To me the easiest way to shoot down FTL warp drives would be to point out the paradoxes that arise from FTL communication.

If you shrink the distance between two points, doesn't that negate the FTL telephone issue? 

To me the easiest way to shoot down FTL warp drives would be to point out the paradoxes that arise from FTL communication.

If you shrink the distance between two points, doesn't that negate the FTL telephone issue? 

I would agree, if you can tunnel between the two points, you don't need to go FTL to get there in a shorter amount of time.

What if the moon orbited the earth in the opposite direction? 

What if the moon orbited the earth in the opposite direction? 

Retrograde!

Though supposing that the moon goes a few cm away from earth, wouldn't it get a few cm closer to earth instead? And eventually collide?

Though supposing that the moon goes a few cm away from earth, wouldn't it get a few cm closer to earth instead? And eventually collide?

I once had a dream that i was orbiting the earth (and it was a very realistic one where I could see the lights on the night side) and the moon kept getting closer and closer and I then realized I was orbiting the moon instead!

Though supposing that the moon goes a few cm away from earth, wouldn't it get a few cm closer to earth instead? And eventually collide?

Exactly right.  The Moon's gravity raises tidal bulges on the Earth, and those bulges also pull back on the Moon.  The Earth's rotation drags the bulges a bit away from the Earth-moon line, so there is a little bit of force pulling the Moon forward in its orbit, accelerating it and causing the orbit to expand outward.  

If the Moon instead orbits backward, then the bulge pulls the Moon opposite its orbital motion, slowing it down and causing it to spiral in.  This would continue all the way until it reaches the Roche limit and is torn apart by Earth's gravity.  In fact this inward spiral to destruction will be the fate of any backwards orbiting moon, and will happen to Mars' moon Phobos (perhaps in a few tens to hundreds of millions of years), and Neptune's moon Triton.

I would agree, if you can tunnel between the two points, you don't need to go FTL to get there in a shorter amount of time.

It actually won't work like that.  For tunneling between distant locations the Schrodinger equation will give the wrong predictions, and we must instead use the relativistic Dirac equation, which obeys the causal rules of relativity.  (So when I did the math of quantum tunneling to Saturn, I was technically using the wrong physics, but it is still fun/insightful for showing how quickly tunneling probabilities go to zero even if you ignore the speed of light).

To me the easiest way to shoot down FTL warp drives would be to point out the paradoxes that arise from FTL communication.

Some will say that the FTL drive does not create paradoxes because there are no closed time-like curves in the metric.  That is true in Alcubierre's original formulation.  However, that was for a single warp bubble moving from A to B.  Alcubierre concluded in his paper that it is "probably not very difficult" to construct a space-time using warp drive that does contain closed timelike curves.  Indeed, it is easy.  All you have to do is complete a closed loop under warp conditions, completing the circuit faster than a light signal can.  This path will be a space-like curve, introducing causal paradoxes.  And that's one good way to conclude that the geometry is unphysical.

However, this does not tell you why, in the sense of what exactly is going wrong. It's similar to pointing out that the presence of closed time-like curves in Kerr's metric for the interior of a black hole suggests that the metric is failing there, but it doesn't tell you what went wrong, and if we stopped there we would miss out on very interesting physics to further our understanding.

That leads to an interesting question: How large is the event horizon at the instant it forms in a black hole?

It depends on how the black hole is formed, and is determined uniquely by the mass of the material collapsing into the region.  For any mass distribution, you can calculate its Schwarzschild radius: 2GM/c².  For most objects that radius is much smaller than the size of the object itself, and so there is no event horizon.  But the moment the mass is enclosed within that radius, an event horizon will form at that location.  The space inside of it will continue to collapse to singularity, while the space outside of it takes on the appearance of a black hole, and an event horizon marks the boundary.

The way this works for a collapsing star (or whatever else forms a black hole) is interesting.  Let's consider a collapsing star with a mass of 25 Suns.  The Schwarzschild radius for 25 Suns is 74 km.  If the whole star collapsed within that size, an event horizon would form there the moment the surface fell through it.  (This never happens in the external universe, but we can use the frame of reference of the star's surface, in which case it happens in finite time).

