Latest news with #ChristosPapadimitriou
Yahoo
18-05-2025
- Yahoo
Home of former Greek rail disaster probe chief attacked
An incendiary device exploded early Sunday outside the apartment block where a former senior state investigator probing Greece's worst train tragedy lives, police said. Athens News Agency reported that unidentified individuals had placed it at the entrance of the and that it had exploded shortly before 03:00 am on Sunday. The explosion caused minor material damage to the entrance and police found the remains of gas cannisters, firecrackers and duct tape. State security is investigating the incident at the building, which according to police sources is where Christos Papadimitriou, until recently head of the state accident investigation agency's railway sector, lives. Papadimitriou resigned early in April, citing "personal and family reasons", having said he had received threats over his work. That followed a row over statements he made about the fireball explosion that followed the devastating train collision of Tempe in 2023. Fifty-seven people, most of them young students, were killed in February 2023 when a passenger train and a freight train collided in Tempe, central Greece, having been allowed to run on the same track. But investigators say some those who died were killed not in the crash but by an 80-metre (260-feet) fireball that came after the collision. - 'Questionable' methodology - On February 27, Papadimitriou presented a report that indicated the "possible presence" of an "unknown fuel" that could have caused the fireball, a conclusion that has divided experts. In subsequent television interviews he said that European train safety officials had pressed for this conclusion to be included in the report, but that it required further investigation. "I received threats because I continued to look into it," he told Skai TV. The fireball theory was based on "questionable" methodology that was "not universally accepted" by experts, he added. Papadimitriou mentioned that there was an alternative theory that involved train engine silicone oils. If that theory turned out to be correct, then "all trains" running in Europe" could be "dangerous", he had warned. The disaster and its aftermath have sparked strikes and hundreds of protests in Greece and abroad this year. It has also led to two votes of no confidence in the conservative government, which the administration overcame. The train's Italian-owned operator Hellenic Train has denied knowledge of any illegal cargo on the freight train. Prosecutors have so taken action against more than 40 people over the accident, including the local station master responsible for routing the trains. A trial into the accident is not expected before the end of the year. Early in April, a bomb exploded outside the Athens offices of Hellenic Train, after anonymous calls to Greek media warned of the attack near one of the capital's busiest highways. No one was injured. kan/jj
WIRED
04-05-2025
- Science
- WIRED
Why Pigeons at Rest Are at the Center of Complexity Theory
May 4, 2025 7:00 AM When pigeons outnumber pigeonholes, some birds must double up. This obvious statement, and its inverse, have deep connections to many areas of math and computer science. Pigeon Home Photograph: Brady Clegg via Shutterstock The original version of this story appeared in Quanta Magazine . They say a bird in the hand is worth two in the bush, but for computer scientists, two birds in a hole are better still. That's because those cohabiting birds are the protagonists of a deceptively simple mathematical theorem called the pigeonhole principle. It's easy to sum up in one short sentence: If six pigeons nestle into five pigeonholes, at least two of them must share a hole. That's it—that's the whole thing. 'The pigeonhole principle is a theorem that elicits a smile,' said Christos Papadimitriou, a theoretical computer scientist at Columbia University. 'It's a fantastic conversation piece.' But the pigeonhole principle isn't just for the birds. Even though it sounds painfully straightforward, it's become a powerful tool for researchers engaged in the central project of theoretical computer science: mapping the hidden connections between different problems. The pigeonhole principle applies to any situation where items are assigned to categories, and the items outnumber the categories. For example, it implies that in a packed football stadium with 30,000 seats, some attendees must have the same four-digit password, or PIN, for their bank cards. Here the pigeons are football fans, and the holes are the 10,000 distinct possible PINs, 0000 through 9999. That's fewer possibilities than the total number of people, so some people must have the same digits. This proof is notable not just for its simplicity, but also for what it leaves out. Many mathematical methods for proving that something exists are 'constructive,' meaning they also show you how to find it. 