# The Naked Scientists Forum

#### AndroidNeox

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##### Re: Question about Relativistic Thought Experiments
« Reply #50 on: 28/10/2013 23:57:45 »
Ask a physicist to present an actual calculated result based on Relativity for the amount of time it would take for some object to pass the event horizon and they won't be able to.
They will, given the relevant frame of reference (i.e. that of an infalling observer). See 'Falling into a black hole' and '.. if I fell into a black hole':
Quote from: berkeley.edu
How long does the whole process take? Well, of course, it depends on how far away you start from. Let's say you start at rest from a point whose distance from the singularity is ten times the black hole's radius. Then for a million-solar-mass black hole, it takes you about 8 minutes to reach the horizon. Once you've gotten that far, it takes you only another seven seconds to hit the singularity. By the way, this time scales with the size of the black hole, so if you'd jumped into a smaller black hole, your time of death would be that much sooner.

Neither of those links present the time it takes for the infalling observer to pass the event horizon, from the perspective of an external observer. This is because the time for the external observer is infinite. Only the time for the infalling observer is finite, but since that time rate slows exponentially, the infalling observer's proper time will not reach the 8 minute mark (from your example) until an infinite amount of time has passed externally.

#### dlorde

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##### Re: Question about Relativistic Thought Experiments
« Reply #51 on: 29/10/2013 10:17:28 »
Neither of those links present the time it takes for the infalling observer to pass the event horizon, from the perspective of an external observer. This is because the time for the external observer is infinite.
That's what both the links say:
"So if you, watching from a safe distance, attempt to witness my fall into the hole, you'll see me fall more and more slowly as the light delay increases.  You'll never see me actually get to the event horizon. My watch, to you, will tick more and more slowly, but will never reach the time that I see as I fall into the black hole."

"Penelope is sitting still at a safe distance, watching me fall into the black hole. What does she see?
Penelope sees things quite differently from you. As you get closer and closer to the horizon, she sees you move more and more slowly. In fact, no matter how long she waits, she will never quite see you reach the horizon.
"
Quote
Only the time for the infalling observer is finite, but since that time rate slows exponentially, the infalling observer's proper time will not reach the 8 minute mark (from your example) until an infinite amount of time has passed externally.
That's why it's such an interesting example; that the two equally valid but different frames of reference result in such entirely different experiences for the observers involved. For one (or all external observers), the fall takes an infinite time; for the other (the infaller), it takes a finite, relatively short time.
« Last Edit: 29/10/2013 10:31:55 by dlorde »

#### AndroidNeox

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##### Re: Question about Relativistic Thought Experiments
« Reply #52 on: 29/10/2013 21:04:06 »
That's why it's such an interesting example; that the two equally valid but different frames of reference result in such entirely different experiences for the observers involved. For one (or all external observers), the fall takes an infinite time; for the other (the infaller), it takes a finite, relatively short time.

Exactly. And this is why Einstein insisted event horizons cannot form.

#### dlorde

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##### Re: Question about Relativistic Thought Experiments
« Reply #53 on: 29/10/2013 21:42:10 »
That's why it's such an interesting example; that the two equally valid but different frames of reference result in such entirely different experiences for the observers involved. For one (or all external observers), the fall takes an infinite time; for the other (the infaller), it takes a finite, relatively short time.

Exactly. And this is why Einstein insisted event horizons cannot form.
Not really; Einstein questioned the whole existence of black holes (see On a Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses):
Quote from: Einstein
The essential result of this investigation is a clear understanding as to why the "Schwarzschild singularities" do not exist in physical reality. Although the theory given here treats only clusters whose particles move along circular paths it does not seem to be subject to reasonable doubt that more general cases will have analogous results. The "Schwarzschild singularity" does not appear for the reason that matter cannot be concentrated arbitrarily. And this is due to the fact that otherwise the constituting particles would reach the velocity of light.

Turns out he was wrong on this one. You can't always be right, even if you're Einstein.

#### AndroidNeox

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##### Re: Question about Relativistic Thought Experiments
« Reply #54 on: 30/10/2013 18:39:14 »
In 1938 Einstein thought rotations would hold matter out but later it was realized rotational energy would radiate as gravitational energy, if through no other means. He always insisted that event horizons cannot form.

There is no difference in Relativity between appearance and reality. It's not an illusion that objects in motion or under acceleration contract. It's not an illusion that time passes more slowly for clocks in motion or under acceleration. It's not an illusion that the infalling object slows to a virtual halt outside the event horizon. Spacetime is stretching without bound. It's a bottomless hole. There is absolutely no theoretical justification for presuming that, in this one case, reality does not obey the same rules it does for the rest of Relativity.

