# What does it mean to say that time stops at the speed of light? Watch

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In special relativity as an objects speed increases, it's time with respect to it's frame decreases and at the speed of light time completely stops.

Imagine I am on a spacecraft and I accelerate and reach the speed of light, in this case let's just say that it's possible for me to do this. Does this mean that events cannot occur inside the spacecraft and I am basically frozen in time?

Would my body still decay? I'm assuming it wouldn't because time is needed for an event to occur.

Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

Imagine I am on a spacecraft and I accelerate and reach the speed of light, in this case let's just say that it's possible for me to do this. Does this mean that events cannot occur inside the spacecraft and I am basically frozen in time?

Would my body still decay? I'm assuming it wouldn't because time is needed for an event to occur.

Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

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#2

wow, some big questions are being asked here. Have you looked at Einstein's postulates that lead to special relativity, if not I suggest you have a quick look. The whole point of SR is that the only true limit is the speed of light (which I'll call SoL to save time). The idea of time not being constant is that it ensure that no matter which reference frame you are in, nothing breaks this limit. Another thing to bear in mind is that as you approach the SoL, your mass increases. Thinking about this in terms of F=ma, or rather a = F/m, this means that a larger and larger force is required to accelerate you by the same amount the closer you approach SoL. At the speed of light, your mass would become infinite, hence it would be impossible to accelerate past c. Therefore only massless objects (photons) can reach c.

With regards to your questions, yes time would go infinitely slowly. This is to ensure that if you threw a ball towards the front of your spacecraft, no observer would see that ball moving faster than SoL. From your perspective, things appear to be normal, but for everyone else looking into the spacecraft from a 'rest frame' the clocks on your ship would have stopped ticking.

Yes your body would still decay. It's all about thinking about who the observer is. If you threw a ball on your spacecraft travelling at SOL, you would see it follow the oath you would expect, at the speed you would expect. But that's because you are in the same reference frame as the ball. By your watch, you would still live to be 100, your hair would go grey etc... However according to me, standing on earth, you would never age, or even move.

With regards to your questions, yes time would go infinitely slowly. This is to ensure that if you threw a ball towards the front of your spacecraft, no observer would see that ball moving faster than SoL. From your perspective, things appear to be normal, but for everyone else looking into the spacecraft from a 'rest frame' the clocks on your ship would have stopped ticking.

Yes your body would still decay. It's all about thinking about who the observer is. If you threw a ball on your spacecraft travelling at SOL, you would see it follow the oath you would expect, at the speed you would expect. But that's because you are in the same reference frame as the ball. By your watch, you would still live to be 100, your hair would go grey etc... However according to me, standing on earth, you would never age, or even move.

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#3

And now for the black hole. Be careful with your use of the word infinite. The gravitational pull of a black hole isn't infinite, or else we would all be falling towards the nearest one. A black hole has finite mass, and hence finite gravity. The whole 'infinitely dense' thing is where it gets complicated, however a maths trick kind of allows infinite density and infinitely small volume to dance out to give finite mass. I do not have the knowledge to answer you question on black holes or dimensions tbh, however if anyone else replies I''d definitely be interested to read their answer.

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But the time of all the atoms inside the ship including my body are at 0 so how can they undergo change? You need time for change to occur. meaning time is literally 0.

From an outsider looking in I would not be moving at all, no matter how long they looked nothing inside the ship would change? I understand that if I was travelling at 90% the speed of light for example then I could still undergo change and if I took a sip of water, for me it would be normal (3 seconds or so) but from an outsider it would take 1 week for me to take that sip or whatever.

So an object with some mass travelling at the speed of light would have infinite mass but does infinite mass mean infinite gravitational density which in turn means infinite gravity pulling the object would pull it faster than light? That is the reason light cannot escape from a black hole in the first place because the inwards pull is faster than the speed of light?

From an outsider looking in I would not be moving at all, no matter how long they looked nothing inside the ship would change? I understand that if I was travelling at 90% the speed of light for example then I could still undergo change and if I took a sip of water, for me it would be normal (3 seconds or so) but from an outsider it would take 1 week for me to take that sip or whatever.

