What’s the real reason you can’t go faster than the speed of light?(bigthink.com)
bigthink.com
What’s the real reason you can’t go faster than the speed of light?
https://bigthink.com/hard-science/real-reason-faster-than-light-speed-spacetime/
5 comments
Nice explanation. I don't get the problem with traveling to stars though. If I could move at the speed of light through space, then I'd be not moving through time, which means the time to travel anywhere, as measured by me would be exactly zero. So I conclude I could reach the center of our galaxy. Is it correct reasoning?
However, the ones that would stay on the Earth would never see me arrive at my destination, as they would have to wait those millions of years. Is it correct?
But then another thing I don't understand in this model is the twin paradox. If I returned back to Earth what would be the Earth time and my local time? Would that agree? Or would I arrive millions years in the future?
However, the ones that would stay on the Earth would never see me arrive at my destination, as they would have to wait those millions of years. Is it correct?
But then another thing I don't understand in this model is the twin paradox. If I returned back to Earth what would be the Earth time and my local time? Would that agree? Or would I arrive millions years in the future?
You're absolutely correct. You can get anywhere you want, in whatever (nonzero) time you would like. You can reach the center of the galaxy in a millisecond, without violating relativity.
And yes, if you returned home, you'd find that tens of thousands of years had passed. (Not millions. It's not that far.)
This explanation is designed to ask you to see if from the point of view of spacetime, rather than asking what it looks like from any particular point of view. You can use the conversion factors to translate.
What those factors tell you is that if they could "see" you, they'd see you moving very, very slowly on your spaceship, because it's always moving at very near the speed of light. They'd see you aging very slowly, and arriving at the center of the galaxy (a thousand generations later) having aged barely at all. Then they'd see you coming back -- again aging very slowly -- and arriving back another thousand generations later. When you stop your ship at earth, they'd watch a clock on your ship suddenly start ticking at a normal rate.
And yes, if you returned home, you'd find that tens of thousands of years had passed. (Not millions. It's not that far.)
This explanation is designed to ask you to see if from the point of view of spacetime, rather than asking what it looks like from any particular point of view. You can use the conversion factors to translate.
What those factors tell you is that if they could "see" you, they'd see you moving very, very slowly on your spaceship, because it's always moving at very near the speed of light. They'd see you aging very slowly, and arriving at the center of the galaxy (a thousand generations later) having aged barely at all. Then they'd see you coming back -- again aging very slowly -- and arriving back another thousand generations later. When you stop your ship at earth, they'd watch a clock on your ship suddenly start ticking at a normal rate.
Thank you, this is very useful. But I have some more doubts. What is that space-time actually? Is there a global single space-time or does everyone has its own? Let's assume the starting point of my travel have coordinates (x: 0, y: 0, z: 0, t: 0). Then I go at near the speed of an light to the center of the Galaxy and come back to the same point a second later (as measured by my local clock). So I arrive at coordinates (0,0,0,1). But for the ones who stayed on Earth this point has a different coordinate (0,0,0,<very large number>). So it looks like it is not a single spacetime and this analogy kinda breaks :(
There is only one space-time, and everybody agrees on it.
Time in space-time is not measured by your clock. It's measured by all clocks that are at rest with respect to each other.
Everybody sees you arriving at that point (0,0,0,<very large number>), including yourself. That's the number on the clock at your destination when you arrived. You took a different route through space-time to arrive there, which is why your wristwatch reads 1.
(We're assuming that the earth clock and the center-of-the-galaxy clock are not moving relative to each other, which isn't quite true but close enough to avoid introducing those complications.)
Since you are the one accelerating and decelerating, you're the one whose wristwatch cannot be trusted. That's the half-assed solution to the twin paradox. A better solution to the twin paradox involves watching the "boosts" from one frame to another and doing the math to calculate what happens. But you do know that the non-moving clock at the origin is always going to the longest route in time, and anything moving relative to it will be moving more in space (and thus less in time).
Time in space-time is not measured by your clock. It's measured by all clocks that are at rest with respect to each other.
Everybody sees you arriving at that point (0,0,0,<very large number>), including yourself. That's the number on the clock at your destination when you arrived. You took a different route through space-time to arrive there, which is why your wristwatch reads 1.
(We're assuming that the earth clock and the center-of-the-galaxy clock are not moving relative to each other, which isn't quite true but close enough to avoid introducing those complications.)
Since you are the one accelerating and decelerating, you're the one whose wristwatch cannot be trusted. That's the half-assed solution to the twin paradox. A better solution to the twin paradox involves watching the "boosts" from one frame to another and doing the math to calculate what happens. But you do know that the non-moving clock at the origin is always going to the longest route in time, and anything moving relative to it will be moving more in space (and thus less in time).
