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Why the Past Still Exists | Leonard Susskind

We usually think of the past as something that no longer exists. It happened — and then it disappeared. But modern physics challenges this intuition in a profound way.

In this video, we explore why the past may still exist — not as memory, but as structure.

Drawing on ideas associated with Leonard Susskind, this documentary examines how relativity and modern spacetime physics reshape our understanding of time. In Einstein’s framework, there is no universal “now.” What is past for one observer may be present or future for another, depending on motion and frame of reference.

This destroys the idea that the past vanishes.

In the spacetime view, the universe is a four-dimensional structure. Events are not erased — they are located. The past is not something that disappeared. It is something that exists in a different region of spacetime.

From this perspective, time does not flow in the way we imagine. The sense of disappearance comes from human experience, not from fundamental physics.

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AI captures particle accelerator behavior to optimize machine performance

Keeping high-power particle accelerators at peak performance requires advanced and precise control systems. For example, the primary research machine at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility features hundreds of fine-tuned components that accelerate electrons to 99.999% the speed of light.

The electrons get this boost from radiofrequency waves within a series of resonant structures known as cavities, which become superconducting at temperatures colder than deep space.

These cavities form the backbone of Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a unique DOE Office of Science user facility supporting the research of more than 1,650 nuclear physicists from around the globe. CEBAF also holds the distinction of being the world’s first large-scale installation and application of this superconducting radiofrequency (SRF) technology.

Temporal evolution of GRB 240825A afterglow provides insight into origins of optically dark gamma-ray bursts

Researchers from the Yunnan Observatories of the Chinese Academy of Sciences have conducted a new study on the temporal evolution of the afterglow from gamma-ray burst GRB 240825A. The study offers new evidence to better understand the physical environment surrounding gamma-ray bursts and provides insights into the mechanisms that govern their afterglow emission. The findings were recently published in The Astrophysical Journal.

Long-duration gamma-ray bursts (LGRBs) are widely believed to form from the core collapse of massive stars, usually occurring in dense star-forming regions. NASA’s Swift satellite detected GRB 240825A on August 25, 2024, and observed an unusually bright optical counterpart.

Early measurements yielded an X-ray afterglow spectral index of 0.79 and a significantly softer optical afterglow spectral index of 2.48, compared with a typical value near 1. Under standard models, a gamma-ray burst is classified as “optically dark” when its observed optical afterglow flux falls below the level predicted from its X-ray spectral index.

Why Gentry Lee Became the Most Important Figure in Space Exploration You’ve Never Heard Of: STARMAN

There are documentaries about history, and then there are documentaries about the people who were quietly in the room when history happened.

STARMAN, the new film from Academy Award–nominated director Robert Stone, belongs firmly in the latter category. It chronicles the life of Gentry Lee—NASA scientist, mission architect, science communicator, and one of those rare figures whose career seems to map directly onto the modern Space Age.

If the Space Age began in 1957 with the launch of Sputnik, then Gentry Lee—born in 1942—has lived his entire adult life shaped by humanity’s reach beyond Earth. More than a witness to that history, Lee has been in the room for many of its defining moments.

As a senior scientist at NASA’s Jet Propulsion Laboratory, Lee served as Director of Science Analysis and Mission Planning for the Viking mission to Mars and the Galileo probe to Jupiter, missions that transformed our understanding of the solar system. Alongside this work, he collaborated with Carl Sagan on PBS’s landmark series COSMOS, narrated Discovery Channel’s ARE WE ALONE?, and co-authored four novels with legendary science fiction writer Arthur C. Clarke.

A Zelig-like figure at the crossroads of interplanetary science and science fiction, Gentry Lee has been everywhere—and worked with everyone—who helped define how we imagine space.

Now the subject of STARMAN, Lee guides us through a lifetime of curiosity, wonder, and exploration. The film is both entertaining and illuminating—and our conversation with him reflects that same spirit.

