ELIF: how is time relative?
In: Physics
An interesting thought I came up with (not saying I’m the only one who thought this but I have never heard anyone else say it) like 20yrs ago…
Why does time seem to go by faster as I get older?
When you are young, let’s say 5yo, a year of your life is 1/5th of your entire life. Seems like a long time, a fifth of your life right? I’m 40ish now so 1/5th of my life would be 8yrs!
Now go back to that 1yr timeframe and apply it to my current age. 1yr is 1/40th of my life. It seems like nothing! If you apply my current aspect of a year back to that 5yo, that is roughly 1.5 months of his or hers life.
1.5mo!
Not the Einsteinian theory of relativity, OP was looking for, but a neat thought experiment on “Why time flys.”
Time is relative to the observer.
To properly explain it like you’re 5 is tricky. Best way perhaps is to look at how speed is relative. Imagine you and a friend are on a fast train. You stand in the gangway between the seats. He throws you a ball. How fast was the ball? To you it felt like a normal ball throwing speed. To an observer looking through the window standing in a field, the ball went 300 mph. That’s what makes things ‘relative’. How fast something goes is relative to where the observer is.
Time is the same. Time is just a way of measuring two events. In the same way that a moving train affects how fast a ball looks depending on whether you are inside or outside the train, it also affects how much time has passed depending on whether you are inside or outside the train.
As a real life example, GPS uses satellites to precisely locate where you are. Because the satellite is moving so fast, they actually go out of sync and so need to be adjusted as the satellite is experiencing time ever so slightly more slowly (but enough to move your GPS co-ordinate very slightly).
Many explanations here, but I am going to shoot for one that helped me understand, from the very famous ancient redditor u/robotrollcall.
Light is a confusing metaphor here. as it it doesnt really explain “time” just observation.
So in a very simplified explanation, here is how I understood it. Everything in the universe is in constant motion, at *exactly* the same speed. the speed we experience this travel, when we standing still, is called time* (see reply to this comment for more explanation), this universal speed is also the same speed as light.
And if you do start to move, or run, etc, you begin to slow your movement through time.
For example, imagine that time were a direction or axis (4th dimension,* see reply to my comment for explanation) – lets say “up”, and universal speed were 100MPH, that means, if you did nothing, you would travel 100miles vertically every hour. However, if you moved forward, you are still going 100 miles per hour, but some portion of that 100 miles you travelled would be in the forward direction, and some portion in the upward direction (time). this means, if you moved forward at the speed of 50MPH, then you also traveled upwards “only 50mph” still a total of 100miles(sort of), and so you moved through “time” at half speed.
Now imagine you were so fast, that you moved “AT THE SPEED OF LIGHT” Foward, well, then, you wouldnt be moving at all in the “up” direction, so you did not “move in the direction of time.” (which explains why light is the same speed as the universal speed limit)
So in short, if you can understand time as a dimension “exactly” like x,y,z, and that there is a constant movement through universe at the same exact speed, and you can only “bend” that constant by moving in a different direction, and every time you move in any direction, you are going to be arriving in the “time” direction slightly slower. And the faster you go, the slower you get there, and why it is “relative” because it just matters how fast you individually move in the universe.
Think of a photon bouncing between two plates. An observer (observer 1) will measure the frequency as a value based on the speed of light. This photon is traveling the same path back and forth (Only movent along a y-axis). Now imagine a second observer (observer 2). One that observes the photon, plates and observer 1 moving as a system perpendicular to the movement of the photon bouncing back and forth. To observer 2 the photon is experiencing movement that lengthens the distance traveled between the two plates (movement in x-axis AND the y-axis). Since a photon can only trave at c, the frequency at which this observer measures the photon will be different AND slower than a photon he measures moving in a simple y-axis pattern within their own reference. Both observers measurements are accurate but different depending on the reference point.
Time is simply a mental construct we project onto motion. It’s an intellectual convenience we use to make sense of reality. Without motion, time would not exist. And without matter, motion cannot exist.
So, for example, historically, people marked the passage of time by the movement of planets. People today use clocks, which are just objects which feature regularly occurring intervals, reliable and consistent enough for us to use to provide context to our lives. But we could just as easily use tides, the motion of the planets, phases of the moon. a heartbeat, weather patterns, etc. Any sort of regularly recurring event – again, time is simply our way of making sense of *motion*.
People have gotten used to accepting time as a physical, independent entity, that has its own substance, only because this has been so useful to us. But time is relative: the closer one gets to the speed of light, the more time slows down. If were you to travel at light-speed for one year, you would age one year, but centuries would have passed for everyone else no traveling at that speed.
Time is simply motion.
Time is relative because the rate at which you see time pass for someone or something depends on how fast the are moving or accelerating, the faster they are moving, the slower time appears to go.
The short version is that light (and anything else with no mass) must be traveling at the speed of light relative to everything, all the time. So when two objects with mass are moving at different speeds something has to give to make this true. Since Velocity = Distance / Time, you can fix this discrepancy by changing how fast time passes, and this happens in reality. (Distances change some too but that’s actually much harder to understand and explain imo so I won’t)
My favorite way to visualize this is picture a photon of light (which you can imagine as looking and generally behaving like a tennis ball for the purposes of this thought experiment) bouncing between two mirrors in a spaceship. To an observer on the ship, it’s moving back and forth at the speed of light, no problem.
