Time is not an absolute?

[Dory]

So, I'm viewing all sorts of physics lecture and someone was trying to explain how time is not an absolute, that being the theory of relativity by Einstein. That the speed of light is constant regardless of the frame of reference. Fine. Okay. Speed of light is max speed. I get it. How does it affect time? When can we say that time is going faster in one reference frame, but not in the other? Is time dependent on light? Does that mean in colder regions time is slower? Is there some sort of "light source" to space where time goes faster there, but slower elsewhere? What am I missing here?

I can only theorize that they mean that light is dependent on time based on where you are, but this is kinda obvious just by the understanding that light has a speed....

I can't wrap my head around the concept of time not being an absolute. Maybe you fella rational-thinkers can help me see the light. (Pun surely intended)

[Gawdzilla Sama]

Ever sit through a sermon? Felt like 20 years instead one hour? Time slowed down for you. (Now replace "subjective" with "relativistic" and you're as close as you'll get without exotic maffs.)

[Mr P]

Ah this one. I've had this discussion a few times already so I'll just blow my own trumpet and refer you to a few previous posts of mine
Here.
Here.
Here.
Here.
And here.

My apologies for the copypasta but I've not had chance to condense them into a single coherent post... and Brian Cox and Jeff Forshaw have already done an infinitely better job than I ever could in their book Why E=mc2.

[GreyICE]

So, I'm viewing all sorts of physics lecture and someone was trying to explain how time is not an absolute, that being the theory of relativity by Einstein. That the speed of light is constant regardless of the frame of reference. Fine. Okay. Speed of light is max speed. I get it. How does it affect time? When can we say that time is going faster in one reference frame, but not in the other? Is time dependent on light? Does that mean in colder regions time is slower? Is there some sort of "light source" to space where time goes faster there, but slower elsewhere? What am I missing here?

I can only theorize that they mean that light is dependent on time based on where you are, but this is kinda obvious just by the understanding that light has a speed....

I can't wrap my head around the concept of time not being an absolute. Maybe you fella rational-thinkers can help me see the light. (Pun surely intended)

It's a construct of each observer being able to say they are not moving, and each observer measuring the same speed of light.

Imagine a ship traveling at 0.9c (90% lightspeed) relative to an observer on earth, who is watching with his telescope.


(E) - Earth (velocity 0)
------------------------------------> >[LLLL]-> (0.9c)
-------------------------------------------> Light (1.0c)

He starts his stop watch, and at the same time fires a very powerful laser. Light moves 299,792,458 m/s, (around 300k kilometers per second). He measures for 10 seconds

Basic equations:
Velocity*Time = Distance
Laser: 300k km/s * 10 s = 3 million kilometers
Ship: 0.9 * 300k km/s * 10 s = 2.7 million kilometers.

Difference: 0.3 million kilometers

Guy on ship sees:
(E) <-------------------------------- (0.9c backwards)
--------------------------------------------------> Laser Light (1.0c forwards)

He starts his stop watch. He measures 10 seconds.
Laser: 300k km/s * 10 s = 3 million kilometers

So in 10 seconds, the laser has moved 3 million kilometers past his stationary ship.

Now do the math to reconcile the two. From Earth, in 10 seconds the laser traveled 0.3 million km past the ship, on the ship in 10 seconds, the light moved 3 million km past the ship.

This raises some issues if time and distance are constants. It's much less plausible than Newton's explanation (both observers agree that the light moves 0.3 million km past the ship, the ship measures the velocity of the light beam as 10% of the velocity measured on earth). But it works, and Newton doesn't.

[devogue]

Ever sit through a sermon? Felt like 20 years instead one hour? Time slowed down for you. (Now replace "subjective" with "relativistic" and you're as close as you'll get without exotic maffs.)

That's philosophy.

[Dory]

It's a construct of each observer being able to say they are not moving, and each observer measuring the same speed of light.

If I float in space with no forces being enacted on me. Let's say I'm at rest, not moving down, up, left or right, but unmoving in one spot in space. Doesn't that make it the best, ideal, absolute 0 reference point?

[GreyICE]

It's a construct of each observer being able to say they are not moving, and each observer measuring the same speed of light.

If I float in space with no forces being enacted on me. Let's say I'm at rest, not moving down, up, left or right, but unmoving in one spot in space. Doesn't that make it the best, ideal, absolute 0 reference point?

Nope, there's no ideal reference points.

