NowThatWeKnow

Even though spacetime is expanding' date=' can we consider it at rest with the universe? [/quote']

 

from what perspective

 

I see the expansion of space and moving through space as different. If you are at rest with the CMB, you are at rest with space. The CMB temperature will be the same in all directions. Space is still expanding though.

 

 

Maybe time moving forward is the expansion we observe?

 

 

 

so are you saying that time and time alone has a physical influence on physical objects and therefore expansion ?

how so ?

 

I start with maybe and end with a question mark. Is it possible that time is the so called fabric of space and causing the expansion? I realize this is NOT mainstream but food for thought.

Thief here...

Then location of observation changes the time value.

No. You can not see in real time. Consider the speed of light and when it will reach your eyes.

...I need to live 1000years to make the observation...

It depends where you are observing from. From point A it would take over 2000 years and from point B it would happen quick.

...What you are imagining and what we are discussing is, I think, a useful thought experiment...

Yes, an abstract thought experiment at best, at least at this time. Who knows though, we may someday have a gravity aware CMB speedodometer connected to an atomic clock. Until then, the fluctuations in speed and gravity because of movement would be difficult to deal with. Today we adjust for SR and GR on our GPS satellites. Tomorrow, absolute universal time.

Martin, A special thanks for your answers in this thread. I went out of my way to spread a few over due thanks to well deserving folks but it is still a no go on the thanks button.

So, even if I was going the speed of light, light would still be going faster than me? because it would be relative to me? Then shouldn't someone on the ground see the light going a speed of 2c ?

"Someone on the ground" would see your headlight at c and you right behind it at almost c. Because of time dilation, you would see your headlight moving away from you at c. Since your second is very long relative to Earth time, the separation speed that seems slow to Earth is fast to you.

If you could generate 1 G of continuous thrust in a rocket your would never quit accelerating but you would also never reach c. You would however go 100,000 light years in just 12 years on your clock. Over 100,000 years will pass on Earth though.

I asked a similar question about supernovae a couple of months ago but it concerned smaller debris, not planets. It seems that I was told that the inertial frames would slow down over distance. Because of space dust and stuff at rest with the solar system and local gravitational fields also at rest.

Where you been Baby Astronaut?

If you accelerate and speed away from someone OR you are in an area of stronger or weaker gravity, you will be in a different time frame and experience time passing at a different rate. Time passage will however seem normal to both parties.

I'm just so confused by this concept of relativity. I watched people on TV like Michio Kaku talk about theoretical time travel well beyond our technology, but the possibility alone is what's strange. How could we even consider time travel if there weren't different points in time for us to travel to? What's more, how can there only be one present time, when we can't even establish an objective present?...

There is only one present time in the universe. No one is living in the past and the future simultaneously just because time may pass at different rates in different frames. No one is living in 2007 and 2010 while we are experiencing our "now".

It is quite possible to travel into the future using speed or gravity. You slow your time down and then return to Earth's frame at the desired year. You can not go back in time since you can not go slower the 0, only forward. At this point we have only accomplished traveling seconds into the future, not years

I'm looking for an explanation of what four dimentional space-time means in terms of understanding the nature of time....

I have been asking about spacetime in another thread so I am also looking for answers. At this point I understand that space and time was connected by man because the math worked out and it was easier then dealing with an extra dimension of time. Most do not think time is the fabric of space but that space is just a metric. However, it seems the jury is still out as far as some type of ether.

In some ways it seems that the speed of light and time are similar because they are both constant, but only relative to the frame they are in. You have "t", proper time, that you experience in your frame. Then you have "T", the time in a different frame then yours. Some will say you have universal time, a clock at rest with the universe CMB and (I think) in the absence of gravity.

Any intelligent woman would surely have been thrilled to have him! Unfortunately he caught typhoid fever and died young.

Interesting read. I never knew that much about Friedmann. His busy life and war must have kept him from marriage as 37 years is enough time to be married several times.

