It was new at the time, anyway. The model being spoken of is the Big Bang, first suggested by Father Georges-Henri Lemaitre in 1927. (The expanding-universe solutions to general relativity had also been derived by Alexander Friedmann in 1922, but he hadn’t emphasized the nature of the intial singularity in such models.) Lemaitre, a Belgian priest who studied at Harvard and MIT, proposed what he called the “Primeval Atom” or “Cosmic Egg” model of the universe, and derived Hubble’s law, two years before Hubble and Humason actually discovered that the universe is expanding. Einstein wasn’t all that fond of Lemaitre’s idea — having been assured by his astronomer friends that the universe was static — but he encouraged Lemaitre in his investigations.
All of which springs to mind because the Modern Mechanix blog has unearthed a Popular Science article from 1932 by Donald Menzel, an astronomer at Harvard, that explains Lemaitre’s ideas. (The time between Hubble and Humason’s discovery and Menzel’s article is somewhat less than the time between the 1998 discovery of dark energy and Richard Panek’s New York Times Magazine article from yesterday.) Menzel’s piece does a great job of explaining the basics of the Big Bang model, long before it was given that name by Fred Hoyle. Indeed, he touches on many of the questions that still arise in a good Cosmology FAQ! For example, he emphasizes that the redshift is due to the expansion of space, not to the Doppler effect.
The case of the universe is analogous, except that the expansion, being of a three-dimensional volume, cannot be visualized. The phenomena are, however, comparable. The nebulae are not running away from us. Their recession is due to expansion of space. This may, perhaps, seem to be quibbling over terms, since it amounts to the same thing in the end. Nevertheless, the distinction is worth keeping. According to the relativity theory, there is a difference between the running away of the nebulae and expansion of the medium in which they are imbedded.
Sadly, he also appeals to the much-hated balloon analogy for the expansion of the universe, although he uses the surface of the Earth rather than the surface of a balloon; in fact, it’s a better choice. And he’s not afraid of diving into the sticky questions, like “What happened before the Bang?”
DR. LEMAITRE’S hypothesis does away with the old query as to the state of affairs before the beginning of things. Going back to the parent atom we may inquire about what happened before the cosmic explosion took place. The answer is: — Nothing. – Computation shows that space would have closed up around the massive atom and, certainly, nothing can happen where there is no room for it to happen. Time has no meaning in a perfectly static world. The age of the universe is to be reckoned from that prehistoric Fourth of July, when space came into existence. Since then, space has been continually expanding before the onrushing stars, sweeping the way for them, forming a sort of motorcycle squadron to make room for the star-procession to follow.
Like many contemporary cosmologists, Menzel is a little more definitive about this than he really should be. When asked “What happened before the Bang?”, the correct answer is really “We don’t know. According to general relativity, space and time do not exist before the Bang, so there is no such thing as ‘before.’ However, we have no right to think that general relativity is correct in that regime, so… we don’t know.” Few people are sufficiently straightforwardly honest to give that answer.
And what about the future?
SO MUCH for the present. What of the future? Einstein and the noted Dutch astronomer, Willem de Sitter, have talked of some future contraction, which might sweep up the stars along with cosmic dust and eventually bring the world back to its original state. Dr. Lemaitre thinks that such a contraction cannot occur. He prefers to believe that the whole universe was born in the flash of a cosmic sky-rocket and that it will keep expanding until the showering sparks which form the stars have burned to cinders and ashes.
We still don’t know the answer to this one, but the smart money is on Lemaitre (and against Einstein, who liked his dice unloaded and his universes compact). Now that we know the universe is not only expanding but accelerating, the simplest hypothesis is that it will keep doing so. To be honest, of course — we don’t know!
Lemaitre passed away in 1966, a year after Penzias and Wilson detected the microwave radiation leftover from the Primeval Atom.
And what’s the real explanation of expanding universe? Do I think right that manifold called “spacetime” has a property that in general two geodesics want to go further and further away from each other?
If so, then what’s wrong with picturing spacetime as a cone with vertex symbolizing big bang?
Although circles on this cone are “expanding” just as bread or spheres, it is fully embedded in a Euclidean space, so nothing is really expanding and one can say “that’s the way it is, this is spacetime and you see that two things that “don’t move” are really further and further apart from each other”.
