How can space travel be like world travel?

[freyar]

Hey, cool! Congrats!

Thanks! It's been a long road.

Barring local gravitational effect, yes. But...

...there's always a but...

...if you want to say space is flat, and spacetime is curved, that implies that time... isn't what most folk think it is, as if that hadn't become obvious already. I was purposefully not being quote so rigorous with my terminology to not muddy the waters with that at the moment.

The fact of the matter is that for most human purposes, talking about space alone is useless. All our discussions are about events and processes that take time.

True enough. I'm trying to avoid being too technical also. My point is that we can actually always think about time as just a normal line (in the rest frame of the "stuff" in the universe), and space is (averaged over long distances) flat also. It's the expansion of space through time that makes the whole spacetime curved. It is a bit subtle of a mathematical point, but it's a very important part of our understanding of cosmology today that the spatial slices of our universe are flat.

Yep, this is how I started imagining in my head. And that thinking took me here, [using the 2D paper analogy]: Why is space only added to the x axis and not also the y axis, (and z)?

I guess we're imagining that the light is moving in the x direction, then. In that case, the x axis, y axis, and z axis all get stretched. That stretches out the wavelength of the light, which is the extent of one cycle of the wave in the x direction, the direction it's moving. The point is that the amplitude of the wave is not any kind of motion in y or z. It's most emphatically not like a guitar string, where the wave is displacement of the string perpendicular to its average direction. The amplitude of the wave is in the electric (and magnetic) field, which is not some kind of extent in space. It's a separate variable, which is not expanding with the universe. So that's why the amplitude doesn't get bigger.

That's actually what's true for a pulse of light with small extent, like a photon. But if you have a big electromagnetic field, like a thermal bath of light filling the universe, the expansion of space spreads the photons out over time. That decreases the strength of that bath of light; this is why the cosmic radiation was hot enough to break nuclei apart in the early universe but is under 3 degrees above absolute zero now.

 

[Bullgrit]

Is there a relationship between the universe expanding and time going forward? Talking about the x, y, and z axes expanding, I thought, isn't t also "expanding." Is there any thought that if the universe stops expanding, and starts contracting that t will reverse along with x, y, and z?

Bullgrit

 

[Umbran]

Yep, this is how I started imagining in my head. And that thinking took me here, [using the 2D paper analogy]: Why is space only added to the x axis and not also the y axis, (and z)?

As noted by freyar, while the wavelength is a length measured in the X direction, and the electric and magnetic fields oscillate in the Y and Z directions, they are not lengths in those directions like the wavelength is. We say say light has a wavelength in meters or centmeters, but the amplitude is not in meters of centimeters.

Is there a relationship between the universe expanding and time going forward?

Not as I think you mean that question, and I understand Relativity, no. We do not experience time "going forward" due to the expansion. It is perhaps more accurate to say there's time to go forward into because of the expansion. Much like there's space to fly a ship through because of the expansion.

Talking about the x, y, and z axes expanding, I thought, isn't t also "expanding."

There is a relationship between space and time - the two are bound together as what we call "spacetime", so you cannot affect one without affecting the other. This is what Einstein's relativity is all about.

Is there any thought that if the universe stops expanding, and starts contracting that t will reverse along with x, y, and z?

That though has been expressed, and we can see the intuitive sense in it. If time starts as zero, and expands as the universe grows, if the universe again shrinks, the extent of the Universe in the "time direction" has to shrink, so things will have to move back from t= a bazillion back towards t=0, essentially moving backwards.

Freyar may be able to correct me if I am wrong, but I think that violates the laws of thermodynamics. There are processes in the universe that are not time-reversible, so that what has been done cannot be undone. If we went from Big Bang, to expansion, to squeeze, to Big Crunch, in the second half of that process, you'd not just see the history of the first half run backwards in the second. You can think of this as the extent of the universe in time shrinking, but the point it shrinks to isn't t=0, but instead t= a bazillion - we crunch in time to the far end, not the beginning.

Which is apt to be a bit moot, for us in this universe, as the current indications and understanding suggests are that we are not going to have that squeeze - it looks like the Universe will continue expanding forever.

 

[freyar]

Is there a relationship between the universe expanding and time going forward? Talking about the x, y, and z axes expanding, I thought, isn't t also "expanding." Is there any thought that if the universe stops expanding, and starts contracting that t will reverse along with x, y, and z?

Not as I think you mean that question, and I understand Relativity, no. We do not experience time "going forward" due to the expansion.

<*snip*>

That though has been expressed, and we can see the intuitive sense in it. If time starts as zero, and expands as the universe grows, if the universe again shrinks, the extent of the Universe in the "time direction" has to shrink, so things will have to move back from t= a bazillion back towards t=0, essentially moving backwards.

Freyar may be able to correct me if I am wrong, but I think that violates the laws of thermodynamics. There are processes in the universe that are not time-reversible, so that what has been done cannot be undone. If we went from Big Bang, to expansion, to squeeze, to Big Crunch, in the second half of that process, you'd not just see the history of the first half run backwards in the second. You can think of this as the extent of the universe in time shrinking, but the point it shrinks to isn't t=0, but instead t= a bazillion - we crunch in time to the far end, not the beginning.

Which is apt to be a bit moot, for us in this universe, as the current indications and understanding suggests are that we are not going to have that squeeze - it looks like the Universe will continue expanding forever.

Umbran again has the essentials mostly right here, though I can't resist mentioning that there are still some big questions here. The microscopic laws of physics can be reversed in time, at least if you reverse all charges and do a mirror reflection at the same time (we believe), so it is a bit weird that time seems to flow in one direction. This really does have to do with thermodynamics and entropy, like Umbran says, and what that gets down to is the many different ways microscopic particles can move. For example, when you break and egg, all the molecules spread out, and it's very unlikely they'll come back into a state where they just happen to flow back together and uncrack the egg. (But it would happen given an infinite amount of time.) This is the essence of entropy and the 2nd law of thermodynamics. But then we see that the early universe has very little entropy, and we have to ask why? If the early universe had a maximal entropy (like we're evolving toward), then there wouldn't really ever change from that maximal entropy state. So why was entropy so low earlier? There are a number of scientists who think about that a lot. One is the cosmologist Sean Carroll, who writes over at the Cosmic Variance blog. If you search through Sean's posts, you'll find some on this topic.

And, yeah, as Umbran says, it looks to us like our universe will just keep expanding at an increasing rate, so all the other galaxies get pushed away from ours. And then all our stars will burn out as we go toward that maximum entropy.

 

[Bullgrit]

[ame=http://www.youtube.com/watch?v=v6o2bUPdxV0&feature=player_embedded]2011 Nobel Prize: Dark Energy feat. Sean Carroll - YouTube[/ame]

Bullgrit