Author's description of twin paradox is incorrect. In fact, the paradox is not described at all. The paradox is that since motion is relative then from both twin's perspective the other twin goes on a journey and ages slowly. So why it is that on returning, only the traveling twin has aged slowly? The answer is that both twins indeed see each other age slowly but for the traveling twin to come back they have to slow down to zero and reverse direction. At that moment the frame is no longer inertial. While turning around, the traveling twin will see the stationary twin age very quickly (enough to catch up with their earth age), so when they meet there's no paradox. For each of them the other has aged as per their observations.
The twin paradox holds in a Pacman universe where there is no change in direction.
The main issue with the twin paradox is that it demonstrates where our euclidian intuition fails us.
There are several interpretations on how to resolve it, but they are all just flawed lenses, intended to help you along a curriculum until you understand the math, and also understand the limitations of intuition.
The 'turn around' explanation is part of that and unfortunately often sold as the ultimate resolution.
This is an extremely detailed explanation of the twin paradox that covers things from multiple angles, even situations that involve no acceleration: https://www.youtube.com/watch?v=vv5d5KHKDVE
The whole site seems very much like it is AI generated. In particular the css is very close to what I've seen ChatGPT generate when I used it to create similar things.
[When you move through space quickly, you move through time slowly]
Why all the “you”? This makes it sound like you would personally experience slowness. Could be clarified to say it’s only according to the perception of others.
What you will experience is the apparent size of the Universe itself changing in proportion to the time dilation along your direction of acceleration. It is necessarily an observable phenomenon due to the constancy of the speed of light.
I was wondering about this too—it's super interesting! Did you create this? Could you add graphs showing acceleration and deceleration? Also, this might be a dumb question, but how does mass factor into the energy calculations? I would love to see graphs that include the multiple stages of travel (acceleration and deceleration) as well as the mass of various kinds of fuel required for different propulsion systems such as chemical rocket, nucular etc.
Relativistic mass increases are an observed effect. So if you're on the relativistic starship (as these are usually called) your mass does not change as your relative velocity does.
But for an at rest observer, your ship's mass would approach infinity as its speed approaches the speed of light. This is the reason the ship would never be observed as hitting the speed of light.
In practical terms this is also why particle accelerators can't just infinitely accelerate the particles - their apparent mass exponentially increases and so too does the amount of energy required to continue to accelerate them.
I really really wish they would call it "apparent mass". It is entirely an observer effect. Its entirely "that ship throws propellant out the back but they don't end up going as fast as Newton thinks it should, so it must have more mass than we think"
If there are many objects moving in many directions, you won't be able to find a reference frame in which the mass of the system is equal to the sum of the rest masses of the objects.
An intelligent alien race, living there, will be able to catch you, don't you watch sci-fi movies? The real question is from what you can get 4.13e+19 Jouls, required to reach Alpha Centauri in about 9 years of traveler's time.
Interesting, although it seems unreasonable that you would be able to constantly accelerate for large distances without an essentially unlimited energy source.
Also, this is using special relativity only. Adding general relativity as you approach massive objects would be interesting.
Tangential, but I want to share the thought experiment that made time dilation click for me.
We know everything (every effect, etc) has a speed-of-light limit.
Imagine a metronome ticking out time. It ticks back and forth.
Put the metronome on a space ship. Now slowly increase the velocity of the space ship. As the space ship speed increases, the "pendulum" weight now has more and more velocity (the space ships velocity plus the back-and-forth velocity). The sum of those velocities cannot exceed the speed of light, so as the spaceship velocity increases, the metronome will tick more slowly (||x+y||<c and x-->c, right?), until, asymptotically, the metronome cannot move along its pendulum swing b/c the spaceship is moving at c.
(The metronome is a proxy for every chemical and physical process going on with you / your spaceship - they electro-chem-quantum-etc tick out their normal evolution, which must cease at c)
It's a clockwork view of the universe that might not be strictly true, but it settles some cognitive dissonance so I'm clinging to it like a life raft.
Maybe it's completely wrong, but I always thought about it as a dimension, and if you view time as the 4th dimension.
