In a couple of weeks I will be participating to a workshop in Munich, Germany, on the current status of string theory, the multiverse, and other cutting edge ideas in fundamental physics.
The event is organized by physicist and philosopher of science Richard Dawid, and its goal is to get some cross-disciplinary talk going among people who vehemently disagree on the future of physics.
I asked Dawid why he invited me, since I’m a biologist and philosopher of biology, not physics, and he said he is interested in outside perspectives and parallels with other fields. Fair enough, it should be fun!
In preparation for the workshop, I wrote an essay over at The Philosophers Magazine Online on the whole debate, which has recently taken a nasty and very public turn.
Lee Smolin a few years ago wrote an influential book, The Trouble with Physics, which called into question the whole string theory operation on grounds that it is leading the fundamental physics community to detach itself from empirical reality. To which Leonard Susskind responded accusing Smolin of being a “Popperazzi,” an obviously derogatory term referring to the philosopher Karl Popper and his idea of falsificationism as a criterion of demarcation between science and pseudoscience.
More recently, George Ellis and Joe Silk wrote an op-ed in the prestigious Nature magazine, dramatically entitled “Defend the integrity of physics,” again criticizing their string and multiverse colleagues. To which cosmologist Sean Carroll responded via Twitter (not exactly a prestigious scientific journal, but much more effective in public discourse) with, and I quote: “My real problem with the falsifiability police is: we don’t get to demand ahead of time what kind of theory correctly describes the world.” The “falsifiability police”? Wow.
As I explain in the essay, I find all of this both amusing and disturbing. Amusing because it turns out that a number of physicists who cavalierly dismiss philosophy (though Sean is not one of them) resort to invoking their (incomplete, misguided) understanding of philosophy of science to score rhetorical points against their colleagues. Disturbing because the very public nature of the debate, and — let’s be frank — the rather crass tone that it sometimes takes, are doing nothing to improve the already troubled public image of science.
Stay tuned for more after Munich…
Footnotes to Plato 
Coel,
“Imagine a long piece of elastic, and imagine an ant walking along the elastic. You then pull the two ends, stretching the elastic. That creates more “space”. That means that it takes the ant longer to get to the other end. But, the walking speed of the ant (judged relative to the elastic directly under its feet at a given time) is not changed.”
You are comparing two metrics. The length of the elastic, versus the walking speed of the ant. Since you say the speed of the ant has not changed, presumably it is the denominator. So there are more ant footsteps/lightyears from one end of the elastic to the other, when it is stretched. Presumably making the elastic the numerator.
Yet you say space is the piece of elastic. What is the basis of the speed of the ant/light, if not space? It seems the speed of the ant/light is just being assumed as a stable constant. How is that? Is there some other form of vacuum that not space, but is the basis of the speed of light? Given that is the constant, it would seem to be a very big assumption.
Massimo,
Keep in mind that epicycles were an observed regularity that were mathematically modeled to a high degree of accuracy. We are at the center of our view of the universe and there is no objective point of reference. To model anything, you need a frame and we are the center point of our own frame. So it is a completely valid mathematical model. The problem was trying to develop a physical explanation for this regular pattern and so they used the concept of a clockwork universe.
Now today, we have General Relativity as a very effective mathematical model to explain gravity. The question that might have to be eventually asked is whether spacetime is a valid physical explanation for this pattern. Or is it like the clockwork universe; An extrapolation from the model that fails to take larger factors into consideration.
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Coel,
To try to further clarify this;
Einstein said “Space is what you measure with a ruler.”
Now as a constant, it would seem that the speed of light is a very fundamental ruler, but according to the theory you are espousing, it doesn’t measure space. So what is it measuring?
According to theory, “the speed of light in a vacuum” is the constant. So what is this “vacuum,” if it is not space?
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Actually, Coel, it depends on what interpretation of quantum theory you accept as to what version of cosmology has things to explain or not. We’ve covered this ground on Sci Sal, including specifically with the one essay about quantum interpretation theories.
