Everard had gone outside his spaceship to repair an errant antenna. His remarkable story is replete with curved spacetime, cats twisting in midair, an imperiled astronaut dog paddling through a vacuum to safety-and Isaac Newton perhaps spinning in his grave. He told me of an amazing experience he had involving some recently discovered aspects of Einstein's theory of general relativity, which I will share with you. Everard, a philosopher and engineer who is carrying on his ancestor's tradition. Not long ago I figuratively encountered one of Mr. In one of these worlds, for instance, the speed of light was 15 kilometers per hour, putting the weird effects of Einstein's theory of special relativity on display if you so much as rode a bicycle. Tompkins, a humble bank clerk who had vivid dreams of worlds where strange physical phenomena intruded into everyday life. Even our galaxy, the Milky Way, has one with a mass of 4 million Suns.Īn accurate black hole model probably needs a unified theory of physics that combines general relativity with quantum field theory.Ī grand unified theory of physics doesn't exist yet, but many physicists are actively looking for one.In a famous series of stories in the 1940s, physicist George Gamow related the adventures of one Mr. When stars runs out of nuclear fuel, gravity will dominate and a black hole may form.Īt the center of most galaxies there is a super massive black hole. This energy balances the force of gravity and prevents stars from becoming black holes. Stars convert mass into energy through nuclear fusion. From an outsider's perspective, objects fall into black holes and never get out, frozen in time.īlack holes are a possible outcome at the end of the life of a very massive star. We can only speculate how black holes might behave, but the time dilation equation suggests very extreme outcomes.Īs a body approaches the event horizon, the passage of time approaches zero. Objects that achieve the high density needed to reach this point are called black holes. The behavior of spacetime at distances below r is undefined. $$ \Delta t_0 = \Delta t_f \sqrt \right) = 0 $$ Light originating inside a time dilated frame of reference has a lower frequency and a different color when observed from outside. You can't feel time dilation inside an accelerating frame of reference,īut observers outside the frame see everything inside as slower. This effect is similar, but different from how relative velocity dilates time in special relativity. General relativity predicts that an accelerating reference frame experiences time dilation. The theory of general relativity suggests a significant change to fundamental aspects of physics,īut there is overwhelming evidence in its favor. ![]() black holes: very dense bodies with gravity so strong that time stops.gravitational time dilation: gravity slows time.Moving sources of gravity make waves in spacetime gravitational lensing: gravity bends the path of light.The pull of gravity is what a straight line through curved space looks like.Īnother example of a straight line through a curved space is the great circle routes around a sphere.Ī full mathematical explanation of general relativity is beyond the scope of this page, but we will explore some of the predictions: This isn't exactly the same math as general relativity, but it's a good analogy.Īs bodies travel through curved space they follow the shortest distance between two points, a straight line. ![]() In the simulation above, classical gravitational potential energy is represented as warping a 2-D surface into a 3rd dimension. Mass is a type of potential energy, so it also curves spacetime. Gravitation occurs when a body tries to move in a straight line through curved spacetime. General relativity describes gravity as the curvature of space and time, or spacetime. In 1915 Einstein completed his theory of general relativity. The equivalence of acceleration and gravityĭrove Einstein towards a new theory that would extend the scope of special relativity to include gravity. On Earth the only way to not accelerate up would be to fall. This means that the surface of the Earth is constantly accelerating upwards. The relativistic point of view would be that the space around the Earth is compressing, but the atoms of the Earth resists this compression. I am accelerating at up at 9.8 m/s² because my chair is pushing me upwards. ![]() Am I at rest? When is a person at rest on Earth? Question: I'm sitting at my desk typing this question right now.
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