BMAONE23 wrote:This started me wondering (a danger I know), that If the farther you get from your localized source of gravity (Earth, Sun, MW Galaxy, Galactic Local Group), the lessening of the influence of gravity on time there is, then this would give rise to the appearance of things moving faster as they moved farther away from your frame of reference WRT your local gravity source.
Could this Time Dilation effect be the cause of the perceived expansion?

No. The effect of gravitational fields on time is very non-linear. The effect is only significant when the fields become very large- as when you are near a black hole's event horizon. For all practical purposes, there is no effect for ordinary fields such as we experience here, or some point in deep space experiences. All that matters is the difference in gravitational potential between two points- and that difference is small between here and the edge of the observable universe.

Is there not a physical difference, in time from the surface of earth to a satellite? How much does that physical difference change when talking about a very heavy object like a the biggest black hole? where do you draw a line from the clock ticking normal to the clock stopping? If the clock ticks normal out side the Event horizon- why does not the stopped space time clock rip a hole in the Fabric of normal ticking space time? Or is that where your new space from nothing is coming from? But as it turns out a speeding photon also has a clock running, A very well known clock. travel at this speed and the universe changes very quickly. How does this not rip a hole in the Fabric of space time? So what do we have, A trillion different Time zones? Hang on, Time is linked with space, so we have to say, A trillion different space time zones. so if time changes, space must change also. You can not go and prove time changes due to mass, then in the same sentence say but it does not change that much for a very big mass. Our Earth is tiny compared to 18 billion s/m. yet we see a difference. And have to change clocks speeds for satellites because of this. That is a physical thing.

swainy wrote:Is there not a physical difference, in time from the surface of earth to a satellite? How much does that physical difference change when talking about a very heavy object like a the biggest black hole? where do you draw a line from the clock ticking normal to the clock stopping? If the clock ticks normal out side the Event horizon- why does not the stopped space time clock rip a hole in the Fabric of normal ticking space time?

All clocks tick the same, and that is normal. All that changes is how a clock in one reference frame ticks with respect to a clock in a different reference frame. That's why relativity is called relativity. No clocks ever stop- that's a limiting case for what is probably an unphysically realizable condition.

We observe time dilation effects from different types of motion (as in your satellite example), and we observe time dilation effects from differences in gravitational potential. These observations precisely match theory. In order for time dilation to be significant, the difference in gravitational potential needs to be very large. Even for a supermassive black hole, you only need to get a few kilometers away and the field is too small to have much effect on time. In the case of satellites, we almost always ignore time dilation effects (which are caused by velocity, not gravity). The cases where it is important are when instruments are very dependent on accurate time, and errors of parts per billion or smaller still need compensation.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com

Chris Peterson wrote:All clocks tick the same, and that is normal. All that changes is how a clock in one reference frame ticks with respect to a clock in a different reference frame. That's why relativity is called relativity. No clocks ever stop- that's a limiting case for what is probably an unphysically realizable condition.

don't you mean nonphysically? But any hows, Give this guy a bell and ask him why he said: ''Time ticks at a different rate at the top of the empire state building than it does on the ground''.

Chris Peterson wrote:We observe time dilation effects from different types of motion (as in your satellite example), and we observe time dilation effects from differences in gravitational potential. These observations precisely match theory. In order for time dilation to be significant, the difference in gravitational potential needs to be very large. Even for a supermassive black hole, you only need to get a few kilometers away and the field is too small to have much effect on time. In the case of satellites, we almost always ignore time dilation effects (which are caused by velocity, not gravity). The cases where it is important are when instruments are very dependent on accurate time, and errors of parts per billion or smaller still need compensation.

swainy wrote:But any hows, Give this guy a bell and ask him why he said: ''Time ticks at a different rate at the top of the empire state building than it does on the ground''. http://en.wikipedia.org/wiki/Brian_Cox_%28physicist%29

Technically, he is wrong. He's expressing a fairly complicated bit of physics in layman's terms. What he means is that if you are at the bottom of the Empire State Building, you will observe the clock at the top keeping incorrect time. And if you are at the top, you will observe the clock at the bottom keeping incorrect time. The reason is that the two clocks are at different gravitational potentials, and GR is perfectly clear how that affects their relative behavior. Neither clock ticks faster or slower in any absolute sense... because there is no absolute reference.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com

Chris Peterson wrote:Technically, he is wrong. He's expressing a fairly complicated bit of physics in layman's terms. What he means is that if you are at the bottom of the Empire State Building, you will observe the clock at the top keeping incorrect time. And if you are at the top, you will observe the clock at the bottom keeping incorrect time.

