In this thought experiment, my twin brother and I are both turning 20 at the airport.
At midnight on our birthday, we are both exactly age 20 years.
He stays put while I get on a 777 and fly around the world. The flight takes me 24 hours and so he waits 24 hours. I arrive and we are both age 20 years plus 24 hours.
If I instead get on an SR-71 and fly around the world at 3x speed of the 777, the flight takes me 8 hours so he waits 8 hours. I arrive and we are both age 20 years plus 8 hours. Clearly, we are both younger in this scenario than the first one.
If I got onto a super plane flying at 0.99x light speed and fly around the world, the flight takes me 1 second. Since I’m so fast, he should also only wait one second. Intuitively, I’m back and we’re both 20 years and 1 second old.
But my understanding of time dilation is that I’m 20 years and 1 second old when I’m back, but he would be much older since I was almost going at light speed.
Why is that? My flight and his wait time should both be much much shorter since I was flying much much faster.
In: Physics
stop thinking about it on a linear line
instead, conceptualize yourself as travelling through both space *and* time simultaneously.
we are all *always* travelling at c through *spacetime*, the faster you travel through one dimension, the slower you travel through the other. and vice versa. so if you’re sitting absolutely still, you are moving through time the fastest. if you are going very fast in space, time is slow for you.
you can visualize this relationship with a spacetime diagram, here’s a couple of videos that explain it, including a physical demonstration
the problem with your examples is that your first two are nowhere near fast enough to notice the discrepancy, and the last one is nowhere near long enough to notice the discrepancy. misleading you into falling back on your medium-sized intuition, when you’re in fact dealing with extremes.
a better real example would be actual measured differences in how quickly time passes between on-the-ground and in-orbit clocks (and people!), even though you have to go faster than a bullet to stay in orbit, the difference at the end is still *tiny*.
a better conceptualization would be if you travelled very far away, let’s say 1 light year away (arbitrary number). if you went at the speed of a commercial jet it would take you however long to get there and back, and it’s slow enough that both you and the person on the ground would subjectively experience *basically* the same amount of time elapsed. but if you were going at close to the speed of light, it would still take you something like 2 lightyears to get there and back from the ground observer’s point of view, but to you the distance is much shorter so it takes you significantly less time to make the same trip. then you end up back home much younger than your sibling.
The weirdest part is that for some reason, c is just the max speed.
“Aging” is what we call all the chemical reactions that add up and degrade our cells. Let’s picture breathing in and out for this. Like we can imagine the air as little balls you breathe in and out. Each cycle ages you by one breath.
The weird part is, you only have a maximum budget for both of these things. Imagine you’re in that space ship going .99x the speed of light and it’s a turn based game. On turns 1-99, you’re spending your motion budget being basically a beam of light. All the “balls” of air that would go in and out of your lungs to spend a breath spent their speed budget going the same direction as the ship at almost the speed of light. On turn 100 they had enough momentum to move “sideways” going in and out of your lungs to spend one breath. You don’t notice this because all the atoms in your brain that do the work of perceiving time are themselves moving in the direction of the ship for almost all of their c budget.
If you could somehow go 1.00c, all your atoms would spend all of their motion (or “turns”) moving in the direction of the ship at the speed of light. They’d have no extra budget to go 1.000000001c needed to have any vector to move “sideways” to chemically react with each other and cause what we’d call aging. So in that sense, going 1c freezes time for atoms relative to each other. You can’t die from lack of oxygen because nothing in your body can react while “frozen”. You can’t perceive that you’re frozen because all your brain neurons and chemicals are frozen. At all lesser speeds, this ratio just keeps working out from your point of view, although people outside your frame of reference see you as closer and closer to stasis the faster you get towards c.
Think of driving towards a mountain, if you change course 45 degrees you’re still getting closer to that mountain but half as fast despite moving at the same speed.
Our universal speed limit c can also be seen as our constant velocity through both time and space. So if we move through space we’re deviating on our course toward the mountain [time] but still moving at c. Likewise if we stay perfectly still in space we’re moving full speed through time.
So the observer in the rocket ship moves really fast (almost exactly equal to c) and travels 30 lightyears from their point of view as instantaneous, they experienced very little time during their journey. The observer on earth spent their budget entirely on time so when the rocket arrives 30 years have passed for them.