In nature a 25 solar mass star doesn't collapse all the way to a black hole (most of the mass is blown off in the supernova), and what instead happens is the event horizon will form as some portion of the core collapses within its own Schwarzschild radius.  For example the condition might be met when 2 solar masses collapses within 5.9km. The horizon then expands as more mass continues to fall into it.  This is neat to see in relativistic simulations like this one:


[youtube]w8LqxjuDQVU[/youtube]

We could also imagine a black hole being formed from light -- the Kugelblitz.  Light is energy and has a mass/momentum associated with it, and so a spherical shell of light fired inward to a central point will also form an event horizon once it gets within its own Schwarzschild radius.  As in the PBS Space-time Kugelblitz challenge video, it is fun to think about why this condition is utterly unavoidable once it occurs.  You cannot prevent the resulting black hole from forming by trying to reflect the light back out, even if the light is reflected perfectly.

I've got more I'd like to say with other questions that have been raised, e.g. by Alex, but this is all I have time for for now.

Though supposing that the moon goes a few cm away from earth, wouldn't it get a few cm closer to earth instead? And eventually collide?

Exactly right.  The Moon's gravity raises tidal bulges on the Earth, and those bulges also pull back on the Moon.  The Earth's rotation drags the bulges a bit away from the Earth-moon line, so there is a little bit of force pulling the Moon forward in its orbit, accelerating it and causing the orbit to expand outward.  

If the Moon instead orbits backward, then the bulge pulls the Moon opposite its orbital motion, slowing it down and causing it to spiral in.  This would continue all the way until it reaches the Roche limit and is torn apart by Earth's gravity.  In fact this inward spiral to destruction will be the fate of any backwards orbiting moon, and will happen to Mars' moon Phobos (perhaps in a few tens to hundreds of millions of years), and Neptune's moon Triton.

I would agree, if you can tunnel between the two points, you don't need to go FTL to get there in a shorter amount of time.

It actually won't work like that.  For tunneling between distant locations the Schrodinger equation will give the wrong predictions, and we must instead use the relativistic Dirac equation, which obeys the causal rules of relativity.  (So when I did the math of quantum tunneling to Saturn, I was technically using the wrong physics, but it is still fun/insightful for showing how quickly tunneling probabilities go to zero even if you ignore the speed of light).

To me the easiest way to shoot down FTL warp drives would be to point out the paradoxes that arise from FTL communication.

Some will say that the FTL drive does not create paradoxes because there are no closed time-like curves in the metric.  That is true in Alcubierre's original formulation.  However, that was for a single warp bubble moving from A to B.  Alcubierre concluded in his paper that it is "probably not very difficult" to construct a space-time using warp drive that does contain closed timelike curves.  Indeed, it is easy.  All you have to do is complete a closed loop under warp conditions, completing the circuit faster than a light signal can.  This path will be a space-like curve, introducing causal paradoxes.  And that's one good way to conclude that the geometry is unphysical, just like the interior geometry of a rotating black hole in Kerr's metric.  

However, this does not tell you why, in the sense of what exactly is going wrong. It's similar to how pointing out that the presence of closed time-like curves in Kerr's metric for the interior of a black hole suggests that the metric is failing there, but it doesn't tell you what went wrong, and if we stopped there we would miss out on very interesting physics to further our understanding.

That leads to an interesting question: How large is the event horizon at the instant it forms in a black hole?

It depends on how the black hole is formed, and is determined uniquely by the mass of the material collapsing into the region.  For any mass distribution, you can calculate its Schwarzschild radius: 2GM/c².  For most objects that radius is much smaller than the size of the object itself, and so there is no event horizon.  But the moment the mass is enclosed within that radius, an event horizon will form at that location.  The space inside of it will continue to collapse to singularity, while the space outside of it takes on the appearance of a black hole, and an event horizon marks the boundary.

The way this works for a collapsing star (or whatever else forms a black hole) is interesting.  Let's consider a collapsing star with a mass of 25 Suns.  The Schwarzschild radius for 25 Suns is 74 km.  If the whole star collapsed within that size, an event horizon would form there the moment the surface fell through it.  (This never happens in the external universe, but we can use the frame of reference of the star's surface, in which case it happens in finite time).

