how time is not linear, please!

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how time is not linear, please!

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Anonymous 0 Comments

The standard view in physics is that time is linear and flows in one direction from past to future. This aligns with our everyday experience and intuition about time. The laws of physics like thermodynamics support the idea that time has a direction and causality flows from past events to future ones.

Anonymous 0 Comments

Disclaimer: I’m not a physicist, just a nerd.

The problem isn’t that time isn’t linear. It is, it goes in one direction instead of jumping around. The problem is that time is relative.

My favorite example: imagine I drive past you in a car at almost the speed of light. Light travels at a set rate in a vacuum (no air), called “c”. Now imagine I turn on the headlights.

What I will see is my lights illuminating the path ahead with light going out at a speed of c. But since to you I’m traveling at almost c and nothing can go faster than c, my headlights will appear to send light out at a speed of almost 0.

Now how can light stand still? It can’t. So how does the universe reconcile this? Well, whenever two forces meet in physics, one of them has to give. So in this case, since light is REALLY stubborn about its speed, the thing that gives in will be fine. So to you I will appear to be traveling at almost c but moving at 0. For me, from your perspective, time has slowed.

Now imagine I do that for a year from your perspective, then stop my car and get out. A year has passed. But inside my car, almost no time passed at all. I just time-travelled forward without aging.

And now you understand the basics of relativity! We know this is true because if we don’t account for it, GPS satellite systems so working. Also you can put a very sensitive click on a ticket and fly it around the earth fast, and when you get back you’ll see it’s out of sync with a clock that started on the ground.

Special bonus: the universe is made of length, width, depth, and time (as far as we can observe). We call this spacetime. We’ve also observed that strong gravity, like the sun, can *bend* spacetime. That’s part of why matter spirals toward a black hole instead of sucking straight in.

Anonymous 0 Comments

One of the main principles underlying the theory of relativity is that the speed of light in a vacuum is the same in every reference frame. That means no matter how fast you’re traveling, the speed of light appears to you exactly the same.

(I’d put a picture, but I can’t it seems, so bear with my explanation- hopefully it’s decent.)

Imagine you set up a clock in the form of a laser that shoots one photon at a time towards a mirror, which bounces the photon back to a receiver next to the laser. Now take a pair of these clocks for person A and B. Person A is going to stay in one place on Earth’s surface. Person B is going to hop on a train. Once they’re both at a constant velocity, they start their clocks, setting up the clocks so that the photon is shot perpendicular to the direction they’re moving (so the clocks are set up vertically, and person B is moving horizontally relative to person A).

Now, person A observes their photon bounce off the mirror and back to the receiver (aka their clock “ticks”) in some amount of time, we’ll call it *t.* They also observe Person B’s photon bounce off the mirror and go back to the receiver. The difference is that person B’s setup is also moving, so in order to hit the mirror and bounce back to the receiver the photon has to travel at an angle, hit the mirror, and bounce back at an angle to hit the receiver (like “leading a shot” on a moving target). Because it’s traveling at an angle the photon has now had to move a greater distance to hit the mirror and go back to the receiver, rather than just going straight up and down. The speed of light is constant in all reference frames, so if it has a greater distance to travel at the same speed, Person A *must* observe that person B’s clock takes longer to “tick” than Person A’s clock.

But from Person B’s perspective, they see their own clock tick in time *t,* because in their reference frame everything is stationary, and in fact Person A is the one moving (same speed, opposite direction), and theirs is the clock that takes longer to tick.

Person B and Person A both started in the same reference frame, observing that both their clocks tick in the exact same time *t*. But Person B *accelerated* relative to Person A, which gave rise to this difference. If Person B then slows down, or *accelerates* in the other direction, their clocks will then tick at the same rate again according to either person. *However*, Person A observed some number of ticks on their clock, say *Ta*, during this experiment, and Person B observed the same number of ticks on their own clock during this experiment, and Person A and Person B both agree that their clocks ticked the same number of times. But remember, Person A observed Person B’s clock ticking at a slower rate throughout the experiment, which means according to Person A, Person B ran the experiment for a longer time than Person A. Yet both *experienced* the same amount of time pass in their own reference frames.

Thus, Person B and Person A have experienced time passing at different rates relative to each other. This is how you get time dilation as seen in Interstellar, where what felt like a year for Person B, who accelerated and decelerated relative to A, felt like multiple years for Person A.
The effect of this at most speeds humans travel is so small to never be noticeable, but if you accelerate until you’re traveling close to the speed of light, it gets very noticeable.

Sorry for the long answer, but hope it’s somewhat helpful in explaining how time is not linear.

Anonymous 0 Comments

Time *is* linear – you’re always progressing from past to future. It’s just relative, meaning you observe it pass differently *for everyone else*. You and I both experience time at a rate of one second per second from our own perspective, but we both observe time ticking faster or slower for the other depending on the circumstance. Both our observations are correct.

