Astrophysics and Cosmology - Discuss the Universe here

[wr250]

Since it's accelerating and expanding you'd never be able to reach beyond the local horizon even if you left at the speed of light today! You're stuck within our local observable part of the Universe.

And you can't say it's expanding into anything! All time and space are measured with respect to the big bang, so the best we can imagine is that it's expanding like a surface. Sorry I can't get more in depth right now (gotta get in to work!) but this article looks interesting and says what I was trying to in a short period of time.

http://www.universetoday.com/1455/podcast-what-is-the-universe-expanding-into/

i have a question, related to the speed of light.
if spacecraft a is moving at the speed of light (c) in one direction and spacecraft b is moving at c in the opposite direction, isnt  the speed of separation 2c ? and therefore no known communication is possible between the 2 craft nor can they see each other as the light from either will not reach the other? (assuming no warpage of space/time faster than light particles etc)
i base this on c being relative to the observer. probably way off base, but wanted to ask.

[Camazotz Automat]

i have a question, related to the speed of light.
if spacecraft a is moving at the speed of light (c) in one direction and spacecraft b is moving at c in the opposite direction, isnt  the speed of separation 2c ? and therefore no known communication is possible between the 2 craft nor can they see each other as the light from either will not reach the other? (assuming no warpage of space/time faster than light particles etc)
i base this on c being relative to the observer. probably way off base, but wanted to ask.

A photon walks into a bar, and sees himself sitting there.

"Impossible!" he screams.

"You and your %@&!$&# thought experiments!" the seated photon screams back.

(Come on out to BEAM SPLITTER's Bar and Grill. Thursday night is lady photons night and every Tuesday night we have the famous impersonator, Elvis Particles. You ain't nothing but the Dog Star.) ***

*** This has been an original "a photon walks into a bar" joke by Cam. If you think it sucked, simply consider yourself part of the test group and thank your god that it wasn't a knock knock joke.

[area51drone]

Agent, what does it mean, in terms of time, when space expands?

[zeebo]

....So it's not known what the topology of the larger Universe is, but inflation suggests that it is finite but unbounded - possibly like the surface of a large sphere. ..

If I'm understanding "unbounded" right, it does sound mathematically distinct from a truly "infinite" universe.  And it would mean theoretically, as M. Kaku says, that the farthest place from you in the universe is the back of your own head.   

Hope that was not too long winded

No way .. this stuff is fascinating!  Thanks for your explanations.

[zeebo]

...The phrase "Crystallized Plutocrat" is pretty damned nifty, btw. I will try not to steal it. But I'm not making any promises.

Here's a few backups I just came up with, in case that one's trademarked.   

Canonized Puma-cat
Carmelized Applecart
Cartoonized Play-Doh Kit

[Tarbaby]

Ahhh! Those are good.

I remember a long time ago there was a group of scientists who thought the universe was toroidal shaped.

[Agent : Orange]

Agent, what does it mean, in terms of time, when space expands?

The way the expansion of space is normally thought of is in terms of a scale factor. This scale factor is time dependent, and it increases with time at a rate that depends on the amount of matter and energy in the Universe, the cosmological constant, etc. The scale factor is related to Hubble's constant.

Here is a good example of what the scale factor means physically: Imagine that you draw a triangle from three galaxies at the corners. Then as time goes on the universe expands and the galaxies get further apart. The scale factor gives you the increase in the area of the triangle as a function of time.

[Agent : Orange]

If I'm understanding "unbounded" right, it does sound mathematically distinct from a truly "infinite" universe.  And it would mean theoretically, as M. Kaku says, that the farthest place from you in the universe is the back of your own head.   

Yes, the same way a sphere is finite but unbounded. You can go around it as many times as you like and never hit a wall or boundary.

