log in or register to remove this ad

 

Faster than light travel

shawnhcorey

Explorer
Again, the warp bubble complicates this. It takes the interior of the bubble outside everybody's light cone - things inside are causally disconnected from the rest of the universe while it exists. If the event that is "staring" effectively happens inside, there's no violation.

But eventually the bubble must be popped. So there are still 2 events outside each other's light cone: creating the bubble and popping it.
 

log in or register to remove this ad

Umbran

Mod Squad
Staff member
My brain is just kinda hurting by the concept that something which causes gravity is the very thing that prevents gravity escaping it. I get stuck in this circular self-referential loop, which basically means I don't understand it.

If anyone who doesn't do tensor calculus tells you they understand General Relativity, they are simply incorrect. So, don't sweat it. Grokking in fullness requires heavy math that not everyone gets exposed to.

The point shawn is making is an issue if you only look at gravity as a particle or a wave - the typical quantum mechanical view. But, as we know, QM and Relativity don't get on so well. And, back in classical mechanics, there's a third thing gravity is - spacetime curvature.

General Relativity is all about calculating that spacetime curvature. And, it does not care about the whole particle/wave thing. From this viewpoint, there is nothing traveling from within the hole to the outside world, so the speed with which it does so is irrelevant.
 

shawnhcorey

Explorer
My brain is just kinda hurting by the concept that something which causes gravity is the very thing that prevents gravity escaping it. I get stuck in this circular self-referential loop, which basically means I don't understand it.

And that is why there is a search for alternatives.

"Black holes do not exist—at least, not as we know them, says renowned physicist Stephen Hawking, potentially provoking a rethink of one of space's most mysterious objects." No Black Holes Exist
 

Umbran

Mod Squad
Staff member
But eventually the bubble must be popped. So there are still 2 events outside each other's light cone: creating the bubble and popping it.

Sure. But you get no paradox, no closing of the curve, unless a round-trip is allowed, which may not be the case for the Alcubierre Drive.
 

shawnhcorey

Explorer
The point shawn is making is an issue if you only look at gravity as a particle or a wave - the typical quantum mechanical view. But, as we know, QM and Relativity don't get on so well. And, back in classical mechanics, there's a third thing gravity is - spacetime curvature.

Curved space-time means the radial distance is increased to infinite. Since everything must travel the speed of light or slower, nothing can escape a black hole, not even gravity.

Some think that the curved space-time detaches itself from the mass and remains in its inertial frame but this has been shown to be incorrect.
 


Umbran

Mod Squad
Staff member
And that is why there is a search for alternatives.

"Black holes do not exist—at least, not as we know them, says renowned physicist Stephen Hawking, potentially provoking a rethink of one of space's most mysterious objects." No Black Holes Exist

From that same article: "I would caution against any belief that Hawking has come up with a dramatic new solution answering all questions regarding black holes," said theoretical physicist Sean Carroll at the California Institute of Technology, who did not participate in this study. "These problems are very far from being resolved."
 

Umbran

Mod Squad
Staff member
If you can do FTL once, you can do it many times.

Moving slower than light, there are places in spacetime, places outside your light cone, that you cannot reach. If you can move at sub-light speed, you can move many times, but there are places you can't get to.

There's no reason, a priori, that moving FTL removes ALL restrictions on travel. It may be that, though you are moving FTL, you could do so many times, but there would still places you cannot get to.
 

shawnhcorey

Explorer
There's no reason, a priori, that moving FTL removes ALL restrictions on travel. It may be that, though you are moving FTL, you could do so many times, but there would still places you cannot get to.

Except that special relativity allows you to change things by merely changing your velocity. If there are 2 events outside each other's light cone, the order in which they happen can be changed by changing your velocity. The past is not fixed; it depends on your point of view.
 

tomBitonti

Adventurer
Not any more than gravity slows down light - which is to say not at all. Things with zero mass (photons and gravitons) move at a fixed speed in vacuum. Rather than slowing down, they lose energy - meaning the their wavelengths get red-shifted. Saying "light cannot escape a black hole" is saying that light originating within the black hole gets red-shifted to nothing.
That's true for light originating at the event horizon. Light originating within the event horizon follows a path that reaches the singularity at the center of the black hole. Any direction in which light can be emitted within a black hole points towards the singularity.
Thanks!
TomB
 


Umbran

Mod Squad
Staff member
That's true for light originating at the event horizon. Light originating within the event horizon follows a path that reaches the singularity at the center of the black hole. Any direction in which light can be emitted within a black hole points towards the singularity.

