so, are they contracted with that moving rod, or just the space in between them?

We need to be a bit careful here. Particles, strictly speaking, have no dimension but what does have a dimension is the quantum wave that is their "essence" and that dimension is the wavelength of that wave. And I think that yes, that wavelength does undergo the relativistic "stretching" in moving frames and in turn this supports the classical notion of a particle's mass appearing heavier in a moving frame.

We have a terminology issue here. When we say "particle" we immediately think of something classical, an entity defined only at a point. But the world is made up of quantum particles and they are not like anything we have an intuition about, especially the ones with half-integral spin (the fermions, which make up matter-as-we-know-it).

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I'm still trying to get my head around a Universe which is only 10^(−35) metres across. Just that comment assumes one can gaze upon it from beyond. But there is nothing outside it. Not even space. Even empty space with no particles at all is something.

The Universe was not only 10^(-35)metres across, I think that was a wrong statement. What observation seems to be saying is that the Universe was denser and hotter in earlier times. The part of the Universe we can observe now (and that's not the whole of it because it's only been around for so long) took up less and less space the further back you go in time.

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thanks for the insightful comments themos.

As is usual for magazine articles, trying to make things accessible often makes things more confusing!

Later on in that article there was a mention of CPT invariance...

CPT invariance means that the Universe looks the same under three transformations taken together. C, charge conjugation, says that nature treats matter and antimatter the same; P, parity inversion, is the idea that nature doesn't discriminate left from right; and T, time reversal, means that at the quantum level events look the same whether time runs forwards or backwards.

But I don't know how this relates to the non-rotation of polarised gamma rays from gamma-ray bursts (GRBs) and the testing for quantum gravity.

The article doesn't really link up the ideas in any coherent way :huh:

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Hey, you think it's tiring reading Alan's articles? He picked up a book on time I'd been reading for weeks at about a page and a half a day and he steadily turned the pages as if it were a James Bond. Then he started to laugh at something and I thought, 'Oh hell, there are jokes in there as well!!'

Olly

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Hey, you think it's tiring reading Alan's articles? He picked up a book on time I'd been reading for weeks at about a page and a half a day and he steadily turned the pages as if it were a James Bond. Then he started to laugh at something and I thought, 'Oh hell, there are jokes in there as well!!'

Olly

I know where you're coming from.

Unfortunately reading articles like the one I mentioned above is not really tiring for me .... more frustrating. There are details that are perhaps being dropped or curtailed not to "overcomplicate" things, but, in fact, has the opposite effect. There was no explicit link of non-rotation of polarised gamma radiation to CPT invariance or quantum gravity. It's not my intention at all to criticise the author or question what has been presented, but I just couldn't make out how one thing related to another. I am genuinely interested in the contents of the article and the idea of using astornomical data to test out the latest ideas and theories.

If anyone has read that article and can fill me in I would certainly appreciate it!

Edited by TonyD
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I don't know if this helps but the way I visualise it is that there was a quantum sea. Perturbations in the sea can form matter. As far as I know the Big Bang wasn't a single event so I imagine several perturbations forming elementary particles and unfolding the physics that we know and love. A bit (ish sort of) like a pop up tent.

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I know where you're coming from.

Unfortunately reading articles like the one I mentioned above is not really tiring for me .... more frustrating. There are details that are perhaps being dropped or curtailed not to "overcomplicate" things, but, in fact, has the opposite effect. There was no explicit link of non-rotation of polarised gamma radiation to CPT invariance or quantum gravity. It's not my intention at all to criticise the author or question what has been presented, but I just couldn't make out how one thing related to another. I am genuinely interested in the contents of the article and the idea of using astornomical data to test out the latest ideas and theories.

If anyone has read that article and can fill me in I would certainly appreciate it!

I haven't read the article, but, prompted by this thread, I have spent some time reading (and hopefully understanding somewhat) the short research journal article on which the polarised gamma-ray CPT stuff is based. Give me a bit more time, a day or so, and I'll have my go at explaining it.

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To start, a bit about dispersion and birefringence in optical media, and about polarization of light, because the CPT - gamma ray - quantum gravity experiment uses a beautiful interplay between these three effects.

First, dispersion. An optical medium, like glass, is dispersive if different frequencies (colours) of light have different speeds in the medium. Equivalently, an optical medium is dispersive if the index of refraction of the medium is different for different frequencies. Glass is a dispersive medium, and a glass prism disperses light into a familiar rainbow pattern.