In nature a 25 solar mass star doesn't collapse all the way to a black hole (most of the mass is blown off in the supernova), and what instead happens is the event horizon will form as some portion of the core collapses within its own Schwarzschild radius.  For example the condition might be met when 2 solar masses collapses within 5.9km. The horizon then expands as more mass continues to fall into it.  This is neat to see in relativistic simulations like this one:


[youtube]w8LqxjuDQVU[/youtube]

We could also imagine a black hole being formed from light -- the Kugelblitz.  Light is energy and has a mass/momentum associated with it, and so a spherical shell of light fired inward to a central point will also form an event horizon once it gets within its own Schwarzschild radius.  As in the PBS Space-time Kugelblitz challenge video, it is fun to think about why this condition is utterly unavoidable once it occurs.  You can't even prevent the resulting black hole from forming by trying to reflect the light back out, even if the light is reflected out perfectly.

I've got more I'd like to say with other questions that have been raised, e.g. by Alex, but this is all I have time for for now.

If the moon got captured around 2 billion years ago instead of it being formed, where would it be today? This would basically be theia never hitting so earth would be smaller, and land masses would probably be different. 

Though supposing that the moon goes a few cm away from earth, wouldn't it get a few cm closer to earth instead? And eventually collide?

Exactly right.  The Moon's gravity raises tidal bulges on the Earth, and those bulges also pull back on the Moon.  The Earth's rotation drags the bulges a bit away from the Earth-moon line, so there is a little bit of force pulling the Moon forward in its orbit, accelerating it and causing the orbit to expand outward.  

If the Moon instead orbits backward, then the bulge pulls the Moon opposite its orbital motion, slowing it down and causing it to spiral in.  This would continue all the way until it reaches the Roche limit and is torn apart by Earth's gravity.  In fact this inward spiral to destruction will be the fate of any backwards orbiting moon, and will happen to Mars' moon Phobos (perhaps in a few tens to hundreds of millions of years), and Neptune's moon Triton.

I would agree, if you can tunnel between the two points, you don't need to go FTL to get there in a shorter amount of time.

It actually won't work like that.  For tunneling between distant locations the Schrodinger equation will give the wrong predictions, and we must instead use the relativistic Dirac equation, which obeys the causal rules of relativity.  (So when I did the math of quantum tunneling to Saturn, I was technically using the wrong physics, but it is still fun/insightful for showing how quickly tunneling probabilities go to zero even if you ignore the speed of light).

To me the easiest way to shoot down FTL warp drives would be to point out the paradoxes that arise from FTL communication.

Some will say that the FTL drive does not create paradoxes because there are no closed time-like curves in the metric.  That is true in Alcubierre's original formulation.  However, that was for a single warp bubble moving from A to B.  Alcubierre concluded in his paper that it is "probably not very difficult" to construct a space-time using warp drive that does contain closed timelike curves.  Indeed, it is easy.  All you have to do is complete a closed loop under warp conditions, completing the circuit faster than a light signal can.  This path will be a space-like curve, introducing causal paradoxes.  And that's one good way to conclude that the geometry is unphysical, just like the interior geometry of a rotating black hole in Kerr's metric.  

However, this does not tell you why, in the sense of what exactly is going wrong. It's similar to how pointing out that the presence of closed time-like curves in Kerr's metric for the interior of a black hole suggests that the metric is failing there, but it doesn't tell you what went wrong, and if we stopped there we would miss out on very interesting physics to further our understanding.

That leads to an interesting question: How large is the event horizon at the instant it forms in a black hole?

It depends on how the black hole is formed, and is determined uniquely by the mass of the material collapsing into the region.  For any mass distribution, you can calculate its Schwarzschild radius: 2GM/c².  For most objects that radius is much smaller than the size of the object itself, and so there is no event horizon.  But the moment the mass is enclosed within that radius, an event horizon will form at that location.  The space inside of it will continue to collapse to singularity, while the space outside of it takes on the appearance of a black hole, and an event horizon marks the boundary.

The way this works for a collapsing star (or whatever else forms a black hole) is interesting.  Let's consider a collapsing star with a mass of 25 Suns.  The Schwarzschild radius for 25 Suns is 74 km.  If the whole star collapsed within that size, an event horizon would form there the moment the surface fell through it.  (This never happens in the external universe, but we can use the frame of reference of the star's surface, in which case it happens in finite time).