So an object with some mass travelling at the speed of light would have infinite mass but does infinite mass mean infinite gravitational density which in turn means infinite gravity pulling the object would pull it faster than light? That is the reason light cannot escape from a black hole in the first place because the inwards pull is faster than the speed of light?

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#5

(Original post by

But the time of all the atoms inside the ship including my body are at 0 so how can they undergo change? You need time for change to occur. meaning time is literally 0.

From an outsider looking in I would not be moving at all, no matter how long they looked nothing inside the ship would change? I understand that if I was travelling at 90% the speed of light for example then I could still undergo change and if I took a sip of water, for me it would be normal (3 seconds or so) but from an outsider it would take 1 week for me to take that sip or whatever.

So an object with some mass travelling at the speed of light would have infinite mass but does infinite mass mean infinite gravitational density which in turn means infinite gravity pulling the object would pull it faster than light? That is the reason light cannot escape from a black hole in the first place because the inwards pull is faster than the speed of light?

**AishaGirl**)But the time of all the atoms inside the ship including my body are at 0 so how can they undergo change? You need time for change to occur. meaning time is literally 0.

From an outsider looking in I would not be moving at all, no matter how long they looked nothing inside the ship would change? I understand that if I was travelling at 90% the speed of light for example then I could still undergo change and if I took a sip of water, for me it would be normal (3 seconds or so) but from an outsider it would take 1 week for me to take that sip or whatever.

So an object with some mass travelling at the speed of light would have infinite mass but does infinite mass mean infinite gravitational density which in turn means infinite gravity pulling the object would pull it faster than light? That is the reason light cannot escape from a black hole in the first place because the inwards pull is faster than the speed of light?

2) This is correct

3) actually I made a mistake here, what I should have said is that F = d(mv)/dt, however as mass is not constant this can't be simplified to f = ma. Look up relativistic mass for some more info. And again,

**relativistic**mass, meaning that it depends on the observer. While photons don't have mass, they do have momentum. Have you done gravity in school yet? If so, have you don't anything on escape velocity? You can actually calculate the radius of a black hole of a given mass by setting the escape velocity to the SoL (which is how we define the event horizon), and this is why light can't escape. You may have heard of gravitational lensing? Light is affected by gravity.

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(Original post by

1)Remember reference frame, to your their appears to be change, but to me everything would be frozen in time

**Darth_Narwhale**)1)Remember reference frame, to your their appears to be change, but to me everything would be frozen in time

If the time inside the spacecraft is 0. That means change cannot occur which means everything inside the spacecraft is completely static. An outsider observer would not see any movement or change because the object being observed cannot change.

For example if time inside the spacecraft is 0 how do you propose the pilot makes a cup of tea for example? It would take him infinitely long which means he would never move.

Imagine it takes 1million events to make a cup of tea, obviously it takes a much larger amount but let's just say 1million. If t = 0 then the first event can never occur.

Can someone confirm?

Or am I misunderstanding something?

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#7

(Original post by

I disagree with what you said here.

If the time inside the spacecraft is 0. That means change cannot occur which means everything inside the spacecraft is completely static. An outsider observer would not see any movement or change because the object being observed cannot change.

For example if time inside the spacecraft is 0 how do you propose the pilot makes a cup of tea for example? It would take him infinitely long which means he would never move.

Imagine it takes 1million events to make a cup of tea, obviously it takes a much larger amount but let's just say 1million. If t = 0 then the first event can never occur.

Can someone confirm?

Or am I misunderstanding something?

**AishaGirl**)I disagree with what you said here.

If the time inside the spacecraft is 0. That means change cannot occur which means everything inside the spacecraft is completely static. An outsider observer would not see any movement or change because the object being observed cannot change.

For example if time inside the spacecraft is 0 how do you propose the pilot makes a cup of tea for example? It would take him infinitely long which means he would never move.

Imagine it takes 1million events to make a cup of tea, obviously it takes a much larger amount but let's just say 1million. If t = 0 then the first event can never occur.

Can someone confirm?

Or am I misunderstanding something?

In reality, anything with mass can never achieve the speed of light and hence an observer would see the ships clock running ever slower but never actually achieving a complete standstill.

Under these conditions, both the ship and stationary observer reference frames, then do not violate Special Relativity.

