The universe is full of wonders, but some of the most fascinating objects are also the darkest. These include black holes and other compact objects that do not emit light themselves but strongly affect the light around them. Even though we cannot see these objects directly, we can study them by observing how they bend and trap light.
In recent years, scientists have achieved something extraordinary. Using a global network of telescopes known as the Event Horizon Telescope (EHT), astronomers captured the first-ever images of the shadows of supermassive compact objects at the centers of galaxies. These images came from the galaxy M87 and later from our own galaxy, the Milky Way, where the object is called Sagittarius A*.
So far, these observations agree well with predictions for black holes, especially rotating black holes known as Kerr black holes. However, the data is not precise enough to rule out other possibilities. One exciting alternative is the idea that these objects could be wormholes—exotic tunnels through spacetime.
In this article, we explain how a study by Angelov and his team explores the role of plasma—a common cosmic material—in shaping the shadows of wormholes. Their work shows that plasma may actually help us tell wormholes and black holes apart.
2. How Gravity Bends Light
More than 100 years ago, Albert Einstein’s theory of general relativity changed how we understand gravity. According to this theory, gravity is not just a force—it is a bending of spacetime caused by mass and energy.
One important prediction of general relativity is that light does not always travel in straight lines. When light passes near a massive object, its path bends. This effect was first confirmed in 1919, when scientists observed that starlight passing near the Sun was slightly deflected during a solar eclipse.
Near very massive and compact objects, such as black holes or wormholes, this bending becomes extremely strong. Light can be deflected many times or even trapped in circular paths. Some light rays fall inward and never reach a distant observer.
3. What Is a Shadow of a Compact Object?
When light from a bright background passes near a compact object, some rays are captured while others escape. To a distant observer, the captured light creates a dark region in the sky. This region is called the shadow of the compact object.
The shadow is not the physical surface of the object. Instead, it is a projection of the paths that light cannot take. The size and shape of the shadow depend on:
The strength of gravity
The rotation of the object
The structure of spacetime around it
Because of this, shadows are powerful tools. By studying them, scientists can learn what kind of object is producing them.
4. Black Holes and Wormholes: What Is the Difference?
Black holes are regions of spacetime where gravity is so strong that nothing—not even light—can escape once it crosses a boundary called the event horizon.
Wormholes, on the other hand, are theoretical objects predicted by general relativity. They act like tunnels connecting two distant regions of space or even two different universes. Some wormholes are not stable or traversable, but later studies showed that, in theory, traversable wormholes could exist under certain conditions.
Unlike black holes, wormholes do not necessarily have an event horizon. Light and matter could pass through them instead of being trapped forever. This key difference affects how light moves around them—and therefore how their shadows look.
5. Why Plasma Is Important in Space
Most early studies of shadows assumed that light travels through empty space. But in reality, space around compact objects is not empty. It is filled with plasma, a hot gas made of charged particles.
Plasma is everywhere in the universe:
In accretion disks around black holes
In jets emitted from compact objects
In interstellar and intergalactic space
Plasma affects light in a special way. Unlike in vacuum, light traveling through plasma depends on its frequency. Low-frequency light, such as radio waves, is especially affected. This is very important because the Event Horizon Telescope observes the universe in radio wavelengths.
6. How Plasma Changes Light Paths
In plasma, light does not behave exactly as it does in empty space. Plasma can:
Slow down light
Bend light differently depending on its frequency
Prevent light from traveling in certain regions
One key concept introduced by plasma is the plasma frequency. If the light frequency is too low compared to the plasma frequency, the light cannot propagate at all. This creates forbidden regions, where light simply cannot travel.
These forbidden regions play a crucial role in shaping or even destroying the shadow of a compact object.
7. The Goal of Angelov and Team’s Research
Angelov and his team wanted to understand:
How plasma affects the shadows of wormholes
How these shadows compare to those of Kerr black holes
Whether plasma makes wormholes easier or harder to detect
To do this, they studied several wormhole models and plasma distributions. They focused on cases where the equations describing light motion can be solved analytically. This allowed them to clearly see how different factors influence the shadow.