'Nonconstructive' proofs, like ones based on the pigeonhole principle, don't have this property. In the football stadium example, knowing that some people must have the same PINs won't tell you what they are. You can always go through the stands asking each person in turn. But is there a simpler way? Questions like this one, about the most efficient way to solve problems, are at the heart of the branch of computer science known as computational complexity theory. Complexity theorists study such questions by lumping problems into classes based on certain shared properties. Sometimes the first step toward a breakthrough is simply defining a new class to unite problems that researchers hadn't previously studied together. That's what happened in the 1990s, when Papadimitriou and other complexity theorists began to study new classes of problems, in which the goal is to find something that must exist because of the pigeonhole principle or another nonconstructive proof. That line of work has led to important progress in disparate fields of computer science, from cryptography to algorithmic game theory. Christos Papadimitriou (inset) and Oliver Korten showed that the empty-pigeonhole principle connects to important problems in math and computer science. Photograph: Columbia Engineering. Inset courtesy of Christos Papadimitriou By January 2020, Papadimitriou had been thinking about the pigeonhole principle for 30 years. So he was surprised when a playful conversation with a frequent collaborator led them to a simple twist on the principle that they'd never considered: What if there are fewer pigeons than holes? In that case, any arrangement of pigeons must leave some empty holes. Again, it seems obvious. But does inverting the pigeonhole principle have any interesting mathematical consequences? It may sound as though this 'empty-pigeonhole' principle is just the original one by another name. But it's not, and its subtly different character has made it a new and fruitful tool for classifying computational problems. To understand the empty-pigeonhole principle, let's go back to the bank-card example, transposed from a football stadium to a concert hall with 3,000 seats—a smaller number than the total possible four-digit PINs. The empty-pigeonhole principle dictates that some possible PINs aren't represented at all. If you want to find one of these missing PINs, though, there doesn't seem to be any better way than simply asking each person their PIN. So far, the empty-pigeonhole principle is just like its more famous counterpart. The difference lies in the difficulty of checking solutions. Imagine that someone says they've found two specific people in the football stadium who have the same PIN. In this case, corresponding to the original pigeonhole scenario, there's a simple way to verify that claim: Just check with the two people in question. But in the concert hall case, imagine that someone asserts that no person has a PIN of 5926. Here, it's impossible to verify without asking everyone in the audience what their PIN is. That makes the empty-pigeonhole principle much more vexing for complexity theorists. Two months after Papadimitriou began thinking about the empty-pigeonhole principle, he brought it up in a conversation with a prospective graduate student. He remembers it vividly, because it turned out to be his last in-person conversation with anyone before the Covid-19 lockdowns. Cooped up at home over the following months, he wrestled with the problem's implications for complexity theory. Eventually he and his colleagues published a paper about search problems that are guaranteed to have solutions because of the empty-pigeonhole principle. They were especially interested in problems where pigeonholes are abundant—that is, where they far outnumber pigeons. In keeping with a tradition of unwieldy acronyms in complexity theory, they dubbed this class of problems APEPP, for 'abundant polynomial empty-pigeonhole principle.' One of the problems in this class was inspired by a famous 70-year-old proof by the pioneering computer scientist Claude Shannon. Shannon proved that most computational problems must be inherently hard to solve, using an argument that relied on the empty-pigeonhole principle (though he didn't call it that). Yet for decades, computer scientists have tried and failed to prove that specific problems are truly hard. Like missing bank-card PINs, hard problems must be out there, even if we can't identify them. Historically, researchers haven't thought about the process of looking for hard problems as a search problem that could itself be analyzed mathematically. Papadimitriou's approach, which grouped that process with other search problems connected to the empty-pigeonhole principle, had a self-referential flavor characteristic of much recent work in complexity theory—it offered a new way to reason about the difficulty of proving computational difficulty. 'You're analyzing the task of complexity theory using complexity theory,' said Oliver Korten, a researcher at Columbia. Korten was the prospective student with whom Papadimitriou had discussed the empty-pigeonhole principle right before the pandemic. He came to Columbia to work with Papadimitriou, and in his first year of grad school he proved that the search for hard computational problems was intimately linked to all other problems in APEPP. In a specific mathematical sense, any progress on this one problem will automatically translate into progress on a host of others that computer scientists and mathematicians have long studied, such as the search for networks that lack simple substructure. Korten's paper immediately attracted attention from other researchers. 'I was quite surprised when I saw it,' said Rahul Santhanam, a complexity theorist at the University of Oxford. 'It's incredibly exciting.' He and others have since built on Korten's breakthrough to prove a flurry of new results about connections between computational difficulty and randomness. 'There is amazing richness to this,' Papadimitriou said. 'It goes to the bone of important problems in complexity.' Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
Al Jazeera
14-04-2025
- Politics
- Al Jazeera
Greek police probe hardline group claims on bombing attacks
