two kinds of expansion.

tommac · #1 (**permalink**) 24-April-2008, 03:46 PM

I know there is a good chance that I am confused about this but I really need someone to clear it up.

Expansion or contraction of space could happen 2 different ways.

1) Space is added so that there was 1 meter and now there is 3 meters 2 meters of space was created. All observers see the new realestate.

2) Space is stretched, 1 meter gets stretched. Locally ( relative to something in the stretched space(-time) 1 meter is still equal to a meter but for an observer outside they would see 3 meters.

In the second stretching I would argue that if space is stretched then time also needs to be stretched relative to the observer OR the speed of light will not stay constant at all points.

In the first time and space are independant. In the second they are related.
Can we agree?

Neverfly · #2 (**permalink**) 24-April-2008, 04:48 PM

Quote:

Originally Posted by tommac

I know there is a good chance that I am confused about this but I really need someone to clear it up.

Expansion or contraction of space could happen 2 different ways.

1) Space is added so that there was 1 meter and now there is 3 meters 2 meters of space was created. All observers see the new realestate.

Ok.

Quote:

Originally Posted by tommac

2) Space is stretched, 1 meter gets stretched. Locally ( relative to something in the stretched space(-time) 1 meter is still equal to a meter but for an observer outside they would see 3 meters.

Incorrect.

Quote:

Originally Posted by tommac

In the second stretching I would argue that if space is stretched then time also needs to be stretched relative to the observer OR the speed of light will not stay constant at all points.

In the first time and space are independant. In the second they are related.
Can we agree?

Cannot agree.

Explain, clearly, why you think that time must be stretched?

Grant H. Answered this repeatedly- the best example he gave was that it took 13.7 billion years for light to travel 40 million light years.
If Time stretched in the manner you describe, it would not have taken 13.7 billion years.

grant hutchison · #3 (**permalink**) 24-April-2008, 05:32 PM

Tommac, will you please, please, please, please quit this business of starting new threads for each tiny new bit of puzzlement that crosses your mind. It's a recipe for confusion, because individuals will inevitably track through your many similar threads in different orders, and answer your questions in different ways, unaware of other answers that have already been given. Most of us now have absolutely no idea where to find any significant previous post that we've read somewhere in this chaotic network you've created.
I, for one, am not going to dodge around after you. I'm prepared to conduct a single conversation on this thread, where we seem to be making some progress.
Otherwise, I wish you luck in your endeavours.

Grant Hutchison

Bob Angstrom · #4 (**permalink**) 24-April-2008, 08:47 PM

Quote:

Originally Posted by Neverfly

Explain, clearly, why you think that time must be stretched?

I think better terms for "stretched" when referring to time would be something like speeds up, accelerates, runs faster, or quickens. Semanticly speaking, you can stretch space but not time since "stretched" refers to an increase in distance. When space stretches, time must quicken if we are to use Einstein’s c as our constant. Meter sticks grow longer relative to the past as space expands while clocks simultaneously run faster. There appears to be some confusion about what you mean by "stretched" time. Would it be safe to say that stretched time means that clocks run faster?

If Einstein’s c is a constant and c=s/t , then "stretching" the value of s requires that we also "stretch" the value of time. The words may be vague but I don’t understand why the concept is so hard to explain.

Quote:

Originally Posted by grant hutchison

Tommac, will you please, please, please, please quit this business of starting new threads for each tiny new bit of puzzlement that crosses your mind.

Good suggestion.

cosmocrazy · #5 (**permalink**) 24-April-2008, 09:27 PM

Quote:

Originally Posted by grant hutchison

Tommac, will you please, please, please, please quit this business of starting new threads for each tiny new bit of puzzlement that crosses your mind. It's a recipe for confusion, because individuals will inevitably track through your many similar threads in different orders, and answer your questions in different ways, unaware of other answers that have already been given. Most of us now have absolutely no idea where to find any significant previous post that we've read somewhere in this chaotic network you've created.
I, for one, am not going to dodge around after you. I'm prepared to conduct a single conversation on this thread, where we seem to be making some progress.
Otherwise, I wish you luck in your endeavors.

