log in or register to remove this ad

 

Faster than light travel

tomBitonti

Adventurer
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?
well, math isn’t evidence. Math is how we organize evidence and create a tractable model for how things work. For black holes, my understanding is that it’s our past experience that guides us. Past experience says that physics doesn’t stop applying in places we can’t see. Then we don’t consider matter that has fallen outside of the visible universe because of cosmological expansion to have left the universe. Likewise, we expect physics to work more or less the same inside a black hole as outside. “Less” is mostly about what happens at the exact center, which has conditions that our understanding of physics can’t describe.
I was agreeing with Umbran’s second statement. (While setting it aside. I think it is irrelevant to my original point.) We can’t measure what happens inside a black hole. Strictly speaking, as an unmeasurable thing, it is not proper material for science.
Thanks!
TomB
 

log in or register to remove this ad

briggart

Explorer
well, math isn’t evidence. Math is how we organize evidence and create a tractable model for how things work. For black holes, my understanding is that it’s our past experience that guides us. Past experience says that physics doesn’t stop applying in places we can’t see. Then we don’t consider matter that has fallen outside of the visible universe because of cosmological expansion to have left the universe. Likewise, we expect physics to work more or less the same inside a black hole as outside. “Less” is mostly about what happens at the exact center, which has conditions that our understanding of physics can’t describe.
I was agreeing with Umbran’s second statement. (While setting it aside. I think it is irrelevant to my original point.) We can’t measure what happens inside a black hole. Strictly speaking, as an unmeasurable thing, it is not proper material for science.
Thanks!
TomB
There is no fundamental reason why matter falling outside the cosmological horizon would be lost to us forever, it depends on the nature of the current accelerated expansion phase. For all we know this could be temporary, similar to the primordial inflationary expansion, and if that is the case at some time in the remote future "we" will get back in touch with that matter. On the other hand, I don't think we have any reasonable idea of how we could get back in touch with matter inside a black hole horizon at all.

This doesn't really affect the overall discussion about FTL, black holes, etc., but there is some ground for looking at cosmological and black hole horizons in different ways.
 

shawnhcorey

Explorer
There is no fundamental reason why matter falling outside the cosmological horizon would be lost to us forever, it depends on the nature of the current accelerated expansion phase. For all we know this could be temporary, similar to the primordial inflationary expansion, and if that is the case at some time in the remote future "we" will get back in touch with that matter. On the other hand, I don't think we have any reasonable idea of how we could get back in touch with matter inside a black hole horizon at all.

This doesn't really affect the overall discussion about FTL, black holes, etc., but there is some ground for looking at cosmological and black hole horizons in different ways.

Einstein's gravity states that the radial distance is increased to infinity. Since nothing can travel faster than light, anything that falls into a black hole will never reach the event horizon. It will fall forever.

This is one of the unresolved anomalies of black holes. Which is why some scientists are looking at alternatives.
 

Umbran

Mod Squad
Staff member
Isn't Physics...pure physics...mostly a math driven thing?

There is no such thing as "pure physics". No science is of value if it doesn't mix in, get its hands dirty, and actually demonstrate itself through experiment or observation of predictions.

Thus, there are things that they can't even observe but which they can see the math predict which are theories.

Beware of what you call "theory", for the common and technical uses vary. For people actually in the field, a "theory" is a model well established and tested by many high quality experiments. A model that has not been tested is an hypothesis.

Not that the world is consistent about this, but it is important. Einstein's General Relativity is referred to a a theory, because it has withstood a whole lot of testing. Meanwhile, so called "String Theories" have not, and are more properly considered hypotheses.

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

Mathematics can be an inspiration, sure. But a mathematical model is not "evidence".
 

Umbran

Mod Squad
Staff member
This is one of the unresolved anomalies of black holes. Which is why some scientists are looking at alternatives.

To quote Tyson - "The Universe is under no obligation to make sense to you."

It is less an anomaly, and more a thing that makes some physicists uncomfortable. Like mathematics, what makes you uncomfortable can be inspiration, but it isn't evidence.
 

shawnhcorey

Explorer
To quote Tyson - "The Universe is under no obligation to make sense to you."

It is less an anomaly, and more a thing that makes some physicists uncomfortable. Like mathematics, what makes you uncomfortable can be inspiration, but it isn't evidence.

If an object is accelerated forever, it will gain infinite kinetic energy. This would violate the 1st law of thermodynamics. Black holes make me more than comfortable.

The universe does not have to make sense but science is making sense of the parts that we can.
 

Umbran

Mod Squad
Staff member
If an object is accelerated forever, it will gain infinite kinetic energy.

Sure, but that's a nonsensical statement, because "forever" is not an attainable moment. At any given time, it has finite KE.

