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Do you believe in life elsewhere?

#325
I was asking a serious question, and I just watched the video, and something doesn't make any sense to me, so if you can, please shed some light on it.

In the video, the guy says that space-time is constantly expanding, which is a pretty simple concept to grasp, but this notion that it's expanding at a speed greater than the speed of light still doesn't make any sense to me, and maybe you can explain it.

Expanding on his road analogy, assume this scenario: You're a person, on Earth, and you're located at the end of a moving sidewalk moving away from you, representing the expansion of space time, and your friend is at some position x distance away, and is coming toward you at the speed of light, and he leaves his luggage at the point he starts from, which represents the source of the photons (I.E. stars). Now, let's assume the moving sidewalk is moving at the speed of light (E.G. the universe is expanding at the speed of light), that would mean that the source of the photons would be travelling away from us, at our reference point, at the speed of light. Now if we crank this up a notch, and the sidewalk is moving now at let's say 3c, which is what it seems like it would have to be to be 39 light years in radius, then the source of the photos, from our perspective, would be moving 3 times the speed of light; which, according to the way that we understand physics now, is impossible.

Am I understanding what he's saying right or is there something missing?
 
#326
yeah... and what i don't understand is:

scientists have pretty much agreed that the speed of light is a finite, unexceedable, outer limit of speed.

so, how the hell can the universe be expanding at a speed greater than the speed of light?

apparently it is, so, does this not mean that the speed of light is not some sacred, finite boundary?

davesignatureII.gif
 
#327
Hmmm... not sure if I'll be able to clarify, but I'll try....

Special relativity has non-intuitive consequences. The short answer is that matter or photons are limited by special relativity to a maximum velocity of c, no matter the velocity of the frame of reference. However, spacetime is not. Thus, if spacetime is expanding at 3c, while a photon is coming toward us at c, it can never arrive at our point in spacetime. If we could measure its velocity, however, it would still be c regardless.

Spacetime is not constrained by the effects of relativity because it isn't a "thing" of any sort, nor is it moving.
 
#330
12bass said:
Hmmm... not sure if I'll be able to clarify, but I'll try....

Special relativity has non-intuitive consequences. The short answer is that matter or photons are limited by special relativity to a maximum velocity of c, no matter the velocity of the frame of reference. However, spacetime is not. Thus, if spacetime is expanding at 3c, while a photon is coming toward us at c, it can never arrive at our point in spacetime. If we could measure its velocity, however, it would still be c regardless.

Spacetime is not constrained by the effects of relativity because it isn't a "thing" of any sort, nor is it moving.
Click to expand...

I understand that.

What I have issue with is that according to the theory posited in that video, the objects on either ends of the expanding space time are having the distance between them increased at a rate that could, potentially be greater than c.


Earth -------- Alpha Centauri (Distance between = 1 light year)
Assuming, just for ***** and giggles that the expansion rate of space time between those two points is 3c, then after 1 light year, you have this:
Earth -------------------------------- Alpha Centauri, which means that the distance between those two points has increased more than is theoretically possible if we assume that the maximum speed attainable is c, i.e. the Earth and Alpha Centauri are moving, relative to each other, faster than the speed of light, away from each other.

I can't see any way where you can increase the space between two points, but and not have motion between them in order to accommodate the extra space.
 
#331
here is a follow up qx-

how far, exactly, must the distance between two objects be for what you said to be true?

i mean, we have light reaching us all the time from very distant stars which must have started out thousands, or even millions, of years ago... so they must not be expanding at 3c from us... right?

so, exactly how far off must light start it's journey to never reach us?

davesignatureII.gif
 
#332
Right... well.... the relative distance between Earth and Alpha Centauri would have changed, but not due to their actual motion in spacetime. It would be because spacetime itself grew larger.

I think that a big part of the problem is that most of us tend to think in terms of Newtonian physics within Euclidean space. Relativity messes with those notions.
 