8. Photon Regions: The Key to Shadows
Shadows are determined by special regions called photon regions. These are areas where light can move in unstable circular or spiral paths around the object.
In vacuum, photon regions are shaped only by gravity. In plasma, however, photon regions depend on both gravity and the plasma distribution.
As plasma density increases, photon regions change shape. Eventually, they may disappear entirely, meaning no shadow can form.
9. Plasma Depending Only on Distance
First, the researchers studied plasma distributions that depend only on distance from the center (the radial direction).
In this case, they found something surprising:
The behavior of the photon region depends mainly on the plasma, not on the spacetime itself.
This means that wormholes and black holes behave very similarly under such plasma conditions. Their shadows shrink in the same way as plasma frequency increases.
While this situation does not help much in telling wormholes and black holes apart, it shows that plasma has a strong and universal influence.
10. Plasma Depending on Direction as Well
Next, Angelov and his team studied plasma distributions that depend on both distance and angle.
Here, the results change dramatically. When plasma has angular dependence:
The photon region becomes highly sensitive to the spacetime geometry
Different wormholes behave differently under the same plasma conditions
Wormholes and black holes show clear qualitative differences
This means angular plasma distributions can act like a magnifying glass, revealing the true nature of the compact object.
11. Shrinking Shadows and Critical Frequencies
As plasma frequency increases, the shadow of every compact object gets smaller. Eventually, it disappears completely. The plasma frequency at which this happens is called the critical frequency.
Angelov and his team found a very important result:
For all wormholes studied, the critical frequency is lower than for the Kerr black hole.
This has a powerful consequence. There are frequency ranges where:
A black hole still has a visible shadow
A wormhole has no shadow at all
If such a situation is observed, it would strongly suggest the object is not a black hole.
12. Shadow Size Differences
Even when both shadows exist, wormhole shadows are:
Always smaller than black hole shadows
Shrink faster as plasma frequency increases
This growing difference makes wormholes easier to distinguish from black holes, especially at higher plasma frequencies.
13. The Role of the Observer’s Motion
In reality, observers are not stationary. Earth moves, galaxies move, and compact objects move. This relative motion causes aberration, which distorts the apparent shape and position of shadows.
Angelov and his team included these effects and found that:
Aberration increases the differences between wormhole and black hole shadows
Wormhole shadows become even more distorted compared to black holes
This further improves our ability to tell them apart.
14. Why Plasma Helps Instead of Hinders
At first, plasma might seem like a complication that makes observations harder. But this research shows the opposite.
Plasma:
Enhances differences between compact objects
Creates unique frequency ranges where wormholes behave differently
Helps break the uncertainty between black hole and wormhole models
Instead of hiding exotic objects, plasma may help us discover them.
15. What This Means for Future Observations
As telescopes become more powerful and sensitive, scientists will be able to observe shadows at different frequencies and with higher precision.
By comparing:
Shadow size
Shadow disappearance
Frequency dependence
astronomers may be able to test whether supermassive compact objects are truly black holes—or something more exotic.
16. Conclusion: Plasma as a Tool to Find Wormholes
The study by Angelov and his team shows that plasma plays a crucial role in shaping the shadows of compact objects. When plasma is taken into account, wormholes and black holes no longer look the same.
Wormholes:
Lose their shadows at lower plasma frequencies
Have smaller shadows than black holes
Show stronger distortions when the observer is moving
These features create strong observational signatures. As our instruments improve, plasma may help us answer one of the biggest questions in modern physics: Do wormholes exist in our universe?
In the darkness of cosmic shadows, plasma may be the key that reveals the impossible.
Reference: Tsanimir Angelov, Rasim Bekir, Galin Gyulchev, Petya Nedkova, Stoytcho Yazadjiev, "Shadows of rotating traversable wormholes surrounded by plasma", Arxiv, 2025. https://arxiv.org/abs/2512.13327
Technical Terms
1. General Relativity
General relativity is Albert Einstein’s theory of gravity.
It says that gravity is not just a pulling force. Instead, massive objects like stars and black holes bend space and time around them. Light follows these bends, which is why gravity can change the path of light.