There is an interesting analogy which came to my mind. Let's say we have $100 each and both decided to buy stock, only I bought e.g. Microsoft shares, and you bought Facebook. Who has more money (i.e. cash)? Hard to tell until we both sell and compare. Similar things happen with inertial frames of references. You can only compare time when you are at the same point in space and have zero relative speed (reminds me of Back To The Future moment when Doc Brown compared watches). E.g. time on the ISS is ticking slower. Really? How do we know that? We put one clock on the ISS (i.e. launch it there), wait some time and bring it back. Yes, it shows less ticks. Is it because ISS is moving? Not really, ISS is following its world line and is roughly an inertial system, so its time is proper. And proper time is always the shortest. We on Earth on the other hand are constantly crossing world lines (unless falling down with 1g of acceleration). Why is it in reverse then? Because we need to accelerate to get there and decelerate returning back. Now imagine that somebody from ISS sends one of their clocks down to the Earth then gets it back. Which clock has less ticks? You guessed it, the one that traveled to Earth and back, because it's our time that is dilated, not their.
One correction: "moving" has nothing to do with it, crossing world lines does. Space ship flying at the speed of light (well, close to it) is also following its world line, so its clock is not "worse" or "better" than the one on the Earth. They both show "proper" time (https://en.wikipedia.org/wiki/Proper_time). When the ship starts to accelerate that's when its time becomes skewed. Also times at two different points of space are not comparable as there is no way to synchronize them, it depends on observer's speed. So "all clocks that are at rest" _always_ show different time if they are not at the same spot (well, they may show "the same" time like a stopped clock shows correct time two times a day).
Thank you. That makes sense!
Suddenly is the wrong word. You will need to decelerate, and that can't be done suddenly. If you decide to fly by your clock will still be ticking at a slow rate (from the point of view of an observer on Earth of course).
If someone can figure out how to go the speed of light, then they certainly can figure out how to decelerate "suddenly." This is all just funsies anyways, right?
Think of it from the earth frame of reference. They watch you travel for a million years there and a million years back. When you arrive two million years have passed in the earth frame of reference. You were traveling at some large fraction of the speed of light so from your frame of reference you traveled for a year there and a year back.
You age 2 years
Earth ages 2 Million years
It's slightly more complicated. The main idea of special relativity theory is that you can't compare "time" at two separate points of "space". E.g. it makes absolutely no sense to compare age of a twin left on Earth with the age of a twin on a moving space ship as long as they are not at the same point in space. So if you really want to compare age you will have to turn your space ship around and return to Earth. Now, that part is covered not by special relativity, but general relativity. Here we need to consider effects of a gravitational field (any acceleration is equal to gravitation). The most interesting question in the twin paradox is what's the difference between space ship and Earth. Why it's the ship which is accelerating, but not the Earth. Why Earth is considered an inertial system (according to the Newton's first law), but a space ship making U-turn is not.
> I don't get the problem with traveling to stars though. If I could move at the speed of light through space
Well, to be a stickler for the laws of physics, you can't move through space at the speed of light. In principal, you can get moving pretty quick though, but what if there's a space pebble between you and those stars? If you're going at half the speed of light, the collision force would be in the atomic bomb range. So that's a potential problem.
Well, to be a stickler for the laws of physics, you can't move through space at the speed of light. In principal, you can get moving pretty quick though, but what if there's a space pebble between you and those stars? If you're going at half the speed of light, the collision force would be in the atomic bomb range. So that's a potential problem.
If a large pebble (or a large piece of space junk) collides with ISS at 16 km/s ISS will be destroyed. That doesn't prevent ISS for being where it is though.
The ISS routinely eats micrometeorites or other tiny debris and also maneuvers to avoid larger trackable junk on occasion. If it was travelling at half the speed of light, it would be a bit more problematic.
We don't know how much of this stuff exists in the interstellar space. It's possible that a probability to hit something there is much less than ISS hitting something big enough to destroy it, even with all radars and maneuvers. So I wouldn't consider it as an argument against interstellar travel, not yet.
I'd say it's one of many potential problems. We've already observed a couple interstellar comets, right? So we actually already know that there are objects out there. Certainly it wouldn't hurt to send some laser accelerated probes out there and see what we find, but human travel seems like pure sci fi.
[deleted]
An intriguingly clean and simple explanation - but my knowledge about the Theory of Relativity is not really up to speed to know if this is physically sound. What this article does not explain, however, is why we cannot travel back in time...
The most likely answer is that the past doesn't exist.
We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well.
If that is the case, when a moment passes it ceases to exist in the same way that a ripple in a puddle of water does. If you wanted to go backwards in time, you would have to take the whole universe with you.
We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well.
If that is the case, when a moment passes it ceases to exist in the same way that a ripple in a puddle of water does. If you wanted to go backwards in time, you would have to take the whole universe with you.
I'm not a physicist so someone please correct me if I'm wrong.
There is no 'past' to return to if you could reverse the arrow of time. I don't think the universe has frames like a video that you can rewind to. Interactions are probabilistic with time going forwards and they would still be probabilistic were time inverted.
Source: I listen to podcasts
There is no 'past' to return to if you could reverse the arrow of time. I don't think the universe has frames like a video that you can rewind to. Interactions are probabilistic with time going forwards and they would still be probabilistic were time inverted.