[Agent : Orange]

i have a question, related to the speed of light.
if spacecraft a is moving at the speed of light (c) in one direction and spacecraft b is moving at c in the opposite direction, isnt  the speed of separation 2c ? and therefore no known communication is possible between the 2 craft nor can they see each other as the light from either will not reach the other? (assuming no warpage of space/time faster than light particles etc)
i base this on c being relative to the observer. probably way off base, but wanted to ask.

Okay, this is really an excellent question. It strikes at exactly the heart of relativity. I will also use a tiny bit of very simple math here to make a nice example. But please don't get scared off! The answer is surprising and it makes a great example of what relativity changes for our understanding compared to out intuitive, Newtonian Universe.

So the question is what does it mean for two objects to travel apart from one another at speeds approaching the speed of light, and can the objects communicate with one another using light rays (or any other electromagnetic signals that travel at the speed of light)?

In our day to day life we live in a low velocity world so we're used to thinking of velocities adding together linearly in the way you did above. This makes sense to us because it works when the velocities are small with respect to the speed of light. So let's say one rocketship has speed u and the other v. They are going directly away from one another. The intuitive Newtonian view that we have is to add the velocities together, such that each one would see the other going at a speed we'll call s, where s=u+v. If you think about it this makes sense. If you've ever thrown anything out of a car window in the direction you're moving you'll notice the thing you threw went in front of the car, because you add the speed of the thrown object and the speed of the car together. This makes sense and is intuitive. Adding speeds in this way is called a Galilean transformation because Galileo was the first one to actually write it down. Using a Galilean transformation we expect each ship to see the other going at u+v.

However, as speeds get closer to the speed of light we find something strange going on. Our notions of distance and time start to change because of two effects - time dilation and length contraction. These effects are too small to be noticed in every day life because we move so slowly. But they begin to become significant as we move with speeds that approach light speed. The change in distance and in time conspire together to keep the speed of light, c, the same for everyone regardless of how fast they are moving. It means that the Galilean transformation law - and our intuition - must be wrong! The correct addition law, given by special relativity, looks like this:
s=(u+v)/(1+u*v)
I used * to show multiplication here, and I'm also using units of speed that are with respect to the speed of light, so 0.99 really means 99% the speed of light. This addition law shows us something surprising. First, if u and v are both small, their product u*v is small so then the denominator is close to 1. This means we get back the Galilean s=u+v for low speeds - our familiar world - as we would hope. The surprising thing about it is that when the opposite happens, as we go close to the speed of light, u and v both get close to 1 and so does s, so we can never see any object moving faster than this speed. In fact, for any two speeds u and v that are both less than the speed of light, the speed they will attribute to the other, s, will also be less than the speed of light. So this is the ultimate speed limit for anyone moving through space-time.

As an example let's say both our ships are moving at 99% the speed of light, so u=0.99 and v=0.99. The naive Galilean transformation and our intuition would say that the resulting speed should be s=1.98 so no signal can get from one ship to the other at the speed of light. However, the relativistic addition law says s=0.9999. So each ship actually sees the other moving at 0.9999 times the speed of light, and since this is slower than the speed of light, a signal CAN make it between them! This means that even though both ships are themselves traveling at 99% the speed of light, observers in each would see the other moving slower than we would expect intuitively!

As an aside, let's talk about the assumption we made above that the speed of light must be the same for every observer. Why must this be so?

This is really an elaborate way of saying the laws of physics must be the same for everyone. Light itself is just the solution to a wave equation made from Maxwell's equations of electricity and magnetism, that travels at the speed of light. These equations are true for all observers, regardless of their state of motion. So it must be that each observer in relative motion with respect to one another find this same solution and the same speed of light. Otherwise, if you could pace a light ray, you would not see it as a wave traveling at c anymore, you would see it as some strange "thing", frozen in time and also inconsistent - there are no charges around to support a static distribution of electric and magnetic fields like that. So then it would mean that Maxwell's equations were wrong for the person moving that fast. But this is not so, and we can measure their effects and predictions at high speeds and we never find anything like this. So, Einstein said, if each observer measures the same speed for these wave solutions it must be that each observer sees the same speed of light, and our intuitive understanding of how velocities add, and what space and time mean MUST BE WRONG. And all of the experiments and tests we can do hold this up - Einstein's theories are now among the most well tested in all of science.

