Score
Title
547
AskScience Panel of Scientists XVII
414
AskScience AMA Series: I am a squid biologist, AMA!
2913
How do surgeons avoid air bubbles in the bloodstreams after an organ transplant?
6247
Why do joints ache so much when you get the cold/flu?
36
Are all massless particles their own antiparticles?
19
What do prion proteins naturally do in the brain/body?
780
What is the relationship between the rate of change of a function and differentiation?
502
If the energy of photons is continuous, and electron's energy levels around an atom are discreet, then how can you ever have a photon that has the exact energy to be absorbed by an electron?
32
How does coding physically work? How does a computer, made up of inanimate parts, understand what to do based on a made up language?
165
Why do "Y" chromosomes only have 3 chromatids?
9
Does the human body make any noticeable 'microadjustments' when exposed to a particular climate for a length of time?
7
Why do sperm cells have a large nucleus if they only carry half the genetic material?
8
What is thought to happen to quarks during the big rip?
8
How we know certain animals can detect specific scents from X distance away? How are we measuring and determining that?
11068
How do scientists studying antimatter MAKE the antimatter they study if all their tools are composed of regular matter?
3
Are there problems in computer science that no algorithm can solve for all inputs?
1
How do spacelike separated measurements of entangled particles work?
5
Does Urine Affect Plant Transpiration?
1
How do you actually use Density Functional Theory?
205
Can an unvaried diet cause the human body to learn to digest a certain (type of) food faster?
6
Is there a correlation between peoples hearing range and the type of music they like?
60
Why do large metal beams or trusses sometimes have tiny connections/joints?
1
How do you measure forces between individual atoms and molecules?
43
Does Supersymmetry include antimatter?
1
Why do some photos of the heavens show stars radiating light in a 'cross' shape instead of evenly in a circle?
3
I've recently been told that cloning different types of animals varies in difficulty. Is this true and if so what the key challenges in cloning different organisms?
1
Why does Nima Arkani-Hamed say we need an infinitely large apparatus to get rid of quantum uncertainty in measurements?
1
[Engineering] How do modern cars calculate fuel economy?
1
What exactly is the Doppler effect?
6
Why don’t everyday movements cause sub-concussive impacts?
3
Does our mother tongue affect our face features in any way?
1
What is physically different about the brain of someone with an exceptional memory?
29
Why is the waste produced in a thorium fuel cycle need storage for only 300 years instead of thousands of years for uranium fuel cycle, even though U233 from Th232 had mostly similar fission products as U235?
40
How did Scott and Amundsen KNOW when they reached the south pole (100 years ago)?
6
How does convection of heat work in space?
3
[Physics] Has there been significant research relating to anti-matter weaponry?
1
How does a paraconformity originate?
7
Can non ear neurons detect sounds?
990
If 2 black holes were close enough that their event horizons were overlapping, could things in that overlapped region escape those black holes?
4
Is it possible for gravity waves to have a particle nature? If so, what would this particle be like? If not, what sets gravitational waves apart from light and matter, which have particle wave duality?
6
Why is this year's influenza outbreak so much deadlier than previous years?
0
What are fingerprints made of ?
20 Midtek First, the Earth and the ship will never leave the observable universe of the other. That is not possible. What you mean to ask is whether the ship can cross the *cosmological event horizon* for Earth. This event horizon is at the farthest possible distance that a light signal emitted *right now* will eventually reach us. Any light signal emitted farther out will never reach us. This event horizon is also shrinking over time. Eventually, the cosmic event horizon will contain only our Local Group of galaxies. [This graphic](https://imgur.com/YScuwrO) shows what you are looking for. The blue line segment indicates the diameter of the cosmic event horizon as it is right now; it is currently about 16 Gly away. (Note how the event horizon diameter shrinks over time.) On this diagram, the path of a light ray is at a 45-degree angle to the horizontal parallel axes. So suppose we send out a light signal right now, and then when it reaches the event horizon it is sent back (idk, by some bored alien or whatever). What does the path of those two light signals look like? Like the two green line segments [in this image](https://imgur.com/BAufBHt). So how far out does that light signal travel? From the axes, it looks to be about 8 Gly. But that's in co-moving distance. So that means the light signal gets to a galaxy that is currently 8 Gly away from us. By the time the light signal gets there, space would have expanded and the light signal would actually be a proper distance away of greater than 8 Gly. How much has the universe expanded by then? The vertical axis on the right gives the *scale factor* which is the ratio of proper distances at that time to proper distances today. If we trace a horizontal line from the end of the first line green line segment to the vertical axis on the right, we see that the scale factor is about *a* = 2.1. So the light signal will be about 8\*2.1 = 16.8 Gly away from us when it reaches the event horizon. [This calculator](http://home.fnal.gov/~gnedin/cc/) seems to say that a scale factor of 2.1 should correspond to an event horizon proper distance of 17.1 Gly, which is about close enough. (These are tough calculations and the calculator