For example imagine just 2 dimensions. If you can only travel at the speed of light you could go at the speed of light in the x-axis, or at the speed of light in the y-axis. But if you want to go diagonally in both dimensions, you have to split your travel speed up between both such that when added together they don't exceed light speed.
If we view time as a dimension, then either you can stay at rest and travel through time at "full speed", or you can travel in the x, y, or z, axis at up to light speed, but in order to do so, you need to "give up" some of your traveling through time dimension. Such that your x+y+z+time "speed" do not exceed the speed of light.
I'm not one to nitpick tools to grok things but I think this could confuse more than help!
Because everything is relative to something else - and your example of the pendulum on a ship is suggestive of a "real" velocity, which does not exist.
I think a far easier scario to imagine is some ship flying away from Earth, and this ship has a magic button to release an impulse enough to give it a 10% of the speed of light boost in speed.
So what happens the tenth, or hundredth time that button is pushed? For those on the ship they would begin to observe (out the window) length contraction and of course time dilation - if they could somehow see Earth, everything would be in fast forward.
And vice versa, what happens on Earth? The ships observed speed would asymptotically approach the speed of light, but never reach it with its "apparent mass" approaching infinity, and thus the amount of velocity boost from each impulse approaching 0.
If our ship travels 20,000 light years in 40 years (from the perspective of those on the ship) then that would take a "real" 20,000 years from the perspective of those on Earth, who for many centuries would be able to track it moving away. If they somehow had a magic eye to look in the ship, things would seem to be going in extremely slow motion.
It's this nature of velocity (and dilation/etc) always and only being relative to something else that's really at the guts of all of this.
"If our ship travels 20,000 light years in 40 years (from the perspective of those on the ship) then that would take a "real" 20,000 years from the perspective of those on Earth"
This part just breaks my brain. I've been reading about this stuff for decades, and it just does not compute. It's also a little funny because there's basically zero incentive for the folks on earth to send astronauts away at near-light speed, as they'd never realize the benefits of the mission.
Yip the reality that the speed of light is fixed, but the 'speed' of time is variable is just so intuitively absurd.
This is the reason that I find things like the Fermi Paradox no more than mildly interesting.
It implicitly assumes we have a sound understanding of the fundamentals of the universe yet each revolutionary discovery we make soundly refutes that assumption.
And with the vast number of 'known unknowns' and an unknowable number of unknown unknowns, it seems ridiculous to imagine we're anywhere near the end of revolutionary discoveries.
This is not correct, I urge you to read a bit about Lorentz invariance, once you understand you will see why your statement does not make sense given special relativity is accurate.
Lorentz invariance means the laws of physics remain the same in all inertial reference frames. Also a spaceship going 99.9999…% the speed of light.
This leads to effects like time dilation, length contraction and the speed of light itself.
The metronome can keep going at any speed independent on the speed of its own reference frame.
Yes, and for that metronome to continue "ticking" according to the normal laws of physics the distance must shrink to accommodate the limits on speed - Length contraction.
But, all the observable effects are the same under my "life raft" mental model: To an outside observer their time has stopped, to an inside observer it has not (I'm just thinking slowly). And the laws of physics are unchanged - as a function of time e.g., the number of clockwork steps per metronome tick.
I realized that my thought experiment is not special or unique. Here's the "light clock" version
interesting thought experiment... am i correct in understanding that for this to apply, the pendulums plane of movement must not be perpendicular to that of the ship itself? additionally, when the pendulum swing's direction is in the opposite direction of the ship, it would still move even in the case where the ship is moving at c, correct?
That's the way I think of it, yes, but I think it generalizes - during the backward swing it would "go", but when swinging toward the front of the ship it would "stop" - forever.
You cannot imagine what will be in the spaceship at the speed exactly c, the "asymptotic" thinking doesn't work here, just like some number sequences or functions don't have limits at some points.
At the speed of 0.9999c the metronome will be ticking exactly the same for an internal observer. An observer from the Earth will notice time dilation, so that ticking cycle will be slower than "normal". If the speed of spaceship remains constant, then the times of back and forth cycles will appear the same from Earth. Simply because same time intervals are delayed by the same amount in this case.
