Tuesday, March 20, 2018

Hawking’s “Final Theory” is not groundbreaking

Yesterday, the media buzzed with the revelation that Stephen Hawking had completed a paper two weeks before his death. This paper supposedly contains some breathtaking insight.

The headlines refer to a paper titled “A Smooth Exit from Eternal Inflation” in collaboration with Thomas Hertog. The paper was originally uploaded to the arXiv in July last year, but it was updated two weeks ago. It is under review with “a leading journal” which I suspect but do not know is Physical Review D. Thomas Hertog gave a talk about this at the conference which I attended last summerYou can watch the video of Hertog’s talk here.

According to The Independent the paper contains “a theory explaining how we might detect parallel universes and a prediction for the end of the world.” Furthermore, we learn, “Hawking also theorised in his final work that scientists could find alternate universes using probes on space ships, allowing humans to form an even better understanding of our own universe, what else is out there and our place in the cosmos.”

In the Sunday Times you can read that the paper “shows how we might find other universes”  and in The Telegraph you find a quote by Carlos Frenk, professor of cosmology at Durham University who said: “The intriguing idea in Hawking’s paper is that [the multiverse] left its imprint on the background radiation permeating our universe and we could measure it with a detector on a spaceship.”

Since the paper doesn’t say anything about detecting parallel universes, I was originally confused whether the headlines were referring to another paper. But no, Thomas Hertog confirmed to me that the paper in question is indeed the paper that is on the arXiv. There is no other paper.

So what does the paper say?

The paper is based on an old idea by Stephen Hawking and Jim Hartle called the “no-boundary” proposal. In the paper, the authors employ a new method to do calculations that were not previously possible. Specifically, they calculate which type of universes a multiverse would contain if this theory was correct. The main conclusion seems to be that our universe is compatible with the idea, and also that this particular multiverse which they deal with is not as large as the usual multiverse one gets from eternal inflation.

It’s not entirely uninteresting if you are into multiverse ideas, because then you need this information to calculate the probability of our universe. But it is also a very theoretical paper that does not say anything about observational consequences.

The only thing that the paper does say is that inflation took place. And inflation predicts that gravitational waves produced in the early universe should leave an imprint in the cosmic microwave background (CMB). This is the CMB polarization signal that BICEP was looking for but didn’t find. There are, however, some satellite missions in the planning that will look for it with better precision.

So how do we detect parallel universes? By detecting the CMB polarization. I do not kid you.

Here’s what Hertog said about this:
“This model predicts that our universe came into existence with a burst of rapid expansion called cosmic inflation. A big bang of this kind amplifies gravitational waves which in turn show up in satellite images of [the pattern of temperature fluctuations in] the cosmic microwave background. Future satellite missions should see this, if the theory is correct.

Observational evidence for the no-boundary model [in the form of gravitational waves from the big bang] would yield strong evidence for a multiverse. This paper provides a step towards a mathematically sound and testable model of the multiverse. That constitutes a significant extension of our notion of physical reality.

Some cosmologists have argued against the multiverse on the basis it can’t be tested. However our model shows that observations in our own universe can provide strong evidence for the existence of other universes. ”
Allow me put this into perspective.

Theoretical physicist have proposed some thousand ideas for what might have happened in the early universe. There are big bangs and big bounces and brane collisions and string cosmologies and loop cosmologies and all kinds of weird fields that might or might not have done this or that. All of this is pure speculation, none of it is supported by evidence. The Hartle-Hawking proposal is one of these speculations.

The vast majority of these ideas contain a phase of inflation and they all predict CMB polarization. In some scenarios the signal is larger than in others. But there isn’t even a specific prediction for the amount of CMB polarization in the Hawking paper. In fact, the paper doesn’t so much as even contain the word “polarization” or “tensor modes.”

The claim that the detection of CMB polarization would mean the multiverse exists makes as much sense as claiming that if I find a coin on the street then Bill Gates must have walked by. And a swarm of invisible angels floated around him playing harp and singing “Ode To Joy.”

In case that was too metaphorical, let me say it once again but plainly. Hawking has not found a new way to measure the existence of other universes.

Stephen Hawking was beloved by everyone I know, both inside and outside the scientific community. He was a great man without doubt, but this paper is utterly unremarkable.


Nicolai F. said...

It is always refreshing to get a non-exaggerated, but precise description of what has actually happened, thank you.
Also, if the paper is based on the no-boundary proposal, has it not been ruled out by now?
I am referring to the paper by Feldbrugge, Lehners and Neil: Turok https://arxiv.org/abs/1708.05104
What is your opinion on that paper?


Michael John Sarnowski said...

Hi Sabine,

Obviously Stephen Hawking is an amazing scientists. Scientists because of who he was and who people thought he was. I don't think it would be a bad thing if his final work didn't at least get people thinking that there might be ways to prove a multiverse, even if his last idea is unremarkable.

If there was a multiverse, wouldn't the pull of gravity from these other universes affect the red shifting of light according to the inverse square law. Therefore, whatever the main driver for red-shifting of light is, there should be some diminishing affect the farther away someone is from that other universe?

Lima Alfa said...

Dear Sabine
Your comments are, as always, the most pertinent end rigorous we can find in the net.
Thanks to let us understand, in the right way, what mainstream information give us to eat every day!

Hamish Johnston said...

Thanks Sabine...I just spent an hour or so looking at the preprint and then looking at the newspaper articles and then getting very confused!

Matthew Rapaport said...

Only specific prediction I've read connecting a specific signal in the CMB to a specific scenario (a bounce) is Penrose's Conforming Cyclic Cosmology

Bill said...

