ELIF: how is time relative?
In: Physics
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 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.
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.
Basically the faster you move, the slower time moves around you. To achieve any real noticeable time dilation you need to achieve massive speeds. As an example if we take the earth moving through space as a constant. a person at the equator will be moving, due to earths rotation, at 1000 miles per hour. The ISS travels around the earth at about 17,000 mile per hour. This 17x speed increase creates a time loss of .007ths of a second every 6 months. This dilation grows exponentially until you get up to lightspeed, where time no longer moves.
A lot of the answers here are explaining why time is relative in an observation of events sense, which isn’t what the OP is going for. I assume they want to know about the theory of relativity, so I’ll give it a go:
Imagine you’re driving a car at 30mph, and another car overtakes you at 60mph. Common experience tells us that someone standing stationary at the side of the road will observe the overtaking car going at 60mph, whereas from your perspective it is going at 30mph.
Now imagine that you are driving a car at 30mph (13.4m/s) and your shine a light. From your perspective the light is travelling at the speed of light ( 299 792 458 m/s). Common experience tells us that the person at the side of the road will see that light travelling at 30 mph + the speed of light (299 792 471 m/s). But that doesn’t happen, both observers see the light travelling at the exact same speed (299 792 458 m/s). Einstein’s theory says that the speed of light can never be broken no matter what frame of reference you measure it from. That means if someone is another car travelling at half the speed of light, the light shone is still travelling at 299792458 m/s relative to them.
So how does this work? Well, speed is distance over time – so for speed to be fixed distance or time have to change within your frame of reference. Turns out both do depending on your reference frame. But for the car, time slows down relative to the guy who is stationary. The effect is very negligible until you reach speeds close to the speed of light.
It’s not just a theory, it’s been verified using atomic clocks.
As to how/why this works? I have no clue, I’m not sure anyone does.
think of spacetime as an X/Y graph
x is space, y is time.
all objects move through spacetime at C (the speed of causality) which is the maximum velocity that the effects of a cause can propagate through spacetime.
if an object is only moving through space, then its velocity through the X axis is C: this is a photon,
if an object isnt moving through space at all, then all of its velocity is through the time axis. (this is you sitting at home bored, time just flies)
but an object that is moving rapidly through the x axis necessarily has to subtract velocity from the Y axis, and the faster you move through space, the slower you move through time, because your “actual” velocity through spacetime is fixed at C.
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.
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.
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