2017-11-26 19:05:52 UTC
"I'm Brian Greene, professor of physics and mathematics at Columbia University and co-founder of the World Science Festival. It's critical that you realize that there are two types of time travel, and they are radically different. Time travel to the future? Definitely possible. We know how to do it because Einstein showed us the way over a hundred years ago. It’s surprising how few people actually really know about this in their bones. He showed that if you go out into space and travel near the speed of light, and you turn around, and you come back, your clock will be ticking off time more slowly. So, when you step off it's going to be the future on planet Earth. You will have time traveled into the future." http://www.businessinsider.com/how-to-time-travel-with-wormholes-2017-11
David Morin, Introduction to Classical Mechanics With Problems and Solutions, Chapter 11, p. 14: "Twin A stays on the earth, while twin B flies quickly to a distant star and back. [...] For the entire outward and return parts of the trip, B does observe A's clock running slow, but enough strangeness occurs during the turning-around period to make A end up older. Note, however, that a discussion of acceleration is not required to quantitatively understand the paradox..." http://www.people.fas.harvard.edu/~djmorin/chap11.pdf
Physics Girl (4:30): "One last question. What's happening to the clocks during the period of acceleration? We still get time dilation, but we have to use a different set of rules from the general relativity. General relativity states that clocks runs slower in accelerated reference frames. So while your twin is turning around, her clock runs slower, and she sees the same thing. She sees your clock running faster than hers, so you're aging quicker. It's during this period of acceleration that you become the older twin."
"At the same time, the twin in the spaceship considers himself to be the stationary twin, and therefore as he looks back towards Earth he sees his brother ageing more slowly than himself. [...] When the twin in the spaceship turns around to make his journey home, the shift in his frame of reference causes his perception of his brother's age to change rapidly: he sees his brother getting suddenly older. This means that when the twins are finally reunited, the stay-at-home twin is the older of the two." https://hubpages.com/education/Twin-Paradox
John Norton: "Moments after the turn-around, when the travelers clock reads just after 2 days, the traveler will judge the stay-at-home twin's clock to read just after 7 days. That is, the traveler will judge the stay-at-home twin's clock to have jumped suddenly from reading 1 day to reading 7 days. This huge jump puts the stay-at-home twin's clock so far ahead of the traveler's that it is now possible for the stay-at-home twin's clock to be ahead of the travelers when they reunite." http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/spacetime_tachyon/index.html
Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this striking example of time dilation proves that time travel (towards the future) is possible. As a gedanken experiment (if we neglect practicalities such as the technology needed for reaching velocities comparable to the velocity of light, the cost of the fuel and the capacity of the traveller to sustain high accelerations), it shows that a sentient being can jump, "within a minute" (of his experienced time) arbitrarily far in the future, say sixty million years ahead, and see, and be part of, what (will) happen then on Earth. This is a clear way of realizing that the future "already exists" (as we can experience it "in a minute")." http://www.bourbaphy.fr/damourtemps.pdf
Neil deGrasse Tyson, Death by Black Hole: And Other Cosmic Quandaries, pp. 123-124: "If everyone, everywhere and at all times, is to measure the same speed for the beam from your imaginary spacecraft, a number of things have to happen. First of all, as the speed of your spacecraft increases, the length of everything - you, your measuring devices, your spacecraft - shortens in the direction of motion, as seen by everyone else. Furthermore, your own time slows down exactly enough so that when you haul out your newly shortened yardstick, you are guaranteed to be duped into measuring the same old constant value for the speed of light. What we have here is A COSMIC CONSPIRACY OF THE HIGHEST ORDER." https://www.amazon.com/Death-Black-Hole-Cosmic-Quandaries/dp/039335038X
Joe Wolfe: "At this stage, many of my students say things like "The invariance of the speed of light among observers is impossible" or "I can't understand it". Well, it's not impossible. It's even more than possible, it is true. This is something that has been extensively measured, and many refinements to the Michelson and Morley experiment, and complementary experiments have confirmed this invariance to very great precision. As to understanding it, there isn't really much to understand. However surprising and weird it may be, it is the case. It's the law in our universe. The fact of the invariance of c doesn't take much understanding." https://newt.phys.unsw.edu.au/einsteinlight/jw/module3_weird_logic.htm
Steve Carlip: "Is c, the speed of light in vacuum, constant? At the 1983 Conference Generale des Poids et Mesures, the following SI (Systeme International) definition of the metre was adopted: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. This defines the speed of light in vacuum to be exactly 299,792,458 m/s. This provides a very short answer to the question "Is c constant": Yes, c is constant by definition! [...] Einstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: "...according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position." Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so. This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity. [...] Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies." http://www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/speed_of_light.html
Lisa Randall, Michio Kaku, Brian Cox, Neil deGrasse Tyson, Brian Greene: "Now, listen carefully. The faster you move, the heavier you get. Light travels at the same speed no matter how you look at it. No matter how I move relative to you light travels at the same speed. No matter who is doing the measurement and no matter what direction you are moving the speed of light is the same. The speed of light is the same no matter what direction or how fast... As you travel faster time slows down. Everything slows down. Everything slows down. Time slows down when you move. Time passes at a different rate. Clocks run slow. It's a monumental shift in how we see the world. It's a beautiful piece of science. It's a beautifully elegant theory. It's a beautiful piece of science. It's a beautiful piece..."