# Old question?

1. Aug 14, 2009

### cary cato

i posted this comment earlier today, but in a wrong part of this forum.

if the fabric of space is expanding (accelarating?), shouldn't the fabric of time be changing proportionaly? wouldn't this make the speed of light a non-constant?

2. Aug 14, 2009

### Staff: Mentor

No and therefore no.

3. Aug 14, 2009

### cary cato

the reason i ask this question here is because i lack the skills to pursue an answer. all i can do is read about others' work in this area.

but, i just don't understand how the fabric of space-time can be changing without time being affected also???

4. Aug 14, 2009

### cary cato

if time is changing along with space, this would throw a lot of the calculations of our known universe out of whack.

5. Aug 14, 2009

### rsr_life

If A was changing proportionally with B, then the C(A,B) would still remain the same. Seems simple enough. Also remember that space and time are equivalent - they are the same when looked at differently.

Anyway, Russ, I'm not sure what the first 'No' was for. Are you implying that space-time is not expanding or that time isn't being affected by an expansion of space? If it's the latter, someone enlighten me too.

The one thing that 'illuminated' my mind as a kid was when I read that Physicists were considering the singularity at the beginning of time - the big bang - was like a singularity within a black hole (this was a while back). Basically, that 'looking back' in time was equivalent to 'looking into' a black hole. My understanding was that just as we believed back in the 90s that space was zipped to a point and then 'ripped' inside a black hole, so was time if we look at the light cone backwards 13-14 Gyears. Somebody should add some details here to help expand the analogy to Cary Cato and correct me if I'm wrong.

6. Aug 14, 2009

### cary cato

thank u for your respond rsr. my limited knowledge to express myself may be confusing things here.

space-time is changing
space is expanding
time is a constant?

7. Aug 14, 2009

### atyy

Spacetime is a single geometric structure (a solution to Einstein's equations relating matter and spacetime geometry).

There are many ways of dividing it into coordinate space and coordinate time.

In the most popular division, coordinate space expands as coordinate time increases.

8. Aug 14, 2009

### Naty1

"...shouldn't the fabric of time be changing proportionaly? wouldn't this make the speed of light a non-constant?.."

Good question, not obvious how to explain it....
but you might consider an analogous situation: Here on earth's surface, in a gravitational field, time runs slower than out in free space, without much gravity present. Yet in each location, local observers see light at speed c.

Another way is to rely on the old observation: light moves at c and only speed c, as observed locally.

Maybe a better way would be to think about new space coming into existance over vast cosmological distances. (within our galaxy, in contrast, gravity holds everything in place) As overall space expands, individual light waves (or individual photons) still move around at speed c; however, those moving towards us do appear red shifted since very distant sources are moving away from us. So the energy of light is decreased, but not it's speed.

Hubble law or cosmological redshift in Wikipedia will likely explain redshift.

Last edited: Aug 15, 2009
9. Aug 14, 2009

### cary cato

thank you guys for your replies. and thanks for your patients (sp?) with me in explaining these concepts.

what does "coordinate time increasing" mean?

time IS changing proportinally to the ongoing change in coordinate space? do we include this continual change for coordinate time in our calculations in the size of the universe? was this concept used in the calculations that showed the universe's expansion to be accelerating?

(i do understand why the GPS clock corrections are made)

10. Aug 14, 2009

### cary cato

Code (Text):
but you might consider an analogous situation: Here on earth's surface, in a gravitational field, time runs slower than out in free space, without much gravity present. Yet in each location, local observers see light at speed c.
thank you for the explanation naty1. i see your point about c always appearing constant.

i am ...way in over my head in this discussion. the subject of time changing with the expansion of space obviously has been well hashed out. i was already aware about some of the local distortions of space-time...GPS, gravitational lensing, that kind of stuff.

i was just curious if you guys in the physics community had considered "time" being changed in the larger picture as the universe expands. kudos to you guys :-)

11. Aug 14, 2009

### atyy

Calendar year increasing means (2009,2010,2011,2012 ...)

Same idea for coordinate time.

12. Aug 14, 2009

### cary cato

ok...you mean going forward. actually, my question about changing time... was the "rate" at which it is going forward. whether the rate is changing along with the rate of expansion of space.

13. Aug 14, 2009

### atyy

"Rate" means compare one thing against another. Rate of expansion of space means "how quickly does space expand as time goes forward?"

Rate of time going forward means you define 2 times and ask "how quickly does one time go forward compared to the other time?"

In the expression "space expands", we choose a particular description of spacetime in which there are 3 space axes and 1 time axis. So you cannot ask "how fast does time go compared to time?"

Actually, there is always something(s) called the proper time(s) of a particular ideal clock(s) (eg. an atomic clock). Atomic clock A takes a particular path in spacetime - it will accumulate its own proper time which is just its clock reading. Atomic clock B takes another particular path in spacetime - it will accumulate its own proper time which is just its clock reading. Atomic clock C ....

Now, suppose we choose space and time coordinates such that coordinate space expands with time. You could ask how does coordinate time change relative to the proper time of a particular clock. If you pick a clock that starts at the big bang and is stationary in space, then coordinate time equals the proper time of that clock. (I think this statement is a bit inaccurate, you have to check what the more correct version is.)

14. Aug 14, 2009

### cary cato

...."If you pick a clock that starts at the big bang and is stationary in space, then coordinate time equals the proper time of that clock."...

atyy...that is my question. would THAT clock (described above and assuming it has not moved since) be counting seconds at the same "speed" NOW as it was immediately after the big bang?

