Space and Physics
PUBLISHED
What do physicists agree on? Well, not much, it appears. This is the finding of a large survey of more than 1,600 physicists focusing on the big open questions of physics.
Most of the participants described themselves as researchers, with a minority identifying as science enthusiasts, as not every physicist is currently in active research but might still follow the field closely.
Regardless, it is very rare that any physicist — active or not — will have no opinion on something. And this bunch of scientists didn't disappoint.
When did time start?
The first question asked was about the Big Bang and the implication of what we consider the beginning of the universe. This is where most physicists agreed. A whopping 68 percent of them believe that the Big Bang is a hot, dense state, but that this doesn't tell us if time started there or not.
A peculiar 5 percent see it as an absolute beginning of time but not in a singularity, while 20 percent instead accept both the singularity scenario and the beginning of time. The remaining people either had other ideas or no opinion.
The next big question was about the extremely early universe. When we look far into the cosmos, there is a puzzling uniformity to what we see, with every part of the universe looking roughly the same. This is unexpected.
To solve this uniformity problem, the hypothesis of cosmic inflation was put forward, which argues that a fraction of a second after the Big Bang, the universe underwent a prodigious expansion. Just over half (51 percent) of those surveyed agreed with this scenario.
The other physicists were spread among the always fun bouncing or cyclic universe hypothesis — the idea that the universe repeats by experiencing cycles of expansion and collapse — or some other weird quirks of physics that are not cosmic inflation.
What’s missing in the universe?
We then get into the big three open questions in cosmology: what is dark matter? What is dark energy? And what the heck is going on with the Hubble Tension?
Dark matter is a hypothetical form of matter needed to explain the rotation of galaxies and their distribution in the universe because they look like they have five times more matter than we can see in stars and gas.
Possible explanations for this are extremely light particles, quite massive particles, changing gravity, or even primordial black holes. The top answer, with 21 percent, was a hybrid of the above suggestions.
Dark energy is needed to explain the accelerated expansion of the universe. It is thought to be either a property of all space-time, the so-called "cosmological constant", or a field that can change over time. While the first of these is still the leading explanation, the latter has found new observational support.
The field, however, is split, with 24 percent of physicists backing the cosmological constant and 26 percent saying it's time-varying.
Which leads us to Hubble Tension. The expansion rate of the universe today can be measured in multiple ways. The two main approaches are using the first light in the universe, the Cosmic Microwave Background, or using supernovae and other stellar objects as distance indicators. The two numbers from both these observations should be the same, but they're not.
So what do physicists think? The winning answer here was the “no opinion” camp, which is truly the most shocking result, and I say that as a physicist.
The rest are split between thinking there are issues in the measurements, changes in dark energy, or some weird quantum gravity effects.
A trip to quantumland
The study of the cosmos pairs well with the study of the microscopic world. Quantum mechanics is particularly relevant to the Big Bang, black holes, and even a fundamental theory of gravity.
The first question here was about quantum mechanics itself, which is really asking about the nature of reality. The leading idea, also reflected in the survey with 36 percent, is the Copenhagen Interpretation which argues that nature is fundamentally probabilistic. A famous alternative is the Many Worlds interpretation, which argues that every observation of a quantum state produces a parallel universe.
The next two questions were about black holes. What happens to matter inside a black hole, and how can we square that with quantum mechanics? Relativity says that nothing can escape a black hole, but quantum mechanics rejects the loss of information.
For the first question, 41 percent of physicists accept that matter that enters a black hole is crushed in the singularity at its core (very few consider a white hole or another universe likely).
For the second question, however, opinions are split. The majority, though, accept that Hawking radiation is the way to solve the information paradox.
Last but certainly not least, we get to quantum gravity. Another big chunk (29 percent) again have no opinion, while the rest are split between string theory (19 percent), gravity not being quantum (18 percent), and loop quantum gravity (13 percent).
Whose constant is it, anyway?
The most philosophical of the questions was about the fundamental constants of the universe. If any of these constants were different, we would not exist. So why are they the way they are?
Just over a quarter of the physicists said that these are simply brute facts, with no need for further explanation. I picture these physicists as the acerbic Miranda Priestly from The Devil Wears Prada: “Please bore someone else with your questions.”
For 20 percent, though, they think that there are a bunch of different universes in the multiverse, and ours is simply one of the many. We are seeing those values because those values allow life.
For 16 percent, the constants are set by principle, meaning that the fundamental theory of the universe doesn’t allow for other values.
Then there’s 9 percent that consider an intelligent designer as the explanation, and 6 percent that believe it is a case of cosmic Darwinism in which a universe like ours is good at making black holes, and inside these new universes are born.
“The most striking result is how few of the ‘standard answers’ in fundamental physics command overwhelming support, with most falling short of a majority,” lead author Niayesh Afshordi, associate faculty member at Perimeter Institute and professor at the University of Waterloo, said in a statement.
“The interesting point is not that physicists are confused. It is that the frontier is genuinely alive.”
Afshordi worked with Phil Harper and the American Physical Society’s Physics Magazine in producing this fascinating report. It is good that there are so many opinions on the questions that have yet to find an answer, after all we don't want to operate with dogma.
The study can be read in its entirety on the ArXiv.
