Physicists have long assumed that the universe is pretty much the same in every direction, and now they have found a new way to test this hypothesis: by examining the shadow of a black hole.
If this shadow is a bit smaller than existing physics theories predict, it may help to prove a far-fetched notion called bumble bee gravity, which describes what would happen if the seemingly perfect symmetry of the universe is not so perfect.
If scientists could find a black hole with such an undershadow, it would open the door to a whole new understanding of gravity ̵
But to understand how this bumblebee idea could fly, let’s dig into some basic physics.
Related: The 18 greatest unsolved mysteries in physics
Looking in the mirror
Physicists love symmetry; after all, it helps us understand some of the deepest secrets in the universe. For example, physicists have realized that if you perform an experiment with basic physics, you can move your test equipment somewhere else and you will get the same result again (that is, if all other factors, such as temperature and gravity, remain the same ).
In other words, no matter where in the room you conduct your experiment, you will get the same result. Through mathematical logic, this leads directly to law of conservation of momentum.
Another example: If you run your experiment and wait a while before running it again, you will get the same result (again, everything else is equal). This temporal symmetry leads directly to the law of conservation of energy – that energy can never be created or destroyed.
There is another important symmetry that forms the bedrock of modern physics. It’s called “Lorentz” symmetry in honor of Hendrik Lorentz, the physicist who figured all this out in the early 1900s. It turns out that you can take your experiment and turn it around, and (everything else being equal) you get the same result. You can also increase your experiment to a fixed speed and still get the same result.
In other words, everything else being equal – and yes, I repeat it often, because it’s important – if you carry out an experiment in total rest and do the same experiment at half the speed of light, you will get the same result.
This is the symmetry Lorentz revealed: the laws of physics are the same regardless of position, time, orientation, and velocity.
What do we get out of this basic symmetry? First, we get Einstein’s whole theory of special relativity theory, which indicates a constant speed of light and explains how space and time are connected for objects running at different speeds.
Bumblebee gravity
Special relativity is so important to physics that it is almost a metatheory of physics: if you want to put together your own idea of how the universe works, it must be compatible with the poems of particular relativity.
Or not.
Physicists are constantly trying to prepare new and improved physical theories because the old ones, like general relativity, which describes how matter twists space-time and the standard model of particle physics, cannot explain everything in the universe, such as what happens in the heart of a black hole. And a very juicy place to look for new physics is to see if any valued notions might not be as accurate in extreme conditions – valued notions like Lorentz symmetry.
Related: 8 ways you can see Einstein’s theory of relativity in real life
Some gravity models claim that the universe is not exactly symmetrical after all. These models predict that there are extra ingredients in the universe that force it to not exactly obey Lorentz symmetry all the time. In other words, there would be a special or privileged direction in the cosmos.
These new models describe a hypothesis called “bumblebee gravity.” It gets its name from the supposed idea that scientists once claimed that bumble bees should not be able to fly because we did not understand how their wings generated lift. (By the way, scientists never believed in it.) We do not fully understand how these models of gravity work and how they could be compatible with the universe that we see, and yet there they are, we stare in the face as viable possibilities for new physics.
One of the most powerful uses of bumblebee gravity models is to potentially explain dark energy —The phenomenon responsible for the observed accelerated expansion of the universe. It turns out that the degree to which our universe violates Lorentz symmetry may be bound to an effect that generates accelerated expansion. And because we have no idea what creates dark energy, this option looks very appealing.
The black shadow
So you have a lively new gravity theory based on some icon-crushing ideas like symmetry violation. Where would you go to test that idea? You will go to the place where gravity is stretched to the absolute limit: a black hole. In the new study not yet peer-reviewed and published online in November 2020 to the pre-print database arXiv, scientists did just that and looked at the shadow of a black hole in a hypothetical universe modeled to be as realistic as possible.
(Keep in mind that first ever picture of black hole M87, produced by Event Horizon Telescope just a year ago? The hauntingly beautiful, dark cavity in the center of the bright ring was actually the “shadow” of the black hole, the region that sucked all the light in behind and around it.)
To make the model as realistic as possible, the team placed a black hole in the background of a universe that accelerated in its expansion (exactly as we observe) and set the level of symmetry violation to match the construction of dark energy, as scientists measure.
They found that in this case, the shadow of a black hole can work up to 10% less than in a “normal gravity” world, providing a clear way to test the bumblebee gravity. While the current image of the black hole M87 is too vague to tell the difference, an effort is underway to take even better pictures of several black holes exploring some of the deepest mysteries in the universe in the process.
Originally published on WordsSideKick.com.
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