Seems the media were breathlessly reporting similar nonsense 90 years ago ("Einstein on verge of major discovery," New York Times). But as an American already living in the parallel Trump universe, I'll make the jump to another one any day.

neo said...

" And inflation predicts that gravitational waves produced in the early universe should leave an imprint in the cosmic microwave background (CMB). This is the CMB polarization signal that BICEP was looking for but didn’t find. There are, however, some satellite missions in the planning that will look for it with better precision."

in light of the debate over inflation by Steinhardt et al, in Pop goes the Universe,

would NOT finding gravitational waves produced in the early universe should leave an imprint in the cosmic microwave background falsify all models of inflation?

Louis Tagliaferro said...

Sabine said, All of this is pure speculation, none of it is supported by evidence.

Where I think theoretical physics errs grossly is the belief among many if they can model their pure speculation mathematically and have it be consistent with some current observations than they must be right.

What disappoints me is the number of extraordinarily intelligent people working in this manner slows real scientific progress unnecessarily. I believe there are some empirically strong foundations that can possibly rebuild the dynamics gravity that require minds who value evidence and strong fundamental foundations over speculation. It seems as time passes there are fewer and fewer of such minds in the field.

Uncle Al said...

"none of it is supported by evidence" Chemistry says enantiomers are utterly indistinguishable absent a chiral background (opposite shoes embedded within a preponderant left foot).

if QM is considered, 50 years of physical theory do not predict the observed universe outside curve-fitted intervals. One hour in commercial equipment (brightspec.com) using a literature-synthesized non-racemic molecule sample fundamentally falsifies curve fits by observing their defective postulate. Reality misses beauty by an emergent part-per-billion, allowing the universe.

Look. Partially resolved C_3-prolate or -oblate chiral tops are literature syntheses. A muck chiral rotor is two undergrad lab steps. Physics is arrogant. Physical theory can be screwed (a joke amidst lab coats, a terror for crossed nablas).

JimV said...

Re: "if I find a coin on the street then Bill Gates must have walked by" - this reminded me of a story from the unauthorized biography "Gates":

Bill Gates was holding up a long line at a Baskin-Robbins (ice cream shop) because he kept searching through his pockets, looking a 25-cent coupon. Finally, the man behind him slapped a quarter (25-cent coin) on the counter, saying, "Here, you can pay me back out of your first million." (Not knowing who Gates was or that he was a billionaire.)

So actually, if you find a coin on the street, you know that Bill Gates did not recently walk by.

(Gates has now become something of a philanthropist, but there are many stories of his cheapness as a young entrepreneur.)

Tam Hunt said...

Thanks for this summary Bee. But if it was that cut and dried in terms of the paper being not only "unremarkable" but apparently pretty idiotic, surely the authors would have recognized this and reviewers will also?

Tom Aaron said...

This is a good article. One that all aspiring science media sudents should read.

Back to Hawking's career...'A Tale of Two Scientists'. One of the top half dozen minds of the 20th century. However, not of the 21st century.

Hank Smith said...

Sabine is not a multiverse fan. She is also not a fan of science fiction posing as theoretical physics.

Sabine Hossenfelder said...


The paper is not "pretty idiotic," it's just not what it's been sold for. It's a study about the shape of universes in a multiverse of a particular kind. It doesn't make any prediction from which you could infer the presence of other universes.

marten said...

The not very logical cosmological term multiverse (no unquestionable definition and no unquestionable existence) should be replaced by another word in order to avoid misunderstanding: examples of real multiverses are my favourite Iliad and Odyssey.

Lee McCulloch said...

Is the CMB polarisation of Electormagnetic moments rather than gravitational wave ones or both?
Would there be anything predictions to be seen in the neutrino background equivalent of the CnB?

John Anderson said...

Hawking has such a media-magnified reputation with the general public that it short-circuits any critical discussion of his life and work. Physics needs a Hall-of-Fame (more than a list of Nobel laureates) which might help put Hawking's career in perspective with the many other great physicists whom the public don't know and maybe never will: Thompson, Dirac, Born, Witten, Hess, Ray Davis, even Maxwell, etc. etc. etc. This is a public outreach concern.

BTW, I think the song choice for Bill Gates' angels is "Smells Like Teen Spirit" (Nirvana 1991) or Weird Al's variant if the Windows operating system is considered.

Unknown said...

Clark Kerr would be proud--he championed the move from "the university" to "the mulitversity."

Sabine Hossenfelder said...


The CMB is electromagnetic radiation; its polarization is caused by gravitational waves (in the early universe). I am guessing the neutrino background would also be polarized, yes, but we haven't measured that to begin with, so I don't think it'll be of much help. Best,


Mark Martens said...

Only recently coming into this space, relieved to find the odd oasis of sane conversation in what otherwise looks to me like flannel and drivel. Construed, as far as an outsider can tell, to complexify and obfuscate to the point where it either preserves the field for complex mathematics, or impresses science funding agencies.

Thank you Sabine Hossenfelder.
Mark Martens, Accidental scientist

Q said...

Thank you for not afraid to slay some sacred cows. No matter how brilliant of a scientist Hawking might have been or what hardships he had to endure, it should have no bearing on our pursuit of truth and on our evaluation of his ideas on their own merits.

johnduffieldblog said...

Good stuff Sabine. I rather think there are more issues with Hawking's physics than many people appreciate.

Jacqui White said...

It's really great to see you present such an unbiased opinion on someone who is a brilliant scientist in his own right but also deserves to be looked at and evaluated in the same way every other scientist is.