[Agent : Orange]

And a quick note, once again things are getting crazy so I will be scarce around here for the next few weeks, but I will try to keep on posting one sentence replies and (hopefully) interesting links once in a while.

[DigitalPigSnuggler]

And a quick note, once again things are getting crazy so I will be scarce around here for the next few weeks, but I will try to keep on posting one sentence replies and (hopefully) interesting links once in a while.

I would like to toss a related idea into the discussion.  When I was young, I came up with all kinds of fanciful plans for powering a spacecraft past the speed of light.  I approached it as an energy supply problem, which in a sense it is, but not in the way that I imagined.

What I learned is that an object acquires mass as it moves faster.  This is due to the equivalency of mass and energy.  An object that is set in motion acquires kinetic energy, and since energy and mass are equivalent, the mass of the object becomes higher than when it is at rest.

This doesn't have much practical effect for most of us in our daily lives, and in fact doesn't really start to show much at speeds below 50% of the speed of light.  At about 90% of the speed of light, though, the effect becomes very significant.  And this becomes the problem: as the object gets closer and closer to the speed of light, the mass of the object gets larger and larger, requiring more and more energy to move it ever faster.  As the speed of the object approaches that of light, its mass approaches infinity -- and it would take an amount of energy approaching infinity to continue to speed it up.

To my disappointment, I realized that it didn't make any difference how big my matter-antimatter rocket engine was.  It couldn't supply enough energy to push my ship past the speed of light.  Nothing could.

That's when I gave up and started drinking instead.

[Agent : Orange]

I would like to toss a related idea into the discussion.  When I was young, I came up with all kinds of fanciful plans for powering a spacecraft past the speed of light.  I approached it as an energy supply problem, which in a sense it is, but not in the way that I imagined.

What I learned is that an object acquires mass as it moves faster.  This is due to the equivalency of mass and energy.  An object that is set in motion acquires kinetic energy, and since energy and mass are equivalent, the mass of the object becomes higher than when it is at rest.

This doesn't have much practical effect for most of us in our daily lives, and in fact doesn't really start to show much at speeds below 50% of the speed of light.  At about 90% of the speed of light, though, the effect becomes very significant.  And this becomes the problem: as the object gets closer and closer to the speed of light, the mass of the object gets larger and larger, requiring more and more energy to move it ever faster.  As the speed of the object approaches that of light, its mass approaches infinity -- and it would take an amount of energy approaching infinity to continue to speed it up.

To my disappointment, I realized that it didn't make any difference how big my matter-antimatter rocket engine was.  It couldn't supply enough energy to push my ship past the speed of light.  Nothing could.

That's when I gave up and started drinking instead.

A bit of fine detail, but the mass increase you talk about is from a very specific definition that's not widely used anymore (see the "controversy" section here, http://en.wikipedia.org/wiki/Mass_in_special_relativity). Proper mass (the mass of a given observer that can be directly measured, see http://en.wikipedia.org/wiki/Invariant_mass) is invariant and does not change (it's the m in mc^2). Relativistic mass can be confusing because it's not true that moving really fast can make something become a black hole (it would have to be a black hole in all reference frames). So it's better to think in terms of energy and invariant mass and clears up a lot of confusion.

But ultimately any way you slice it you are right and your alcoholism is indeed justified - it does take an infinite amount of energy to get anything up to even approach light speed.

[Tarbaby]

Yes, dPS, but as your fuel supply approached infinite mass at least you would have an infinite supply of fuel.