and author of the graph may be using slightly different approximations. I am also estimating distances using the axes in the graph, and so I can just be way off. How long does it take for the light signal to get there? Ehhhh, that's a tough calculation and not one you can read off from this chart since the time axis is in units of *conformal time*, not *cosmological time*, which is how we usually think about time. If you trust that the scale factor at the time the light reflects back is about a = 2.1 and you trust [this calculator](http://home.fnal.gov/~gnedin/cc/), then the light signal reaches the event horizon after about 12 Gyr (that's 12 billion years) after it was emitted from Earth. Strictly speaking it takes an infinite amount of time to come back if it is reflected right from the horizon exactly. So if the light is reflected just before the horizon, then the time until it comes back can be arbitrarily long by just reflecting the signal at distances arbitrarily close to the horizon. So what about your spaceship? Well, the spaceship has to travel slower than light speed. In the graphs I have provided, the path of the ship is not exactly a straight line, even if it travels at constant speed. But the slope of its path must always be greater than 1. Which means the path of the spaceship must lie within the triangle formed by the two green line segments and the x = 0 vertical line in the graphic. In any event, the distances and times gives above are upper bounds and a spaceship can reach those bounds arbitrarily close by just traveling faster. Finally, as for the direction the ship or light signal travels, all of these calculations assume the light/ship is emitted radially outward and then is reflected (or comes back) radially inward. That is, the paths are directly out and directly back.
19 0 Midtek First, the Earth and the ship will never leave the observable universe of the other. That is not possible. What you mean to ask is whether the ship can cross the *cosmological event horizon* for Earth. This event horizon is at the farthest possible distance that a light signal emitted *right now* will eventually reach us. Any light signal emitted farther out will never reach us. This event horizon is also shrinking over time. Eventually, the cosmic event horizon will contain only our Local Group of galaxies. [This graphic](https://imgur.com/YScuwrO) shows what you are looking for. The blue line segment indicates the diameter of the cosmic event horizon as it is right now; it is currently about 16 Gly away. (Note how the event horizon diameter shrinks over time.) On this diagram, the path of a light ray is at a 45-degree angle to the horizontal parallel axes. So suppose we send out a light signal right now, and then when it reaches the event horizon it is sent back (idk, by some bored alien or whatever). What does the path of those two light signals look like? Like the two green line segments [in this image](https://imgur.com/BAufBHt). So how far out does that light signal travel? From the axes, it looks to be about 8 Gly. But that's in co-moving distance. So that means the light signal gets to a galaxy that is currently 8 Gly away from us. By the time the light signal gets there, space would have expanded and the light signal would actually be a proper distance away of greater than 8 Gly. How much has the universe expanded by then? The vertical axis on the right gives the *scale factor* which is the ratio of proper distances at that time to proper distances today. If we trace a horizontal line from the end of the first line green line segment to the vertical axis on the right, we see that the scale factor is about *a* = 2.1. So the light signal will be about 8\*2.1 = 16.8 Gly away from us when it reaches the event horizon. [This calculator](http://home.fnal.gov/~gnedin/cc/) seems to say that a scale factor of 2.1 should correspond to an event horizon proper distance of 17.1 Gly, which is about close enough. (These are tough calculations and the calculator and author of the graph may be using slightly different approximations. I am also estimating distances using the axes in the graph, and so I can just be way off. How long does it take for the light signal to get there? Ehhhh, that's a tough calculation and not one you can read off from this chart since the time axis is in units of *conformal time*, not *cosmological time*, which is how we usually think about time. If you trust that the scale factor at the time the light reflects back is about a = 2.1 and you trust [this calculator](http://home.fnal.gov/~gnedin/cc/), then the light signal reaches the event horizon after about 12 Gyr (that's 12 billion years) after it was emitted from Earth. Strictly speaking it takes an infinite amount of time to come back if it is reflected right from the horizon exactly. So if the light is reflected just before the horizon, then the time until it comes back can be arbitrarily long by just reflecting the signal at distances arbitrarily close to the horizon. So what about your spaceship? Well, the spaceship has to travel slower than light speed. In the graphs I have provided, the path of the ship is not exactly a straight line, even if it travels at constant speed. But the slope of its path must always be greater than 1. Which means the path of the spaceship must lie within the triangle formed by the two green line segments and the x = 0 vertical line in the graphic. In any event, the distances and times gives above are upper bounds and a spaceship can reach those bounds arbitrarily close by just traveling faster. Finally, as for the direction the ship or light signal travels, all of these calculations assume the light/ship is emitted radially outward and then is reflected (or comes back) radially inward. That is, the paths are directly out and directly back.