Noob question about relativity

[ydoaPs]

It is exactly the behavior he criticized me for that he exhibited

And, you still fail to grasp the point. If you don't like people being an ass to you, then people probably don't like you being an ass to them.

 

This was an attempt to get you to see that. But perhaps you're beyond saving.

 

especially in the context of his being wrong.

No, it's not. You're still wrong. Even in the context of the twin paradox you're still wrong. We only know there is no gravitational field in the twin paradox because we stipulated that there wasn't one. Having an accelerometer doesn't make us know that one is accelerating. The readings on the acceleratometer would be perfectly consistent with an observer in a gravitational field.

 

Your claim wasn't indexed to the twin paradox, but rather was an absolute universally generalized claim. Even in the context of the twin paradox, you're still wrong. "No, acceleration (proper acceleration , more correctly said) is absolute. You can measure it with an accelerometer, so you know exactly who's accelerating and who's not.".

 

Accelerometer readings, despite the name, do not generally imply acceleration over gravity. The twin paradox is entirely consistent with one of the inertial twins passing through a gravitational field. Their trajectory curves such that they meet their twin and they 'feel the acceleration'. You're still wrong.

 

So, what we have here is:

1) You still fail to comprehend the point of the exchange.

2) You still fail to comprehend that you are in fact incorrect.

[imatfaal]

!

Moderator Note

 

Renewed discussion on members posts, style, and attitudes split off to the trash.

 

[michel123456]

That is false, clocks tick at the same rate all over the universe: 1 second per second. You simply repeat the same misconceptions over and over in a slightly different way.

 

 

 

 

Yes, all observers agree on the age of the universe. This is the second time you are being told that. You seem to have a hard time accepting it.

This statement answers a question of another thread, as if there IS indeed a universal Now across the universe.

Or do I understand very badly the consequences of "all observers agree on the age of the universe"

[MigL]

Why do you think that " all observers agree on the age of the universe" implies a universal 'now' ?

 

Take two rulers, one marked in centimeters and one marked in inches.

You can be at the '10' mark on either, but that is not the same distance, because the scales are different.

 

Clocks only tick at 1 sec per sec, in their own frames, all over the universe.

And since there is no universal frame, there is no universal now.

[md65536]

Why do you think that " all observers agree on the age of the universe" implies a universal 'now' ?

Because "agreement" implies some form of common ground in the measurement. It doesn't mean much to say that everyone agrees on the age of the universe, just at different times. Everyone on Earth agrees that the sun is overhead, right??? (According to their own clocks, at noon.) No, agreement doesn't mean "measured at different times"...

 

Take two rulers, one marked in centimeters and one marked in inches.

You can be at the '10' mark on either, but that is not the same distance, because the scales are different.

 

Clocks only tick at 1 sec per sec, in their own frames, all over the universe.

And since there is no universal frame, there is no universal now.

I don't think this is applicable because the age of the universe isn't measured using just any local clock. Any local clock can measure 10 billion years passing, but that doesn't mean every observer can say "The universe is 10 billion years old" when their local clock marks that time. This is because the age of the universe is measured according to "privileged" clocks. A clock orbiting a mass in a deep gravity well at relativistic speeds wouldn't be used to measure the age of the universe.

 

By the way I think we're mixing up some concepts from some different threads, and I'm trying to see that the different concepts aren't actually contradictory. Not sure if I understand it yet.

 

 

"No there is no universal now" means (as best as I can summarize) that there's no general concept of simultaneity in GR, and in SR there is simultaneity but it is generally not agreed upon in different frames.

 

"Everyone agrees on the age of the universe" means that the universe is homogeneous, and everywhere you can find similar observers (including only inertial observers in low-strength gravitational field) whose clocks can all be said to tick at the same rate, in all of their frames. The "age of the universe" refers to the general homogeneous areas, not the "lumps" or the individual different possible inertial frames. This does mean choosing a "privileged" frame, but it's a reasonable choice because---assuming the universe is homogeneous---it is applicable almost everywhere. Eg. Earth's own clock has been moving in various orbits and in a gravitational field, but with low speed and low gravity, and a local clock agrees pretty closely with a "universal time", at least to the precision we can estimate the age of the universe. When speaking of the age of the universe, if you get mixed up between Earth time and universal time, it's not going to make a difference. However, deep in a gravity well, or orbiting at high speed, the difference could be significant.

