Why can't time be constant for everything in the universe?

[arknd]

I am not well versed in physics, which would probably explain why such a stupid question is being asked here, but I just can't seem to wrap my head around time being slower around certain masses, and at velocities relative to observers.

 

If one person is close to a black hole, why is that 1 second slower than one on earth? Logically, it doesn't seem like it should move slower. I guess if you view space and time as a fabric, and gravity bends that fabric, then time is adjusted due to gravity, but why does space and time have to be a fabric, theororetically?

 

Or if a person is traveling at the speed of light away from an observer, if they are both holding a stopwatch, a second should pass by between the two equally. Yet I hear people talking about how if a person traveled fast enough away from earth, then came back, they would have aged much less than people on earth even though the same amount of time should have passed between everything theuniverse.

[elfmotat]

We know that the rate of time is affected by gravitation and motion from experiment. Indeed every time you use a GPS system you are directly confirming the predictions of relativity, because without taking into account the affects of time dilation our GPS systems would be drastically off.

 

You should stop talking about what should happen, or what seems logical to you. Nature doesn't care about your naive biases.

[swansont]

In situations where you can ignore gravity, it boils down to the speed of light being the same for any observer. Because of that, length and time can no longer be absolutes. Once you realize that length is no longer absolute, there are further implications for geometry not being absolute, and so when you add gravity you get this result.

 

To get into more detail requires knowing some of the physics background.

[arknd]

When in your physics life did you get the background to learn about adding gravity to the concept? Would calculus level physics be required? Because there is no calculus level phsyics where I go yet, and I really enjoy learning about things like this.

[swansont]

AFAIK General relativity is, at best, an elective in most undergraduate programs. It's typically a graduate level course.

[howlingmadpanda]

Keep in mind the way we measure time is kept by spacial movements such as gravity and rotation, so an 'hour' here will be different than an 'hour' on say Mars.

Also note that we can demonstrate with the GPS systems that timing has much to do with relativity and space time dilation as the formula to calculate space time dilation is needed to properly use and align GPS systems and keep them in sync with communications and electronics on Earth.

[elfmotat]

When in your physics life did you get the background to learn about adding gravity to the concept? Would calculus level physics be required? Because there is no calculus level phsyics where I go yet, and I really enjoy learning about things like this.

 

You need to learn the basics of vector calculus and linear algebra before you start tackling subjects like GR. SR can be done by (mostly) algebra alone, so if you want to learn about basic time dilation due to relative velocity then it shouldn't be too difficult.

Edited February 5, 2013 by elfmotat

[maskman`]

to arkn

well it all boils down to a very interesting fact that the speed of light is constant no matter what and that the gravitational force doesnt slow the speed of light but in fact it bends it. they found this out both mathematically and experimentally.

imagine two cars moving towards each other. now they would reach each other faster because the relative velocities add up. however imagine a person light sec away from light that is about 3 hundred million metres. since the speed of light is constant light will reach him in one sec. however if the person moves towards the light the relative velocity does not add up because of the speed of light is constant. however the distance to be covered by the light has decreased. S=C/t the relative velocity is constant. and the distance has decreased. to maintain a balance it would take more time for light to reach the user. now it would only make sense that the time has has slowed down for the user.

for your question on how things slow down in black holes we need to understand two things.

1) that acceleration in the space is simliar to that of the gravitational field within a planet.

that means that when your in a rocket accelerating in space or a rocket in the earth gravitational field at rest. you would not know the difference.

2) understand how a person standing at a top of a accelerating rocket feels different time to a person stand at the bottom.

at rest

lets say that a person on the rocket at the top is at a certain distance from a person at the bottom. say 1 light sec is this distance. the people have 2 stop watches. a person 1 gives a signal of light to the bottom. person 2 recieves it after one sec. the time interval is one sec. person 1 sends the second signal at time=2. person 2 recieves signal at time=3. conclusion the person receives the signal at equal intervals at which it is send.

at constant velocity

person 1 sends signal. person 2 receives signal earlier than 1 second say at t= 1.98 because of constant velocity. person 1 sends signal at time =2. person 2 receives the signal earlier than 1 second by the same amount say at 2.98. time interval in which it is send ie sec is equal to the time interval the other person receives it .

however in a accelerating rocket .

person 1 sends the signal. person 2 reclieves the signal ealier than 1 sec say t=1.98. person 1 sends the signal at t=2. person 2 receives the signal earlier than 1 sec earlier but even quicker at the velocity is more for an accelerating rocket. say 2.97. the time interval in which the signal is the send is larger than the time interval in which the signal is recieved. that is trying to say that you age more quickly at the top of the rocket than at the bottom.

 

well this is pretty significant in itself but it is even more significant to know, that this works in terms of the earths gravitational field as well, according to the two points discussed above. a person at the top of the mountain ages more faster than the person near the sea level. this is also proven by experiment. this is because the earths gravitational field plays the same effect as a accelerating rocket.

regards hope this helps.

source : stephen hawking brief history of time

a level physics student

Edited February 9, 2013 by maskman`

[swansont]

!

Moderator Note

 

HeadAsplode's response has been split http://www.scienceforums.net/topic/72903-misinformed-about-modern-physics/

 

 

[Przemyslaw.Gruchala]

We know that the rate of time is affected by gravitation and motion from experiment. Indeed every time you use a GPS system you are directly confirming the predictions of relativity, because without taking into account the affects of time dilation our GPS systems would be drastically off.

 

Calculate how drastically.

