At the start of the 20th century, physicists had a problem: The speed of light was always the speed of light.

If you threw a baseball out of a train going 20 mph, it would travel the speed at which you threw it plus 20 mph, just as Isaac Newton’s laws predicted. However, if you aimed a flashlight out of a train going 20 mph, the light would travel the speed of light—no more, no less—no matter your perspective. And according to Newton’s picture of the universe, that didn’t make any sense.

“We didn’t have a theory that would explain why light was special,” says Lia Medeiros, a NASA Einstein fellow at Princeton University.

The key turned out to be something Albert Einstein would soon propose: the idea of spacetime.

The concept was revolutionary. “For common-day experience, as well as most experiments, space and time being separate is totally fine,” says Daniel Holz, a professor of physics and astrophysics at the University of Chicago. “But if you want to make a general statement about how the universe works, then you really need to view them as one object.”

A matter of perspective

In 1905, building on existing experimental and theoretical work, Einstein published the theory of special relativity. Among other things, the theory combined space and time into a single entity that he called spacetime.

“Spacetime is a necessary consequence of the fact that all observers measure the same value for the speed of light,” says Scott Hughes, a professor of physics at the Massachusetts Institute of Technology. “Einstein took the question ‘What if the speed of light is just the same to everyone?’seriously. And spacetime grew out of that thought experiment.”

It all starts with the concept of different frames of reference. How a person experiences the world depends on their individual frame of reference. Two people standing together on a moving train will perceive one another as stationary. But an observer standing outside the train will perceive both of those people as in motion, chugging along at the speed of the train. Zoom out even farther, and another observer floating in space will perceive the person standing outside the train as in motion as well, spinning along with the Earth while in orbit around the sun, which in turn is flying through the galaxy.

What Einstein realized is that something similar happens with time: Different people will experience the passage of time differently, depending on their frame of reference. The key to understanding how this works is the universal speed of light.

Imagine a single quantum of light, a photon, bouncing up and down between two mirrors that are facing each other. Traveling at the speed of light, the photon should bounce at regular intervals, like a steadily ticking clock.

A person standing on a moving train with this photon clock will see the photon moving up and down in a line. To a person standing outside the moving train, on the platform, however, the photon will seem to move in a different way. Not only will the photon bounce up and down, it will also move forward with the train.

From one frame of reference—on the train—the photon follows the shortest possible path, a straight line. From another—on the platform—it follows a stretched-out zig-zag path instead.

The puzzle Einstein faced becomes apparent if you imagine two photon clocks, one sitting stationary on the platform, and the other whizzing by in the train.

If the speed of light is constant regardless of the frame of reference, then to the person on the train, the photon clock next to them will tick more quickly, while to the person on the platform, that same clock on the train will tick more slowly. This effect is called time dilation. A similar thought experiment, with the photon clock tipped on its side, shows that objects are more compact along the direction of the train’s motion, an effect called length contraction.

This works out mathematically. It’s only when you combine the different measurements from the different frames of reference of space and time that all the observers will agree on the result, suggesting that space and time are inextricably linked.

“If you allow both space and time to change in a connected way, then everyone agrees that light moves at the speed of light,” Holz says. “Once you combine them, everything kind of follows naturally. The equations are very beautiful and elegant.”

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posted by f.sheikh

( Interesting to read synopsis of panoramic view of American civil War which encroached Mexico and Canada. This Civil War has not ended and is still raging in political arena. f.sheikh).

“Some excerpts; A mountain of historical studies testifies to enduring interest in the American Civil War, a conflict still politically relevant in a nation riven over how to remember it. Those doubting that there is anything fresh to say about the bloodiest event in the republic’s history should read Pulitzer Prize winner Alan Taylor’s brilliant, panoramic account of the conflict. Applying a wide continental lens, he explores this crux of United States history and how it shook neighbouring Mexico and Canada. In all three settings, liberals and social and political conservatives were involved in parallel struggles to build a modern nation. After a French invasion, the creation of a short-lived monarchy and a devastating civil war, the Liberal Party leader Benito Juárez returned to power in Mexico. Fearing the growing power and rapacity of the United States, meanwhile, Canadians navigated internal divisions to create a continental confederation. And in the United States, the pulsing heart and geographical centre of events, Abraham Lincoln’s Union forces subdued the reactionary and rebel slave power to achieve emancipation and the constitutional basis for a more liberal and democratic nation.”

