Andromeda sounds far away because 2.5 million light-years sounds far away. It lands with authority. It feels exact. It gives you the comfort of a labeled universe, one where galaxies, stars and planets sit at fixed intervals inside a giant cosmic grid.
Might be time to ease up on that confidence.
Einstein, Juan Maldacena, Mark Van Raamsdonk, Brian Swingle and other physicists have pushed modern physics into a place where distance stops looking like a simple fact and starts looking more like a useful habit of thought. The old picture still works for everyday life, and for plenty of astronomy. But at deeper levels, distance gets slippery. It depends on motion. It depends on gravity. It depends on how you define the measurement. And in some of the boldest theoretical work, it may emerge from quantum entanglement rather than existing as a basic ingredient of reality.
That is a startling claim, but it does not come out of nowhere. It grows out of a series of cracks in common sense, and once you follow those cracks, the floor starts to shift.
Andromeda is how far away?
Take the familiar line that the Andromeda galaxy is about 2.5 million light-years away. That sounds like a clean statement about space. In practice, it is already tangled up with time.
A light-year is not some separate cosmic unit floating above the others. It is the distance light travels in one year. So when you say Andromeda is 2.5 million light-years away, you are also saying the light reaching you took 2.5 million years to arrive.
That detail matters more than it first appears to.
The light in your telescope left Andromeda millions of years ago. By the time it got here, Andromeda had not been politely standing still, waiting to be measured. It had been moving. So the number attached to the galaxy is not really telling you where it is “right now” in any easy everyday sense.
Things get even messier once cosmic expansion enters the discussion. The distance the light traveled is not identical to the current distance between Earth and the galaxy that emitted it. Those are different ideas. So are the distance at the moment the light was emitted, the distance implied by travel time, and the current distance after expansion. Click To Continue Reading
Quantum experiment shows events may have no fixed order
by Sam Jarman, Phys.org
For the first time, a team of physicists in Austria has carried out an experiment that appears to verify the principle of indefinite causal order: an idea that suggests that timelines of events can exist in multiple orders at the same time. Led by Carla Richter at the Vienna Center for Quantum Science and Technology, the researchers hope their result could finally allow physicists to verify a key prediction of quantum theory. The results have been published in PRX Quantum.
Rules of causality
The basic principle of cause and effect underpins everything that happens in the classical world: for an event to occur, it must be triggered by another event in its past. Yet in the quantum world, physicists have long suspected that these rules may not always apply. Click To Continue Reading
Quantum Time Travel
Simulations of time travel send quantum metrology back to the future
By Unnati Akhouri Physics World
Have you ever wished you could go back in time and change your decisions? If only knowledge from today could travel back in time with us, we could alter our actions to our advantage. For now, such time travel is the stuff of fiction, but a trio of researchers have shown that by manipulating quantum entanglement, one can, at least, design experiments that simulate it. Read More