However let’s now say this ship is moving past the Earth at 90% the speed of light. From the perspective of someone on the earth this photon is now zigzagging back and forth because the mirrors are moving past you along with the ship. You don’t really need to even be able to do the math here to know that if the photon is bouncing up and down at the speed of light AND moving sideways at the same time, it would have a total velocity higher than the speed of light from your perspective while being still at the speed of light for the person in the ship. *Since this photon is still of course moving at the speed of light from your perspective you would find if you calculated it without time dilation it would be moving slower than the speed of light from the perspective of the person on the ship.* This obviously is a problem, the speed you see and would expect the other observer to see are different from the values they would observe.
Since the ship is moving past you at 90% the speed of light though, there is a significant amount of time dilation. Anyone on this ship would seem to be moving in slow motion to you so that if you did the math you would find from their perspective the photon is in fact bouncing up at down at exactly the speed of light.
Now since motion is totally relative, this dilation occurs at everyone’s perspective, including for the person sitting on the ship looking at Earth. Now this might be a little confusing, if they are moving slowly relative to you, and you are moving slowly relative to them, then time is being lost somewhere right? Well yes but also no, part of the solution is that since time and space are very much connected just the fact that they are moving affects this problem, but explaining that concisely is above my pay grade. The bigger thing that affects this is acceleration, acceleration is not actually relative, and any acceleration something undergoes will have a large, much more one sided effect on time dilation, and as I understand it will make up for this lost time if they decelerate back to your velocity.
It’s kind of hard to put succinctly, but the basic notion is, we have an intuition that everyone can agree that an event, say, a light turning on, occurred at a particular place, and at a particular time. We also intuit that if I think two lights came on at the same time, that everyone else will also agree they came on at the same time.
But the universe doesn’t actually work that way. Everyone with the same velocity will agree on when and where something happened, and will agree on which happened simultaneously, but other observers at a different velocity won’t agree. They’ve got a different view of the universe, in which events happened at different times, in different places, and may see that things we thought were simultaneous weren’t. Both are just different descriptions of the same underlying reality, and you can convert from one to the other using a bit of maths called the Lorentz transform.
All we really mean by time being relative is I might say that events A and B were 5 seconds apart by my measurements, and your measurements say that A and B were 6 seconds apart. Both are accurate descriptions of reality, it just means that you’re going faster than me; a little bit more than 40% of the speed of light faster. At speeds much much slower than the speed of light, like the ones we live our lives at, the difference is so small we never notice.
This will be hard if you are 5 but there is a simple thought experiment, one that PBS Spacetime did on the topic. Say you are sitting on a photon clock, and your buddy is sitting on a photon clock facing you. The photon clock is a photon that bounces between two mirrors that you happen to be sitting on. They are bouncing away, as long as you and your friend are going at the same speed, these clocks will appear to be in sync. Say you shove your buddy a bit ahead if you, his clock will appear to slow down. That photon isn’t just moving up and down, it is also moving in the same direction you are going. So the closer you get to the constant C (186,282 miles a second) the longer that photon will appear to take bouncing between the mirrors. Except, if that is the clock you are sitting on time appears to be the same as it ever was. Once you hit C then the photon will not longer bounce, you have traded all the ‘time’ for speed. At C there is no time, everything is *right now*.
Interestingly, the opposite happens (all time, no speed) at the event horizon of a black hole. We have no real human experience to relate this too, which is why it is so hard for a 5 year old to understand. We are dealing with very abstract concepts that require us to take perspective of multiple different points of reference at differing velocities. That is why it took hundreds of years between Newton and Einstein, not that relativity wasn’t an unknown concept to people, but it is so mind-bending that it took another seminal genius to sort it out.
Time is essentially a way of measuring change (or ‘movement’)
If nothing ever changed you’d have no way of knowing that time was passing.
Time is relative to the frame of reference.
Bad analogy, but kind works. New year’s eve is celebrated at different times for the first calendar day. Why does this happen? We set are clocks to how the sun cross the sky. So, a person in China will celebrate the new year 13 hours before us, because there sun sets before ours.
Einstein noticed a problem in classical physics. The basic rules would no longer apply. For example, you can approximate a car speed based of yours. You know your speed is 60 MPH and the driver driving faster so you could say the other driver speed is 70 MPH.
However, if you are in a spaceship traveling at the speed of light and fire a missile, what is the speed of the missile? This is where you need to change you reference frame.
You set your frame as not moving, this allows you to calculate the speed of the missile. Your calculations show that the missile is traveling at a third the speed of light.
Meanwhile, an observer sees the spaceship fire the missile and wants to know the speed of the missile. He uses two stars as indicator for distance and calculate the missile was fired at 100MPH.
Both observer are correct for their reference frames. This why time is relative.
By the way seed is calculated by taking the distance and dividing by the time.
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