In nature a 25 solar mass star doesn't collapse all the way to a black hole (most of the mass is blown off in the supernova), and what instead happens is the event horizon will form as some portion of the core collapses within its own Schwarzschild radius.  For example the condition might be met when 2 solar masses collapses within 5.9km. The horizon then expands as more mass continues to fall into it.  This is neat to see in relativistic simulations like this one:


[youtube]w8LqxjuDQVU[/youtube]

We could also imagine a black hole being formed from light -- the Kugelblitz.  Light is energy and has a mass/momentum associated with it, and so a spherical shell of light fired inward to a central point will also form an event horizon once it gets within its own Schwarzschild radius.  As in the PBS Space-time Kugelblitz challenge video, it is fun to think about why this condition is utterly unavoidable once it occurs.  You cannot prevent the resulting black hole from forming by trying to reflect the light back out, even if the light is reflected perfectly.

I've got more I'd like to say with other questions that have been raised, e.g. by Alex, but this is all I have time for for now.

Wow Thanks Wat- and I though the black hole electron was interesting- now we have a light black hole too
About FTL or pseudo-FTL, so I guess we are back to other dimensions then.  The only way to do it without breaking the laws of physics is to have other dimensions to tunnel through to make the distance shorter (like burrowing through the Earth.)

Though supposing that the moon goes a few cm away from earth, wouldn't it get a few cm closer to earth instead? And eventually collide?

Exactly right.  The Moon's gravity raises tidal bulges on the Earth, and those bulges also pull back on the Moon.  The Earth's rotation drags the bulges a bit away from the Earth-moon line, so there is a little bit of force pulling the Moon forward in its orbit, accelerating it and causing the orbit to expand outward.  

If the Moon instead orbits backward, then the bulge pulls the Moon opposite its orbital motion, slowing it down and causing it to spiral in.  This would continue all the way until it reaches the Roche limit and is torn apart by Earth's gravity.  In fact this inward spiral to destruction will be the fate of any backwards orbiting moon, and will happen to Mars' moon Phobos (perhaps in a few tens to hundreds of millions of years), and Neptune's moon Triton.

I would agree, if you can tunnel between the two points, you don't need to go FTL to get there in a shorter amount of time.

It actually won't work like that.  For tunneling between distant locations the Schrodinger equation will give the wrong predictions, and we must instead use the relativistic Dirac equation, which obeys the causal rules of relativity.  (So when I did the math of quantum tunneling to Saturn, I was technically using the wrong physics, but it is still fun/insightful for showing how quickly tunneling probabilities go to zero even if you ignore the speed of light).

To me the easiest way to shoot down FTL warp drives would be to point out the paradoxes that arise from FTL communication.

Some will say that the FTL drive does not create paradoxes because there are no closed time-like curves in the metric.  That is true in Alcubierre's original formulation.  However, that was for a single warp bubble moving from A to B.  Alcubierre concluded in his paper that it is "probably not very difficult" to construct a space-time using warp drive that does contain closed timelike curves.  Indeed, it is easy.  All you have to do is complete a closed loop under warp conditions, completing the circuit faster than a light signal can.  This path will be a space-like curve, introducing causal paradoxes.  And that's one good way to conclude that the geometry is unphysical, just like the interior geometry of a rotating black hole in Kerr's metric.  

However, this does not tell you why, in the sense of what exactly is going wrong. It's similar to how pointing out that the presence of closed time-like curves in Kerr's metric for the interior of a black hole suggests that the metric is failing there, but it doesn't tell you what went wrong, and if we stopped there we would miss out on very interesting physics to further our understanding.

That leads to an interesting question: How large is the event horizon at the instant it forms in a black hole?

It depends on how the black hole is formed, and is determined uniquely by the mass of the material collapsing into the region.  For any mass distribution, you can calculate its Schwarzschild radius: 2GM/c².  For most objects that radius is much smaller than the size of the object itself, and so there is no event horizon.  But the moment the mass is enclosed within that radius, an event horizon will form at that location.  The space inside of it will continue to collapse to singularity, while the space outside of it takes on the appearance of a black hole, and an event horizon marks the boundary.

The way this works for a collapsing star (or whatever else forms a black hole) is interesting.  Let's consider a collapsing star with a mass of 25 Suns.  The Schwarzschild radius for 25 Suns is 74 km.  If the whole star collapsed within that size, an event horizon would form there the moment the surface fell through it.  (This never happens in the external universe, but we can use the frame of reference of the star's surface, in which case it happens in finite time).