The problem with using increasingly fast airplane flights this way is that the “clock” of all the space and matter on the plane runs slower than the clock of everything back on the ground. Given a VERY fast and very long flight (completely hypothetical), a banana you bought at the airport would be fresh when it landed while all the other bananas from its bunch would have spoiled. The stationary bananas would age; the traveling banana would not.
The difference doesn’t really add up unless you’re traveling a LONG way/time moving at a VERY fast speed. For trips that start on Earth and go up and back down, we’re talking about billionths of a second in difference. For a near-light speed trip to the Moon and back, you would experience about 3 seconds less than your twin on Earth.
A simple way to think of it.
You’re always moving through time, t, and space, (x, y, z) at a speed of “10”. You can’t go any faster or slower than 10.
So if you’re completely stationary, you’re moving at a speed of “10”. Broken down it would look like this:
– Moving in the X direction at a speed of 0.
– Moving in the Y direction at a speed of 0.
– Moving in the Z direction at a speed of 0.
– Moving in the T direction at a speed of 10.
0+0+0+10 = 10
Lets say you start moving forward in the x direction.
– Moving in the X direction at a speed of 0.1
– Moving in the Y direction at a speed of 0
– Moving in the Z direction at a speed of 0
– Moving in the T direction at a speed of 9.9.
0.1+0+0+9.9 = 10
Still a constant speed of 10, but because you’ve started moving in the x direction, you’re speed in the T direction has decreased.
So now you can see, the fastest you move in space, the slower in time you move.
If you’re moving at a speed of 6 in the x direction, then you’re speed in the T direction must be 4 – much slower than someone who is stationary.
I’ve come back to the subject many times in my life trying to better understand it. I guess it’s just hard for me to understand how 2 seemingly identical organisms, here being twins, could age at different speeds? For some reason it’s just not clicking. I’m sure my inability to fully comprehend it all is related to how I view time.
The earth has mass and is moving, but it’s minimal enough that we can pretend these have no effect. We will pretend that the airport where you start is in an inertial reference frame. Inertial reference frames are usually easier to deal with, except when it comes to time dilation because it’s impossible to tell one inertial reference frame from the other which leads people into unintuitive (seemingly contradictory) conclusions.
Luckily, however, one of you gets in a plane which experiences significant acceleration. While accelerating (and decelerating), you are not in an inertial reference frame. This means we have a way point in the sand that makes it easier to compare. We can guarantee there is something about your experience which differs from your twin’s. Namely, time dilation.
While you are accelerating, your clock will slow down compared to anyone in an inertial frame of reference, but not to you. Your clock is your source of truth. Your body slows down, your metabolism slows down, your heart rate slows down, the clock ticking on the shelf slows down, even the very slight rusting of the fuselage between repairs slows down. From your perspective, everything is normal because it all slows down together. You can’t measure any change in clock speed.
This means, if your twin measures the plane moving for two ticks of his clock, your slower clock might actually only tick once. (The exact ratio, of course, depends on speed and duration of acceleration). So your brother aged 2 seconds during the flight, but you only aged 1 second. Your twin is now older than you by one second.
Remember the other side of time dilation is length contraction. So from your twin’s perspective, you are going a certain speed. That is a certain distance in a certain amount of time. Miles per hour means miles÷hours. From your perspective, you can measure your speed by looking out the window and seeing how much earth you fly over in how much time. Remember, it takes less time for you because your clock is slower… However, the earth will appear to shrink by the same factor. You will travel less distance in less time, but they cancel out perfectly and you measure the same speed.
I could try to explain relativistic time dilution but I actually don’t understand what you are asking.
The best explanation for this that I’ve seen for people to get it is the train scenario. Think of it like this, there is a train going bye at the speed on light and you have a you see a passenger shine a light from floor to ceiling. To the passenger they just see the light shine up and the time traveled is short. The observer on the outside though sees the light travel at an angle with a length that is longer than the line to the ceiling. It basically takes a longer amount of time to see the light than it does for the passenger and that’s why time moves faster for the passenger or the observer slower than the passenger if you prefer.
Time dilation technically is also happening in your other 2 examples. In the first example you were 20 years, 24 hours, and 0.000000000000000000000025 seconds years old, and your brother is exactly 20 years and 24 hours old.
The difference between all 3 is scale. At the speeds of a jetliner or even the SR-71 is so slow it’s basically standstill when compared to light speed. But if you’re at 99% of light speed, the effects of time dilation becomes observable.
Latest Answers