What color is antimatter?

Question

Salt (NaCl) is white. But what about anti-salt? Assuming it didn't collide with the regular matter and annihilate, and assuming I could look at it, would it also be white? Same for any material/substance. If you replaced the matter (protons, electrons, neutrons) with antimatter (anti-protons, positrons, anti-neutrons), would they be the same color, or would the color be different in any way (e.g. inverted)? Would an anti-rose be red, or what? My guess is the color would be unchanged.

Answer 1

This is a matter of a recent, as well as ongoing research.

So far, only antihydrogen is available in amounts that some very advanced spectroscopy is possible to run on.

So far, the results are that hydrogen and anti-hydrogen are spectrally identical - down to the accuracy of the measurements.

The accuracy in these measurements is many orders of magnitude better than the human eye spectral discrimination, meaning that if we get enough anti-hydrogen and force it to glow its red Balmer spectral series, the red hue will be exactly the same as in the normal hydrogen.

We are yet to see light from heavier anti-atoms, but we keep trying.

The modern theoretical consensus is that we will probably not find any difference at all.

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p.s. Even marginal (from the current standpoint) theories that DO expect spectral differences, predict only minor differences between normal- and anti- substances, on the order of magnitude of less than what we see between different isotopes. These differences, should they exist, will also be way below human eye color precision.

Answer 2

Antimatter has the same electromagnetic properties as matter (due to CPT symmetry), so its atoms and solids exhibit identical energy levels and band structures, meaning an anti-object would have the same color as its matter counterpart—though forming and observing bulk antimatter is practically impossible because it annihilates upon contact with normal matter.

Edit: From a basic quantum‐electrodynamics perspective, antimatter has the same electromagnetic properties as matter and would reflect/absorb light in the same way—so, to the naked eye, an ‘anti‐metal’ should look identical in color to an ordinary metal. However, the full Standard Model is chiral, and it exhibits discrete‐symmetry violations (e.g., C and P), as shown in experiments such as the 1956 Wu experiment and 1964 kaon‐decay studies. Consequently, at extremely high precision, one would detect a slight difference in the photon spectrum from matter versus antimatter, meaning that—strictly speaking—matter is not exactly the same color as antimatter, though the difference is too small to be observed in practice.

Answer 3

Atomic (or molecular) transitions in anti-matter are the same as those in matter with one caveat:

They emit, absorb, reflect anti-photons, so the question reduces to: what color are anti-photons?

To the PDB! (https://pdg.lbl.gov/2022/listings/rpp2022-list-photon.pdf) where we see that the photon's quantum numbers are:

$$ I(J^{PC}) = 0,1(1^{-\,-}) $$

which means that the photon is its own anti-particle: an anti-photon is a photon.

So anti-salt looks like salt, same for anti-gold (w.r.t gold), and anti-stars or anti-galaxies or anti-CMB...and so on.

Answer 4

Probably the other effective answers are true enough at low energies in the setting of your question that they suffice. But for future reference, let me give a more-fundamental answer.

Quantum electrodynamics has emergent charge conjugation and parity symmetry, which is why it makes sense to talk about antiparticles at lowest order. Indeed this is quite a good emergent symmetry, so the measurement you could make with your eye of the color of an anti-metal would likely be exactly the same as that of a metal.

But these are not fundamental symmetries. Electromagnetism is a low-energy description of the physics of the Standard Model, and the Standard Model is a chiral theory. The emergent discrete symmetries of parity and charge conjugation are violated explicitly in this more fundamental theory, as we learned with the Wu experiment in 1956 and with neutral kaon decays in 1964. The Standard Model does satisfy the combined symmetry CPT (charge conjugation, parity, and time-reversal) as another answer says, but this is true of every Lorentz-invariant quantum field theory and does not address this question.

The discrete symmetry violation of the Standard Model, which we have measured through those processes, necessarily enters in every process at some order in perturbation theory. So the real answer is that while at lowest order the color would appear the same, if you very precisely measured the spectrum of the photons reflected by some material and by some anti-material, you could tell them apart. Ultimately, matter is not the same color as antimatter.