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New experiments suggest Earth’s core contains up to 45 oceans’ worth of hydrogen

Scientists have long known that Earth’s core is mostly made of iron, but the density is not high enough for it to be pure iron, meaning lighter elements exist in the core, as well. In particular, it’s suspected to be a major reservoir of hydrogen. A new study, published in Nature Communications, supports this idea with results suggesting the core contains up to 45 oceans’ worth of hydrogen. These results also challenge the idea that most of Earth’s water was delivered by comets early on.

Because of the extreme conditions in Earth’s core and its distance from the surface, analyzing its composition presents difficulties. Additionally, many techniques are inadequate for resolving hydrogen because it is the lightest and smallest element. Earlier estimates relied on indirect methods, such as inferring hydrogen composition from lattice expansion in iron hydrides. These difficulties have led to highly uncertain estimates of hydrogen in the core, spanning four orders of magnitude.

The team involved in the new study took a different approach, using laser-heated diamond anvil cells to simulate high-pressure, high-temperature core conditions, up to 111 GPa and around 5,100 Kelvin. The team placed core-like iron samples and hydrous silicate glass, representing Earth’s early magma oceans, in the diamond anvil cells to induce melting, similar to conditions in the core.

Space mining without heavy machines? Microbes harvest metals from meteorites aboard space station

If humankind is to explore deep space, one small passenger should not be left behind: microbes. In fact, it would be impossible to leave them behind, since they live on and in our bodies, surfaces and food. Learning how they react to space conditions is critical, but they could also be invaluable fellows in our endeavor to explore space.

Microorganisms such as bacteria and fungi can harvest crucial minerals from rocks and could provide a sustainable alternative to transporting much-needed resources from Earth.

Researchers from Cornell and the University of Edinburgh collaborated to study how those microbes extract platinum group elements from a meteorite in microgravity, with an experiment conducted aboard the International Space Station. They found that “biomining” fungi are particularly adept at extracting the valuable metal palladium, while removing the fungus resulted in a negative effect on nonbiological leaching in microgravity.

Subaru observations suggest an intrinsic gap in NGC 5466’s tidal stream

Astronomers from the National Astronomical Observatory of Japan (NAOJ) and elsewhere have used the Subaru Telescope to perform deep imaging observations of a distant globular cluster known as NGC 5466. The observational campaign yields important information about the structure of the cluster’s tidal stream. The new findings were published February 4 on the arXiv preprint server.

In general, stellar tidal streams are the result of tidal interactions between a central galaxy and lower mass systems such as satellite galaxies or globular clusters (GCs). Therefore, they could keep the memory of their progenitors’ chemical and dynamical information, even after a few billion years.

Bennu asteroid reveals new origins for life’s amino acids

“Our results flip the script on how we have typically thought amino acids formed in asteroids,” said Dr. Allison Baczynski.

Did the ingredients for life as we know it exist in the early solar system? This is what a recent study published in the Proceedings of the National Academy of Sciences hopes to address as a team of researchers investigated new evidence for how amino acids, the known building blocks of life, ended up in the asteroid Bennu, which is estimated to have formed during the early days of the solar system billions of years ago. This study has the potential to help scientists better understand the early solar system, how life might have formed on Earth, and potentially elsewhere.

For the study, the researchers analyzed samples of asteroid Bennu that were retrieved and returned to Earth by NASA’s OSIRIS-REx mission in September 2023. The goal of the study was to ascertain the origins of the amino acids that had previously been identified in Bennu samples, which could help scientists gain insights into the origins of life in the early solar system. To accomplish this, the researchers used novel methods for measuring the amount of amino acids while comparing these findings to the carbonaceous meteorite Murchison.

In the end, the researchers discovered that glycine, one of the simplest amino acid molecules, with Bennu was formed in early ices in the early solar system while the glycine found in Murchison was formed in a protoplanetary body, which formed later in the history of the solar system. Additionally, the researchers found that certain aspects of the protein-building amino acid, glutamic acid, found in the Bennu samples, experienced significant changes during its formation and evolution.

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