I'm assuming that ben_w is referring to the blue shift in the CMB that you will see in front of you. And pair production is the process of turning sufficiently high energy light into matter.
But I've never given thought to the fact that for some observers the microwave background are low energy photons, and for others, they could be high enough in energy to produce electron-positron pairs. Seems like another one of those relativity related paradoxes. Only fast-moving observers (relative to the CMB) are seeing constant pair production from the CMB photons?
> Seems like another one of those relativity related paradoxes. Only fast-moving observers (relative to the CMB) are seeing constant pair production from the CMB photons?
Both observers agree about pair production.
Pair production obeys conservation of momentum and energy — although the equations allow both to be conserved with two photons interacting, the practical effect here is CMB photons interacting with the spaceship hull: the observer in the CMB-rest-frame will see very low energy photons doing this with a high-velocity hull, and the spaceship frame will see very high energy photons doing this with a stationary (to them) hull.
That said, I'm told that accelerating observers can, during the acceleration, disagree about what particles exist around them: https://en.wikipedia.org/wiki/Unruh_effect
(Hmm, and now I'm wondering if the Unruh effect is really as described — if different quantum fields contribute at different rates, or have cut-offs at different scales, that would automatically also create something that, if you squint, will look a bit like an approximation of MOND… sadly, this is beyond my competence at this time).
Author's description of twin paradox is incorrect. In fact, the paradox is not described at all. The paradox is that since motion is relative then from both twin's perspective the other twin goes on a journey and ages slowly. So why it is that on returning, only the traveling twin has aged slowly? The answer is that both twins indeed see each other age slowly but for the traveling twin to come back they have to slow down to zero and reverse direction. At that moment the frame is no longer inertial. While turning around, the traveling twin will see the stationary twin age very quickly (enough to catch up with their earth age), so when they meet there's no paradox. For each of them the other has aged as per their observations.
The twin paradox holds in a Pacman universe where there is no change in direction.
The main issue with the twin paradox is that it demonstrates where our euclidian intuition fails us.
There are several interpretations on how to resolve it, but they are all just flawed lenses, intended to help you along a curriculum until you understand the math, and also understand the limitations of intuition.
The 'turn around' explanation is part of that and unfortunately often sold as the ultimate resolution.
This is an extremely detailed explanation of the twin paradox that covers things from multiple angles, even situations that involve no acceleration: https://www.youtube.com/watch?v=vv5d5KHKDVE
The whole site seems very much like it is AI generated. In particular the css is very close to what I've seen ChatGPT generate when I used it to create similar things.
The whole website and the way FAQ is worded give AI generated vibes
The wording is misleading:
[When you move through space quickly, you move through time slowly]
Why all the “you”? This makes it sound like you would personally experience slowness. Could be clarified to say it’s only according to the perception of others.
What you will experience is the apparent size of the Universe itself changing in proportion to the time dilation along your direction of acceleration. It is necessarily an observable phenomenon due to the constancy of the speed of light.
How do you stop so you don't go whizzing past Alpha Centauri at near light speed? :)
I was wondering about this too—it's super interesting! Did you create this? Could you add graphs showing acceleration and deceleration? Also, this might be a dumb question, but how does mass factor into the energy calculations? I would love to see graphs that include the multiple stages of travel (acceleration and deceleration) as well as the mass of various kinds of fuel required for different propulsion systems such as chemical rocket, nucular etc.
Relativistic mass increases are an observed effect. So if you're on the relativistic starship (as these are usually called) your mass does not change as your relative velocity does.
But for an at rest observer, your ship's mass would approach infinity as its speed approaches the speed of light. This is the reason the ship would never be observed as hitting the speed of light.
In practical terms this is also why particle accelerators can't just infinitely accelerate the particles - their apparent mass exponentially increases and so too does the amount of energy required to continue to accelerate them.