 

http://www.jb.man.ac.uk/~jpl/cosmo/RW.html

 

 

"All observers agree on the age of the universe" means "All observers can agree on a set of clocks that can meaningfully be called synchronized (even in GR) and which can be set so that time t=0 represents the beginning of the universe." ... But definitely not "all observers agree on the locally measured time since the beginning of time."

Edited May 24, 2015 by md65536

[Mordred]

I would say anyone measuring the age of the Universe and recognizes the influences of GR. Would seek a common reference point. Cosmology today utilizes a well known homogeneous and isotropic point. That being the CMB. However if you look at time in Cosmology. You have three versions.

 

Proper time, conformal time or cosmic time. All three forms require calculations accounting for GR influences.

 

None of the three uses strictly our clocks as per se.

 

So in regards of a common moment of now. That would depend on whether each observer uses similarly accurate measurements and calculations.

http://en.m.wikipedia.org/wiki/Conformal_time#Conformal_time_and_the_particle_horizon

 

http://en.m.wikipedia.org/wiki/Cosmic_time

 

If you look close enough at the metrics one could say we use density as our clock. (Wish I could remember the article I learned that expression from lol) however it's always stuck with me.

 

Oh if you choose the moment now (same homogeneous and isotropic density ) and extrapolate backwards this is look back time (forgot one).

Edited May 24, 2015 by Mordred

[MigL]

Consider the expanding universe, balloon analogy, md65536.

Say it starts from an extremely small size and expands outward, like an inflating balloon, with the surface of the balloon representing the present.

Now this balloon is going to be lumpy, as local space-time curvature determines the local expansion rate ( can I say passage of time ?).

 

At any point on this balloon surface an observer can measure the time to the beginning as the radial distance from the centre.

All of these radial distances are going to be different, i.e. no universal 'now', but since the local scale is also different from others, they are all going to measure 13.8 billion.

 

This is the third different attempt at trying to explain myself. The previous two iterations confused things even more so I scrapped them.

Hope this makes some sense.

Edited May 24, 2015 by MigL

[michel123456]

Consider the expanding universe, balloon analogy, md65536.

Say it starts from an extremely small size and expands outward, like an inflating balloon, with the surface of the balloon representing the present.

Now this balloon is going to be lumpy, as local space-time curvature determines the local expansion rate ( can I say passage of time ?).

 

At any point on this balloon surface an observer can measure the time to the beginning as the radial distance from the centre.

All of these radial distances are going to be different, i.e. no universal 'now', but since the local scale is also different from others, they are all going to measure 13.8 billion.

 

This is the third different attempt at trying to explain myself. The previous two iterations confused things even more so I scrapped them.

Hope this makes some sense.

I suppose that the interpretation of the balloon analogy is that the entire Universe lies on its surface.

In the interior of the balloon there "was" the Universe in the past, and outside of the balloon there "will be" the Universe in the future. IOW the only one Universe that "actually exists" is the surface of the balloon, there "is" nothing inside and there "is" nothing outside. In this sense, there "is" a universal Now and that "is" the surface of the balloon.

 

I understand that the statement "all observers agree on the age of the universe" corresponds to that point of vue.

Edited May 24, 2015 by michel123456

[md65536]

Consider the expanding universe, balloon analogy, md65536.

Say it starts from an extremely small size and expands outward, like an inflating balloon, with the surface of the balloon representing the present.

Now this balloon is going to be lumpy, as local space-time curvature determines the local expansion rate ( can I say passage of time ?).

 

At any point on this balloon surface an observer can measure the time to the beginning as the radial distance from the centre.

All of these radial distances are going to be different, i.e. no universal 'now', but since the local scale is also different from others, they are all going to measure 13.8 billion.

 

This is the third different attempt at trying to explain myself. The previous two iterations confused things even more so I scrapped them.

Hope this makes some sense.

It does, thanks, but I don't think it's right.

 

I'm getting hung up on your explanation because you used the word "present" to describe the surface of the balloon, which is everywhere. How is that different from a universal 'now'?