“Panoramic in scope, Taylor’s book regularly shocks the reader with its descriptions of the brutality of warfare (‘arms shot off – legs shot off. Eyes shot out – brains shot out … everything shot to pieces and totally maimed for all after life,’ a Union surgeon grieved in the early stages of the war). Taylor concedes that Union and Confederate generals abided by a code of conduct that precluded total war against white civilians. But it did not extend to non-white peoples. To the horrors of battlefield carnage must be added Americans’ indiscriminate slaying of Indians and Mexicans, as well as Confederates’ murder of black troops in slaughter pens and rampaging whites’ violence against black civilians in the Union’s draft riots. The self-proclaimed ‘fiends from hell’, William Quantrill’s Raiders, threw victims, including a black baby, into burning buildings. In Mexico, frustrated and vengeful French troops played football with the heads of decapitated guerrilla prisoners. The victory of American liberals was scarcely cost-free. “

American Civil Wars: A Continental History, 1850–1873

By Alan Taylor

W W Norton 577pp £33.99

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(Source: Adobe Firefly.)

Some excerpts; “

For many decades after the Founding, American identity was indeed based on ancestry. Americans struggled mightily to move beyond ancestry to an identity based on ideas alone. 

We can trace changing ideas of American identity by the evolution of requirements for voting and citizenship. The first sentence of the Constitution refers to “We the People of the United States of America,” but does not define who “The People” are. It was in fact quite restrictive. At the moment of the Constitution’s ratification, only white men who owned property qualified as full rights-bearing members of the political community. The property qualification was lifted in most states by the 1820s, but the country moved dangerously towards civil conflict over the question of race and whether one American could hold another American as a chattel slave. It took a civil war that killed over 600,000 Americans to settle that question; in the war’s wake, the country ratified the Fourteenth Amendment that stated:

All persons, born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State in which they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.

For the first time black men could vote, and African Americans were elected to office both in the states and at a federal level. But shamefully, these rights were progressively taken away as the Southern states were readmitted to the Union after 1876, and the country looked away as legal segregation and restriction of voting rights for black people were imposed. Throughout this period, women did not have the right to vote either; their citizenship was codified only by passage of the Nineteenth Amendment after World War I. It was not until the passage of the Civil Rights and Voting Rights Acts in the 1960s that the promise of the Fourteenth Amendment was finally realized and women and racial minorities were accepted as full rights-bearing citizens, even if they faced continued discrimination on a social level.

It is important to step back and understand what was going on as a result of these changes. American citizenship and therefore American identity were initially based not simply on ideas, but on ascriptive characteristics like social class, race, and gender (“ascriptive” meaning things you are born with and have no control over). The full promise of the Declaration’s assertion of human equality was not formally implemented until the Civil Rights era. In other words, American identity was made creedal over time, by stripping out those other qualifications based on ancestry. Getting to a creedal identity was therefore a huge achievement, one that required war, death, struggle, and nation-wide mobilization.  

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posted by f/sheikh

“Today, Holiday is revered as one of the most influential musical artists of all time. Time magazine named her 1939 recording of “Strange Fruit” the song of the 20th century. “In this sad, shadowy song about lynching in the South,” Time wrote in 1999, “history’s greatest jazz singer comes to terms with history itself.”

“Abel Meeropol, a New York City teacher and songwriter who used the pen name Lewis Allan, wrote “Strange Fruit” after seeing a photograph of a lynching that shocked and haunted him: “Black body swinging in the Southern breeze / Strange fruit hanging from the poplar trees.”

Sixty-five years ago, on July 17, 1959, Billie Holiday died at Metropolitan Hospital in New York. The 44-year-old singer arrived after being turned away from a nearby charity hospital on evidence of drug use, then lay for hours on a stretcher in the hallway, unrecognized and unattended. Her estate amounted to 70 cents in the bank and a roll of bills concealed on her person, her share of the payment for a tabloid interview she gave on her deathbed.

Strange Fruit song with lyrics

Lyrics: Abel Meeropol (Lewis Allen) ⁽³⁾

Music: Billie Holiday and Sonny White ⁽⁴⁾
Year: 1937⁽⁴⁾
Genre: Blues
Country: USA

Southern trees bear a strange fruit,
Blood on the leaves and blood at the root,
Black body swingin’, In the southern breeze.
Strange fruit hangin’ from the poplar trees.

Pastoral scene of the gallant South,
The bulging eyes and the twisted mouth,
Scent of the magnolia sweet and fresh,
Then the sudden smell of burning flesh.
This song was originally posted on protestsonglyrics.net
Here is the fruit for the crows to pluck,
For the rain to gather, for the wind to suck,
For the sun to rot, forthe trees, tree to drop,
Here is a strange and bitter crop.

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posted by f.sheikh