Matter vs. Spacetime: A Matter of Choice
A scale that weighs not a body, but the very space around it. That's how Einstein's equations work in general relativity: they link the curvature of spacetime to what resides within it.
When Particles Whisper Goodbye
Two charged spheres collided and flew apart in different directions. You'd think it was all over — but the light from their farewell still wanders through space. In the ordinary world, this is a well-known effect: an accelerated charge always radiates.
Heat from the Void: How to Catch the Glow of Acceleration
In a room, air touches the skin, and we feel its temperature. But if your body suddenly accelerated at a colossal rate, heat would come even from absolute emptiness. This is not fiction but a strict consequence of quantum field theory: for an accelerating observer, the vacuum becomes a warm bath.
The Gravitational Echo of a Black Hole
At the centers of many galaxies lurk black holes — regions with gravity so strong that even light cannot escape. Stars often orbit them, and sometimes one of those stars is a pulsar, a cosmic lighthouse sending out strictly periodic radio pulses. When such a pulsar moves behind a black hole, its signals reach us with a delay.
How to Hear the Warmth of the Void
The vacuum is not as empty as it seems. If an observer moves with acceleration, the vacuum around them heats up: this was predicted by William Unruh back in the 1970s. The effect is akin to black hole radiation, but in flat space.
Gravitational Waves Leave an Eternal Imprint
Gravitational waves are the very trembling of space, born when black holes or other massive bodies collide. Their main oddity isn't that they pass through us, but that they permanently alter the curvature of spacetime and the distances between everything they touch.
How Gravity Learns from Quantum Mechanics: The Invisible Tuning of Forces
A guitar string decides for itself how tightly to stretch when a musician plays a chord in different octaves. That's roughly the picture physicists paint when they try to unite gravity, the weakest but most pervasive force, with the quirky laws of the microcosm.
Gravity on a Cosmic Ruler
Take two heavy balls and place them a meter apart: they attract each other with a force that weakens by a factor of four when you double the distance. Newton discovered this law, and it works in a room, in the solar system, in a galaxy. But does it still hold when we're talking about hundreds of millions of light-years?
Astronomers found a way to test this directly.
A Star's Dance Puts Gravity's Symmetry to the Test
Star S2 completes an orbit around the black hole at the Milky Way's center every 16 years, reaching speeds of 7650 kilometers per second. Its orbit is a stretched ellipse, and at its closest approach, the star comes within a distance 120 times the Earth-Sun separation.