There’s still problem of imagining existence of this cone outside of Euclidean space but I don’t believe there is any analogy that could change that aspect.
What I like about the balloon is that it works for many different topics that I cover in an intro course, without reinforcing common misconceptions. The only trick is that you have to get the students to accept the rules of flatland, i.e. that the balloon analogy only works if you’re a 2-d being with no direct way to experience (rather than infer) the higher 3rd dimension. If you can do that, then they get that the notion of a “center” of the expansion is ill-defined, they get that there is no edge to the expansion, they get that there is no preferred place in space, they get that the space is not expanding into some pre-existing space that they could perceive, they get that marks on the balloon (i.e. galaxies) get dragged apart by the expansion of space without needing to physically move through the space, they get the idea that space can be curved, and they get the idea that there are things you can do in your 2-d space to infer the higher-dimensional curvature (though yeah, you have to toss out the closed finite universe bit, but that’s small potatoes compared to what you get in return). I usually introduce the notion when talking about black holes, and then when talking about gravitational lensing, and so when they finally get to the expansion of the universe, it’s a reasonably comfortable analogy. Let’s see you get that from a loaf of raisin bread!
And besides, raisins in baked goods are disgusting.
Well, I like both metaphors. But the raisin bread analogy raises another question. As space expands, what happens to extended objects like raisins and galaxies? Do they expand as well, together with space? I guess that’s why photons redshift, i.e., expand, but what about cosmologists? Are they expanding, too?
Just to clarify things…
From Animal House
—————————————
Larry: [to Jennings, while high] Okay. That means that our whole solar system could be, like one tiny atom in the fingernail of some other giant being. [Jennings nods] This is too much! That means one tiny atom in my fingernail could be–
Jennings: Could be one little tiny universe.
Larry: Could I buy some pot from you?
—————————————-
I think we can move beyond the balloon/raisin bread stuff.
e.
You’ll get no argument from me on this point. Those in Elliot’s state might take exception.
You’re probably right that the balloon is more versatile as an analogy, but, as Allyson’s question shows, it’s easy to run into misunderstandings when you’re working with a 2d analogy. So adding chocolate-chip bread (let’s say) to your explanation can help.
Allyson: you’re most certainly not the first person to wonder about this, and you don’t sound stupid. In case Julianne’s attempt didn’t take, I’ll try to explain too. For the balloon analogy to “work,” you have to imagine yourself as a two-dimensional being living on the surface of the balloon. You have no experience of 3d space off the balloon. So, as far as your 2d mind knows, your space isn’t expanding “into” anything. It has no edge, no outside, so it’s just expanding–everything is getting further from you with time. Similarly, it’s conceivable that our expanding universe is actually growing like a balloon in some higher dimensional space, but, being three dimensional, it’s not obvious how we’d test this, so it’s not something we concern ourselves with too much (well, some people do). We just note that it’s perfectly possible to have an expanding space that isn’t expanding “into” anything (like the balloon), and get on with our lives.
Hi brian,
I have been posting under the name “Brian.”
anon – My understanding of the term “cosmological constant” is that it means that the “dark energy” in, say, a cubic kilometer of space is the same regardless of the postion of that cubic kilometer in space and time, whereas quintessence theories allow the energy density to differ at different places and times. If this understanding is incorrect, I would welcome comments by those more knowledgeable than I.
There are various active and proposed projects to measure how the rate of expansion of the observable universe has changed over the course of time. These will increasingly favor either a cosmolgical constant or some form of quintessence. But while such measurements would describe the expansion, they, by themselves, would produce something akin to Kepler’s laws of planetary motion in that they would enable us to specify the rate of expansion as a function of time (and perhaps, God forbid, location) but would not provide an underlying theory, such as Newton’s Laws or General Relativity.
I thought that by “a hypothesis to test,” someone meant a plausible underlying theory as to some sort of “cause” or mechanism. As the art6icle points out, however, a “cause” can sometimes be little more than a …?
I agree with the defenders of the balloon. Most of all because it is more then an analogy. It’s actually mathematically correct. The (intrinsic) geometry of a balloon expands according to the very mathematics that govern our 4Dworld.