I really really wish they would call it "apparent mass". It is entirely an observer effect. Its entirely "that ship throws propellant out the back but they don't end up going as fast as Newton thinks it should, so it must have more mass than we think"
If there are many objects moving in many directions, you won't be able to find a reference frame in which the mass of the system is equal to the sum of the rest masses of the objects.
if you can accelerate at 1g you can also decelerate at 1g
An intelligent alien race, living there, will be able to catch you, don't you watch sci-fi movies? The real question is from what you can get 4.13e+19 Jouls, required to reach Alpha Centauri in about 9 years of traveler's time.
If you use fission, Uranium 235 has 83.14 TJ/kg. So you would need about 500 metric tons of it.
>An intelligent alien race, living there, will be able to catch you, don't you watch sci-fi movies?
The aliens on alpha Centauri are 12 ft tall blue hunter-gatherer humanoids. Not much of a help for your parking problems.
yeah, I'd love to see an option to include a deceleration step and what the time debt looks like from there.
Interesting, although it seems unreasonable that you would be able to constantly accelerate for large distances without an essentially unlimited energy source.
Also, this is using special relativity only. Adding general relativity as you approach massive objects would be interesting.
Tangential, but I want to share the thought experiment that made time dilation click for me.
We know everything (every effect, etc) has a speed-of-light limit.
Imagine a metronome ticking out time. It ticks back and forth.
Put the metronome on a space ship. Now slowly increase the velocity of the space ship. As the space ship speed increases, the "pendulum" weight now has more and more velocity (the space ships velocity plus the back-and-forth velocity). The sum of those velocities cannot exceed the speed of light, so as the spaceship velocity increases, the metronome will tick more slowly (||x+y||<c and x-->c, right?), until, asymptotically, the metronome cannot move along its pendulum swing b/c the spaceship is moving at c.
(The metronome is a proxy for every chemical and physical process going on with you / your spaceship - they electro-chem-quantum-etc tick out their normal evolution, which must cease at c)
It's a clockwork view of the universe that might not be strictly true, but it settles some cognitive dissonance so I'm clinging to it like a life raft.
Maybe it's completely wrong, but I always thought about it as a dimension, and if you view time as the 4th dimension.
For example imagine just 2 dimensions. If you can only travel at the speed of light you could go at the speed of light in the x-axis, or at the speed of light in the y-axis. But if you want to go diagonally in both dimensions, you have to split your travel speed up between both such that when added together they don't exceed light speed.
If we view time as a dimension, then either you can stay at rest and travel through time at "full speed", or you can travel in the x, y, or z, axis at up to light speed, but in order to do so, you need to "give up" some of your traveling through time dimension. Such that your x+y+z+time "speed" do not exceed the speed of light.
I'm not one to nitpick tools to grok things but I think this could confuse more than help!
Because everything is relative to something else - and your example of the pendulum on a ship is suggestive of a "real" velocity, which does not exist.
I think a far easier scario to imagine is some ship flying away from Earth, and this ship has a magic button to release an impulse enough to give it a 10% of the speed of light boost in speed.
So what happens the tenth, or hundredth time that button is pushed? For those on the ship they would begin to observe (out the window) length contraction and of course time dilation - if they could somehow see Earth, everything would be in fast forward.
And vice versa, what happens on Earth? The ships observed speed would asymptotically approach the speed of light, but never reach it with its "apparent mass" approaching infinity, and thus the amount of velocity boost from each impulse approaching 0.
If our ship travels 20,000 light years in 40 years (from the perspective of those on the ship) then that would take a "real" 20,000 years from the perspective of those on Earth, who for many centuries would be able to track it moving away. If they somehow had a magic eye to look in the ship, things would seem to be going in extremely slow motion.
It's this nature of velocity (and dilation/etc) always and only being relative to something else that's really at the guts of all of this.
"If our ship travels 20,000 light years in 40 years (from the perspective of those on the ship) then that would take a "real" 20,000 years from the perspective of those on Earth"
This part just breaks my brain. I've been reading about this stuff for decades, and it just does not compute. It's also a little funny because there's basically zero incentive for the folks on earth to send astronauts away at near-light speed, as they'd never realize the benefits of the mission.
Yip the reality that the speed of light is fixed, but the 'speed' of time is variable is just so intuitively absurd.