I'm aware.

Sorry, I do more editing of my comments for a broader audience than some of the rest of you, and that was a level of complexity I didn't think relevant for this discussion.
 

Umbran

Mod Squad
Staff member
Except that special relativity allows you to change things by merely changing your velocity. If there are 2 events outside each other's light cone, the order in which they happen can be changed by changing your velocity. The past is not fixed; it depends on your point of view.

But, again - sub-light, there are places you CANNOT get to. If they are outside your light cone when you start, there is NO WAY to ever get there.

Some of the reading I've done on the Alcubierre drive suggests that it may have similar, but far more complicated, aspects. There may be regions of spacetime that it cannot reach, such that the closed curve is impossible.

Whether it is true? Moot, until they bring the energy requirements down by 30 orders of magnitude or so. That's unlikely to happen in any of our lifetimes, so I am happy to toy with the possibility. If you aren't happy to do so... that's your choice.
 



tomBitonti

Adventurer
I'm aware.

Sorry, I do more editing of my comments for a broader audience than some of the rest of you, and that was a level of complexity I didn't think relevant for this discussion.
Sure, but this is important to understanding how things work inside a black hole. There are a lot of misconceptions that are cleared up knowing that all movement within a black hole at less than or equal to the speed of light points to the center of the black hole.
Thanks,
TomB
 

Umbran

Mod Squad
Staff member
Sure, but this is important to understanding how things work inside a black hole.

Except, of course, it isn't. For the majority of readers here, it is a curiosity, and nothing more. It doesn't even fit into the science literacy needed to make reasonable decisions for day-to-day life in a technological culture.

There is a strong argument that since by definition these things cannot be observed, they fall into the pile of "non-falsifiable" items that aren't even science, per se.
 

tomBitonti

Adventurer
Except, of course, it isn't. For the majority of readers here, it is a curiosity, and nothing more. It doesn't even fit into the science literacy needed to make reasonable decisions for day-to-day life in a technological culture.
There is a strong argument that since by definition these things cannot be observed, they fall into the pile of "non-falsifiable" items that aren't even science, per se.
Well sure. What can be said about the interior of a black hole is arguably philosophy, not physics. One simple alternative view of black holes is that they have no interior. No currently conceivable experiment can measure anything about the interior. The point is important for anyone wanting to deepen their understanding of things. Special relativity is a subject that extends beyond black holes. But the real problem is that the statement is quite misleading. I don't think it should be provided even casually, as it leads folks to incorrect thinking. For example, it seems to allow a chain of in-falling devices, each with an emitter and a receiver, relaying signals outwards, while removing the incremental red-shift of the prior device.
Thanks!
TomB
 

GreyLord

Hero
Well sure. What can be said about the interior of a black hole is arguably philosophy, not physics. One simple alternative view of black holes is that they have no interior. No currently conceivable experiment can measure anything about the interior. The point is important for anyone wanting to deepen their understanding of things. Special relativity is a subject that extends beyond black holes. But the real problem is that the statement is quite misleading. I don't think it should be provided even casually, as it leads folks to incorrect thinking. For example, it seems to allow a chain of in-falling devices, each with an emitter and a receiver, relaying signals outwards, while removing the incremental red-shift of the prior device.
Thanks!
TomB

Isn't Physics...pure physics...mostly a math driven thing? Thus, there are things that they can't even observe but which they can see the math predict which are theories.

In such a way, couldn't the math be part of the evidence regarding the interior of a black hole or special relativity?
 

shawnhcorey

Explorer
In such a way, couldn't the math be part of the evidence regarding the interior of a black hole or special relativity?

You can prove anything with math; it all depends on your assumptions. Nobody knows what the interior of a black hole is like, so the assumptions run wild. I read one article where gravity was reversed inside the event horizon. Everything was pushed out to the event horizon. The math was good but the assumptions were debatable.
 

An Advertisement

Advertisement4

Top