What they do is correct the data so that it is as if it were measured by an observer at rest. This is not hard to do because the solarsystem speed is small... If I climb out of my gravity valley and walk over and descend down into yours then if it's at the same level then the clocks at each of our houses are the same.

Since it is unlikely that any two locations would be at rest with the CMB and they would be in their own gravity frames, adjustments would be necessary to build a clock to show exact universal time. In most cases these adjustments would be very small unless you were at relativistic speeds or near a black hole.

If two different locations both had their universal clocks set up with adjustments compensating for there local speed and gravity, would you consider these two clocks synchronized? Allowing them to share the same "now" and simultaneous events, even if they were not aware it because of delays in communication?

Edit - Would adjustments in universal time because of gravity be made considering the absence of gravity or mean gravity? It sounds like you were using "way out in space" gravity where it is minimal.

Wouldn't the same be true of the expansion of space?

Wait one second. We need to back up. You said

1 new second = 1/2 old seconds...(and)...If you have two years where you had one before

No wonder my head hurt. You have time getting shorter in one place and longer in the other place.

If "1 new second = 1/2 old seconds" you have time dilation and dilation is the act of expanding. I believe time dilation allows you to travel further in less time. You also have length contraction so you see space as contracted and I guess it really is relative to your frame.

I forgot what I was trying to figure out so I will go read Martin's post.

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Well that is a respectable question, let's see if I can give it an adequate answer. It may take several days and several installments and/or help from others like Swansont etc, but we'll see...

 

...Did you see the photograph of Friedman they have in Wikipedia? Does that look like a guy who once held the high altitude ascent world record? Or more like an extreme egg-headed schoolteacher. My hero!...

Friedman looks like an "egg-headed schoolteacher". I wonder if he got married?

This post was great for explaining things in a way I can understand. I will put together a summary of what I have learned so far and you can critique it. It is getting late and there are obligations in the morning but I should have some time later in the day tomorrow. I feel good about what I am learning and we can fire up the calculators later. Thanks

Edit - I got the same message as you about spreading thanks around.

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Universal time in the Friedman model

I understand a universal time (and "now") can be compared and equal at any location in the universe as long as you are at rest with the CMB at that location. The best way for you to determine if you are at rest is a consistent temperature of the CMB in all directions. Because of the motion of the Milky Way we are not at rest with the CMB. Our technology does not allow us to become at rest but our knowledge does allow us to mathmatically adjust for it. It sounds to me like the bottom line is being in the same (at rest) frame so universal time will work.

First question:

Since the Milky Way is not at rest and we would have to mathmatically adjust for it, wouldn't that be coming up with a universal time or "now" while frame mixing? Would it be a problem that 1 second in each frame would not have the same duration? Since the differences are small in most cases should we just say close enough knowing there is nothing we can do about it?

I know that our universe is only somewhat uniform and can not be considered a true homogeneous and isotropic environment. Your words were "Fred is an idealization (perfect uniform matter distribn)". Would uniformly spaced galaxies of the same size and at rest meet that criteria? It would seem that for true uniformity you would have to break down all matter to it's smallest component and distribute it evenly accross the universe (no galaxies, no stars)

Second question

Similar to the first but using gravity. The variable gravity, depending on your location, would make for a variable frame. In most cases the time differences would be small so should we ignore them and again say close enough? Realizing that it would take a true homogeneous and isotropic universe for a true universal time and "now" we just have to accept things for what they are.

... From my understanding there are magnetic fields for just about anything with mass? And normally the more mass, the larger the field and strength of it? ...

This is not my area of expertise but I thought the proper material was essential for a magnetic field. I thought that the strength of Earths magnetic field was influenced by it's active core more then it's size. Other expert thoughts will be interesting.

Edit - Are you thinking about a gravitational field rather then magnetic field?

Space and time are in a way related via the speed of light. If time were to contract (or is that expand? eg if 1 new second = 1/2 old seconds) with space left unchanged, then when we use light along with a clock to measure distance, than distances would be greater. I guess this makes more sense when you measure stuff in light-years. If you have two years where you had one before, then you have two light-years where you had one before -- space expanding.