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Coel,
“That means that it takes the ant longer to get to the other end.”
Might we agree that, “Space expands, relative to the speed of light?”
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brodix,
*IF* you were going to use the speed of light as a ruler, *then* you would indeed be making an assumption that the speed of light was constant, and, yes, this would then not be independent of that assumption.
BUT, there are plenty of other measures of distance that are INDEPENDENT of BOTH the cosmological expansion of space, and of the speed of light.
As I’ve explained to you before, cosmology does not use the speed of light as a ruler (that’s just another of your misconceptions; the cosmological distances are way too big for that to be useful); rather, the main distance indicator that cosmology uses is a combination of the inverse-square law and standard-candle methods.
Science is always a matter of tying *multiple* lines of evidence together. And here we can use those other lines of evidence to check *both* the expansion of space, and the constancy of the speed of light.
Again, the test is whether the overall model fits the observations, and it does, indeed it does so very well. And, once again, if you have some other conception of cosmology that you think is better, then feel free to show — with explicit and detailed calculations of actual values — how it explains all the cosmological observations. Until you’ve done that you have nothing.
As I’ve said, the “tired light” scenario you’re suggesting was first proposed in the 1930s by Zwicky, and for a couple of decades it was a serious alternative. Eventually it fell out of favour because, as better and better observations came along, it could not be made to fit them, and so it is now a dead dodo. It really is not the case that no cosmologists have thought about this stuff!
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Just a point about “adding to or subtracting from the infinite”: If there were, for example, a computer traveling in a Malament–Hogarth spacetime, an actual-infinite number of compute steps could be completed in a finite program-time period, and the program could go on to additional steps beyond that. That’s one way to (hypothetically) physically “adding to infinity”.
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Hi Massimo,
The string theorists also think that string theory is testable – they just don’t know how yet. So it still seems to be in the same category as energetic causal sets.
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Coel,
If we agree that the speed of light is independent of the expansion of space, then why is the premise of spacetime being used as the basis for explaining this expansion, given that one of its primary arguments is the speed of light being constant to the dimensions of the space?
I can certainly accept the possibility of all other galaxies flying away from us, given I’m only going on the information I hear, but it would seem that if there is this apparently stable dimension of space, as defined by the speed of light, then the expansion would seem to be occurring within it, not of it.
The only problem with that would be that we appear as the center of this expansion and thus the center of the universe, with all other galaxies apparently flying directly away from our position and at a rate increasing proportional to their distance from us.
“Again, the test is whether the overall model fits the observations, and it does, indeed it does so very well.”
Other than having to add Inflation, Dark Matter and Dark Energy to fill various gaps, it all fits together perfectly.
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Robin,
“The string theorists also think that string theory is testable – they just don’t know how yet”
Right. For others in the physics community that seems to amount to the same thing, though.
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Simple. You put the string theory on a scale, then in the water. If it floats, it’s a witch.
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Now if they were to get rid of the physical strings and just have the wiggling part, then you would have vacuum fluctuation and all you need is the vacuum and a little instability.
Then it could coalesce as mass and disperse as radiation. So the physicality would be emergent as form and gravity as the coalescing of it.
Might be easier to flesh out than how the entire universe worth of energy emerged from a dimensionless point, 13.8 billion years ago.
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brodix,
The trouble is I don’t know what that’s supposed to mean. What does “the premise of spacetime being used as the basis for explaining this expansion” mean? What does ” the speed of light being constant to the dimensions of the space” mean?
And what does “this apparently stable dimension of space, as defined by the speed of light” mean? Most of what you write might make sense to you, but it doesn’t make sense to me.
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Coel,
I can only assume you have read the history of the development of the cosmology over the last hundred years. I think it was back in the twenties, when they realized all those other galaxies are redshifted directly proportional to distance and there is no apparent lateral motion to match the redshift, they realized it made us appear to be at the center of the universe. So then it was switched from just being a conventional expansion to saying space itself is expanding, based on spacetime.