Grant Hutchison

Tommac please listen to Grant and just take a step back, we are all struggling to keep up. I enjoy most of the questions you pose because we have similar mis-understandings and are both keen for answers & solutions! But i have just spent the last 2 hrs reading through threads posted on Q&A and ATM and we seem to be going back and forth. Please m8!!

speedfreek · #6 (**permalink**) 24-April-2008, 10:00 PM

Quote:

Originally Posted by Bob Angstrom

The words may be vague but I don’t understand why the concept is so hard to explain.

Because people (including myself) keep giving vague descriptions to something that may be impossible to describe with words. We inevitably end up confusing him, especially if someone writes something that can be misinterpreted or is just plain misleading.

Quote:

I think better terms for "stretched" when referring to time would be something like speeds up, accelerates, runs faster, or quickens.

It would, but unfortunately we are talking about the expansion of the universe over time, where time does not speed up. At least I think the OP might be referring to the expansion of the universe. He might also have mixed in something about the Lorentz transformation from SR, I'm not sure. Whatever, time doesn't accelerate. Wherever or whenever you experience a second in this universe, whatever you are doing, a second is a second and is the same length as everybody else's seconds. Observers in different places simply experience different amounts of seconds depending on what they are doing.

Quote:

When space stretches, time must quicken if we are to use Einstein’s c as our constant.

No, c is always measured as c in meters per second. Light emitted at a distance of around 40 million light years away travelled 13.7 billion light-years to reach us here. Meters are always the same length and so are seconds. New meters were being introduced between that light and this point in space as the light moved.

Quote:

Meter sticks grow longer relative to the past as space expands while clocks simultaneously run faster.

No. Meter sticks and clocks stay the same whilst the distance in meters between co-moving coordinates increases.

Quote:

There appears to be some confusion about what you mean by "stretched" time. Would it be safe to say that stretched time means that clocks run faster?

So there does. It would not be safe to say that, as time isn't thought to be stretching anywhere, due to cosmic expansion. Even on a purely conceptual level, if time were to "stretch", then a second would now be longer so time would be running slower, not faster. But it isn't.

Now if we are talking pure SR, pure relative motion between two observers in different inertial frames, one might see the other as length contracted along the direction of motion, and see the others time running slower than their own. The other would see them as contracted and time-dilated in the same way, looking shorter along the direction of motion, and with their time running slower. In the end it they would work out (if they met up) that both ships were always as long as they had ever been and both had experienced time at the normal rate, but one had experienced less seconds than the other.

In GR, you experience different amounts of seconds, relative to another observer, depending on the gravitational conditions around you. But all seconds are the same length.

You might like to think of it like time is running faster or slower, but in science, seconds and meters stay the same throughout, you just get more or less of them!

Tim Thompson · #7 (**permalink**) 24-April-2008, 11:40 PM

Quote:

Originally Posted by tommac

2) Space is stretched, 1 meter gets stretched. Locally (relative to something in the stretched space(-time) 1 meter is still equal to a meter but for an observer outside they would see 3 meters.

An obvious question arises: Exactly how does one go about the task of looking, in order to "see" 3 meters instead of 1 meter, from a distance? This goes to the heart of the question: How do you know that two different observers will measure two different distances?

Quote:

Originally Posted by tommac

In the second stretching I would argue that if space is stretched then time also needs to be stretched relative to the observer OR the speed of light will not stay constant at all points.

And I will refute that argument. It is a simple matter of basic physics, for all waves including light, that velocity = wavelength*frequency. So let space stretch. If the velocity in fact remains constant, then the necessary consequence of that would be that the product of wavelength and frequency would have to remain invariant. That means that the stretching of space could stretch the wavelength (which gets larger) while the frequency gets smaller accordingly. This is in fact exactly what is observed in cosmological redshifts. Therefore, observation implies that the speed of light in fact remains constant, throughout the entire range of observed redshifts, which implies (in the standard cosmology) all the history of the universe excepting perhaps the first ~800,000,000 years.