#333
Metric expansion of space - Wikipedia, the free encyclopedia

The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space. In other words, the universe is not expanding "into" anything outside of itself, although a frequently used analogy is the expansion of the surface of an expanding rubber balloon. If this analogy is used, this surface should be seen as an intrinsic manifold.
Click to expand...

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be expanding away from each other at a speed greater than the speed of light. Since the parts of the universe cannot be seen after their speed of expansion away from us exceeds the speed of light, the size of the entire universe could be greater than the size of the observable universe.

It is also possible for a distance to exceed the speed of light times the age of the universe, which means that light from one part of space generated near the beginning of the Universe might still be arriving at distant locations (hence the cosmic microwave background radiation). These details are a frequent source of confusion among amateurs and even professional physicists. Interpretations of the metric expansion of space are an ongoing subject of debate.
Click to expand...
 
#334
12bass said:
Right... well.... the relative distance between Earth and Alpha Centauri would have changed, but not due to their actual motion in spacetime. It would be because spacetime itself grew larger.

I think that a big part of the problem is that most of us tend to think in terms of Newtonian physics within Euclidean space. Relativity messes with those notions.
Click to expand...

So what you're saying is that in order for this idea to work, space-time isn't a plane, the way Einstein described it as, it's more like a Stretch Armstrong doll, correct?
 
#335
12bass said:
Right... well.... the relative distance between Earth and Alpha Centauri would have changed, but not due to their actual motion in spacetime. It would be because spacetime itself grew larger.

I think that a big part of the problem is that most of us tend to think in terms of Newtonian physics within Euclidean space. Relativity messes with those notions.
Click to expand...

yeah but if spacetime is expanding at 3c (eg) then light from alpha centauri should never reach us and that is obviously not the case.

davesignatureII.gif
 
#336
Lonesomedave said:
yeah but if spacetime is expanding at 3c (eg) then light from alpha centauri should never reach us and that is obviously not the case.

davesignatureII.gif
Click to expand...

That's an example number, but it raises and interesting point, because going off this quote from wikipedia, and accepting the concept of the metric expansion of space, then what's to keep me from saying the universe is 110 billion light years across?

Wikipedia said:
Since the parts of the universe cannot be seen after their speed of expansion away from us exceeds the speed of light, the size of the entire universe could be greater than the size of the observable universe.
Click to expand...
 
#338
sloasdaylight said:
So what you're saying is that in order for this idea to work, space-time isn't a plane, the way Einstein described it as, it's more like a Stretch Armstrong doll, correct?
Click to expand...

Right.... relativity changes spacetime into something malleable depending upon velocity (SR) and gravitation (GR). Earlier conceptions of space were based upon Euclidean geometry, where space was thought to be like an almost infinite number of identical cubes. Non-euclidean space has non-intuitive consequences (but appears to be correct for our Universe).

Lonesomedave said:
yeah but if spacetime is expanding at 3c (eg) then light from alpha centauri should never reach us and that is obviously not the case.
Click to expand...

Yep... that was a hypothetical.
 
#339
12bass said:
Right... well.... the relative distance between Earth and Alpha Centauri would have changed, but not due to their actual motion in spacetime. It would be because spacetime itself grew larger.

I think that a big part of the problem is that most of us tend to think in terms of Newtonian physics within Euclidean space. Relativity messes with those notions.
Click to expand...

Also, going back to this. If we accept that a photon travels at the speed of light, c, then in order for that photon to never reach us, the distance it's travelling, along whatever plane or membrane or Stretch Armstrong doll it's on, must also be increasing by the speed of light, in order to keep the photon from not slowly inching it's way closer to us.
 
#340
12bass said:
Right.... relativity changes spacetime into something malleable depending upon velocity (SR) and gravitation (GR). Earlier conceptions of space were based upon Euclidean geometry, where space was thought to be like an almost infinite number of identical cubes.
Click to expand...

Ok, so accepting this, what you're essentially saying is to think of it as if it as a Stretch Armstrong doll, Earth is at the nose, and Alpha Centauri is at the knee. In order to get from one to the other, you still have to travel from nose to knee, but the space between them is going to be different depending on the time you do it, and the expansion rate?
 

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