Yet that does overlook the fact that the premise of General Relativity is that when the frame is contracted, due to gravity or acceleration, the measure of the speed of light slows accordingly, so that it remains constant to the dimensions.
This was in the popular descriptions of the evolution of the theory I read back in the seventies. I don’t really know how it is being taught today.
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brodix,
So all those cosmologists of the time, including people like Einstein, just overlooked a big blunder in their thinking, and every cosmologist since has done the same, and now you are the only one to have spotted this blunder — and yet you can’t state what it is in terms that I can understand?
Again, what does that mean? What does “remains constant to the dimensions” mean?
Any why is there some big inconsistency between the basics of General Relativity, and the cosmological expansion? You do realise, I presume, that General Relativity *predicts* an expanding space?
The field equations of General Relativity lead to the Friedmann equation (as first shown by Alexander Friedmann, and repeated since then in all cosmological textbooks), and independently George Lemaitre showed that the solution to this was an expanding (or contracting) universe. All of that is in textbooks and has been learnt by every generation of cosmology students since then, lots of them very able mathematicians.
So, all of these people (Einstein, Lemaitre, Friedmann, Eddington, Hawking, etc) think that General Relativity **predicts** expanding or contracting space, and you are now suggesting that they’ve all just done their maths wrong, and that General Relativity is actually *inconsistent* with expanding space?
OK, if you think that, then let’s see your working. Start from the Einstein’s field equations of general relativity, and then find solutions for our universe, and then show that static-space solutions are the only ones allowed. Be sure to point out explicitly where the maths by all those listed above goes wrong.
brodix, the only statement by you in this thread that makes any sense is your self-description of yourself as a “delusional crank”.
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Coel,
I’m not trying to explain the psychology of the evolution of the theory. I’m just trying to point out what I see as inconstancies. People do have a strong tendency to rationalize.
I’ve heard that “General Relativity predicts an expanding universe” for years now and it’s not true. General Relativity predicted that gravity would/should cause space and the universe to collapse. Which is why Einstein added the Cosmological Constant. And it is the CC which would be an expansion to balance the contraction of space.
I would add I never thought to question any of this, until reading in Hawking’s A Brief History of Time, that “Omega=1.” Which stood for the theory that overall space should be very close to flat, for the universe to be as stable as it appears. This meaning that the expansion is balanced by gravity. Which is exactly what Einstein proposed for the CC.
Now it was my own thought at the time that if they are balanced, why is there any additional expansion for the universe as a whole to keep expanding? Wouldn’t the fact the the measures of space collapsing inter-galactically are balanced by the measures of space expanding intra-galactically make it all cancel out? Since then experiments by COBE and WMAP have apparently shown the overall space to be flat. The only reason since provided, that I’ve heard, for how space continues to expand, is that Inflation blew it up so fast and far that it only appears flat locally, much like the surface of the planet only appears flat.
Which leads me to see it as some sort of cycle, of collapsing mass and expanding radiation. Basically a cosmic convection cycle.
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brodix,
Either expanding *or* contracting solutions are compatible with General Relativity. (Since the theory is time symmetrical.) The one thing you can’t have in General Relativity is a *static* universe.
Thus, your claim that there is an inconsistency between GR and an expanding universe is just false.
I note that you haven’t taken up my challenge to show the mistakes in the mathematics by Friedmann and Lemaitre (and loads of theorists since then) who showed that GR leads to solutions that are expanding or contracting. Now, why am I not surprised by that?
This is another of your gross misconceptions. It is not true that “space [is] collapsing inter-galactically” and being “balanced by … space expanding intra-galactically”. I’ve previously tried to explain that to you, yet you take no notice and just repeat the same misconceptions. You are not only a delusional crank, you are a totally incorrigible delusional crank.
