Score
Title
260
AskScience Panel of Scientists XVIII
624
AskScience AMA Series: IAmA restoration ecologist focused on restoring oysters to the NY Harbor in New York City. AMA!
969
How does sunscreen stop you from getting burnt?
2844
How are drugs like antidepressants (who’s effects aren’t immediately apparent) developed?
5494
In the last 5-10 years, there’s been tremendous efforts made by many of the first world countries to curb carbon emissions. Have we made a dent?
8
Why does the space shuttle's transonic transition end so abruptly (see linked video)?
30
How do spacewalking astronauts get rid of body heat and CO2 they generate?
3
How does a catalyst affect activation energy?
3122
Why doesn't microwave energy escape through the holes in the screen of a microwave oven?
5
Why does the United States generally have a colder climate than most of Western Europe despite being at the same lattitude, and some parts even being further south?
15
How does the mind make up a physical feeling you've never experienced before while dreaming? For example, a virgin having a wet dream or having a foot amputated?
2
How often do collisions in LEO happen?
4
Why do people start to shrink once they get past a certain age?
3
Do bees have spatial memory when foraging for food?
7
How do we know the composition of Earth's core?
3
Suppose in an X speaking country, a child is raised by 2 parents; one speaks Y language around them, the other speaks Z language. What's the science behind this kid learning to differentiate all three languages and eventually being able to speak all 3, as opposed to if they were only exposed to 1?
25
Why does alcohol kill bacteria?
22
Why is the output power greater than the input power for a microwave?
5
Why do 3 polarising filters, at certain angles, allow some light through when two are at right-angles?
0
What happens do humans blood brain barrier as we age?
5
How do plants react to opiates? If I were to, say, use fentanyl as fertilizer, would the plant die? And what about other popular types of drugs, such as coke?
9
When can you consoder a organism dead?
2
Is chemical rocket exhaust usually a plasma?
12
Does hydraulic fracturing contribute to increased seismic activity(earthquakes) and how likely is it for the mix of chemicals+water ,that are used to displace the shale, to contaminate wells and body’s of water?
7
Why do car wheels look like they're rolling backward when moving fast?
4
Does everything rotate in space? If so, why?
2
Why don't lakes with streams flowing into other lakes eventually completely drain into the other lake?
9
Do animals have a really good sense of time or is it confirmation bias by humans?
10
What is the difference between an imaginary friend and a hallucination? What about Tulpas?
4
Where do scientists come up with temporal timelines for dinosaurs?
4987
This may be a stupid question, but what defines GMO. Is it simply changing a plant through cross pollinating (at its simplest level) such as Mendel, or does GMO mean laboratory tested and genetically altered through a laboratory?
14
What defines an “oil”?
136
Why is an Alpha particle denoted as a Helium atom?
0
When you pull at a slinky from the top, why does the bottom take a second to go with it?
3
a spider learns to build a net from other spiders or is it instinct ?
6
Why do molecules interact with others when they both have 8 valence electrons?
5
Is it more energy efficient to put cold water in a kettle, or hot water?
8
Do the planets in our solar system all orbit the sun on the same plane?
1
Which regions of the brain show diminished cell count with aging?
8
If Radiation is cumulative in our body, is Bluetooth (even with BLE) harmful to humans if we are constantly exposed to it with cellphones, smartwatch, etc. and also from other's electronic devices around us?
0
Can you feel the cold in space?
188
Are there any "weird" uses for heavy/transuranic elements?
22 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.
22 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.