I'm trying to remember if the speed of light constant limits the universes rate of expansion. No matter how good our new giant telescope is we would never see the front wave of the big bang because the universe is expanding as fast or faster than the speed of light so the light rays would never get here. Unless we could cheat by folding space. Which reminds me, I meant to ask Steller about that. He or she has a thread about it. But I could see AO telling me know the speed of light limitation does not relate to the rate of universe expansion because it is inside the bubble and does not pertain to physics outside the bubble. Or he may say it does. It's like the Schrodinger's cat enigma. I can't predict what AO is going to say.

[wr250]

Okay, this is really an excellent question. It strikes at exactly the heart of relativity. I will also use a tiny bit of very simple math here to make a nice example. But please don't get scared off! The answer is surprising and it makes a great example of what relativity changes for our understanding compared to out intuitive, Newtonian Universe.

So the question is what does it mean for two objects to travel apart from one another at speeds approaching the speed of light, and can the objects communicate with one another using light rays (or any other electromagnetic signals that travel at the speed of light)?

In our day to day life we live in a low velocity world so we're used to thinking of velocities adding together linearly in the way you did above. This makes sense to us because it works when the velocities are small with respect to the speed of light. So let's say one rocketship has speed u and the other v. They are going directly away from one another. The intuitive Newtonian view that we have is to add the velocities together, such that each one would see the other going at a speed we'll call s, where s=u+v. If you think about it this makes sense. If you've ever thrown anything out of a car window in the direction you're moving you'll notice the thing you threw went in front of the car, because you add the speed of the thrown object and the speed of the car together. This makes sense and is intuitive. Adding speeds in this way is called a Galilean transformation because Galileo was the first one to actually write it down. Using a Galilean transformation we expect each ship to see the other going at u+v.

However, as speeds get closer to the speed of light we find something strange going on. Our notions of distance and time start to change because of two effects - time dilation and length contraction. These effects are too small to be noticed in every day life because we move so slowly. But they begin to become significant as we move with speeds that approach light speed. The change in distance and in time conspire together to keep the speed of light, c, the same for everyone regardless of how fast they are moving. It means that the Galilean transformation law - and our intuition - must be wrong! The correct addition law, given by special relativity, looks like this:
s=(u+v)/(1+u*v)
I used * to show multiplication here, and I'm also using units of speed that are with respect to the speed of light, so 0.99 really means 99% the speed of light. This addition law shows us something surprising. First, if u and v are both small, their product u*v is small so then the denominator is close to 1. This means we get back the Galilean s=u+v for low speeds - our familiar world - as we would hope. The surprising thing about it is that when the opposite happens, as we go close to the speed of light, u and v both get close to 1 and so does s, so we can never see any object moving faster than this speed. In fact, for any two speeds u and v that are both less than the speed of light, the speed they will attribute to the other, s, will also be less than the speed of light. So this is the ultimate speed limit for anyone moving through space-time.

As an example let's say both our ships are moving at 99% the speed of light, so u=0.99 and v=0.99. The naive Galilean transformation and our intuition would say that the resulting speed should be s=1.98 so no signal can get from one ship to the other at the speed of light. However, the relativistic addition law says s=0.9999. So each ship actually sees the other moving at 0.9999 times the speed of light, and since this is slower than the speed of light, a signal CAN make it between them! This means that even though both ships are themselves traveling at 99% the speed of light, observers in each would see the other moving slower than we would expect intuitively!

As an aside, let's talk about the assumption we made above that the speed of light must be the same for every observer. Why must this be so?