This is the reason that I find things like the Fermi Paradox no more than mildly interesting.
It implicitly assumes we have a sound understanding of the fundamentals of the universe yet each revolutionary discovery we make soundly refutes that assumption.
And with the vast number of 'known unknowns' and an unknowable number of unknown unknowns, it seems ridiculous to imagine we're anywhere near the end of revolutionary discoveries.
fair enough! Perhaps I arrived at this mental model due to an incorrect understanding of this result:
https://www.emc2-explained.info/The-Light-Clock/
This is not correct, I urge you to read a bit about Lorentz invariance, once you understand you will see why your statement does not make sense given special relativity is accurate.
Lorentz invariance means the laws of physics remain the same in all inertial reference frames. Also a spaceship going 99.9999…% the speed of light.
This leads to effects like time dilation, length contraction and the speed of light itself.
The metronome can keep going at any speed independent on the speed of its own reference frame.
Yes, and for that metronome to continue "ticking" according to the normal laws of physics the distance must shrink to accommodate the limits on speed - Length contraction.
But, all the observable effects are the same under my "life raft" mental model: To an outside observer their time has stopped, to an inside observer it has not (I'm just thinking slowly). And the laws of physics are unchanged - as a function of time e.g., the number of clockwork steps per metronome tick.
I realized that my thought experiment is not special or unique. Here's the "light clock" version
https://www.emc2-explained.info/The-Light-Clock/
interesting thought experiment... am i correct in understanding that for this to apply, the pendulums plane of movement must not be perpendicular to that of the ship itself? additionally, when the pendulum swing's direction is in the opposite direction of the ship, it would still move even in the case where the ship is moving at c, correct?
That's the way I think of it, yes, but I think it generalizes - during the backward swing it would "go", but when swinging toward the front of the ship it would "stop" - forever.
You cannot imagine what will be in the spaceship at the speed exactly c, the "asymptotic" thinking doesn't work here, just like some number sequences or functions don't have limits at some points.
At the speed of 0.9999c the metronome will be ticking exactly the same for an internal observer. An observer from the Earth will notice time dilation, so that ticking cycle will be slower than "normal". If the speed of spaceship remains constant, then the times of back and forth cycles will appear the same from Earth. Simply because same time intervals are delayed by the same amount in this case.
Fascinating that if you could accelerate at 1.5g, it would only take you 18 years to reach the edge of the visible universe
And with that kind of gamma factor, the CMB will start evaporating you via positron-electron pair production.
I didn’t understand this. What is it about the gamma factor that will cause the cosmic microwave background to evaporate you?
I'm assuming that ben_w is referring to the blue shift in the CMB that you will see in front of you. And pair production is the process of turning sufficiently high energy light into matter.
https://en.wikipedia.org/wiki/Pair_production
https://www.imperial.ac.uk/news/149087/scientists-discover-t...
But I've never given thought to the fact that for some observers the microwave background are low energy photons, and for others, they could be high enough in energy to produce electron-positron pairs. Seems like another one of those relativity related paradoxes. Only fast-moving observers (relative to the CMB) are seeing constant pair production from the CMB photons?
Correct.
> Seems like another one of those relativity related paradoxes. Only fast-moving observers (relative to the CMB) are seeing constant pair production from the CMB photons?
Both observers agree about pair production.
Pair production obeys conservation of momentum and energy — although the equations allow both to be conserved with two photons interacting, the practical effect here is CMB photons interacting with the spaceship hull: the observer in the CMB-rest-frame will see very low energy photons doing this with a high-velocity hull, and the spaceship frame will see very high energy photons doing this with a stationary (to them) hull.
That said, I'm told that accelerating observers can, during the acceleration, disagree about what particles exist around them: https://en.wikipedia.org/wiki/Unruh_effect
(Hmm, and now I'm wondering if the Unruh effect is really as described — if different quantum fields contribute at different rates, or have cut-offs at different scales, that would automatically also create something that, if you squint, will look a bit like an approximation of MOND… sadly, this is beyond my competence at this time).
In the reference frame of the Earth a starship would be seen ramming in to stationary dust and long wavelength light.