Astronomy Question of the Week

[Da6onet]

So who likes watching Cosmos?

If you're interested in learning more about how everything in the universe works, I thought I could post questions once a week (more often if people want) to see who can come up with the correct answer first. Most will be conceptual, though for those with a little more math/physics experience, I can throw in problems that require algebraic manipulation, trig, and/or a little light calculus.

We'll pretend I asked the following question on Sunday for this week 5/11-5/17

On November 1 at 8:30 p.m. you look toward the eastern horizon and see the bright star Bellatrix rising. At approximately what time will Bellatrix rise one week later, on November 8?

[Seoni]

I'm estimating at possibly 7:30 in the morning, and I got this information with its right ascension and declination.

[Da6onet]

I'm estimating at possibly 7:30 in the morning, and I got this information with its right ascension and declination.

The right ascension and declination give it's position relative to the equator and prime meridian on Earth so you can look for it in the same spot in the sky (adjusting for things like time zone and seasons). The link I posted for Bellatrix was just for fun facts (also so you'd know it was a star and not just a Harry Potter character!). You can assume at very far distances, say 250 light years (77 parsecs, 2.4 × 10^18 meters), Bellatrix does not move relative to the background over short time periods (a week is short on stellar motion time scales).

The 1st hint I'll give is that an Earth day as most people know it is not a 360 degree rotation.

[Da6onet]

ANSWER
Because the Earth's synodic day (3) is 4 minutes longer than it's sidereal day (2), stars rise about 4 minutes earlier each night. Thus, a week later, Bellatrix will rise at about 8:02pm (8:00 pm is close enough) on November 8.




NEW QUESTION
So I just saw a trailer for a new Chris Nolan movie - Interstellar.

http://youtu.be/zSWdZVtXT7E


This brings up a good question for this week.

Under what circumstances can you travel faster than the speed of light? Are there examples of this in nature?

[Zero]

If you were on a space ship travelling the speed of light and then walked forward, you would be moving faster than the speed of light. Not sure about examples in nature...

[Da6onet]

If you were on a space ship travelling the speed of light and then walked forward, you would be moving faster than the speed of light. Not sure about examples in nature...

Einstein's special relativity says that to an observer outside the light speed train, time proceeds normally but you and the train are both infinitely thin. Since you traverse no extra distance, you have no additive velocity. Don't worry, in your frame of reference you appear to be walking normally across the train car, but the outside world appears to be stopped.



1st hint General Relativity

[Longboard]

This brings up a good question for this week.

Under what circumstances can you travel faster than the speed of light? Are there examples of this in nature?

Isn't the theory that light travels faster then itself at a black hole or event horizon. I think it is something close to that.

[Tbone]

This brings up a good question for this week.

Under what circumstances can you travel faster than the speed of light? Are there examples of this in nature?

Isn't the theory that light travels faster then itself at a black hole or event horizon. I think it is something close to that.

I believe a black hole is where the gravity is so strong that light is unable to escape the pull. The event horizon is simply the point where that pull becomes so strong. I don't think that gravity speeds up light. It's not faster, but stronger. I could be wrong.

As far as I knew, any attempt at going faster than the speed of light would result in everything relative slowing down. For example, if the speed of light was 60 MPH and you were on a bus going that speed and tried to throw a ball, relative to the ball, the bus would slow down below 60 MPH since the ball cannot exceed that.

[Broin]

Universal expansion shows that it is possible that some galaxies are moving away from us so rapidly that the light from them may me never reach us. Kind of like two cars traveling down the road in the same direction. One car has a head start on the other and is moving faster so that the car behind will never catch up and see the car in front of it. It's possible I suppose with universal expansion  that we'll all be traveling faster than light someday

[Lithium]

If an object is outside of space-time and it is being pushed by the expansion of space-time, the space-time in theory can expand faster than light. This likely happened in the first trillionth of a trillionth of a second after the big bang when the universe expanded faster than light.

[NoCry]

comets baby; comets.

[Da6onet]

Isn't the theory that light travels faster then itself at a black hole or event horizon. I think it is something close to that.

Light as we are defining it here are photons (that behave like waves/rays or particles depending on the situation). While some interesting things happen at the event horizon (such as Hawking radiation), the important idea there is escape velocity is exactly c (where c is the speed of light in a vacuum). Beyond the event horizon no information escapes, so it is fair to say that physics as we define it for our universe does not apply for anything inside this radius. That said, outside the black hole, all mass and energy moving through space is governed by laws of physics and thus cannot exceed the speed of light.

I believe a black hole is where the gravity is so strong that light is unable to escape the pull. The event horizon is simply the point where that pull becomes so strong. I don't think that gravity speeds up light. It's not faster, but stronger. I could be wrong.