Greek police say they are investigating claims by an unknown hardline group that it was responsible for a couple of bomb blasts that rocked Athens over the past year. A police spokeswoman told Greek media on Monday that the authorities were examining a statement issued by Revolutionary Class Struggle that it was behind an explosion on Friday near the offices of railway operator Hellenic Train last week, as well as an attack at the Ministry of Labour last year. The group posted a statement on the website on Sunday, which dedicated both the attacks to 'the Palestinian people and their heroic resistance'. 'The claim is being examined with great attention by the relevant officers so that they can gather information,' police spokeswoman Constantina Dimoglidou told Action 24 TV. She said it was too early to draw conclusions about the group's identity but police were investigating possible links with other cases in the past. The Revolutionary Class Struggle statement blamed Hellenic Train, a unit of Italy's Ferrovie dello Stato, for a rail accident in 2023 near the Greek village of Tempi. The crash, which killed 57 people, has led to numerous protests over deficiencies in the railway system and claims of government corruption. The group's statement branded them demonstrations against 'state capitalist crime', and sought to link them to Palestine and the war in Gaza 'since they took place within a country with an active role in the American-Zionist genocidal war'. The statement also said the group was behind a 2024 attack on the Labour Ministry. Neither attack resulted in any injuries as warnings were sent in advance. Hellenic Train, the operator of passenger and cargo train services, was once a subsidiary of state-owned Hellenic Railways. In February, Greece's Air and Rail Accident Investigation Authority (HARSIA) said the 2023 accident was due to chronic safety gaps and that they still needed to be addressed to prevent a repeat. Christos Papadimitriou, the head of HARSIA, resigned on Wednesday, a move expected to further stall efforts to shed light on the country's worst rail disaster. A judicial investigation is expected to conclude this year.
Al Arabiya
09-04-2025
- Politics
- Al Arabiya
Lead investigator of 2023 deadly Greek train crash resigns
The supervisor of Greece's national investigation into a 2023 train crash resigned on Wednesday, a move expected to further complicate efforts to shed light on the country's worst rail disaster that killed 57 people. According to opinion polls, most Greeks view the crash as emblematic of the neglect of the country's railways in recent decades and also of a persistent failure by the state to address safety concerns. The crash has prompted angry protests, fueled further by a lack of trust in institutions. Christos Papadimitriou, the head of the railway division at Greece's Air and Rail Accident Investigation Authority (HARSIA), stepped down days after a top court prosecutor ordered a probe into HARSIA's findings. In February, HARSIA said the safety gaps, which failed to prevent the head-on collision of a freight train and a passenger train on February 28, 2023, had not yet been fixed. It also found that a fireball that followed the collision could not have been caused by train equipment, generating doubts about the freight train's cargo as well as political wrangling. HARSIA decided this week to remove the section which refers to the causes of the fireball, after at least one of the foreign universities cited in its report said it had neither reviewed nor authorized the content. Greece's Supreme Court prosecutor ordered a probe into the developments to determine if there was an attempt to influence a judicial investigation which has been underway since 2023. HARSIA, an independent authority, was only set up in late 2023. It launched its probe in March 2024, more than a year after the crash, which meant it had to rely on others for much of its information. 'I tried to serve the public interest... in a difficult situation,' said Papadimitriou in his resignation letter, standing by HARSIA's main findings.
Yahoo
09-04-2025
- Politics
- Yahoo
Lead investigator of 2023 deadly Greek train crash resigns
ATHENS (Reuters) - The supervisor of Greece's national investigation into a 2023 train crash resigned on Wednesday, a move expected to further complicate efforts to shed light on the country's worst rail disaster that killed 57 people. According to opinion polls, most Greeks view the crash as emblematic of the neglect of the country's railways in recent decades and also of a persistent failure by the state to address safety concerns. The crash has prompted angry protests, fuelled further by a lack of trust in institutions. Christos Papadimitriou, the head of the railway division at Greece's Air and Rail Accident Investigation Authority (HARSIA), stepped down days after a top court prosecutor ordered a probe into HARSIA's findings. In February, HARSIA said the safety gaps, which failed to prevent the head-on collision of a freight train and a passenger train on February 28, 2023, had not yet been fixed. It also found that a fireball that followed the collision could not have been caused by train equipment, generating doubts about the freight train's cargo as well as political wrangling. HARSIA decided this week to remove the section which refers to the causes of the fireball, after at least one of the foreign universities cited in its report said it had neither reviewed nor authorised the content. Greece's Supreme Court prosecutor ordered a probe into the developments to determine if there was an attempt to influence a judicial investigation which has been underway since 2023. HARSIA, an independent authority, was only set up in late 2023. It launched its probe in March 2024, more than a year after the crash, which meant it had to rely on others for much of its information. "I tried to serve the public interest... in a difficult situation," said Papadimitriou in his resignation letter, standing by HARSIA's main findings.