Yes, Nobody knows what time is, how it works, how it interacts with space. Pretty much nothing is known about Time.

swainy wrote:Yes, Nobody knows what time is, how it works, how it interacts with space. Pretty much nothing is known about Time.

Not true. Time is well understood, and how space and time interact is described by GR and backed by many independent lines of observation. The fact that time and space can behave in very unintuitive ways does not mean that either are not well understood.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com

swainy wrote:Yes, Nobody knows what time is, how it works, how it interacts with space. Pretty much nothing is known about Time.

Not true. Time is well understood, and how space and time interact is described by GR and backed by many independent lines of observation. The fact that time and space can behave in very unintuitive ways does not mean that either are not well understood.

Chris, this guy is waiting for your call.

Click to play embedded YouTube video.

If we do not know what time it is, how do we know what space time expansion is?
tc

swainy, what that guy is saying is merely that we would surely like to know more about time than we do now. it's like any other question of "do we know what X is" kind, that should actually be "how much do we know about X", but 1st way makes for better tv shows.

take any other example, such as X="number", and try to think what kind of answers you could give now, 5 years ago, 10 years ago, etc (I assume you're not the guy in his 60s, who did not learn a thing about numbers for last 30 years). you should see that as you go back in time, you know less and less about "true nature of numbers", yet at any point you have perfectly well-defined concept of number that you can operate with (do simple, or not so simple calculations).

swainy wrote:Chris, this guy is waiting for your call. http://www.youtube.com/watch?v=km89X8zoJH0
If we do not know what time it is, how do we know what space time expansion is?

As I said, he's wrong. Now I don't mean that as a physicist he has any misunderstanding. What I'm saying is that in order to attempt to convey something of the mystery and wonder of the Universe to a non-scientific audience, he is expressing these ideas somewhat unscientifically. That's pretty normal and reasonable for somebody trying to popularize science, but you have to realize that a lot of his arguments are more philosophical than anything else. This Youtube video does not carry the same weight as a scientific paper!

Both space and time are well described by GR. What that means is that we have solid theory in place that lets us explain what we observe, and accurately predict what we haven't yet observed. Scientifically, that is the very definition of "well understood". It doesn't mean that we fully understand these things, but that's far different from having a poor understanding.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com

Fast forward the program to the part where he says: ''Einstein kind of knew what time it was, in 1916, for about 6 months''.
Is that not part of GR? But he still did not know what time was.

I would not say, that ''biging'' up the science mystery goes for making science easier to understand for the general public. not for me any hows. Just makes it frustrating for those of us who do partly understand, What you guys are talking about.

swainy, you should know that Chris is of the kind of guys who thinks, if we have formula that fits observational data with RMSE less than 1%, we do know everything we need about thing in question but then again, if you will find the data which makes RMSE worse, he will reconsider.

swainy wrote:Is there not a physical difference, in time from the surface of earth to a satellite? How much does that physical difference change when talking about a very heavy object like a the biggest black hole?

Black holes are a complex case because the gravitational time dilation becomes infinite at the horizon. The event horizon marks the point where the simplified idea of "time ticks at different rates at different places" falls apart utterly and you need another mental model in order to wrap your mind around the GR equations. (If you can find one that works for you. Otherwise there's only the equations themselves).

However, in more peaceful circumstances, such as within the solar system, the "different timescales at different points in space" can be a helpful approximation. (Just don't forget that it is merely an approximation). In such situations it is easy to estimate the magnitude of the effect, because the logarithm of the "speed of time" at each point turns out to be proportional to the "gravitational potential" that the Newtonian theory would give us. Remembering just that fact, a little dimensional analysis gives a ballpark estimate of the magnitude of the effect:

Say we want to know how much slower a clock on the surface of Earth goes relative to one that floats far from Earth's gravity, but the same realtive to the Sun, and not too close to other planets to feel much of their gravity:

Look up or calculate the escape velocity at the location of the first clock relative to the Earth's gravity. Let's say 11 km/s in round numbers.

Square it (because the gravitational potential is about energy, and energy goes as the square of velocity at non-relativistic velocities). This gives 121 km²/s².

Divide by c² to get a dimensionless number. Here, about one part in 8 billion.

Multiply with some constant factor on the order of unity (i.e. small integral power of 2 and/or pi) which I have not bothered to look up since we're just doing ballpark estimates.