The model you present, where observable reality is “an illusion” that is causally irreconcilable with the “real” reality. It violates GR because it’s non-causal.

What the thought experiment shows us is not just what something will look like. It is the paths of information. It defines, for all observers, observable reality. And, observable reality has no causal violations.

Relativity isn't just a theory. It's a complete logical argument. It's a flawless set of statements. The rules Einstein used in developing the theories are essential to the reality of Relativity. It's not possible to use Relativity to argue that event horizons can form because their formation presumes reality does not follow the physical rules that Einstein relied on to develop it. All of Einsteins thought experiments he used to develop his theories from which he derived his equations depended upon the fact that physical reality is entirely consistent with its observable nature.

There are no viable arguments for asserting event horizons can form. If you know of any, I'd be interested in seeing them. Just an unending repetition of "because it's just so" isn't science. It wasn't until years after Einstein was dead that the idea sprang up and I can't find an explanation. Einstein would have stomped on it.

#### dlorde

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##### Re: Question about Relativistic Thought Experiments
« Reply #55 on: 30/10/2013 21:24:01 »
There is no difference in Relativity between appearance and reality. It's not an illusion that objects in motion or under acceleration contract. It's not an illusion that time passes more slowly for clocks in motion or under acceleration. It's not an illusion that the infalling object slows to a virtual halt outside the event horizon. Spacetime is stretching without bound. It's a bottomless hole. There is absolutely no theoretical justification for presuming that, in this one case, reality does not obey the same rules it does for the rest of Relativity.
Yup, I agree with all that.

Quote
The model you present, where observable reality is “an illusion” that is causally irreconcilable with the “real” reality. It violates GR because it’s non-causal.
Ah, no. I explicitly said the two reference frames were equally valid, despite the apparently contradictory experiences. There is only one reality; different observers experience it differently.

Quote
There are no viable arguments for asserting event horizons can form. If you know of any, I'd be interested in seeing them. Just an unending repetition of "because it's just so" isn't science. It wasn't until years after Einstein was dead that the idea sprang up and I can't find an explanation. Einstein would have stomped on it.
Are you suggesting black holes can't exist? A black hole has an escape velocity > c. The event horizon is the distance from it at which the escape velocity becomes c. If event horizon can't form, you won't have a black hole...

#### AndroidNeox

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##### Re: Question about Relativistic Thought Experiments
« Reply #56 on: 31/10/2013 18:55:08 »
The model you present, where observable reality is “an illusion” that is causally irreconcilable with the “real” reality. It violates GR because it’s non-causal.
Ah, no. I explicitly said the two reference frames were equally valid, despite the apparently contradictory experiences. There is only one reality; different observers experience it differently.

Quote
There are no viable arguments for asserting event horizons can form. If you know of any, I'd be interested in seeing them. Just an unending repetition of "because it's just so" isn't science. It wasn't until years after Einstein was dead that the idea sprang up and I can't find an explanation. Einstein would have stomped on it.
Are you suggesting black holes can't exist? A black hole has an escape velocity > c. The event horizon is the distance from it at which the escape velocity becomes c. If event horizon can't form, you won't have a black hole...

Second comment, first: I agree that, under general relativity, black holes are inevitable. But, I am certain that the contemporary model of what black holes are (which includes event horizons and singularities) cannot form, in finite time.

Now, regarding causality. While observers in different reference frames inevitably don't see the world the same way, they can always agree on whether an event takes place. e.g. "Was the coin tossed and it came up heads?" They might not agree on time or location or even sequence for different events, they will agree on the events, themselves. This is fundamental to all science and natural philosophy: Causality.

For any matter to have yet fallen to an event horizon within our universe would not only violate the apparent and observable reality predicted by general relativity, but it would violate causality.

This is why I've proposed a thought experiment in which a mirror is dropped toward an event horizon and a laser beam is bounced off of it. If matter can actually fall through an event horizon, then there will be a point in time where the laser beam stops coming back but I can still see the mirror:
• If the beam can stop while the mirror appears to still hang there, then matter can fall to an event horizon and Relativity is fundamentally wrong.
• If the beam continues to return forever, though it will be redshifted by the motion of the falling mirror, which I could compensate for by perpetually increasing the source laser's frequency (but the returning light will not be shifted by gravity because it took a round-trip) then matter cannot fall to an event horizon and Relativity passes another test.

#### The Naked Scientists Forum

##### Re: Question about Relativistic Thought Experiments
« Reply #56 on: 31/10/2013 18:55:08 »