Are you not moving relative to the earth? But the earth is rotating around the sun, and is thus in motion (and you even have the right to say so on earth). Are you not moving relative to the sun? But the sun is in a large rotating Galaxy, and is constantly moving about center. Are you not moving relative to the center of the Milky Way Galaxy? But the Milky Way Galaxy is moving relative to all the other galaxies.

Einstein formulated a very simple solution, and it works - you have the right to define yourself as unmoving if you're not accelerating. This is not a 'belief,' it is how the universe acts - motion is only relevant in terms of other objects.

And, no matter what, you measure the same speed of light. Everywhere. If you go sailing by me in a space ship while I am floating in space, and you think I'm traveling at 0.9c, and I think you are traveling at 0.9c, we are both right and we both receive the same answers to the constant - the speed of light. Same speed of light. Everywhere.

This is why it's the called the theory of relativity. Before Einstein, people wanted things to be constant. Distance, time, mass, etc. They're not, it's lightspeed by which all of those are measured. It is not that the speed of light is some constant, like Avagadro's number. It is that the speed of light defines distance, mass, and time. The units of distance, time, and mass are 'c' and the units we use in every day life are derived units from that base quantity.

[Dory]

It's a construct of each observer being able to say they are not moving, and each observer measuring the same speed of light.

If I float in space with no forces being enacted on me. Let's say I'm at rest, not moving down, up, left or right, but unmoving in one spot in space. Doesn't that make it the best, ideal, absolute 0 reference point?

Nope, there's no ideal reference points.

Are you not moving relative to the earth? But the earth is rotating around the sun, and is thus in motion (and you even have the right to say so on earth). Are you not moving relative to the sun? But the sun is in a large rotating Galaxy, and is constantly moving about center. Are you not moving relative to the center of the Milky Way Galaxy? But the Milky Way Galaxy is moving relative to all the other galaxies.

Fine, but what if you get out of galaxies, to the emptiness of space, in between galaxies, where you're just floating still there. Isn't that the ideal absolute 0 reference point?

[GreyICE]

Fine, but what if you get out of galaxies, to the emptiness of space, in between galaxies, where you're just floating still there. Isn't that the ideal absolute 0 reference point?

How do you know you are still? How do you know that you are not moving 1 million miles per hour? What tells you that you have no velocity?

Nothing. You have no ability to measure your own velocity. There are no ideal reference points, no fixed observers. It's a construct to make life easier for high school teachers. There's quite a few of these that have slid into popular culture (electrons making these nice orbits around a nucleus, for instance). But it's just a thing to make life easy for kids when there's a problem in the book.

[mistermack]

What we experience as time, is actually the rate at which physical things happen.
The swing of a pendulum, or the decay of an atom.
All the energy that makes up matter is moving internally at the speed of light. It can't go any faster. So if the object moves, and the internal processes didn't slow down, they would be moving faster than the speed of light, which can't happen.
So phyical processes (ie time) slow, when an object moves.

Imagine a clock made from two paddles, and a ping pong ball in space.
The ping pong ball is moving at the speed of light, and it takes one second to cross the gap. If the whole thing moves, the ping pong ball has to go further, diagonally. As the speed stays the same, the speed of light, the trip now takes longer. So your clock has slowed down.
If the speed increases to the speed of light, the ping pong ball can never reach the other paddle, and so your clock has stopped altogether.
In other words, your "second" measured by the ball got longer and longer, and reached infinity when the speed reached the speed of light.
( that can't be done of course, but it shows why time should slow ).

I have no problem with time slowing, it's perfectly logical.
Relativity having no favoured frame of reference, I still can't accept.
But it actually doesn't matter, because nothing would be any different if there was a favoured, or "real" frame of reference. As we are made of what we are measuring, it's impossible to ever see an absolute anything. So relativity is what we observe, it's what works.

[GreyICE]

Everything would change if there's a real frame of reference! The speed of light would have to be measured next to the universal frame of reference, not your personal frame of reference!

No time dilation. No distance decreasing. No mass increase. Gravity and acceleration would be distinguishable from eachother.

A universal frame of reference destroys the theory of relativity entire. It doesn't exist. There are only different personal frames. All personal frames agree when they are in the same frame, all may disagree when they are in different frames, but are reconciled when they are brought into the same frame. For instance, in the spaceship example, lets say the spaceship turns around, heads back (/catches up) to earth, and lands. Both earth and the spaceship agree on what they'll find out, even though they disagree on the starting state of the frames. A universal frame of reference lets them agree on the starting state.