This is really an elaborate way of saying the laws of physics must be the same for everyone. Light itself is just the solution to a wave equation made from Maxwell's equations of electricity and magnetism, that travels at the speed of light. These equations are true for all observers, regardless of their state of motion. So it must be that each observer in relative motion with respect to one another find this same solution and the same speed of light. Otherwise, if you could pace a light ray, you would not see it as a wave traveling at c anymore, you would see it as some strange "thing", frozen in time and also inconsistent - there are no charges around to support a static distribution of electric and magnetic fields like that. So then it would mean that Maxwell's equations were wrong for the person moving that fast. But this is not so, and we can measure their effects and predictions at high speeds and we never find anything like this. So, Einstein said, if each observer measures the same speed for these wave solutions it must be that each observer sees the same speed of light, and our intuitive understanding of how velocities add, and what space and time mean MUST BE WRONG. And all of the experiments and tests we can do hold this up - Einstein's theories are now among the most well tested in all of science.

yes i understand the above; and worked out the answer to my question.
my question was ca 2 ships traveling at (not near, at)  the speed of light (c) away from each other communicate .
since the formula is s=(u+v)/(1+u*v):  s=(1+1)/(1+1*1) ; s= 2/(1+1) ;s=2/2; s=1 therefore no communication can occur by any means we know of  (short of warping space/time or creating wormholes etc). unless other forces come into play at c that im unaware of.

[Agent : Orange]

yes i understand the above.

The short answer is no, it's not twice the speed of light. But I wanted to give a more meaningful response than that.

[DigitalPigSnuggler]

your alcoholism is indeed justified

It's about time that someone agrees with me about that.

[Agent : Orange]

Yes, dPS, but as your fuel supply approached infinite mass at least you would have an infinite supply of fuel.

I'm trying to remember if the speed of light constant limits the universes rate of expansion. No matter how good our new giant telescope is we would never see the front wave of the big bang because the universe is expanding as fast or faster than the speed of light so the light rays would never get here. Unless we could cheat by folding space. Which reminds me, I meant to ask Steller about that. He or she has a thread about it. But I could see AO telling me know the speed of light limitation does not relate to the rate of universe expansion because it is inside the bubble and does not pertain to physics outside the bubble. Or he may say it does.

There is no constraint on the rate at which space can expand. As long as nothing within that space moves faster than the speed of light locally you're okay. Galaxies can become separated in a way that signals can't be passed between them because of the expansion of the Universe.

In regards to spaceships traveling in opposite directions think of it like this: if the spaceships go for a long enough time (billions of years) then the distance between them is going to be large enough to start being affected by the large scale expansion of the Universe. Then their signals will get redshifted out of the observable range and any further communication between them will be impossible.

It's like the Schrodinger's cat enigma. I can't predict what AO is going to say.

lol

[Agent : Orange]

It's about time that someone agrees with me about that.

haha

[wr250]

The short answer is no, it's not twice the speed of light. But I wanted to give a more meaningful response than that.

i just edited that post lol .
and im editing this one. i am aware that no object with mass can attain the speed of light, only the appearance of c or faster to an observer.

classic example: which is faster, driving 20 miles to get over the mountain (fold in space-time) , or taking the 1.5 mile tunnel (wormhole?)  through it?

[DigitalPigSnuggler]

I'm trying to remember if the speed of light constant limits the universes rate of expansion.

It does not.  In fact, according to what seems to be a pretty solid theory, the universe underwent a titanic rate of expansion during a tiny fraction of a second shortly after the big bang.  Not the physical stuff like particles, but space itself.

No matter how good our new giant telescope is we would never see the front wave of the big bang because the universe is expanding as fast or faster than the speed of light so the light rays would never get here.

That's not really the problem.  The universe was suffused with light until about 380K years after the big bang, so anything earlier than that is opaque to us.

It's like the Schrodinger's cat enigma. I can't predict what AO is going to say.

True dat.  He said he was going off to work, so I tried to post something smart and he was still lurking.  You can't trust these goddamn physicists.

[wr250]

True dat.  He said he was going off to work, so I tried to post something smart and he was still lurking.  You can't trust these goddamn physicists.

he is both here and not here at the same time.

[Agent : Orange]

I exist in a superposition of trolling and lurking states.

[zeebo]

It's like the Schrodinger's cat enigma. I can't predict what AO is going to say.