In nature a 25 solar mass star doesn't collapse all the way to a black hole (most of the mass is blown off in the supernova), and what instead happens is the event horizon will form as some portion of the core collapses within its own Schwarzschild radius.  For example the condition might be met when 2 solar masses collapses within 5.9km. The horizon then expands as more mass continues to fall into it.  This is neat to see in relativistic simulations like this one:


[youtube]w8LqxjuDQVU[/youtube]

We could also imagine a black hole being formed from light -- the Kugelblitz.  Light is energy and has a mass/momentum associated with it, and so a spherical shell of light fired inward to a central point will also form an event horizon once it gets within its own Schwarzschild radius.  As in the PBS Space-time Kugelblitz challenge video, it is fun to think about why this condition is utterly unavoidable once it occurs.  You can't even prevent the resulting black hole from forming by trying to reflect the light back out, even if the light is reflected out perfectly.

I've got more I'd like to say with other questions that have been raised, e.g. by Alex, but this is all I have time for for now.

If the moon got captured around 2 billion years ago instead of it being formed, where would it be today? This would basically be theia never hitting so earth would be smaller, and land masses would probably be different. 

I often wonder what would be the dominant species on the planet and which group of animals would eventually achieve technological sophistication if that big asteroid never hit and giant reptiles were still roaming the planet.

I'm not 100% sure that causality should be considered an unbreakable "law" of physics, because (as an example) we can use MWI to clean up the paradoxes.  The other requirements are more stringent, but I don't view causality as a "law of physics."  (Just like we can have different arrows of time, going in different directions in different universes and yet they can all still be forward relative to their own universe.)

Some of these oddities may just be mathematical abstractions but they're still fun to think about!

What's the type of fluid flow in Jupiter's atmosphere that creates these clouds formations?

I think it's just called turbulent flow, which is what leads to the vortices and mixing.  The other type of flow is "laminar", where the fluid flows in smooth sheets without breaking up into turbulence.

Are you sure there isn't more to it?

This looks very similar to what I see in Jupiter's atmosphere (near the poles) but I'm pretty sure there's a specific name for the flow pattern associated with Rayleigh-Banard Convection and turbulence other than turbulent flow. Some type of fluvial instability I think.
https://upload.wikimedia.org/wikipedia/ ... 170525.jpg

collide with a planet or star

Which is death in all the best ways

This question started off from a discussion in the Gameplay forum about being able to destroy a planet with a push of a button. Would a ship powered by an Alcubierre warp drive be able to destroy a planet by ramming it at FTL speeds? Would it be able to make a star go nova? How would you even measure the kinetic energy of a ship traveling in a warp bubble?

Some of the counterarguments we were speculating about include: Would the pressure of a planet's atmosphere on the warp bubble cause it to collapse? Would interacting with a planet's gravity well cause the warp bubble to collapse? If the warp bubble failed before the ship impacted the ground, you'd expect that it would only crash with the intrinsic speed that it had in the flat-space region inside the bubble.

Would a ship powered by an Alcubierre warp drive be able to destroy a planet by ramming it at FTL speeds?

No.

But the required power source would.  Any ship with the capability to do warp travel would have the energy reserves to create relativistic weaponry or simply directed energy weapons that could roast a planet.

Would it be able to make a star go nova?

No.  Hot plasma would get inside of the warp bubble and the ship would cook.

How would you even measure the kinetic energy of a ship traveling in a warp bubble?

This is a question for Watsisname, but I don't think that applies since the only kinetic energy in the system would be that of the ship inside the bubble which is STL.

Would the pressure of a planet's atmosphere on the warp bubble cause it to collapse?

Maybe.  In regards to gameplay in SE, probably will be impossible to use a warp drive inside a planets atmosphere.  Given the requirements for negative mass/energy/vacuum pressure being inside of an atmosphere probably wouldn't be conducive to creating a stable warp bubble.

Would interacting with a planet's gravity well cause the warp bubble to collapse?

Probably not since it would be localized and gravity on such small scales is fairly weak, and definitely not in regards to gameplay.

If the warp bubble failed before the ship impacted the ground, you'd expect that it would only crash with the intrinsic speed that it had in the flat-space region inside the bubble.

Even if it crashed with the bubble engaged the only kinetic energy is that of the ship inside the bubble.

Is is possible for winged spacecrafts to fly(or should I call this atmospheric surfing?) in gasgiant's thin upper atmosphere and come back to orbit safely again?