And yet another of your gross misconceptions. The concepts of space being *flat* (no general-relativistic curvature) and space being *static* (not expanding or contracting) are totally different. You seem to be suggesting (as best I can make out, which, I admit, is not very far) that if space is “flat” then it should also be “static” — that is just wrong. You just have layer upon layer of misconceptions, multiple layers deep.
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Coel, brodix,
you two never give up, do you? Comments closing in a few hours…
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I say, let’s nationalize cosmology.
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Hi Massimo,
Well, one never really knows whether someone is genuinely interested in learning and getting at the truth of the matter, or whether it is a hopeless case.
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Coel,
Unfortunately I’m stopping through from the riding and hopefully get a reply in later.
There are lots of angles to explore here, though I would say personal insults do not add to the argument.
Yes, math can be used to model lots of things. As I pointed out to Massimo, epicycles were a completely legitimate modeling of the cosmos from a geocentric frame. The problem was assuming it reflected some deeper mechanical process. Similarly, based on modeling of mass/gravity, you have contraction. The question is whether there is an opposite model of radiation which would show expansion and if these might be related in some larger model.
While I’m sure most of those following this debate will probably agree that I can’t be right, I don’t think I’m presenting a completely opaque argument, that there might be some cyclical relationship between the two, even if one might disagree with it. So hopefully someone not entirely confused can chime in.
Best and thank you for the discussion.
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General Relativity Theory* and soon Quantum Field Theory can be written in SageMath/SageManifolds (Python-based). Exercise: Write a program that automatically creates new theories in SageMath and compares them to how well they work with scientific Big Data collections.
* http://luth.obspm.fr/~luthier/gourgoulhon/fr/present_rec/sagemanifolds-CoCoNuT-13.pdf
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Just to add a note to the last;
The expansion of intergalactic space is based on the redshift of the spectrum of light that has traveled across it. Basically the waves are stretched. This creates the assumption that the space is expanding.
General Relativity argues that gravity is the contraction of space, based on the relationship of mass.
Now consider the rubber sheet and bowling ball analogy of gravity; That the ball distorts/curves inward the sheet/space around it.
Does this mean the sheet would otherwise be flat in the areas where there is no ball?
Consider another thought experiment; Say the sheet is covering water and that as the ball pushes down in one point, the sheet is correspondingly pushed up elsewhere to compensate.
The result is that while it is curved inward at the mass points, it is curved outward in the open areas, but distributed over a larger area, on the space where there is no ball.
So the overall effect is that, yes, space is both curved inward and curved outward, but in an overall balance.
Now when we measure light from distant galaxies, it necessarily can only travel those routes with the least interference by massive objects and other physical material, therefore the most “curved outward.”
Also consider the effect of gravitational lensing; That light passing close to massive objects is magnified, as well as distorted.
So my argument is this redshift is a form of lensing, rather than the entire universe having expanded from a point.
Keeping in mind that we do appear as the center of this expansion, which would be quite normal for an optical effect.
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brodix, at this point I’ve given up — especially as you are making no attempt to defend your previous assertions, such as the claim that there is an inconsistency between General Relativity and an expanding universe, despite the fact that every theoretical cosmologist since Friedmann and Lemaitre has regarded an expanding (or contracting) universe as the natural solution to Einstein’s field equations of general relativity.
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Coel,
It would seem we have both laid out our arguments, or rather I’ve laid out my argument and you have offered up various appeals to authority and a few insults, so it would seem neither of us has convinced the other.
It will be interesting to see how far the multiverse movement gets, before fading into irrelevance.
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Coel,
Looking back over your last comment, my primary argument was there is an inconsistency between General Relativity and an expanding universe, as opposed to expanding space, since the expanding universe is premised on the speed of light being distinct from the dimensions of the space of this universal expansion, which even you acknowledged. So if you don’t see the problem, it would seem to be your confusion, not mine.
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