His explanations are clear, until we "open the box" and take a look - then we can't predict whether or not our brains will comprehend them.   

[area51drone]

The way the expansion of space is normally thought of is in terms of a scale factor. This scale factor is time dependent, and it increases with time at a rate that depends on the amount of matter and energy in the Universe, the cosmological constant, etc. The scale factor is related to Hubble's constant.

Here is a good example of what the scale factor means physically: Imagine that you draw a triangle from three galaxies at the corners. Then as time goes on the universe expands and the galaxies get further apart. The scale factor gives you the increase in the area of the triangle as a function of time.

I'm not so sure I like your example, it seems too simplistic.   Are you saying that only with the expansion of space is there an increase in time?   If one could contract space, would time go in reverse?

Here are two more questions I'm not sure you have answers to, but I'm curious what you might say.   Does time exist outside of space?   What is outside of space?

[zeebo]

...Does time exist outside of space?   What is outside of space?

Oooh these are good.  Looking forward to the answers. 

[Tarbaby]

I think we've had this earlier, they have no idea what is outside the space-time bubble. Could be an infinite ocean of lime Jell-O. Just like they have no idea what is inside a black hole.

[area51drone]

I think we've had this earlier, they have no idea what is outside the space-time bubble. Could be an infinite ocean of lime Jell-O. Just like they have no idea what is inside a black hole.

If they can say there might be infinite universe bubbles, then they might have an idea.  Also, I'm curious in respect to time, which I don't believe is generally talked about.    What even is the definition of "space" according to an astrophysicist?   How would one find and know that they've reached the edge of it?     

I think my questions on the contraction of space are especially valid.   If space is expanding, as well as time, then if you can contract space somehow, why wouldn't it have the effect of reversing time within that space?

[zeebo]

... I think my questions on the contraction of space are especially valid.   If space is expanding, as well as time, then if you can contract space somehow, why wouldn't it have the effect of reversing time within that space?

Seems to me like you have a point, if time is one of the inextricable dimensions of spacetime, then warping space should warp time as well (although I don't think it would warp backwards, reversing time's arrow, time would rather just speed up or slow down, relative to other regions.)

One thing I remember from a Hawking book I read years ago btw, was that according to the math, that the Big Bang not only created space, but created time, so that time did in fact have a beginning, and there was nothing "before" it.   

[Tarbaby]

Well, yes. That was my understanding, that time and space are inextricable parts of the same thing. Like, for example, a tapestry has a front and a back. That's why at a certain point they started calling it space-time. Related are the components of gravity and mass.(Mess in it's two states of matter/energy)

I think what makes "time" so confusing is that to our consciousness we know it's relative. A moment for one person may seem like several minutes well to another person 2 feet away it may seem like a millisecond. So, a long time ago they started asking is there any absolute rate of time.

I can understand what a51d is saying above but I agree with zeebo, The rate of time may be variable but I don't think you can reverse it.

[area51drone]

I can understand what a51d is saying above but I agree with zeebo, The rate of time may be variable but I don't think you can reverse it.

There are a fair number of questions regarding this topic.  Is the expansion or contraction of space the same thing as traveling through it?   If it is, then you can use relativistic notions as long as space is not expanding or contracting faster or slower than the speed of light, I would think.   But supposedly space is expanding faster than the speed of light, right?  What then?  What if space is contracting faster than the speed of light?

So, if you were a point on the edge of the universe, would you be carried along with the wave?     Or is it that from a point (coordinate) in space, space is expanding all around that by the definition of time passing - ie the universe will forever expand around that point as time expands?

I guess this begs the question too - suppose the universe did contract as it has been theorized in the past (I think they've said the universe will continue to expand, but let's ignore that if that's the case).   Time wise, as the reverse of the big bang continues, how will that affect time to someone at the edge of space?

It's a huge question, and deserves a much better response than what AO initially responded with.  I understand that you're busy AO, but hey man, inquiring minds want to know!