To get people to understand that the embedding of the surface is irrelevant is crucial but can again be done in the context of 2D geometry by pointing out the difference between intrinisic and extrinsic curvature (roll up a sheet of paper, still the same! on the sheet of paper you don’t notice any stretching! Doesn’t work with a sphere, etc…). But really this is the only caveat of the balloon.
I have been using 2D surfaces as lay-toy explanaitions for GR for a while with more success then with anything else. Especially for explaining how we can do physics with geometry (conceptually). I start out with pointing out that if you draw time and space as directions on a bit of paper stuff moves between points along the shortes connection between them. If they accept this then the idea of modifying the way things move by putting dents on the surface and changing what the shortest connection is becomes a fairly natural generalization (insert equivalence principle here). And then one can point out that for people on the surface extrinsic curvature is irrelevant since it doesn’t change what the shortest path is, so embedding is not important (mention klein bottle i.e. that it’s possibly even not embeddable in 3D). Then tell them that it’s really complicated because “matter tells spacetime how to curve and spacetime tells matter how to move”.
IMO this captures one of the central conceptual insights of GR.
At this point I can start explaining QM and how it’s probabilistic interpretation seems at odds with this geometric picture where there is no good notion of “now” and then they will truly and verily regret to have asked what this Quantum Gravity thing is.
I like raisins, plain and in baked goods.
“And what’s the real explanation of expanding universe?”
I’m sure you are familiar with Pythagoras’ theorem. You note that nowhere in that formula does anything depend on time.
Einstein’s big discovery was that this is actually wrong: the correct version of P’s theorem actually DOES contain a function of time. Thus the distance between two *stationary* objects naturally becomes a function of time. This time-dependence is what we call the expansion of the universe. That’s all there is to it. No balloons. No raisins. Though I too like raisins.
“The nebulae are not running away from us.”
Sad that people understood this in 1932 but there are still professional physicists who don’t get this simple point.
Returning to the main subject of the blog, does not the title provide a counter example of Richard Dawkins thesis that, as God does not exist, a scientist would recognise the fact?
Garth
Whether one uses the “dough and raisin” analogy, or the “balloon” analogy, there’s a fundamental problem:
Without introducing the concept of a topology change, there can be no real explanation of what happens when you reach the planck scale.
This is what makes the analogies so imperfect and what most people find so hard to grasp.
Garth: The history of astronomy is full of examples of significant contributions from the Jesuit priests, such as Lemaitre, above, and Secchi, who invented spectroscopy. You can scroll down to see more people like him in other scientific fields. Today, you will not find a shortage of their refereed papers in the astrophysics journals, as they continue to be excellent scientists. To the Jesuits, they are ‘exploring the mind of God’. Here’s one example of that perspective.
Lemaitre wasn’t a Jesuit. It’s a common assumption though–because he was a PhD (twice over). But in terms of his clerical status, he was an ordinary diocesan cleric.
For a truly hilarious ‘bad’ bio of Lemaitre, check out Dan Brown’s Angels and Demons. He describes Lemaitre as a monk who wanted to prove the existence of God…
I like the balloon analogy as a general starting point. If you imagine yourself on the surface it provides a logic for understanding expansion and how there can be no center or edge. However if one is actually in flatland, I think it would be just as difficult to explain to the other inhabitants, how their space was ‘expanding’ or curved.
The problem is that in our normal daily experience of the 3d world we use a perceptual ‘ruler’ that is only two dimensional, not three. If for example, our normal 3d world was expanding at a meter per second, we wouldn’t be able to perceptually tell this was occurring. A xyz ‘ruler’ would also be expanding and there would still be N units between two ‘things’, you need one more dimension for a proper perspective.
Question to Alex Nichols, can you explain a little further on:
Amara,
I was well aware of that fact!
So, is it Richard Dawkins who is Deluded?
Garth
John Farrell- thanks for the correction (bad assumption on my part).
Garth- “Is Dawkins Deluded?”