"If 1 new second = 1/2 old seconds" you would have time segments getting shorter over time, so time is contracting (or is it expanding because it is getting faster?). Therefore it would take longer to cover the same distance making a constant space seem larger. So, the $64,000 question is, 1. Is space expanding? or 2. Is time contracting? The number of miles per light year would seem to remain constant in space so either could be happening. However, Here on Earth where the surface is not expanding, we would have a discrepancy comparing miles per hour. Me thinks space is expanding.

I had to think about that for a minute and I may change my mind as time contracts (or expands). Are you trying to make my head hurt?

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Martin, I have a strong interest in cosmology and relativity but my math background is limited so some of the equations are about as useful as an ASCII dump on my PC monitor. I did take a couple years of algebra and geometry but after that I see F-16s over my head. That is one reason I pursue the different calculators so vigorously as they help me grasp the big picture.

Well the answer is easy actually. All over the universe the people can just hold up a thermometer and take the temperature of the CMB sky, and if you and I happened to measure the same temp, then we were part of the same 'now' at that moment. Those two EVENTS were synchro---the events of taking the temp, even if we were billions of lightyears apart.

What does the temperature being the same mean? That you are at rest with the CMB and/or you are in the same frame? Does gravity have an impact on this 'now'? I understand this is approximate because of matter distribution. Is that because inconsistent matter distribution causes gravity to not be uniform?

Can we do this one short step at a time so my questions are not wasted on irrelevant issues? As you have time of course. I do appreciate your continued help.

ok the amount of light years away for this scenario is irrelevent...I want to know if someone...-as corrected-....65 million light years away were to view us would they be staring at dinosaurs...and if so...WHY in the universe would they try to communicate with them...thats like us finding microbes on mars and trying to communicate with them ...oh BTW if the universe is 13 billion years old....need we not just point hubble in the right direction and see the big bang (if it happened)(along the same line of thought as to reason that half the stars in the sky arent there anymore)

Your eyes see the reflection of light off of an object. You see what happened at the time the light left the object. If you were 65 million years out in space and it was possible to see that far, you may be looking at a T-Rex having dinner.

It is not likely that distant civilizations first observe life visually and then try to communicate with them. That would not be practical at the speed of light. Here on earth we just listen for signals from space that may have come from another advanced civilization. What we observe looking out into space happened long long ago and there is no way for us to know what is happening in real time.

We can see just after the big bang looking in any direction. The Cosmic Microwave Background Radiation is as close as we can get to seeing the big bang and we currently believe it was not from a singularity.

There are some sticky threads at the top of the page that cover some of the basics concerning cosmology and relativity. They may bring you up to speed quicker then asking random questions.

As mentioned, the Milky way galaxy is about 100,000 light years accross and one of our neighbors, the Andromeda galaxy, is about 2.5 million light years away. You would have to see Earth form 65 million light years away to see the dinosaurs.

Depending on their frequency, mass less photons make up radio waves, visible light, infrared light, x-rays, Gama rays etc and travel at the speed of light. Communicating at the speed of light would be painful over long distances. Don't wait up for an answer.

Our views of what is possible with science today would leave even advanced technology at a disadvantage for communicating or traveling to distant stars. If the human race does not self destruct, we will probably figure something out some day to make many things possible.

I do not have a PHD in physics or anything else so this is the layman's view after hanging around here for awhile and paying attention.

... our galaxy is only a few hundred million lightyears across...

You mean a few hundred THOUSAND lightyears across. right? Actually, Wikipedia says 100,000 lightyears across, depending on where you define the border.

you might what to take a look at quantum general relativity.. it predicts violent flunctuations in the geometry of spacetime at distances in order of plancks length.. an interesting subject..

Quantum general relativity? I participate her for fun, not torture. I still get panic attacks when I see "energy stress tensor".

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Could you just as well say time is contracting?

