Register to reply

What is space?

by Philip7575
Tags: space
Share this thread:
phinds
#73
Dec16-11, 12:20 PM
PF Gold
phinds's Avatar
P: 6,353
Quote Quote by Tanelorn View Post
phnds, no need to be abrasive, I am not suggesting anyone overlooked anything or was being stupid!
Anyway I was hoping Chalnoth could give me his thoughts on this. thanks.
You're right ... I was snippy and I apologize. I'm always taken aback when someone comes up with an idea that just could not possibly have been overlooked and then rather than say something like "it seems to me that .... so why isn't that the case", they ask it as though they think no one has ever thought of it before.
Tanelorn
#74
Dec16-11, 12:22 PM
Tanelorn's Avatar
P: 728
I mostly expect to be completely wrong or missing a fundermental understanding when I ask these kind of questions, but at least I am still asking, which I think is a good thing.
phinds
#75
Dec16-11, 12:51 PM
PF Gold
phinds's Avatar
P: 6,353
Quote Quote by Tanelorn View Post
I mostly expect to be completely wrong or missing a fundermental understanding when I ask these kind of questions, but at least I am still asking, which I think is a good thing.
I couldn't agree more that asking is a good thing, I was just put off by the particular way you asked, and again I apologize for being snippy.
Drakkith
#76
Dec16-11, 02:22 PM
Mentor
Drakkith's Avatar
P: 11,890
Quote Quote by Tanelorn View Post
"If you were traveling at the speed of light and turned a flashlight on, what would happen to the light?
Relative to you, the light from your flashlight would still be moving at 3 x 10^8 m/s.
To allow this to happen, your perception of time slows down the faster you move and you gain more mass (E=mc^2)."
You cannot travel at the speed of light. Traveling arbitrarily close to c your perception of time would be vastly slower compared to "stationary" observers and distances would appear to you to be length contracted. However, you do not gain mass, you gain energy.


I believe the above is correct. So my question is, at the beginning of the BB when all particles were moving very fast, would time effects like the above example effect our estimates for the rate of inflation, or even our estimates for the age of the universe since particles were moving very fast for quite a while? I am very uncertain of which relativitic frames of reference apply in this case.


Thanks!
Are you referring to inflation? In that case I am unsure of the effects that expansion of space has with regards to relativity. Does time dilation apply between areas that are moving apart at .5c thanks to expansion or inflation?
Imax
#77
Dec18-11, 01:19 AM
P: 186
If space was infinite before the Big Bang event and the entire mass/energy of the Universe was confined to a small volume and that volume expanded something like an explosion at the BB event, then it could be difficult to postulate a homogeneous and isotropic Universe. The outer regions of the Universe could see a very lower mass/energy density in the direction of expansion, as compared to a higher mass/energy density in the opposite direction. Given this scenario, itís possible that the Universe wouldnít be homogeneous and isotropic.

Seems to me that one way of having a homogeneous and isotropic Universe is if space is finite (i.e. compact).
phinds
#78
Dec18-11, 02:14 AM
PF Gold
phinds's Avatar
P: 6,353
Quote Quote by Imax View Post
If space was infinite before the Big Bang event and the entire mass/energy of the Universe was confined to a small volume
You can't have it both ways. Either space was infinite OR it was confined to a small volume.

If the U was infinite at its inception, that does NOT mean that it could not have expanded exactly as it did.
Drakkith
#79
Dec18-11, 02:17 AM
Mentor
Drakkith's Avatar
P: 11,890
Quote Quote by Imax View Post
If space was infinite before the Big Bang event and the entire mass/energy of the Universe was confined to a small volume and that volume expanded something like an explosion at the BB event, then it could be difficult to postulate a homogeneous and isotropic Universe. The outer regions of the Universe could see a very lower mass/energy density in the direction of expansion, as compared to a higher mass/energy density in the opposite direction. Given this scenario, itís possible that the Universe wouldnít be homogeneous and isotropic.

Seems to me that one way of having a homogeneous and isotropic Universe is if space is finite (i.e. compact).
The problem is that to the best of our knowledge the big bang was NOT an explosion like we normally think of. Whatever the size of the universe, infinite or finite, whatever, the big bang was the starting point. This includes spacetime.
Tanelorn
#80
Dec19-11, 08:54 AM
Tanelorn's Avatar
P: 728
In my earlier unanswered question I was asking if special relativity type effects have to be included in estimates for either the rate of inflation or even the age of the universe?

So I had a look around to see if photons which begin their journey in a higher density medium, with lower gravitational potential to a modern day lower density medium with higher gravitational potential, might undergo a red shift, and came up with the following:



Source: http://en.wikipedia.org/wiki/Self-cr...onal_red_shift

Gravitational red shift
The local conservation of energy, and the consequential variation in rest mass, demand that gravitational mass is treated under the de Broglie wave theory. Mass is defined by the DeBroglie frequency of that particle. The red shift caused by the curvature of space-time, a time dilation expressed by the metric component, is suffered not only by the photon but also by the atom with which it interacts and is thus undetectable. The red shift that is detectable is caused by the increase in rest mass that fundamental particles undergo when raised to the higher level. Gravitational red shift in this theory is interpreted not as a loss of gravitational potential energy by the photon but as a gain of gravitational potential energy by the apparatus measuring it. The red shift predicted is hence equal to the difference in Newtonian potential and thus identical with that of GR as confirmed in the Pound-Rebka experiment.