If that sounds weird think of relativity in a more familiar context. If you’re driving down the road and look down at the cup in your cupholder then from your perspective the cup isn’t moving. But to someone standing on the side of the road as you drive by they *do* see your cup moving. Both observations are correct. This misalignment in perspective – the cup is moving and the cup isn’t moving – is what forms the basis of relativity and it affects how we observe the passage of time for others.

Anonymous 0 Comments

time moves fast and time moves slow. Like when I make you sit at the table until you eat your brussell sprouts. Time feels like it is moving slowely then. When your having fun at the park and I say we have to leave in 15 minutes. It feels like 15 minutes is 1 minute so it feels like time is moving fast.

Anonymous 0 Comments

That’s just it. Time **is** linear. But like others have pointed out, time is relative.

If you imagine time like a flowing river. The river will forever flow in one direction; but you’ll have spots with nothing in its way and the river flows faster, and there’ll be spots with things in its way (rocks, boulders, fallen trees, branches, etc) that will slow it down.

Things with mass create distortions in the fabric of space (however big or small). Planets, moons, stars, black holes, even you and me create those distortions; and those distortions slow down the flow of time. The more mass the greater the distortion, and the slower the flow of time is.

If you were very close to a high-mass object, let’s say a black hole, a far away observer would see you moving very slowly; but for you the observer would look like were moving very fast

Anonymous 0 Comments

Time is linear, but it is also *relative*.

Click your fingers on one hand.

Now click your fingers on the other hand.

Those two events have a certain “time-like separation” between them. There is a specific length of time between them happening.

But that time depends on perspective (similar to how how if you look at something far away it may appear really small, but if you get closer it looks bigger). Those two events might take place 3 seconds apart from your point of view, but from another (perfectly valid) point of view they might be 2 seconds apart, or 4 seconds apart. And this is a real, physical effect, not an illusion difference in how things appear.

The time between events is *relative* – it depends on who you ask, but in a predictable, understandable way (if you know what perspective someone is looking at it from you can work out what they would see – like with looking at buildings far away).

In certain cases we can even mess with the order events happen in. Two events might happen in one order from one perspective but the opposite order from a different perspective (although there are limits on this – the events have to be “space-like separated”, meaning that there is no perspective where they happen in the same space; you can’t get anything from one to the other).

The two ways we get different perspectives are by moving at different speeds (if someone is moving faster than you their clocks will run slow from your perspective), and by being deeper in gravitational wells (if someone is closer to a massive object than you their clock will run slow from your perspective).

This doesn’t just happen with time, but also with space (and both together, not just one or the other). Things moving faster than you are squished in the direction of travel (you can fit a 5m ladder inside a 3m shed if it is moving fast enough – >4/5c), and space is squished together deeper in gravitational wells.

Anonymous 0 Comments

Everyone here seems to be proving that time moves forward and it’s relative. They keep claiming their proving that’s it’s **linear** but I haven’t see anyone accomplish that. Linear means that it moves at a constant rate. Time could move at a non constant rate and still only move forward and be relative. Can anyone prove that time is linear? Your job is to prove that the rate at which time passes never changes.

Anonymous 0 Comments

For a full answer, with so little maths that a five year old truly could have a go at understanding it, read The Order of Time by physicist Carlo Rovelli. It’s magical, yet completely scientifically sound, stuff. And a captivating read.

Anonymous 0 Comments

First we have to start with something called a “reference frame.” It refers to a perspective with a certain velocity and acceleration (an inertial frame is one with zero acceleration). Within your own reference frame your velocity is zero. You don’t experience yourself moving, you experience everything around you moving.

There is no special reference frame, all of them are equally valid, so we can’t say that one is “correct” and the others are modifications.

Within each reference frame, time moves at a constant rate, ie it is linear. Because none of these frames are more valid than another, we consider them all to be correct, the only objective truth is something all frames can agree on (this is known as an invariant, the starting invariant being all reference frames agree that light moves at the same speed). Because they all agree time is linear, even though they all experience a different line, we consider time to be linear.

Another example is the charge invariant, all frames agree on the charge of a proton. However, charge density is not an invariant, because frames can’t agree on the volume of a proton. This is what causes magnetism, different observers see a different charge density based on their velocity, and so experience a different electric force. That discrepancy manifests as the magnetic force in classical physics.

You as an observer can experience “nonlinear time” by changing your reference frame, either velocity or acceleration. But at any given instant you are experiencing a linear time, just jumping to different lines.

This last part is a bit beyond ELI5. Mathematically you can plot the “world line” of a frame in 4 dimensional spacetime. When you change frames and experience time dilation, it can be represented by a rotation of the coordinate system. When a line is rotated it remains a line.

Fun fact: different parts of your body experience time at different rates. Because your head and feet experience different amounts of acceleration due to gravity, and move at a different velocity around the earth’s axis, they are in different reference frames.