Moreover, there's going to be a point in that descent in which the object fails to be a classical, or even relativistic object. Long before forever, it will spaghettify into bits that are better described as quantum objects, and that simple classical description will no longer hold. And, eventually, we'd expect that it would cease to be a separate object from the star that collapsed - the quantum objects having gained enough energy to do something like tunneling to the singularity, come into the region where spacetime is more like a quantum foam, or the like - and there's nothing to accelerate any more.

This would violate the 1st law of thermodynamics.

The Laws of Thermodynamics only hold for closed systems. The interior of a black hole is certainly open at the event horizon, and may well be open at the singularity.
 
Last edited:

tomBitonti

Adventurer
Einstein's gravity states that the radial distance is increased to infinity. Since nothing can travel faster than light, anything that falls into a black hole will never reach the event horizon. It will fall forever.

This is one of the unresolved anomalies of black holes. Which is why some scientists are looking at alternatives.
Well, what is the state of the universe at a particular time? Is it based on our awareness of an event based on when a signal from that event reaches us? Or do we consider that time has elapsed since the event created the signal that we perceived? The sense that there is a universal “now” doesn’t work in special relativity.
That said, I struggle with this same question.
It is true that we never see a crossing of the event horizon. The signal is stretched out over longer and longer time and red shifted away. But see the preceding paragraph.
Also, the infinite distance problem was, I thought, a problem of the coordinate system being used. A more careful choice of coordinate systems avoids infinities at the event horizon. (That is at the edge of my understanding. I’m thinking others could explain this last point better.)
Thanks!
TomB
 

Umbran

Mod Squad
Staff member
Also, the infinite distance problem was, I thought, a problem of the coordinate system being used. A more careful choice of coordinate systems avoids infinities at the event horizon.

So, spacetime at and across the event horizon is smooth. There is no physical discontinuity. Signals trying to escape to the distant stars get further and further red-shifted as you approach the event horizon, but that's not a physical infinity. It only seems like one to a distant observer.

The only real "infinity" in a black hole is at the singularity itself, which we don't get to look at.
 

shawnhcorey

Explorer
Sure, but that's a nonsensical statement, because "forever" is not an attainable moment. At any given time, it has finite KE.

Moreover, there's going to be a point in that descent in which the object fails to be a classical, or even relativistic object. Long before forever, it will spaghettify into bits that are better described as quantum objects, and that simple classical description will no longer hold. And, eventually, we'd expect that it would cease to be a separate object from the star that collapsed - the quantum objects having gained enough energy to do something like tunneling to the singularity, come into the region where spacetime is more like a quantum foam, or the like - and there's nothing to accelerate any more.



The Laws of Thermodynamics only hold for closed systems. The interior of a black hole is certainly open at the event horizon, and may well be open at the singularity.

The pint is that the KE is forever increase, with no known source. That violates the 1st law.

The quanta cannot tunnel into the singularity. They are too far away. The probability of them doing so is almost zero.

The universe is a closed system because we can see a fixed distance into the past. All the energy we see back to the big bang is finite. Or would be if there weren't any black holes.

Well, what is the state of the universe at a particular time? Is it based on our awareness of an event based on when a signal from that event reaches us? Or do we consider that time has elapsed since the event created the signal that we perceived? The sense that there is a universal “now” doesn’t work in special relativity.
That said, I struggle with this same question.
It is true that we never see a crossing of the event horizon. The signal is stretched out over longer and longer time and red shifted away. But see the preceding paragraph.
Also, the infinite distance problem was, I thought, a problem of the coordinate system being used. A more careful choice of coordinate systems avoids infinities at the event horizon. (That is at the edge of my understanding. I’m thinking others could explain this last point better.)
Thanks!
TomB

The problem with the coordinate system is that a flat-space system is used. But space is not flat. It is impossible to get close to the event horizon. It is infinitely far away.

So, spacetime at and across the event horizon is smooth. There is no physical discontinuity. Signals trying to escape to the distant stars get further and further red-shifted as you approach the event horizon, but that's not a physical infinity. It only seems like one to a distant observer.

The only real "infinity" in a black hole is at the singularity itself, which we don't get to look at.

Signals from above the event horizon get red-shifted. There are no signals from the event horizon or below it. Nothing can escape the event horizon.
 

Umbran

Mod Squad
Staff member
The pint is that the KE is forever increase, with no known source. That violates the 1st law.

The quanta cannot tunnel into the singularity. They are too far away. The probability of them doing so is almost zero.

The probability of tunneling is almost zero, at any particular moment. But there's two things to remember:

1) the probability increases as energy increases. So, as you like to keep noting that the energy goes to infinity, so, then, does the probabilty approach certainty, because the barrier height is fixed.

2) Long before that certainty, though, we note that almost zero probability at any particular moment means a certainty over "forever". So, eventually, it happens.