[Dory]

Fine, but what if you get out of galaxies, to the emptiness of space, in between galaxies, where you're just floating still there. Isn't that the ideal absolute 0 reference point?

How do you know you are still? How do you know that you are not moving 1 million miles per hour? What tells you that you have no velocity?

Nothing. You have no ability to measure your own velocity. There are no ideal reference points, no fixed observers. It's a construct to make life easier for high school teachers. There's quite a few of these that have slid into popular culture (electrons making these nice orbits around a nucleus, for instance). But it's just a thing to make life easy for kids when there's a problem in the book.

Makes sense. I'm with you.

What we experience as time, is actually the rate at which physical things happen.
The swing of a pendulum, or the decay of an atom.
All the energy that makes up matter is moving internally at the speed of light. It can't go any faster. So if the object moves, and the internal processes didn't slow down, they would be moving faster than the speed of light, which can't happen.
So phyical processes (ie time) slow, when an object moves.
would be any different if there was a favoured, or "real" frame of reference. As we are made of what we are measuring, it's impossible to ever see an absolute anything. So relativity is what we observe, it's what works.

(couldn't follow the clock example, but I got the first one)

Are you saying that for faster things, time moves slower? Time is kinda like the equilibrilizer of the speed of light. The speed of light can't go over, but time will slow down to balance it? Amazing, then! I'm shocked.

Let's say, for the sake of argument, a planet is rotating close to the speed of light (yes, yes, I know, heavy objects don't make it close to the speed of light, but bear with me)... and let's say there's oxygen and food there (see where I'm going with it eh?), let's say we can live there without getting a headache from the super spinning rate. Could it possibly mean, that, in theory, we would be aging more slowly since time is slower?

[Xamonas Chegwé]

It's a construct of each observer being able to say they are not moving, and each observer measuring the same speed of light.

If I float in space with no forces being enacted on me. Let's say I'm at rest, not moving down, up, left or right, but unmoving in one spot in space. Doesn't that make it the best, ideal, absolute 0 reference point?

Nope, there's no ideal reference points.

Are you not moving relative to the earth? But the earth is rotating around the sun, and is thus in motion (and you even have the right to say so on earth). Are you not moving relative to the sun? But the sun is in a large rotating Galaxy, and is constantly moving about center. Are you not moving relative to the center of the Milky Way Galaxy? But the Milky Way Galaxy is moving relative to all the other galaxies.

Fine, but what if you get out of galaxies, to the emptiness of space, in between galaxies, where you're just floating still there. Isn't that the ideal absolute 0 reference point?

No. As you are sitting there, at your point in space, every galaxy, star, planet and clump of intergalactic dust is moving relative to you. BUT, it is equally valid to say that you are moving relative to each of them. If a lump of cosmic debris collides with you at 1000 km/s, it is equally valid to say that it is stationary and you slammed into it at 1000 km/s as the other way around. There are no fixed points in the universe.

From your point of view, the light from the sun takes 8m 19s to travel the 1.496×108km from the sun to your eye. From the lights POV, that journey is instantaneous and the distance travelled is 0!

[GreyICE]

Dory, yes, time slows down (and distance decreases, V = D/T (and light speed is constant) relative to the speed you're traveling.

But this wouldn't make you age slower. You'd still age one year for every year that you lived. It's just that if someone leaves your frame of reference and come back, they might end up with a different calender than you - different day, different year, different clock.

To examine your super-spinny planet, lets give it a second frame of reference, another planet that is spinning at a normal pace that is nearby. Yes, the people on super-spin would age slower than the people on slow spin.

However, both of them would agree that super-spin was undergoing acceleration, so they'd both agree on what the time dilation should be.

P.S. Light doesn't get to have a point of view, for the same reason that it doesn't get to have a mass.

[mistermack]

OK, forget pingpong balls. Imagine you are on a football field with two friends.
Your friends are ten metres apart, and you are running as fast as you can back and forth. The three of you constitute a clock, and each time you touch one of your friends, that's a unit of time, a tick.
Now your friends start to walk along, in parallel directions.
You can't run any faster, but you have to cover more ground, so it takes longer for you to cross between the two. Your "clock" has slowed.
If your friends speed up, and reach your top speed, you can never cross between them, so your clock has stopped.
So the faster your clock moves, the slower it ticks.