Meanwhile, we know that according to special relativity, the speed of light should be constant for all inertial frames of reference. Since special relativity is strongly supported by observation (What is the experimental basis of Special Relativity?), then one must take seriously the requirement that the speed of light be constant. Time dilation specifically requires this constancy. Observation indicates that time dilation works as predicted on Earth (Muon Experiments in Relativity; Bailey, et al., 1979, Coan, Liu & Ye, 2005; Field, 2006). Observation also indicates that time dilation works in the same way, which implies the same speed of light in a local inertial frame, in high redshift environments of type Ia supernovae (Foley, et al., 2005; Blondin, et al., 2008), and apparently for gamma ray bursts (i.e., Chang, 2001). This is direct observational evidence that the speed of light has not changed over most of the history of the universe. This is consistent with the analysis of cosmological redshifts shown above.

So not only in there no observational support for the claim that the speed of light does vary as the universe expands, there is positive observational support to the contrary.

tommac · #8 (**permalink**) 25-April-2008, 02:50 AM

yes stretched time means clocks run faster

Quote:

Originally Posted by Bob Angstrom

I think better terms for "stretched" when referring to time would be something like speeds up, accelerates, runs faster, or quickens. Semanticly speaking, you can stretch space but not time since "stretched" refers to an increase in distance. When space stretches, time must quicken if we are to use Einstein’s c as our constant. Meter sticks grow longer relative to the past as space expands while clocks simultaneously run faster. There appears to be some confusion about what you mean by "stretched" time. Would it be safe to say that stretched time means that clocks run faster?

If Einstein’s c is a constant and c=s/t , then "stretching" the value of s requires that we also "stretch" the value of time. The words may be vague but I don’t understand why the concept is so hard to explain.

Good suggestion.

tommac · #9 (**permalink**) 25-April-2008, 02:52 AM

I disagree.

Quote:

Originally Posted by speedfreek

Because people (including myself) keep giving vague descriptions to something that may be impossible to describe with words. We inevitably end up confusing him, especially if someone writes something that can be misinterpreted or is just plain misleading.

It would, but unfortunately we are talking about the expansion of the universe over time, where time does not speed up. At least I think the OP might be referring to the expansion of the universe. He might also have mixed in something about the Lorentz transformation from SR, I'm not sure. Whatever, time doesn't accelerate. Wherever or whenever you experience a second in this universe, whatever you are doing, a second is a second and is the same length as everybody else's seconds. Observers in different places simply experience different amounts of seconds depending on what they are doing.

No, c is always measured as c in meters per second. Light emitted at a distance of around 40 million light years away travelled 13.7 billion light-years to reach us here. Meters are always the same length and so are seconds. New meters were being introduced between that light and this point in space as the light moved.

No. Meter sticks and clocks stay the same whilst the distance in meters between co-moving coordinates increases.

So there does. It would not be safe to say that, as time isn't thought to be stretching anywhere, due to cosmic expansion. Even on a purely conceptual level, if time were to "stretch", then a second would now be longer so time would be running slower, not faster. But it isn't.

Now if we are talking pure SR, pure relative motion between two observers in different inertial frames, one might see the other as length contracted along the direction of motion, and see the others time running slower than their own. The other would see them as contracted and time-dilated in the same way, looking shorter along the direction of motion, and with their time running slower. In the end it they would work out (if they met up) that both ships were always as long as they had ever been and both had experienced time at the normal rate, but one had experienced less seconds than the other.

In GR, you experience different amounts of seconds, relative to another observer, depending on the gravitational conditions around you. But all seconds are the same length.

You might like to think of it like time is running faster or slower, but in science, seconds and meters stay the same throughout, you just get more or less of them!

tommac · #10 (**permalink**) 25-April-2008, 02:53 AM

Quote:

Originally Posted by Tim Thompson

An obvious question arises: Exactly how does one go about the task of looking, in order to "see" 3 meters instead of 1 meter, from a distance? This goes to the heart of the question: How do you know that two different observers will measure two different distances?

You have a redshift without a change in distance.

tommac · #11 (**permalink**) 25-April-2008, 03:01 AM

Quote:

Originally Posted by Tim Thompson

And I will refute that argument. It is a simple matter of basic physics, for all waves including light, that velocity = wavelength*frequency. So let space stretch. If the velocity in fact remains constant, then the necessary consequence of that would be that the product of wavelength and frequency would have to remain invariant. That means that the stretching of space could stretch the wavelength (which gets larger) while the frequency gets smaller accordingly. This is in fact exactly what is observed in cosmological redshifts. Therefore, observation implies that the speed of light in fact remains constant, throughout the entire range of observed redshifts, which implies (in the standard cosmology) all the history of the universe excepting perhaps the first ~800,000,000 years..