I’m an atheist, I can’t disagree too much with Dawkins, however his firebrand style of presentation on this topic could be better because he’s turned some people off. On the other side, I appreciate the Jesuits alot (and some are my friends) and I think that they could be important communication bridges to show fundamentalists the value of science. I also wonder if Dawkins has spent any time around the Jesuits. I suspect that they would get along very well (they might even make him laugh).
But was it understood in 1932 that general relativity would eventually need to be replaced by a theory of quantum gravity, and that GR is likely to become significantly inaccurate at the Planck scale? Obviously a good physicist would consider the possibility that GR only worked in certain limits just like Newtonian gravity, but I’m wondering whether Menzel would have had any good sense of what these limits were likely to be, or why GR would be more likely to break down near the moment of the Big Bang than in any other circumstances.
Amara, you’re welcome. And good points. I agree. Sadly, there are not as many Jesuit scientists as they’re used to be.
I usually cite the balloon example in the last few days of an undergrad course when im talking about spacetime topology (if theres time). Pointing out that just b/c we have a good grasp on the important dynamics for measuring things, theres still a good deal of wiggle room left in the background.
re 40: Question to Alex Nichols, can you explain a little further on:
“Without introducing the concept of a topology change, there can be no real explanation of what happens when you reach the planck scale.”
Both the “dough & raisin” and the balloon analogies rely on describing the expansion of a 3-d manifold.
However, there is a distinction between inflation and the big bang.
You can explain inflation using a manifold, but as I understand it, if you project the process back to a planck-scale big-bang event, even this description breaks down. This follows strictly from the equations of Quantum Mechanics and Special Relativity.
The problem of course, is what leads to the debates going on between String theorists and Loop Quantum Gravity proponents at the moment.
I tend to think that underlying it, is a problem of philosophical conceptualisation, as much as mathematics.
Something along the lines that expansion of the universe and contraction are in someways dual/indistinguishable, which T-dualityin string theory points towards.
My own take on it is: – there’s no such thing as nothing.
See this succinct paper for the problem: –
http://arxiv.org/PS_cache/gr-qc/pdf/9610/9610066.pdf
Alex Nichols re 46:
Thank you very much for the PDF link.
‘Popular accounts, and even astronomers, talk about expanding space. But how is it possible for space … to expand? … ‘Good question,’ says [Steven] Weinberg. ‘The answer is: space does not expand. Cosmologists sometimes talk about expanding space — but they should know better.’ [Martin] Rees agrees wholeheartedly. ‘Expanding space is a very unhelpful concept’.’
— New Scientist, 17 April 1993, pp32-3.
Spacetime contracts around masses; the earth’s radius is contracted by 1.5 mm radially (the circumference or transverse dimension is unaffected, hence the fourth dimension is needed to keep Pi constant via curvature) by its gravitation. Time is also slowed down.
This is pretty obvious in cause – exchange radiation causes radial contraction of masses in general relativity, just as in special relativity you get contraction of moving masses. Take the Lorentz contraction, stick the Newtonian escape velocity into it, and you get Feynman’s simplified (1/3)MG/c^2 formula for gravitational radial contraction in general relativity (you have to put in the 1/3 factor manually because a moving object only has contraction in one dimension, whereas the contraction is shared over 3 dimensions in GR). The justification here is that the escape velocity is also the velocity acquired by an object falling from an infinite distance, so it is velocity corresponding to the kinetic energy equivalent to the amount of gravitational potential energy involved.
It’s obvious that spacetime is contracted by gravitation. Expanding space really just refers to the recession of masses, i.e., expanding volume.
All the experimentally or observationally confirmed parts of general relativity mathematically correspond to simple physical phenomena of exchange radiation in a Yang-Mills quantum field theory. (Ad hoc theorizing to model observations is not observational confirmation. E.g., dark energy speculation based on redshift observations, isn’t confirmed by the observations which suggested the speculation. A better model is that whatever exchange radiation causes quantum gravity when exchanged by receding masses, gets some kind of redshift like light due to the recession of masses, which weakens gravitational effects over large distances. OK, I know you don’t want to know all the correct predictions which come from this physics, so I’ll stop here.)
So it appears that ten digits of pi are all you need in my Newtonian 3d world.
So my friend who lives on an Earth that is about 6,000 years old is possilbly completely correct with his pi = 3 idea.
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