Firstly, I hope this is acceptable science and that it is ok to post this, I am finding that posting genuine science questions here is getting somewhat risky of terse replies.

So does this effect, if real, make any contribution to CMBR red shift and thus estimates for the age of the universe? Oh - and I am sure that if it does, that it has already been included.




I also saw at the end of the wikipedia article a section on Dark Matter which looked interesting. Is this view accepted science?
Tanelorn
#81
Dec21-11, 11:50 AM
Tanelorn's Avatar
P: 728
This page explains the concept of Gravitational Red Shift very well:

http://en.wikipedia.org/wiki/Gravitational_redshift





At the bottom there is also a discussion on gravitational time dilation:


Gravitational redshift versus gravitational time dilation

When using special relativity's relativistic Doppler relationships to calculate the change in energy and frequency (assuming no complicating route-dependent effects such as those caused by the frame-dragging of rotating black holes), then the Gravitational redshift and blueshift frequency ratios are the inverse of each other, suggesting that the "seen" frequency-change corresponds to the actual difference in underlying clockrate. Route-dependence due to frame-dragging may come into play, which would invalidate this idea and complicate the process of determining globally agreed differences in underlying clock rate.

While gravitational redshift refers to what is seen, gravitational time dilation refers to what is deduced to be "really" happening once observational effects are taken into account.



So is there anyone here who can speak to the level of contribution of gravitation red shift to the CMBR redshift and also the possible effect of gravitational time dilation on the estimated age of the Universe?
Imax
#82
Dec21-11, 11:29 PM
P: 186
Quote Quote by phinds View Post
You can't have it both ways. Either space was infinite OR it was confined to a small volume.
It'is two different things. If you postulate that space is flat Euclidean and it can expand to infinity in all directions, it doesn’t preclude the possibility that the entire mass/energy of the U could occupy a finite volume near the Big Band event. What I’m trying to point out is that the BB event is unlikely to be something like an explosion, something you could expect from detonating dynamite. If this was the scenario, then it could be difficult to have a model of the U that was homogeneous and isotropic.

We are embedded within the BB event. A compact space model of the Universe can allow for closed space-like and closed light-like geodesics. A compact space model can explain a U that is homogeneous and isotropic.

And to all PF participates, Merry Christmas and Happy New year!
Drakkith
#83
Dec22-11, 12:43 AM
Mentor
Drakkith's Avatar
P: 11,890
Quote Quote by Imax View Post
It'is two different things. If you postulate that space is flat Euclidean and it can expand to infinity in all directions, it doesn’t preclude the possibility that the entire mass/energy of the U could occupy a finite volume near the Big Band event.
If I'm not mistaken, it isn't that it can expand to infinity, it's that it IS infinite. Or might be.
phinds
#84
Dec22-11, 12:54 AM
PF Gold
phinds's Avatar
P: 6,353
Quote Quote by Imax View Post
... it doesnít preclude the possibility that the entire mass/energy of the U could occupy a finite volume near the Big Band event.
I contend that if it starts off not infiite it ends up not infinite. Do you disagree? How do you get from finite to infinite in a finite amount of time? Seems like a good trick to me
phinds
#85
Dec22-11, 12:55 AM
PF Gold
phinds's Avatar
P: 6,353
Quote Quote by Drakkith View Post
If I'm not mistaken, it isn't that it can expand to infinity, it's that it IS infinite. Or might be.
well, my point is more that it EITHER is or it isn't, and you can't have both. If it isn't at the start, then it isn't now and if it is now then it was at the start.
Drakkith
#86
Dec22-11, 01:19 AM
Mentor
Drakkith's Avatar
P: 11,890
Quote Quote by phinds View Post
well, my point is more that it EITHER is or it isn't, and you can't have both. If it isn't at the start, then it isn't now and if it is now then it was at the start.
Agreed.
Imax
#87
Dec24-11, 12:44 AM
P: 186
Quote Quote by phinds View Post
I contend that if it starts off not infinite it ends up not infinite. Do you disagree?
No. Absent some kind of bizarre event, something like quantum symmetry breaking, then a U that was finite near the BB should be finite now. If space was infinite then, it should be infinite now.

My inclination is a U with a compact space (i.e. finite).


Register to reply

Related Discussions
Topological space, Euclidean space, and metric space: what are the difference? Calculus & Beyond Homework 9
How does the space station survive the damages caused by debris in space? Astronomy & Astrophysics 6
Would an energy diffraction ring in five space form a Minkowski space? Differential Geometry 2
Solution space of linear homogeneous PDE forms a vector space?! Calculus & Beyond Homework 2