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(Original post by

That is why one cannot apply the concept of achieving light speed to something with mass.

In reality, anything with mass can never achieve the speed of light and hence an observer would see the ships clock running ever slower but never actually achieving a complete standstill.

Under these conditions, both the ship and stationary observer reference frames, then do not violate Special Relativity.

**uberteknik**)That is why one cannot apply the concept of achieving light speed to something with mass.

In reality, anything with mass can never achieve the speed of light and hence an observer would see the ships clock running ever slower but never actually achieving a complete standstill.

Under these conditions, both the ship and stationary observer reference frames, then do not violate Special Relativity.

I imagine it like a treadmill, the belt is pulling you back faster than you can run, except it seems to be pulling you back faster than light.

Any physics graduates can answer in detail? With equations if possible.

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#9

Gravity is not infinite inside a black hole, or we would have some serious problems. When a star collapses into a black hole, the mass of the star does not suddenly increase infinitely, it remains (approximately) the same as it was. It's just that density changes.

Your analogy is also completely incorrect. A treadmill moves at a certain speed, whereas gravity is a force, not a velocity.

Your analogy is also completely incorrect. A treadmill moves at a certain speed, whereas gravity is a force, not a velocity.

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#10

(Original post by

In special relativity as an objects speed increases, it's time with respect to it's frame decreases and at the speed of light time completely stops.

Imagine I am on a spacecraft and I accelerate and reach the speed of light, in this case let's just say that it's possible for me to do this. Does this mean that events cannot occur inside the spacecraft and I am basically frozen in time?

Would my body still decay? I'm assuming it wouldn't because time is needed for an event to occur.

Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

**AishaGirl**)In special relativity as an objects speed increases, it's time with respect to it's frame decreases and at the speed of light time completely stops.

Imagine I am on a spacecraft and I accelerate and reach the speed of light, in this case let's just say that it's possible for me to do this. Does this mean that events cannot occur inside the spacecraft and I am basically frozen in time?

Would my body still decay? I'm assuming it wouldn't because time is needed for an event to occur.

Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

So in this situation, if you're travelling at the speed of light. You'll appear frozen to me while it will be the opposite way round for you.

I also don't think you will be able to see that your time is slower by looking at a clock for example. You would have to observe someone/thing else travelling at a far lower speed.

This video shows it better than my explanation https://www.youtube.com/watch?v=aZrjMmMBa_8

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(Original post by

Events can occur normally from your perspective, to anyone else observing at a lower speed you'll seem slower to them.

So in this situation, if you're travelling at the speed of light. You'll appear frozen to me while it will be the opposite way round for you.

I also don't think you will be able to see that your time is slower by looking at a clock for example. You would have to observe someone/thing else travelling at a far lower speed.

This video shows it better than my explanation https://www.youtube.com/watch?v=aZrjMmMBa_8

**Kravence**)Events can occur normally from your perspective, to anyone else observing at a lower speed you'll seem slower to them.

So in this situation, if you're travelling at the speed of light. You'll appear frozen to me while it will be the opposite way round for you.

I also don't think you will be able to see that your time is slower by looking at a clock for example. You would have to observe someone/thing else travelling at a far lower speed.

This video shows it better than my explanation https://www.youtube.com/watch?v=aZrjMmMBa_8

(Original post by

Gravity is not infinite inside a black hole, or we would have some serious problems. When a star collapses into a black hole, the mass of the star does not suddenly increase infinitely, it remains (approximately) the same as it was. It's just that density changes.

Your analogy is also completely incorrect. A treadmill moves at a certain speed, whereas gravity is a force, not a velocity.

**Darth_Narwhale**)Gravity is not infinite inside a black hole, or we would have some serious problems. When a star collapses into a black hole, the mass of the star does not suddenly increase infinitely, it remains (approximately) the same as it was. It's just that density changes.

Your analogy is also completely incorrect. A treadmill moves at a certain speed, whereas gravity is a force, not a velocity.

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#12

(Original post by

With all due respect I don't think you have any idea what you're talking about. I already said that time is relative to the observer but when time = 0 there can be NO change, to ANY observer.

Yes but gravity does pull with a force which gives an object velocity. Things would not move without a force.