As far as I knew, any attempt at going faster than the speed of light would result in everything relative slowing down. For example, if the speed of light was 60 MPH and you were on a bus going that speed and tried to throw a ball, relative to the ball, the bus would slow down below 60 MPH since the ball cannot exceed that.

Correct. An interesting effect of this is that if we threw Longboard into a blackhole, we would see him appear to slow down as he approached the event horizon, then stop at the event horizon. The photons that allow us to see him would be continuously red shifted and fade from view. Doesn't answer the question though! :-)

Universal expansion shows that it is possible that some galaxies are moving away from us so rapidly that the light from them may me never reach us. Kind of like two cars traveling down the road in the same direction. One car has a head start on the other and is moving faster so that the car behind will never catch up and see the car in front of it. It's possible I suppose with universal expansion  that we'll all be traveling faster than light someday

If an object is outside of space-time and it is being pushed by the expansion of space-time, the space-time in theory can expand faster than light. This likely happened in the first trillionth of a trillionth of a second after the big bang when the universe expanded faster than light.

ANSWER
Objects that are within spacetime are limited to the speed of light. Spacetime itself could give two shits about speed "limits" and expands (or contracts) at whatever rate it wants, like when it grew in size by 10^50 around between 10^-40 to 10^-30 seconds after the big bang started. Today we can observe that there are galaxies with a redshift of z>1.4 (where z = 1.4 is a redshift due to moving away at c). An analogy would be two slugs on an expanding balloon (where the ballon is spacetime and the slugs are galaxies. The slugs are moving less than the speed of light in their frame of reference, but the balloon can expand very quickly, thus the slugs are moving apart faster than c.

What's troubling to some cosmologists is that we observe that all galaxies outside our local cluster (where gravity trumps dark energy), are actually accelerating away from us (and the further away, the faster the acceleration). Thus at some point in the far future we will not be able to observe anything beyond our local cluster, the galaxies will appear to freeze in space, red shift and fade from view. It possible/likely that this has already happened and why our observable universe isn't necessarily the whole thing.

There are two leading theories of moving through the universe that don't violate special relativity (moving faster than c through space) and in fact are predicted by general relativity.

1. Connect two points in space via a worm hole. This is hypothosized in general relativity as one possible way supermassive black holes form and gain so much mass so quickly (10^6-10^9 solar masses in a period of 10^5 years or so). It's not technically moving outside space time since the tunnel is just two highly warped regions of space time connecting.

2. Make use of the flexibility of spacetime (expansion/contraction). An Alcubierre warp drive, utilizing negative energy could contract the spacetime immediately in front of it while expanding it immediately behind. The ship itself would not actually be moving, rather it would be "surfing" on a spacetime wavefront.

Broin has the most correct answer so he is this week's smart ass!

[Broin]

FUCK YA!!!  Told ya I'm not just a contract killer!!!!

[Lithium]

Very confused. Broin's answer failed to make any mention of space-time. His 'answer' was simply a statement that two objects can be moving apart faster than light relative to one another's position--but neither object is traveling faster than light.

I only posted today because of how wrong the other answers were and I made clear mention of space-time being the only viable theory of faster than light travel. And even the space-time theories are thin at best.

[Da6onet]

Broin's phrase "universal expansion" and subsequent wording correctly explains the natural example of superluminal separation. Your phrasing that spacetime can expand faster than the speed of light is just another way of saying it using the term spacetime, which is why I included your quote, but Broin posted first and you had some slightly mis-worded information in your post.
It is worth clarifying a few things.
-normal energy/matter as we know it doesn't move outside spacetime, they move with spacetime. Outside of spacetime would indicate moving in some extra dimension (in sci fi this commonly referred to as things like subspace, hyperspace, etc.) String theory theorizes this but no observational data backs it up.
-dark energy is the currently accepted name for the force expanding the universe (that is to say it fits mathematically but we don't understand what it is exactly).
-while the universe did expand rapidly during the inflationary period, it is still expanding today and is actually acceleratng it's rate of expansion-- Hubble's constant is not actually constant.

I would also argue that Einstein's theory of general relativity explains a great deal of how the universe works and there is a large quantity of evidence--binary pulsar period changes, satellites being framed dragged by Earth, and more recently gravity wave detection in the cosmic microwave background. These are all observed effects of spacetime behavior that fit predicted models.
I will admit the two ways of human spaceflight I mentioned are very far-fetched, but again, predicted by the same mathematical models. Real life physicists are trying to prove existence of these ideas (like string theory and WIMPS) and it's neat when they do (like the higgs boson, like negative energy). A theory is only thin until it's verified :-)