4 ms·

Q1. Does the future already exist? I realize this might be purely philosophical, but if I leave Earth, and fly through space near c kph, and return home a few h

• the_gipsy 5y ago
  Q1b. Are you the same person you were yesterday?
  • subroutine 5y ago
    I think yes, and the answer is more concrete for "oneself", since the molecules that make-up oneself are all accelerating and decelerating with you/as you in your own reference frame.
    • mewse 5y ago
      I mean, apart from cells and etc dying and being replaced as we age. ;) Truly, the Ship of Theseus was inside us all along. (Which is super convenient, considering how often we step into the same river twice!)
• ww520 5y ago
  Regarding Q2, spacetime curvature is the grid itself. Think of it as "space" in the simple term where objects including black hole travel through. Black hole's gravity warps the surrounding spacetime. Think of it as the "space" is stretched out by the black hole's gravity. Regarding time, there's no universal global coordinate for time in General Relativity. Time is based on each observer's time coordinate. When time stops inside a black hole, it means time appears to stop in an outside observer's time coordinate. This is because the spacetime curvature inside a black hole has been stretched so much that the stretched curvature approaching infinite long. See the pulled down funnel [1]. An object traveling in no faster than light speed in the infinitely long curvature takes forever and its time appears frozen to an outside observer. But it's an illusion to the outside observer. The observer is seeing the light imprint emitted from the object, not the real object itself. See the diagram below. BH << o. EH. . . . .......... >> P BH is the black hole singularity and o is the falling object toward BH. EH is the event horizon. P is the observer. The dots are the light photons emitted from the object along the way. The photons are traveling away from o and BH toward P. When the photons reach P, P can measure o's movement and time. Imagine the object o has a blinking light beacon at its tail, blinking every second by its clock, i.e. on for half of a second and off for half of a second. You can see the light every second and measure the object's time. Since the speed of the light is constant, the photon traveling away from BH will take longer and longer to move across the stretched out space, as shown by the spaced out dots between EH and P. As o approaching EH, P will see the blinking light slows down because the photons take longer to move across the stretched space. P would conclude that o's time is slowing down. Passed EH, the photon is not coming out because the space is stretched longer than it can cover in its constant speed. At EH, the photon can still come out but at a very slow pace because the space is stretched matching the speed of light. The beacon is not blinking because the light stream been stretched to infinitely long. Object o appears frozen as its time has stopped by P's measurement of the blinking rate. But what P sees is the long stream of photons stretched out when o approaching EH. The object o has long gone in its forever falling across the infinitely stretched spacetime. Only its light imprint before EH is being observed as frozen. [1] https://www.sciencenews.org/wp-content/uploads/2017/05/051217_EC_blackhole-curved-universe_main.jpg https://www.sciencenews.org/wp-content/uploads/2017/05/05121...
• awb 5y ago
  Q1. > basically make people whatever age I want by varying my speed. No more so than making the Earth change shape by moving your physical location. > Since the same 1 hour passes for me no matter the age for them I chose, it seems like I'm selecting an already existent future from a stack, not fast-forwarding the present I left. Yeah these are more philosophical questions rather than a testable hypothesis. It’s an interesting way of viewing the world, but if you’re talking about a future “existing” scientifically and materially in concert with the present, then you’d need to devise an experiment to be able to enter or interact with that future beyond just waiting for that future to exist. Otherwise time travel isn’t even necessary for this thought experiment. You could say the morning already exists and fall asleep and wake up and boom it’s morning time. But in reality all of the ticks of time happened between when you fell asleep and when you woke up, you just weren’t able to observe all of them at the same speed as someone who stayed awake all night.
  • caf 5y ago
    You can even make this clear in the "leave Earth in a fast ship and return" scenario - have the people on Earth send you a birthday card by radio every year that you're away, on your birthday in their reference frame. You'll receive those ticks of time as you travel (most of them on your way back).
• RogerL 5y ago
  I suggest stop giving time a privileged meaning, and think of it as just one of four dimensions. John Wheeler starts his book on Special Relativity[1] by talking about surveying. You and I survey a building (plot of land, whatever) separately because we want to check each other's work. At then end we look at our data and disagree on every single coordinate except the origin. Ah, you're a crappy surveyor, I conclude. No, you are, you reply. Then for whatever reason I ask what distance you compute for the distance from the origin to the corner of the desk. 3.183 meters. Huh, exactly same number I compute. Okay, what about the distance from the far corner of the room to the southmost window? 18.45 meters. Me too! A bit more chatting and it turns out you used true North as the y-positive direction, and I used magnetic North. Opps. Our frames are just rotated in respect to each other. Now, is it freaky and weird that I say the speaker is 2.78 meters in the y direction, and you say it is 2.619? No, we are just using different frames of reference. There is no 'reality' to any given y direction or coordinate. It is arbitrarily chosen, as our the units (I could use yards instead of meters, and have an entirely different number yet). OTOH, what is real, and invariant, are distances. That's a physically real thing. hence we always comput the same distance between any two points, despite using different coordinates for our (x,y) tuple. If you want to be mathy about it we say the metric is s^2 = x^2 + y^2. Pythagoras, in other words, in a Euclidean space. Well, we don't live in space, we live in spacetime, where time is a dimension. When we move at different speeds relative to each other our 4-D coordinate systems are rotated relative to each other. That includes time. So, if you rotate yours relative to mine, travel for awhile (time and space!), well, you will end up with different coordinates for x, y, z, and t. It's no odder than if you and I travel 'North' in your car, but you use true North and I use magnetic we end up in different places on the globe. In 4D space what is 'real' is not coordinates or time, but events, and what is constant is the interval between events. Just like what is 'real' in 2D Euclid space is not some arbitrary y-direction, but the distance between two objects. Distance is invariant in 2D space, event intervals (space and time) are invariant in 4D Minkowski space (the space we live in absent of gravity). There's a bit of handwaving in there, but that's pretty much the physics; any 7th grader can do it. The main part that will lead to bad conclusions is that the metric in Minkowski space uses a negative number for time; so s^2 = x^2 - c^2 t^2. That's hyperbolic, so if you use intuition from Euclidean space you may conclude that in some instance distance will contract when it expands, or vice versa. So, finally, to your Q1, if I travel magnetic North, is the position I reach on the "same" Earth as the one where you use true North? Feels like a weird question that misses the point, right? Same Earth, just a different location than you expected because my frame was rotated wrt yours. Note that every experiment we have ever carried out bares this out. Accelerate a clock, bring it to Earth, that clock is younger (I'm ignoring general relativity's effects here, but the experiments don't). Measure how long a very short living particles live that are created by other particles crashing into our atmosphere, and they live exactly as much longer as SR would predict. Etc. did that clock "select" a different version of you? No, it just travelled a different path in 4D spacetime than you, and hence ended up at different coordinates. To go deeper into that I'd have to introduce "proper time", but since the ending x,y,z are the same are your x,y,z, can you see that intuitively it must be the t coordinate that changed? [1] https://www.eftaylor.com/spacetimephysics/ https://www.eftaylor.com/spacetimephysics/ This book is released under CC, free to download and share, and utterly fantastic. All you need is junior high math to master the material.