**AishaGirl**)With all due respect I don't think you have any idea what you're talking about. I already said that time is relative to the observer but when time = 0 there can be NO change, to ANY observer.

Yes but gravity does pull with a force which gives an object velocity. Things would not move without a force.

Gravity is a force, a force which acts on a mass and gives it an acceleration. Yet another basic principle of physics: if something is moving and no forces act on it, it is still moving.

Also with regards to your previous comment about gravity on earth, that is also wrong. The Earth's gravitational field is described by the equation:

GPE = (GM)/r^2 the units of GPE are obviously N/kg

and therefore the force between two masses (M and m) is:

F = (GMm)/(r^2)

At the surface of the Earth, the field is approximately 9.81 N/kg, which causes an acceleration of, of course, 9.81 m/s^2 (you'll notice the units for the two quantities I've mentioned are actually the same, however they represent two different ways of thinking about mass, as an inertial quantity, and as a gravitational one)

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(Original post by

There is no such thing as time = 0.

**Darth_Narwhale**)There is no such thing as time = 0.

You say that there is no such thing as t=0 but this is just wrong. Photons do not experience time.

I already showed you the time dilation equation and I understand all this is hypothetical for travelling in spacecraft but I want to know at what speed to objects travel in a blackhole? Do they travel at the speed of light? Do they travel faster than the speed of light?

What is the strength of the gravitational pull on a given object falling into a black hole and how does the objects velocity increase as the object becomes 1 dimensional.

I also asked if you could link some equations describing these events... I do not know enough about this to know what to search for which is why I am asking here. There must be some equations which describe the relationship because an objects velocity and the Schwarzschild radius?

I don't understand the equations of relativity so some explanations would be great.

Sorry if I came across as arrogant, I don't mean to be. I just want a detailed answer with calculations from someone who has specifically studied this topic, if possible.

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#14

(Original post by

Indeed I do not know for certain how it works which is why I posted, I was expecting people who actually knew about it to reply.

You say that there is no such thing as t=0 but this is just wrong. Photons do not experience time.

**AishaGirl**)Indeed I do not know for certain how it works which is why I posted, I was expecting people who actually knew about it to reply.

You say that there is no such thing as t=0 but this is just wrong. Photons do not experience time.

Another way to see this is the following: A photon's energy is pc, where p is the momentum, and this energy formula only applies to things at the speed of light with zero mass. Without energy, the photon cannot exist. Any speed other than c, and the energy formula reverts to gamma m c-squared. For light m = 0, so light has no energy at any other speed than c, so at any other speed it does not exist. This includes its rest frame. A rest frame for a photon cannot exist because if it did, the photon would not exist and then there is no rest frame.

What can be said is that for a photon, everything is in one point in spacetime. It probably is more correct to say that there is zero spacetime interval for a photon than time interval. What that implies is that the entire life of a photon occurs simultaneously in one place. At light speed, you die and are you are born at the same time, in the same place.

Again, that applies only if you are travelling at c. If you are an observer of light, and you are travelling at less than c, then as long as you do not accelerate or try and change reference frame, Newton's idea of time applies perfectly well. You measure a time interval. According to you light measures a time interval of zero, but you will measure some non-zero interval. There is no need to bring in Lorentz transforms in the matter as long as we are talking about what you see. Only when trying to calculate what light sees that we call on the Lorentz transforms.

I already showed you the time dilation equation and I understand all this is hypothetical for travelling in spacecraft but I want to know at what speed to objects travel in a blackhole? Do they travel at the speed of light? Do they travel faster than the speed of light?

What is the strength of the gravitational pull on a given object falling into a black hole and how does the objects velocity increase as the object becomes 1 dimensional.

Outside a black hole, you could try and approximate the acceleration withe following equation:

Where M is the black hole's mass, r is the distance from the black hole and R is the Schwarzschild radius. It holds until you get to the Schwarzschild radius, then it stops working because we don't know how it works beyond there.

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#16

**AishaGirl**)

In special relativity as an objects speed increases, it's time with respect to it's frame decreases and at the speed of light time completely stops.

Imagine I am on a spacecraft and I accelerate and reach the speed of light, in this case let's just say that it's possible for me to do this. Does this mean that events cannot occur inside the spacecraft and I am basically frozen in time?

