It you’ve ever dreamed of taking off on a TARDIS vacation, that might be scientifically impossible.
You may be surprised that it isn’t the laws of physics which have determined that it is now less likely than ever for anyone to shoot chronologically backward. What will probably crush your dreams of petting a live dinosaur or meeting Houdini in the flesh is the recent discovery of a weird atomic nucleus that could have a lot to say about which direction time itself travels in.
To understand why time machines may forever stay in science fiction, you need to get up close and personal with atomic nuclei. It was previously assumed that the nucleus of an atom could either be spherical, like most nuclei, or distorted slightly into a disc or football-shaped. The shape and distortion are determined by the number of protons and neutrons in an atom (it makes no difference if the atom is naturally occurring or a lab-created isotope), but whatever the case, these shapes are all symmetrical.
Symmetry in particle physics is explained by CP-symmetry. It merges C-symmetry, or charge symmetry, and P-symmetry, or parity, which are the two types thought to exist in the universe.
C-symmetry means that the physics of an atom remain the same even if you reverse the charge. Mess with a hydrogen and anti-hydrogen atom in the same way, and they should both respond identically, because they are still two versions of hydrogen atom, just in matter and antimatter form. P-symmetry allows the spatial coordinates of a system to invert at the point of origin. Think of a graph with coordinates x, y, and z marked on the right side. If you mark –x, -y, and –z on the left, the path of those coordinates is going to be the inverse of whatever appears on the right.
CP-symmetry is both of these suggestions at work. The way an antimatter particle of a particular element spins and decays should be the exact inverse of what the matter particle of that element is doing.
"In particle physics, if you have a particle spinning clockwise and decaying upwards, its antiparticle should spin counterclockwise and decay upwards 100 percent of the time if CP is conserved," astrophysicist Ethan Siegel told It Starts With a Bang. “If not, CP is violated.”
Matter and antimatter supposedly emerged in equal amounts during the Big Bang. So why does there seem to be so much matter everywhere but the only antimatter we know of has to be messed with in a lab? Some scientists have considered the possibility of CP violation to prove the existence of antimatter, except nothing in physics can really explain it yet. That would actually need evidence.
Because so many theories in physics rely on symmetry, the pear-shaped nucleus first discovered in the isotope Radium-224 and then Barium-144 challenged everything scientists thought they knew about the universe. More protons in the narrow end of the nucleus distribute charge unevenly. This violation of CP symmetry might not only demystify the uneven distribution of matter and antimatter, but could also be telling us that the forward-pointing nucleus indicates the only direction in which time flows. Bummer.