2. Compact Object
A compact object is something very massive squeezed into a very small space.
Examples include:
Black holes
Neutron stars
Wormholes (theoretical)
Because they are so dense, they strongly affect nearby light and matter.
3. Gravitational Lensing
Gravitational lensing happens when light bends as it passes near a massive object.
Just like a glass lens bends light, gravity bends light in space. This can:
Change the position of stars
Create multiple images
Produce rings or shadows
4. Shadow of a Compact Object
A shadow is a dark area seen against a bright background.
It forms because some light rays fall into the compact object or get trapped and never reach the observer.
The shadow tells us about the object’s gravity and structure.
5. Kerr Black Hole
A Kerr black hole is a rotating black hole.
It is described by:
Its mass
Its rotation speed
Most real black holes in space are expected to rotate, so Kerr black holes are very important in astronomy.
6. Wormhole
A wormhole is a hypothetical tunnel through space and time.
It could connect:
Two distant parts of the same universe
Two different universes
Some wormholes could, in theory, allow light or matter to pass through them.
7. Traversable Wormhole
A traversable wormhole is a wormhole that can be crossed safely.
Light—and possibly even objects—can go in one side and come out the other without being destroyed.
8. Stationary and Axisymmetric
These terms describe symmetry in spacetime:
Stationary: The object does not change with time
Axisymmetric: The object looks the same when rotated around a central axis
Rotating black holes and rotating wormholes usually have these properties.
9. Plasma
Plasma is a hot gas made of charged particles (electrons and ions).
It is very common in space and exists in:
Accretion disks
Jets
Interstellar space
Plasma affects how light travels, especially radio waves.
10. Plasma Frequency
The plasma frequency is a number that tells us how plasma affects light.
If light’s frequency is:
Higher than the plasma frequency → light can travel
Lower than the plasma frequency → light cannot travel
This is why plasma can block or change shadows.
11. Dispersive Medium
A dispersive medium is a material where light speed depends on frequency.
Plasma is dispersive, meaning different colors (frequencies) of light bend differently.
12. Photon
A photon is a tiny particle of light.
Photons carry energy and move at the speed of light in vacuum.
13. Photon Region
The photon region is an area near a compact object where light can move in unstable circular paths.
If photons fall slightly inward, they get trapped.
If they move slightly outward, they escape.
This region creates the shadow.
14. Hamilton–Jacobi Equation
The Hamilton–Jacobi equation is a mathematical tool used to describe motion.
In this study, it helps scientists track how light moves through curved space and plasma.
When it can be “separated,” calculations become much easier.
15. Separable Equations
An equation is separable when it can be split into simpler parts.
This allows scientists to solve complex problems step by step instead of all at once.
16. Forbidden Region
A forbidden region is an area where light cannot travel because of plasma.
If the plasma frequency is too high, light is blocked completely in that region.
17. Critical Frequency
The critical frequency is the plasma frequency at which the shadow disappears.
Above this value, light cannot reach the observer in a way that forms a shadow.
18. Accretion Disk
An accretion disk is a spinning disk of hot gas and plasma around a compact object.
It emits light and feeds matter into the object.
19. Aberration
Aberration is a visual distortion caused by motion.
If the observer is moving, the shadow:
Changes shape
Shifts position
This is similar to how rain appears slanted when you run.
20. Observer
An observer is the person or telescope detecting light.
In astronomy, the observer’s motion affects how objects appear.
21. Event Horizon
The event horizon is the boundary of a black hole.
Once something crosses it, it can never escape—not even light.
Wormholes usually do not have event horizons.
22. Spacetime
Spacetime combines space and time into a single structure.
Mass and energy bend spacetime, and light follows these bends.
23. Metric
A metric is a mathematical description of spacetime.
It tells scientists how distances and times behave near massive objects.
24. Exotic Compact Object
An exotic compact object is a theoretical alternative to black holes.
Examples include:
Wormholes
Gravastars
They behave differently from black holes but can look similar without detailed observations.
25. Observational Signature
An observational signature is a clear feature that helps identify an object.
In this study, differences in shadow size and disappearance act as signatures for wormholes.
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