What happens when a black hole seems steady, yet its magnetic “compass” turns on its head? Welcome, curious minds of FreeAstroScience.com. Today we travel to M87*, the first black hole ever imaged, and we sift through fresh, peer-reviewed evidence that its near-horizon plasma is more restless—and more surprising—than we thought. Stick with us to the end; you’ll leave with a clear picture of what flipped, what stayed rock-solid, and why it matters to you.
What did the Event Horizon Telescope actually see between 2017 and 2021?
Let’s start with the anchor. The famous bright ring around M87* stayed the same size across multiple years. The Event Horizon Telescope (EHT) measured a persistent diameter of 43.9 ± 0.6 microarcseconds at 230 GHz—just what General Relativity predicts for lensed emission around a supermassive black hole shadow. That’s the “Einstein still works” part. The twist? The polarized light—the part that reveals magnetic structure—changed year to year, including a flip in the pattern’s handedness (helicity) in 2021. In plain terms: the ring stayed steady; the magnetic map didn’t.
If you prefer a quick mental image for polarization, think about 3D movie glasses. Each lens selects light oscillating in a specific direction. Space does this too. Strong magnetic fields near a black hole imprint directions onto light, giving us a tell-tale polarization pattern we can read from Earth.
Here are the key facts from the new, multi-epoch EHT analysis:
- Three epochs: 2017, 2018, and 2021 at 230 GHz.
- Ring: Bright, asymmetric, 43.9 ± 0.6 μas, stable across years.
- Polarization fraction: Peaks near ~15% in 2017, but ~5% in 2018 and 2021.
- Magnetic swirl: The ring’s spiral polarization pattern changed, and in 2021 its EVPA helicity flipped.
- 2018 gamma-ray flare: Despite a near-contemporaneous flare, the 2018 and 2021 images look surprisingly similar.
- Jet hints: 2021 coverage provided the first EHT constraints on jet emission outside the ring, on scales ≲ 1 mas.
And because context helps: M87 sits about 55 million light-years from us and hosts a ~6.5-billion-solar-mass black hole. The black hole powers a ~5,000-light-year jet that lights up the spectrum and shapes its galaxy’s environment.
So why would the polarization swirl reverse, and what does it change?
Short answer: the plasma is alive. Magnetic fields thread the inflowing, super-hot gas. That gas emits synchrotron light, which is naturally polarized. Turbulence, magnetic reconnection, and Faraday rotation—the twisting of polarization as light passes through magnetized plasma—can all reshape the pattern we see. EHT’s detailed analysis notes that the EVPA helicity flip in 2021 could reflect changes within the magnetized accretion flow or an external Faraday screen along the line of sight. Both are plausible; neither is settled. That’s science doing its job.
To keep the math friendly, this is the essence of what we measure:
Linear polarization fraction: m = √(Q2 + U2) / I
Electric-vector position angle (EVPA): χ = ½ · arctan(U / Q)
Here, I, Q, U are the Stokes parameters. When m drops (from ~15% to ~5%), the signal is either more tangled or more strongly depolarized, often by Faraday effects. When χ changes or its helicity flips, the global field geometry or the foreground “twister” has changed in sign or structure. GRMHD simulations actually predict such variability at horizon scales on week-to-year timescales. The new work uses seven independent imaging algorithms and cross-checked calibration pipelines to make sure the flip is real, not a processing glitch.
To see how the hardware boosts confidence, check the 2021 upgrades:
- NOEMA (France) and Kitt Peak 12-m (Arizona) joined the array.
- The Greenland Telescope improved efficiency and, by 2021, ran at 64 Gbps.
- Result: much better (u, v) coverage, stronger constraints on structure outside the ring.
That denser Earth-sized “virtual dish” tightens the imaging net. The ring’s size staying put while the polarization map changes is the aha moment: spacetime is stable; plasma is wild.
Three EHT epochs at a glance
| Year | Array highlights | Polarization peak | EVPA / Helicity | Ring diameter | Notable notes |
|---|---|---|---|---|---|
| 2017 | Original EHT campaign | ~15% | Spiral pattern; baseline for comparison | ~44 μas | First black hole image; bright south |
| 2018 | Higher data rate; GLT onboard | ~5% | Pattern evolves vs. 2017 | ~44 μas | Gamma-ray flare near epoch; image still similar to 2021 |
| 2021 | +NOEMA, +Kitt Peak; better coverage | ~5% | Helicity flips | 43.9 ± 0.6 μas | First EHT constraints on extended jet at ≤1 mas |
Data summary from the 2025 A&A analysis and supporting context.
What about the jet? M87’s jet is a natural lab for relativistic magnetohydrodynamics. It influences star formation and spreads energy across thousands of light-years. Getting the near-horizon magnetic map right matters, because that’s where the jet is launched and collimated. By picking up structure outside the ring in 2021, the EHT starts to bridge the last mile between the black hole’s engine and the kiloparsec-scale jet we see in radio and optical images.
How do we know the magnetic fields are really there? Because polarization is picky. It disappears if fields are tangled on small scales, and it swings if plasma changes upstream. Across 2017–2021, the EHT team also used closure quantities—combinations of interferometric data that cancel out most instrument errors—to ensure the signal’s astrophysical. That’s how we separate what the sky did from what the antennas or atmosphere did.
What does this mean for Einstein and for us? We can say two things at once, and not be contradictory:
- The ring size is stable and matches GR’s expectations for a ~6.5 × 10⁹ M⊙ black hole. Gravity’s large-scale geometry holds.
- The polarized plasma is dynamic, with patterns that evolve, fade, and even reverse helicity. Magnetic weather near the event horizon is real.
That duality is the quiet beauty here. Space-time is the stage. Plasma writes the script—sometimes right-to-left.
Key takeaways (for the train-scrolling brain)
- Steady ring, changing swirl. Size holds; polarization flips.
- Lower polarization in 2018/2021. More depolarization or more tangled fields.
- Upgraded array in 2021. NOEMA + Kitt Peak sharpen the view; jet constraints appear.
- M87 is close and huge. ~55 Mly away; ~6.5 billion Suns. Jet spans ~5,000 ly.
- Uncertainty acknowledged. Helicity flip may be accretion-flow change or Faraday screen.
Related terms you might search (and now understand) Event Horizon Telescope; horizon-scale variability; EVPA; Faraday rotation; magnetically arrested disk (MAD); GRMHD simulations; 230 GHz polarization; VLBI; NOEMA; Kitt Peak 12 m; M87 jet; supermassive black hole shadow.
Written for you, by FreeAstroScience.com We built this guide for you. At FreeAstroScience, we explain complex ideas with simple words and honest nuance. We want you to keep your mind switched on—because the sleep of reason breeds monsters. And because the Universe is kinder to people who keep asking “why?”.
Conclusion M87* didn’t move the goalposts of gravity; it remixed the magnetic playbook. A stable shadow tells us the geometry is right. A flipping polarization pattern tells us the engine is alive. Between those two truths, we learn how black holes launch jets and heat galaxies. Keep following the evidence, and the cosmos keeps giving back. Come back to FreeAstroScience.com soon—we’ll keep the lights on, and the questions sharper.
Sourcing and dates The multi-epoch polarization analysis, including the EVPA helicity flip and 43.9 ± 0.6 μas ring, appears in Astronomy & Astrophysics on September 4, 2025.
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