 

I think it's incorrect that the same 13.8 billion year age that is measured everywhere uses different distance scales in different places. I think the assumption of homogeneity on the largest scales is what allows us to speak of a universal age in the first place, and it implies that the scale everywhere (ignoring smaller scale "lumps") is the same.

 

In the balloon analogy I think it would be spherical (representing large scale homogeneity), with lumps. Everywhere on the ideal sphere, it would be measured as 13.8 billion, on the same scale. The lumps would have different scales, and could have wildly different measurements, but "no universal now" means that you couldn't have a single balloon surface where every point including the lumps are all reading 13.8B (or any consistent surface at all that includes all the lumps, I think).

 

 

I think the way out of this confusion is that the "present" balloon-surface/age-of-universe represents the large scale universe with a chosen reference frame, and the "no universal now" is talking about the lumps and different reference frames. They're talking about two different things.

 

 

 

I tried googling this and found explanations of what I'm trying to describe, but not what you're describing. Do you have a reference?

https://www.google.ca/#q=is+the+universe+the+same+age+everywhere%3F

 

From http://physics.stackexchange.com/questions/11390/is-the-entire-universe-the-same-age :

When you consider what a distant object is like now, you have to be careful to specify what reference frame you're talking about, because different frames lead to different "nows."

 

At each point in spacetime, there is a reference frame that seems like the "most natural" one to use, namely the one in which the expansion of the Universe looks roughly the same in all directions. Cosmologists tend to use that reference frame to define and synchronize their clocks. That is, the time coordinate at any point in spacetime is, by definition, the amount of time that would have elapsed according to an observer who'd been at rest in that reference frame since the Big Bang. That time coordinate is often called "cosmic time."

 

[...] I'm ignoring things that are due to small-scale inhomogeneities and thinking about large scales, on which the Universe is approximately homogeneous.

 

-- Ted Bunn

Edited May 24, 2015 by md65536

[Akolaad]

Is it possible that there is some kind of organism somewhere on some planet orbiting a gravity well and that organism is more than 13.82 Billion years old according to a local clock? Is it possible also that there could be a planet somewhere with organisms on it which, according to a local clock are only 1 billion years since the big bang? That's what I was trying to ask before. It seems to me that this could be possible. But I was hoping other people would weigh in on it.

[md65536]

Is it possible that there is some kind of organism somewhere on some planet orbiting a gravity well and that organism is more than 13.82 Billion years old according to a local clock? Is it possible also that there could be a planet somewhere with organisms on it which, according to a local clock are only 1 billion years since the big bang? That's what I was trying to ask before. It seems to me that this could be possible. But I was hoping other people would weigh in on it.

The clock in the gravity well would tick slower than clocks farther from the mass, so that wouldn't allow an organism to age faster. As for the second question, I'd say definitely yes, both SR and GR allow that. An object orbiting a black hole might find that the rest of the universe has aged 13.8 billion years while it has only aged 1 B.

 

As I understand it, the fastest a clock can possibly tick is when it is inertial and in the absence of a gravitational field. This is pretty close to describing a clock used to measure cosmic age. (Perhaps a clock in a void ticks slightly faster???) It seems it would be a problem if anything was older than the universe: http://en.wikipedia.org/wiki/Cosmic_age_problem

 

 

By the way, your questions probably only make sense in context of some choices of frame of reference, hence all the talk of cosmic age and universal 'now'. You'd complained that this isn't the best place to ask questions... perhaps so, but this isn't Yahoo Answers or a class with teachers who have answer keys, it's a forum where everyone can discuss.

[MigL]

Your last paragraph is the key md65536, the problem is the differing frames.

Yes to a third frame, the frame in a deep gravity well will experience time at a slower rate than a frame in perfectly flat space-time ( inertial and no gravity ), but they will all measure the passing of time as one sec per sec, in their own frame. In effect, the scales of time passage are different in each frame. In each frame they measure the age of the universe to be the same, 13.8 Byrs, but the seconds making up those 13.8 Byrs are not equivalent across frames.

 

Say an astronaut became trapped by the gravity of a black hole 12.8 Byrs ago. He would then only have aged 1 Byrs since then, but he will still 'see' the rest of the universe having aged 12.8 Byrs, and so will measure the time to the origin ( big bang ) as 13.8 Byrs because of the different scale.