 

GPS satellites orbit is 26560 km,

Earth radius is 6370 km,

so satellite is at height 20190 km.

with speed 3.9 km/s

[swansont]

Calculate how drastically.

 

GPS satellites orbit is 26560 km,

Earth radius is 6370 km,

so satellite is at height 20190 km.

with speed 3.9 km/s

 

There are well-documented effects. The gravitational effect causes the satellite clocks to speed up about 45 microseconds/day, while the kinematic effect slows them by about 7 microseconds/day, so they run about 38 microseconds fast. The clocks are adjusted to compensate for this.

[JrPahoodneyMan]

Time is probably an effect of gravity. The further you get from the Earth, the slower time is.

[swansont]

Time is probably an effect of gravity. The further you get from the Earth, the slower time is.

 

The opposite has been observed. Time slows when you are deeper in a gravitational potential.

[Markus Hanke]

Why can't time be constant for everything in the universe?

 

Because, in the general case, you cannot cover the entirety of a Lorentzian 4-manifold with a single coordinate chart. What that basically means is that both space and time are local phenomena only; observers at different points on the space-time manifold may thus not agree on clock readings and ruler measurements. In fact, even the very notion of comparing such readings in different frames of reference is at best pretty problematic since there is no universal coordinate chart.

[Dekan]

I've studied the above posts, as best I can. Yet they don't dispel this feeling:

 

That right now - at this exact moment - an event is happening on Earth, and an event is happening in the

M.31 Andromeda Galaxy, and the two events are simultaneous.

 

 

[Markus Hanke]

I've studied the above posts, as best I can. Yet they don't dispel this feeling: That right now - at this exact moment - an event is happening on Earth, and an event is happening in the M.31 Andromeda Galaxy, and the two events are simultaneous.

 

And how do you decide that they are simultaneous, given the finite speed of light and non-uniform and non-stationary curvature of space-time between here and there ?

[PureGenius]

Two simultaneous points in time separated by vast space is not a possible variable for one would have to know the exact time differential between galaxys as in relative speeds and rotational speeds affecting the immidiate momententum of that specific event ie time this is beyond current sciences. Also the moment we experience now from say 100 light years away is already in the fast distant past . As in viewed from 10000 years in the future if this helps dispel some of your confusion about the nature of time and why its difficult to understand .

Edited April 29, 2013 by PureGenius

[Markus Hanke]

Two simultaneous points in time separated by vast space is not a possible variable for one would have to know the exact time differential between galaxys as in relative speeds and rotational speeds affecting the immidiate momententum of that specific event ie time this is beyond current sciences. Also the moment we experience now from say 100 light years away is already in the fast distant past . As in viewed from 10000 years in the future if this helps dispel some of your confusion about the nature of time and why its difficult to understand .

 

Exactly, that is part of a wider problem when talking about simultaneity of two spacially separated events on a general Lorentzian 4-manifold. There just isn't any "universal" frame of reference against which we can decide simultaneity, and attempting to do so in the local coordinate charts will lead to problems.

[krash661]

I sometimes have thoughts that time has it's own speed/velocity and possibly faster than light.

 

but it's just a thought.

[robomont]

i may be off target here but basically gravity is not even through outer space so time cannot be either.as mass is clumped up into stars planets and gasses.so time is distorted.the way the pros put it.if you surround yourself with a mountain of gold then you should outlive your class mates.

[swansont]

i may be off target here but basically gravity is not even through outer space so time cannot be either.as mass is clumped up into stars planets and gasses.so time is distorted.the way the pros put it.if you surround yourself with a mountain of gold then you should outlive your class mates.

 

You probably can't surround yourself with enough mass to make a noticeable difference on Earth

[robomont]

center of the sun with some kind of radiation shielding?

anybody want to run the numbers?how much mass or kinetic energy would it take?

say to double life span?200+-years old.

[Markus Hanke]

center of the sun with some kind of radiation shielding?

anybody want to run the numbers?how much mass or kinetic energy would it take?

say to double life span?200+-years old.

 

You see, here's the problem - if you consider a spherical shell of mass surrounding a ( spherical ) cavity, then Birkhoff's theorem implies that the geometry of space-time in the interior of the shell is actually Minkowskian. What that means is that, if you sit inside a hollow space surrounded by a shell of mass, you experience no gravitational time dilation at all as compared to an observer at infinity.

[swansont]

You see, here's the problem - if you consider a spherical shell of mass surrounding a ( spherical ) cavity, then Birkhoff's theorem implies that the geometry of space-time in the interior of the shell is actually Minkowskian. What that means is that, if you sit inside a hollow space surrounded by a shell of mass, you experience no gravitational time dilation at all as compared to an observer at infinity.

 

There would be no gravity inside the shell, but the gravitational potential is the important term, and that's not zero.

[Markus Hanke]

There would be no gravity inside the shell, but the gravitational potential is the important term, and that's not zero.

 

So far as gravitational time dilation is concerned, the important term is the metric tensor. In the case of the cavity within the shell, we simply have the Minkowski tensor [math]\eta_{\mu \nu}[/math], which represents a completely flat region of space-time.

Btw, the Newtonian potential of the gravitational field in the cavity is constant at all points, and equal to the potential at the surface of the shell. This represents an extremum of the potential energy function; the numerical value - and thus whether this is a minimum or a maximum - is simply a matter of convention. The important point is that a stationary observer inside the shell will, as compared to a stationary observer outside the shell, age faster, due to the fact that his region of space-time possesses no curvature. Observers outside the shell are in a region of curved space-time, and thus age more slowly due to gravitational time dilation.

Edited May 25, 2013 by Markus Hanke