15. Aug 14, 2009

### cary cato

oddball idea.... "time" has been speeding up since the creation of space-time during the big bang.

i hope i am not confusing everyone with my profound ignorance (seriously). time has a direction. it is moving forward. has time been accelerating since the big bang (ie...never been a constant rate of change)???

Last edited: Aug 14, 2009
16. Aug 14, 2009

### cary cato

another way of putting it is....has time has been speeding up (like running a movie faster and faster) since the big bang... due to the continual decreasing density of the universe (expansion)???

would this idea change our understanding of space-time?

Last edited: Aug 14, 2009
17. Aug 14, 2009

### cary cato

i thank u guys for hearing me out. if anyone would like to comment on this idea (pro or con), i would love to hear the feedback. cheers :-)

Last edited: Aug 14, 2009
18. Aug 14, 2009

### cary cato

(reposted question on the new page)

another way of putting it is....has time been speeding up (like running a movie faster and faster) since the big bang... due to the continual decreasing density of the universe (expansion)???

would this idea change our understanding of space-time?

19. Aug 14, 2009

### atyy

You need to define your terms. Speeding up compared to what?

In the movie example you give, there are two things changing - (i) the movie frames (ii) a clock going forward - the movie is speeding up relative to the clock.

20. Aug 15, 2009

### cary cato

atyy...i am sorry for the confusion and i thank you for your patience.

1) if we didn't adjust the atomic clock on a GPS to measure time faster, relative to a similar clock on earth, positioning would be off.

2) i am asking....if the above is true, why do we consider the rate of change of time in the universe to be a constant...since the beginning...while the universe has been continually expanding and continually decreasing in mass density?

(i guess "inflation" is an example of space-time's expansion speeding up, but it is assumed to have happened over a finite time only, right?)

again, i thank you for your patience.

Last edited: Aug 15, 2009
21. Aug 15, 2009

### Naty1

Not a proper way to think about space and time as far as is known. You are likely thinking of the density of mass, right?? which does apparently decrease due to increaing volumeas space expands.....but IF the universe is infinite already, then the "expansion" we observe is irrelevant...infinity plus one is still infinity. Gravitational potential depends on mass density, energy density and pressures as well...

As far as we know, time has been quite steady ever since just after the big bang and is rather uniform over interstellar distances in the cosmos because overall the cosmos is rather uniform. Since energy cannot be created or destroyed, the total energy (including mass equivalent) is rather uniform in the cosmos. Another way to think about this is that as far as is known the radioactive half life of a given material was the same a few billion years ago as today and is expected tro be the same a few billion years from now.

You can also consider that time in a big box passes at the same rate as a larger box...so volume alone does not seem to change how fast time passes....

On the other hand, if you approach say a black hole local time as observed from a distance will vary due to incredibly high gravitational potential. Your metabolism slows down as observed from a distance. So time is not exactly the same everywhere either. But each local observer sees a fixed unvarying measure of time....and the speed of light.
As Richard Feynmann likely would have said "This is an altogether fascinating phenomena."

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22. Aug 15, 2009

### cary cato

...."As far as we know, time has been quite steady ever since just after the big bang and is rather uniform over interstellar distances in the cosmos because overall the cosmos is rather uniform."...

one question....if you substitute "variable time" for "steady time" in the equations, would the results make any sense at all? (time as a function of space "mass" density, assuming the universe isn't infinite)

i am not trying to get someone else to do my homework for me. this is just an idea i came up with and i lack the math skills to look at it on paper. my scientific verbage aint that great either.

but i sincerely appreciate you guys' time to discuss this with me.

23. Aug 15, 2009

### atyy

There are several sorts of "time dilation", which are answers to different sorts of experimental questions.

In GPS, suppose you have two clocks, which tick at the same rate when they are together. You place them at different heights in the earths's gravitational field. Every time one clock ticks, it sends a pulse of light to the other clock. The time interval, as judged by the ticks of the receiving clock, between the reception of pulses will be different from the time interval between its ticks.

In the expanding universe, suppose you have two clocks together at the big bang ticking at the same rate. They are "stationary" in space, but space is expanding so they end up far apart. Every time one clock ticks, it sends a pulse of light to the other clock. The time interval, as judged by the ticks of the receiving clock, between the reception of pulses will be different from the time interval between its ticks.

The above phenomena are called "red/blue shifts". In the time dilation of the twin paradox, there are red/blue shifts. There are 3 sorts of red/blue shifts - Doppler, gravitational and cosmological. I don't think this classification of types red/blue shifts is "fundamental" - what is fundamental is the underlying theory of general relativity, and asking questions which correspond to well defined experiments.

24. Aug 15, 2009

### cary cato

thank you for your response atyy.

...."what is fundamental is the underlying theory of general relativity, and asking questions which correspond to well defined experiments.".....

this is gonna take me a while, but i am going off to try and find some specific examples of where my idea of "time varying with the mass-density of space" may apply.

(and thanks to the others here who have tried to help me)

25. Aug 17, 2009

### cary cato

i'm back. i looked at the equations and decided it would be easier for me to re-phrase my question instead (no snickering until i finish :-)

the fabric of spacetime has been expanding since the big bang. assume the rate of expansion of spacetime has been, and still is constant (recent observations conflict with this assumption). assume the time-component of spacetime has not been a constant, but instead much slower at the beginning than now, as seen from an observer?

if these assumptions are correct, would our interpretation of what we see through the telescope change ...say for example, those dimmer than expected supernova explosions???

or maybe, would it change our interpretation of why we see red shifts greater at greater distances???