This is proof of number 2 not number 1. If space expanded so that 1 meter stretched to 3 meters but time didnt stretch. then the speed of light would travel 1 meter in 1 unit and 3 meters in 1 units. So speed of light is not constant. your refute ... is what I have been trying to say ... maybe I am not saying it correctly. But your proof is correct for #2

Bob Angstrom · #12 (**permalink**) 25-April-2008, 06:23 AM

Quote:

Originally Posted by speedfreek

Light emitted at a distance of around 40 million light years away travelled 13.7 billion light-years to reach us here. Meters are always the same length and so are seconds. New meters were being introduced between that light and this point in space as the light moved.

If we ignore the possibility that introducing "new meters" to meters makes no sense, do you understand that one would need to introduce one "new second" for every 300,000,000 "new meters". Or as previously stated, stretching space requires that we stretch time if c is to remain constant.

Quote:

Originally Posted by speedfreek

In GR, you experience different amounts of seconds, relative to another observer, depending on the gravitational conditions around you. But all seconds are the same length.

I didn't know Einsteins clocks all ran at the same speed. They just add or subtract seconds. This is enlightening.

Quote:

Originally Posted by speedfreek

You might like to think of it like time is running faster or slower, but in science, seconds and meters stay the same throughout, you just get more or less of them!

This is an amazingly astute observation I'll have to write it down.

speedfreek · #13 (**permalink**) 25-April-2008, 05:20 PM

Quote:

Originally Posted by Bob Angstrom

If we ignore the possibility that introducing "new meters" to meters makes no sense, do you understand that one would need to introduce one "new second" for every 300,000,000 "new meters". Or as previously stated, stretching space requires that we stretch time if c is to remain constant.

It is quite simple really. If the photons from the CMBR that we receive today were emitted when the surface of last scattering was only 40 million light years away, and both meters and seconds "expanded" with the expansion of the universe, then according to that thinking the universe is now only 40 million years old and 40 million light years in radius....

Those photons took 13.7 billion years to reach us, so if light travels at around 300,000,000 meters per second, it must have travelled many more meters and taken many more seconds in order for it to take that long to reach us.

So distances increased (thus more meters were added) and seconds kept ticking away at the same speed (thus more seconds passed). If a distance was once only 1 meter and is now 2 meters, you have more meters, not expanding meters.

Bob Angstrom · #14 (**permalink**) 25-April-2008, 05:31 PM

Quote:

Originally Posted by tommac

So speed of light is not constant. your refute ... is what I have been trying to say ... maybe I am not saying it correctly. But your proof is correct for #2

This is the way I read it too. Tim Thompson's excellent "refutation" of your statement concludes by supporting your position.

tommac · #15 (**permalink**) 25-April-2008, 05:39 PM

Quote:

Originally Posted by Neverfly

Grant H. Answered this repeatedly- the best example he gave was that it took 13.7 billion years for light to travel 40 million light years.
If Time stretched in the manner you describe, it would not have taken 13.7 billion years.

huh? I need to read this again. What are you basing this on? what is the proof that light travelled 13.7 years to traverse 40 million light years of space? relative to who?

I will read whatever you send me on this. Please show the post or sources of this claim. I may have skipped over this by accident.

Neverfly · #16 (**permalink**) 25-April-2008, 05:40 PM

Quote:

Originally Posted by Bob Angstrom

This is the way I read it too. Tim Thompson's excellent "refutation" of your statement concludes by supporting your position.

Not quite.
Tim Thompson clarified what Tommac had said. Tommac did get some things correct but others incorrect. So it appears to support Tommacs statements- but actually it stands on its own.
Tommac got some parts right but not others.

Now, Tommac is obviously intelligent and I've noticed he will put out some profound things- If he could just settle his pace down to an effective learning curve- he could accomplish quite a lot.

Bob Angstrom · #17 (**permalink**) 25-April-2008, 06:27 PM

Quote:

Originally Posted by speedfreek