[MarianaM (OP)]

Richard has been puzzling over the following:

As a neutron star gets closer and closer to an event horizon of a large black hole we understand that gravitational energy is lost as the spiralling pair loose energy by buckling space-time. It’s intuitive that the neutron star rotates around the black hole faster and faster and thus the resultant chirp detection from LIGO. So far so good.

The problem comes for me when, as we are told, that if a friend is in a space craft and they head towards the event horizon, our perception of the space craft as we are viewing it and the rate at which the radio waves of our friend is talking to us gets slower and slower, and at the event horizon itself we perceive the space craft never actually gets past this boundary but gets stuck in time as the space part gets so large the time part needs to get so small.

So how do we get the chirp? Why not a lowering frequency chirp and why do we not see any stars stuck at the event horizon?

Any thoughts?

[Janus]

Basically, as the neutron star spirals in, the orbital period decreases, or seen another way, the frequency of  its orbits increases. The frequency of the gravitational waves depends on the orbital frequency.  And while time dilation effects do increase as you get closer to the BH event horizon, they do not begin to have a stronger effect than the increase in orbital frequency until the neutron star gets very close to the event horizon.  In other words, the increased frequency due to having a faster and faster and faster orbit, overcomes the time dilation effect until the very last moments.  You see a rise in frequency and intensity and then the quick drop off as time dilation finally wins the day.  You get a "chirp".

The upshot here is that the scientists doing these observations know full well how to take everything into account when calculating what to expect from such an event in terms of gravitational waves measured.   Different types of collisions will produce different chirps, and thus the type of chirp you measure gives you the info you need to determine the nature of the event.

You won't see stars "stuck" on the event horizon for a couple of reasons.  One is that they will be torn apart by tidal forces and interaction with other matter (all the x-ray radiation we see coming from BHs is due to collisions between matter outside of the event horizon). The other that is once matter gets really close the event horizon, any light it may give off will be gravitational red-shifted to the point that it is no longer in the detectable frequency range. It, in effect, becomes "invisible".

[evan_au]

To add to the comments from Janus:
A neutron star is not necessarily the best example of this effect
- A neutron star is likely to be torn apart when it gets close to a stellar-mass black hole. Rather than an asymmetric "point source" producing periodic gravitational waves, the amplitude and frequency will die away as the neutron star gets torn into an extended accretion disk (which is more symmetric=less gravitational waves).
- So it's probably better to look at the collision of black holes.
- The singularity at the center can't be torn apart by the nearby black hole.
- But the event horizon can be distorted by an adjacent black hole

As well as light being red-shifted, gravitational waves are also red-shifted.
- LIGO has a fairly limited frequency response - about 50Hz to 1kHz
- So a red-shift of 10:1 would take a 400Hz merger into an undetectable 40Hz

Even more important, LIGO is straining at the limits of dredging recognisable chirps out of a very noisy measurement.
- A 10:1 red shift also implies a 10:1 reduction in power.
- For most LIGO events, that will take it below the background noise into undetectability

When looking at gravitational waves from black hole mergers, it comes in two phases:
- A "chirp" of increasing frequency and amplitude, as the two black holes get closer and spin more quickly
- A "ringdown" phase, where the event horizons have touched, and this asymmetrical dumbbell shape then settles down into a symmetrical sphere (a slightly squashed sphere if the new black hole is spinning)

A 10:1 (or higher) red shift only occurs fairly close to the event horizon.
- When two black holes collide, we are talking about the merger of two objects that are perhaps 10 or 20km across
- Most of the size of these objects is outside the narrow band of high red shift
- And the approach velocity is such that they will cross this narrow band very quickly
- So although the gravitational wave pulse will be distorted, it won't hang on forever
- And we won't be able to detect it forever (just a couple of milliseconds during the ringdown phase)

[AustinnEp]

Standing 299792458 m from a flashlight, both inside of a spaceship going close to the speed of light, how long does it take for you to see the light if you are in the front of the spaceship and the flashlight is in the back? light going 299792458 m/s

[Colin2B]

Standing 299792458 m from a flashlight, both inside of a spaceship going close to the speed of light, how long does it take for you to see the light if you are in the front of the spaceship and the flashlight is in the back? light going 299792458 m/s

The light will travel at exactly the same speed, relative to you, as it would do if you and the flashlight were on the good spaceship earth.

Does your question have any relevance to the topic?
Like most of your other posts, I doubt it. Take a break.

[Janus]

Standing 299792458 m from a flashlight, both inside of a spaceship going close to the speed of light, how long does it take for you to see the light if you are in the front of the spaceship and the flashlight is in the back? light going 299792458 m/s

It depends on whether you, the one standing inside the ship, are measuring the time (in which case it will be 1 sec), or it is being measured by someone who measures the space ship as moving relative to themselves.
So, for example, someone who measures the ship as moving at  299792457 m/sec relative to themselves would measure the light as taking ~6.8 hrs to reach the front of the ship.( conversely, if the light was going from front to back, you in the ship would still measure it as taking 1 sec, but the person "outside" (not moving with the ship) would measure it as taking only 0.00005 sec.)