The universe is a closed system because we can see a fixed distance into the past. All the energy we see back to the big bang is finite.

We don't see all the way back to the big bang. As you look back in time, the universe eventually becomes opaque before then. We also don't even see the entire universe. We only see a visible universe, limited by lightspeed. There's no indication I'm aware of that the physical universe is not infinite in extent, meaning that the energy overall may well be infinite. There's not a lot of win to be had in discussing the thermodynamics of a system with infinite energy...

We also don't see into black holes. Beyond the event horizon is, for many intents and purposes, outside the visible universe, and possibly outside the physical universe.

Or would be if there weren't any black holes.

Interesting point - I said the interior of the black hole is thermodynamically an open system. That's true. However, that open state is one-way. The only things that can come out of a black hole are the mass/energy, charge, and information that went in, and that only by Hawking radiation. Other than that, black holes are Las Vegas - what happens inside them stays inside them. That'll be important in a minute.

Upthread, I mentioned that I sometimes edit and use simplified language to keep things simple an comprehensible. Well, now we have to be more accurate as we invoke the First Law of Thermodynamics. It does NOT say, "you cannot have infinite kinetic energy". It says that, in a closed system, energy cannot be created nor destroyed. However, we note that it can be transformed.

So, let us consider a closed system that contains one black hole, and one rock. We drop the rock into the black hole. And you say, well, that rock now accelerates infinitely toward the singularity, develops infinite kinetic energy, and breaks the first law!

Then I ask - what was the rock's potential energy of position at the start of the experiment? I answer for you - the rock starts infinitely far from the singularity, so its potential energy of position was infinite! No energy is being created!

Moreover, we can be even more strict, and note that the First Law really says that you cannot get more energy (as work) out of a closed system than you put into it. And now Black Hole Las Vegas comes up. I cannot extract any of that kinetic energy from inside the black hole. Inside the event horizon, all world lines point inexorably inward. There is no path out for that energy. Thus, the First Law still holds.


Signals from above the event horizon get red-shifted. There are no signals from the event horizon or below it. Nothing can escape the event horizon.

Yep. Exactly. For that reason, that which goes on inside the event horizon cannot violate the First Law of Thermodynamics.
 

tomBitonti

Adventurer
If an object is accelerated forever, it will gain infinite kinetic energy. This would violate the 1st law of thermodynamics. Black holes make me more than comfortable.
Note that a thing can (theoretically) accelerate forever tending to a finite velocity. As long as the amount of acceleration decreases over time. A point falling towards the event horizon of a black hole, once it gets close enough, to a distant observer, actually appears to decelerate forever, slowing due to special relativity and the relative position of the observer as the horizon is approached.
Thanks!
TomB
PostScript: Just found this, which has some interesting insights:
"A former student once called the ergosphere the place where little children come from, because nothing can remain at rest there. "
 
Last edited:

shawnhcorey

Explorer
Note that a thing can (theoretically) accelerate forever tending to a finite velocity. As long as the amount of acceleration decreases over time. A point falling towards the event horizon of a black hole, once it gets close enough, to a distant observer, actually appears to decelerate forever, slowing due to special relativity and the relative position of the observer as the horizon is approached.
Thanks!
TomB
PostScript: Just found this, which has some interesting insights:
"A former student once called the ergosphere the place where little children come from, because nothing can remain at rest there. "

Note the word "appears". Yes, if you assume space is flat, it appears to decelerate. But space is not flat. In reality, the object is constantly accelerating and the acceleration keeps increasing.
 




Umbran

Mod Squad
Staff member
No, you have no stated where all that energy is coming from.

In a very real sense, it is a figment of your imagination, and doesn't exist, so it does not "come from" anywhere. There is no finite time at which this energy can be observed in the universe, which makes it a non-physical fiction, a bedtime story to scare children. Where does the energy for the body of Peter Pan come from?

The above is technically correct, but I expect it won't satisfy you. So, I have another, equivalent phrasing.

In an imaginary sense (and, I mean this in terms of the actual math - it is quantum mechanical, so imaginary numbers are involved) the energy is borrowed from the vacuum, and returns to the vacuum when the falling item joins with the singularity (which, despite your protestations, it does do), leaving the overall system of rock + hole with the same mass-energy it started with. This is the usual source for internal, non-physical energies that crop up in math from time to time.
 

shawnhcorey

Explorer
An object falling into a black hole will be accelerated forever. That means its KE is always increasing. That energy must have a source.

A black hole starts as a very large star. Anything falling into the star has a maximum speed when it hits the surface. This means it has a maximum KE.

Then the star goes supernova and leaves a black hole. There is no limit to the KE an object falling into a black hole can acquire. So where did this energy come from?
 

An Advertisement

Advertisement4

Top