Next, polarization. At the location of an observer, light waves “vibrate” in the plane perpendicular to the line between the light source and observer. If light vibrates along a fixed line in this plane, the light is said to be linearly polarized. If light vibrates in this plane along a line that rotates uniformly, the light is said to be circularly polarized, right-circularly polarized if the rotation is clockwise, left if the rotation is counterclockwise. Crucial observation: linearly polarized light can always be considered to be a combination of right-circularly polarized and left-circularly polarized light.

Finally, birefringence. An optical medium, e.g., quartz, is said to circularly birefringent if right- and left-circularly light travel at different speeds in the medium. Suppose linearly polarized light enters a medium that has circular birefringence. If this light is observed at some fixed point in the medium, linear polarization will still be observed, but along a line that makes a fixed angle (fixed rotation) with respect to the original line of linear polarization. This is because: linear polarization is a combination of left- and right-circular polarization; left- and right-circularly polarized light have different speeds in the circularly birefringent medium.

On to quantum gravity. Some quantum theories of gravity (possibly) predict that spacetime behaves like an optical medium that has both dispersion and circularly birefringence. Circularly birefringence treats left and right differently, and thus violates P (parity) in CPT. Not only does spacetime have dispersion and birefringence, it has them in such a way that the amount of birefringence, the difference in speeds between right- and left-circularly polarized light, is frequency-dependent.

A gamma-ray burst produces gamma-rays that have a range of frequencies, and that are largely linearly polarized, with the original line of polarization frequency independent. We don’t know the original line of polarization, but this doesn’t matter. We observe this linearly polarized radiation after it has traveled a long distance through spacetime, which has circularly birefringence, so we should observe linear polarization along some line. Because the amount of birefringence is (according to some classes of quantum gravity theories) frequency-dependent, the direction of the line for linear polarization that we observe should be frequency-dependent.

A recent paper states that to an extremely great degree, the line of polarization for gamma-ray bursts (remember, these bursts of radiation contain a range of frequencies) is frequency-independent. This constrains the amount of possible CPT invariance in possible quantum gravity theories.

The technical paper at which I looked is (full pdf at upper right)

http://arxiv.org/abs/1208.5288

This paper is short and condensed, and it took me some time to unpack (what I think are) some of the main ideas. I might not have been very successful in communicating these ideas, so questions are welcome.

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Thanks so very, very much, George. This is a really marvelous and interesting explanation and has linked the themes that I just wasn't able to see before.

Thanks for the link to the PDF too.

Tony

I might not have been very successful in communicating these ideas, so questions are welcome.

No you have done a very successful job!

As and when I have questions on this and other related concepts, I know where to turn :grin:

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In terms of the Universe being "small" I suppose there are two ways of looking at that:

- first, in terms of curvature of space-time. when the universe was about a foot across that could mean that the ends of your yardstick would be almost touching (oh, and probably beginning to char a little). "Outside" is as meaningless as being north of the north pole - so that is all there is...

- second, in terms of horizon - when the universe was a foot across that simply means that even if there was a big-bang, small pop, wild party or super telescope sale going on another foot away a foot is as far as you can see and it might as well not be there. "Outside" is as meaningless as seeing the Hubble up for sale on ebay ... (but buyer collects)

- as for the other dimensions, if they had expanded "significantly" as well then you wouldn't be here to notice - there would be no stable planetary orbits for one thing, so astronomy would not have got off to a particularly good start and Forums like this wouldn't exist. Fortunately, we were lucky and ended up in an almost perfect universe, where things are 3+1 dimensional, the sky is clear some nights, and there is just enough space in those other hidden ("shrunk") dimensions to hide all the ugly cables, gaffer-tape and clockwork that are probably needed to keep this apparently elegant Universe ticking.

P

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And when we get to the end of the Universe, even now, what do we find? A brick wall? Or do we "push the Universe out" as we go? Or do we involuntarily curve around (thinking we are going straight) and ultimately approach where we left from behind, following some kind of geodesic?

... or can you never get there because the space between SGL's DIY rocket-ship and the "end" is expanding all the time, and even if you get to where the end is now (expecting to see quasars with just binoculars), you would probably find a sparse scattering of dull stars, some worn-out galaxies with zimmer-frames and a lot of old guys complaining that astronomy used to be so much more fun. Brick wall (brick roof?) would take a lot of repointing and graffiti removal though.... perhaps thats what happens to sinners ("Hell" is supposed to be "equally distant from everywhere")...

P

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