Source: I listen to podcasts
My admittedly ignorant take on it is that time moves in one direction, just at varying speed depending on how quickly you move. Much in the same way that distance travelled is always positive, regardless of direction. Traveling back in time would be like traveling a negative distance.
Weirdly, it seems that the equations for time work in either direction. Questions of causality aside it might be possible to get something to move backwards in time. Probably not anything living though. Or rather not anything you’d like to keep living.
You are correct. The equations do work in either direction.
Special relativity does not, in itself, forbid you going backwards in time. But it does tell you what would be involved in going backwards in time that way. They involve things like imaginary mass and imaginary energy -- things that pop out of factor sqrt(1-v^2/c^2) when v>c.
As far as we can tell, imaginary mass and imaginary energy are nonphysical. Relativity doesn't say it has to be that way, but nobody has ever seen them and they don't seem to exist.
There are hints of it in the Standard Model, which define mass and energy in terms of even powers of real numbers. Again, that doesn't prove that other things can't exist, only that we've never seen them, and would expect the universe to look very different if they did exist.
It might have something to do with the configuration of the universe at very early times, the low-entropy state that is the reason that the universe is interesting (and, in particular, capable of having us in it). But that is an open question.
Meanwhile, special relativity is sufficient to rule out time travel unless you want to make some assumptions that mass and energy are very different from anything we've ever seen them do.
Special relativity does not, in itself, forbid you going backwards in time. But it does tell you what would be involved in going backwards in time that way. They involve things like imaginary mass and imaginary energy -- things that pop out of factor sqrt(1-v^2/c^2) when v>c.
As far as we can tell, imaginary mass and imaginary energy are nonphysical. Relativity doesn't say it has to be that way, but nobody has ever seen them and they don't seem to exist.
There are hints of it in the Standard Model, which define mass and energy in terms of even powers of real numbers. Again, that doesn't prove that other things can't exist, only that we've never seen them, and would expect the universe to look very different if they did exist.
It might have something to do with the configuration of the universe at very early times, the low-entropy state that is the reason that the universe is interesting (and, in particular, capable of having us in it). But that is an open question.
Meanwhile, special relativity is sufficient to rule out time travel unless you want to make some assumptions that mass and energy are very different from anything we've ever seen them do.
People who actually understand this should definitely correct me. My understanding of the analogy is that your velocity through time would have to cross the 0 point (i.e. positive flow through time, 0, negative flow through time). However, 0 is when you're traveling at the speed of light through space and not traveling at all through time. I'm assuming there's either no energy left in the time direction to push you or something about the 4d geometry makes it impossible because a negative time direction would require somehow inverting all the space in the universe (crossing some infinite asymptote) and that's not physically possible.
One thing that helped me along is also the explanation that between two points in space, a straight line is the shortest distance. Take a different path and it will always be longer. Now, a straight line between two points in spacetime is the... longest time, which, at least to me, is counter intuitive. (note: i'm far from an expert in relativity)
> When Einstein’s theory was pushed harder, what was revealed was that every object travels through spacetime at a single speed — the speed of light.
Nope, don't push it that hard. Four-velocity is a nice mathematical abstraction, but has nothing to do with "speed" and "travels" as we know them.
Nope, don't push it that hard. Four-velocity is a nice mathematical abstraction, but has nothing to do with "speed" and "travels" as we know them.
I wonder if fortune telling is real. Not the tarot kind but using the power of the mind to travel across spacetime and seeing visions is.
Nothing in physics forbids information moving backward in time, provided it has no effect on whether the event communicated will happen after it arrives.
So, information about an earthquake or asteroid strike, or even a plane crash if you don't prevent it, is fine. You can choose not to get on the plane, based on the information. But if you did stop the plane taking off, what physical event would there be to communicate?
That is not to say information does routinely go back, or does just because somebody wants for it to. The universe appears not generally to care what people want. But if any does go back, and organic systems could pick it up, it would be adaptively advantageous to be receptive to it.
Again, just because it could happen, in principle, doesn't mean it does. It would depend on some biological structure being just incidentally sensitive to the signal, and then refined by selection. It would be a Rube Goldberg sort of process, but study of parasite lifecycles will cure you of skepticism as to nature's tolerance for dodgy complexity and marginal usefulness.
So, information about an earthquake or asteroid strike, or even a plane crash if you don't prevent it, is fine. You can choose not to get on the plane, based on the information. But if you did stop the plane taking off, what physical event would there be to communicate?
That is not to say information does routinely go back, or does just because somebody wants for it to. The universe appears not generally to care what people want. But if any does go back, and organic systems could pick it up, it would be adaptively advantageous to be receptive to it.
Again, just because it could happen, in principle, doesn't mean it does. It would depend on some biological structure being just incidentally sensitive to the signal, and then refined by selection. It would be a Rube Goldberg sort of process, but study of parasite lifecycles will cure you of skepticism as to nature's tolerance for dodgy complexity and marginal usefulness.
I'd be curious to see the equations that would describe these mechanisms
Maybe fortune tellers' insight is that it's as real as you believe it to be?
- Steven Wright