I would be interested in knowing your reasoning with "time is contracting"? Or is that saying that time expanding and time contracting are equally unrealistic?

You may have noticed that the Hubble rate, which today is 71, is on the order of a million back at z=1000-----around when the CMB light started out. and at z=6 the Hubble rate is about tenfold bigger than it is today.

Yes, I was paying attention to the Hubble constant. I figured it would drive you nuts that I did not include it in my chart, as I figured you would consider it one of the most important #'s given.

I don't quite understand what you are asking, so feel free to rephrase the question. Spell it out in simple language.

My question came from what you said in your first post in this thread.

Well, the Friedman model has an absolute time. And it has an absolute present moment and an absolute space (the slice of spacetime corresponding to Now.)

I wanted you to elaborate on what the Friedman model says "absolute time" and "Now" is.

I keep seeing a universal Greenwich mean or zulu time that all frames can relate to. Maybe a clock in totally flat space (remove the effects of gravity and static w.r.t. the CMB). I am sure my views are not scientific and would like to hear about what you were referring to in the above quote. Thanks

Life's a bitch and then we die.

That can be proven through observation and mathamatically using statistics. That constitutes good science, right?

...I have a tail end question, would any of said theories encompass a BH or BB as transition points? That a BB would function as returning everything to a universal center for a bounce, etc...

The current mainstream theories do not believe the BB was from a singularity. However, that is not saying a BH was not involved in some way. I may not have understood what you were asking though.

The Morgan calculator is interesting. The information is easy to read but trying to define a trend can be a real brain teaser. Sort of like mixing frames if you aren't careful.

I made a little chart with some of the info trying to to make it easier to get the big picture.

This still does not answer my questions about absolute time and the Friedman model.

I am a little confused because first you say

the Friedman model has an absolute time.

Then you say

The full GR theory has no chance of an absolute time.

Then you say

They are very compatible!

The Friedman model says we have a homogeneous and isotropic universe. Does that make time absolute in the universe regardless of location? and speed? and gravity? It seems I am still missing something (not unusual).

Have you tried Morgan's calculator? You should. Google "cosmos calculator".

Interesting calculator. What I think I am seeing is that the universe expansion was slowing until it was 2.66 billion years old (z=2.5) and then the expansion started accelerating. Is there a known reason for this? The calculator states all speeds are because of the expansion of space and not actual movement.

Edit - After looking at the calculator again the numbers started playing tricks on me. I am not certain what I am seeing but will look at it again after some sleep.

Has anyone really been far even as decided to use even go want to do look more like?

Msot hree olny seapk Eglsinh in wlel dhisneed steneecs.

Martin, Thanks for your response. I like the way you step outside the box in your thinking to give clarity to your answers (for beginners), but without actually stepping out of the box.

I don't think anyone says "spacetime is expanding". The visual metaphor that some scientists like to use is that space is expanding.

Since science uses spacetime as one dimension, and space is expanding, I was trying to tie time to the expansion. It seems that since the expansion is not constant, that may be a bad idea. I guess we could pick any one point in the expanding metric and have that point be at rest w.r.t. the CMB.

Well, the Friedman model has an absolute time. And it has an absolute present moment and an absolute space (the slice of spacetime corresponding to Now.)

This has been a topic of discussion here on SFN often so could you please elaborate on this. I have additional thoughts and questions but additional information here may keep me from saying more foolish things then necessary.

The full GR theory has no chance of an absolute time.

So the Friedman model and GR are not compatible?

Even though spacetime is expanding, can we consider it at rest with the universe? Maybe time moving forward is the expansion we observe? The rate of time should be constant in the universal frame with spacetime at rest with the CMB and the universe itself.

When you move through spacetime you will observe time as you always have. However, depending on your speed, you may be moving accross two minutes of spacetime in one minute. While you observed time as usual, You have moved through time so your clocks will not agree with absolute spacetime.

Make a few modifications for GR and this would give us an absolute reference frame for time and speed. Isn't the only thing stopping this our unwillingness to adapt it?