The result is the fraction of the time recorded by one clock that will be missing from the other.

makc wrote:swainy, you should know that Chris is of the kind of guys who thinks, if we have formula that fits observational data with RMSE less than 1%, we do know everything we need about thing in question :D but then again, if you will find the data which makes RMSE worse, he will reconsider.

Or to put it in simpler terms, I respect scientific thinking <g>.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com

makc wrote:but then, the question comes, why the formula has to be that, and not something else. what would you call that question? meta-science?

It is really a philosophical question whether there is any "meaning" to a formula that describes some aspect of nature. You might easily imagine two completely different formulas which both give the same answer, matching some observation. For example, you could apply Newtonian mechanics or GR to some observations, with equally good results. So does one or the other, or neither, actually describe the black box that is nature itself? Nobody can answer that question, and I think most scientists would argue that it doesn't matter, and leave the deeper discussion to philosophers. Science is about being able to describe nature, and any formula that can do that, and accurately make predictions, is satisfactory from a purely scientific standpoint. The actual "rules" used by nature (if such things even exist) may be intrinsically unknowable, but that doesn't mean we can't completely understand the behavior of the Universe.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com

Henning Makholm wrote:Black holes are a complex case because the gravitational time dilation becomes infinite at the horizon. The event horizon marks the point where the simplified idea of "time ticks at different rates at different places" falls apart utterly and you need another mental model in order to wrap your mind around the GR equations. (If you can find one that works for you. Otherwise there's only the equations themselves).

The rest just proves the above to be theoretically correct.

Here's a good one for you. Where does space time stop and the Event horizon start? Or is this not the case? does space time fall in too?

@Chris: but it is quite easy to construct the situation where this is wrong I mean if we take two formulas f(x)=x and f(x)=sin(x) we will not see much difference as long as x we deal with are, say, in 0...1e-5 range, but once we go to somewhere near pi/2, the difference will be quite apparent. so, wouldn't it be better to know that we deal with sine in advance?

makc wrote:@Chris: but it is quite easy to construct the situation where this is wrong I mean if we take two formulas f(x)=x and f(x)=sin(x) we will not see much difference as long as x we deal with are, say, in 0...1e-5 range, but once we go to somewhere near pi/2, the difference will be quite apparent. so, wouldn't it be better to know that we deal with sine in advance?

Yeah, deal with sin in advance. all your troubles are over.

swainy wrote:Here's a good one for you. Where does space time stop and the Event horizon start? Or is this not the case? does space time fall in too?

The event horizon is a subset of spacetime. More precisely, it is the boundary between two regions of spacetime, but the boundary itself is also in spacetime.

Those two regions that the event horizon separates are purely abstractly defined. It is not as if spacetime works differently inside the horizon than outside it. Indeed, nothing special is happening at the event horizon. Spacetime there behaves exactly the same as spacetime everywhere else. Its special significance is purely a property of how it fits together with other pieces of spacetime into the global picture.

swainy wrote:Here's a good one for you. Where does space time stop and the Event horizon start? Or is this not the case? does space time fall in too?

The event horizon is a subset of spacetime. More precisely, it is the boundary between two regions of spacetime, but the boundary itself is also in spacetime.

Those two regions that the event horizon separates are purely abstractly defined. It is not as if spacetime works differently inside the horizon than outside it. Indeed, nothing special is happening at the event horizon. Spacetime there behaves exactly the same as spacetime everywhere else. Its special significance is purely a property of how it fits together with other pieces of spacetime into the global picture.

I am sorry, I do not agree with this. On the pretext, I understand, S.Hawking stated my time spent orbiting a black hole is not what an observer would see. Time runs differently. My 5 years becomes 10 years when i return home.

You must realize i can only go with the latest information updates. And what my mind can theorize. And i must take everything we know into account.

makc wrote:@Chris: but it is quite easy to construct the situation where this is wrong :? I mean if we take two formulas f(x)=x and f(x)=sin(x) we will not see much difference as long as x we deal with are, say, in 0...1e-5 range, but once we go to somewhere near pi/2, the difference will be quite apparent. so, wouldn't it be better to know that we deal with sine in advance?

Of course it is better if you know that the function is a sine. But that presumes it is possible to know this. With nature, such knowledge may be impossible. That is, the "function" that describes a particular behavior may be inherently unknowable, or indescribable. The formula we develop to describe a behavior may be an approximation, or it may be an absolutely perfect match to reality, but it still may have nothing in common with the "real" function underneath it all. And as I said before, this may be more a question of philosophy than it is of science.

Chris

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Chris L Peterson
Cloudbait Observatory http://www.cloudbait.com