Would my body still decay? I'm assuming it wouldn't because time is needed for an event to occur.

Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

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#17

Every object travels at the speed of light in spacetime. Since photons do not travel in time, they travel at the speed of light in space. We, however, do travel in time, and so cannot travel at the speed of light in space. If we were to do so, in order to keep our spacetime speed the same, time would have to stop.

AishaGirl

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#18

(Original post by

Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

**AishaGirl**)Also at the centre of a black hole the gravity becomes infinite and the mass is condensed into an infinitely small 1 dimensional point, does this mean the mass at the centre of a black hole ends up travelling faster than light as it has infinite gravitational pull? What does it even mean to say something is 1 dimensional? Isn't it impossible to have a 1 dimensional object considering the very building blocks for matter exists in 3 dimensions?

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#19

(Original post by

It is difficult to answer this sort of question, mainly because there no way we could actually see what what a photon sees. To actually know what a photon experiences, we would have to be in the rest frame of the photon. But the idea of rest frame of a photon is paradoxical: A fundamental postulate of SR is that light always, always travels at c. A rest frame is one in which the speed of the photon is zero. But how can the speed of a photon simultaneously be c and 0? It can't so we cannot conclude anything about what photons can see.

Another way to see this is the following: A photon's energy is pc, where p is the momentum, and this energy formula only applies to things at the speed of light with zero mass. Without energy, the photon cannot exist. Any speed other than c, and the energy formula reverts to gamma m c-squared. For light m = 0, so light has no energy at any other speed than c, so at any other speed it does not exist. This includes its rest frame. A rest frame for a photon cannot exist because if it did, the photon would not exist and then there is no rest frame.

What can be said is that for a photon, everything is in one point in spacetime. It probably is more correct to say that there is zero spacetime interval for a photon than time interval. What that implies is that the entire life of a photon occurs simultaneously in one place. At light speed, you die and are you are born at the same time, in the same place.

Again, that applies only if you are travelling at c. If you are an observer of light, and you are travelling at less than c, then as long as you do not accelerate or try and change reference frame, Newton's idea of time applies perfectly well. You measure a time interval. According to you light measures a time interval of zero, but you will measure some non-zero interval. There is no need to bring in Lorentz transforms in the matter as long as we are talking about what you see. Only when trying to calculate what light sees that we call on the Lorentz transforms.

Objects cannot travel at the speed of light or higher. Thus they must travel at a lower speed that that of light. A black hole cannot accelerate them beyond the speed of light thus they must gain velocity less than that of light speed. Their mass on the other hand increases gradually as they gain energy.

Naturally the above paragraph is nothing but speculation. We have absolutely no idea of what could happen in a black hole because there is no way for us to make measurements in a black hole and return them to the outside. In fact, according to us, nothing ever falls into the black hole. It simply gets closer and closer, and falls in slower and slower and then just stops. Time dilation due to gravity is so large that time stops for the black hole. Now I know what I said above for light: that we would still see light with a non-zero time. The difference is that light has a constant speed, but here the object accelerates and this breaks the symmetry of the situation above: You know who is falling in and not.

Outside a black hole, you could try and approximate the acceleration withe following equation:

Where M is the black hole's mass, r is the distance from the black hole and R is the Schwarzschild radius. It holds until you get to the Schwarzschild radius, then it stops working because we don't know how it works beyond there.

**dbs1984**)It is difficult to answer this sort of question, mainly because there no way we could actually see what what a photon sees. To actually know what a photon experiences, we would have to be in the rest frame of the photon. But the idea of rest frame of a photon is paradoxical: A fundamental postulate of SR is that light always, always travels at c. A rest frame is one in which the speed of the photon is zero. But how can the speed of a photon simultaneously be c and 0? It can't so we cannot conclude anything about what photons can see.

Another way to see this is the following: A photon's energy is pc, where p is the momentum, and this energy formula only applies to things at the speed of light with zero mass. Without energy, the photon cannot exist. Any speed other than c, and the energy formula reverts to gamma m c-squared. For light m = 0, so light has no energy at any other speed than c, so at any other speed it does not exist. This includes its rest frame. A rest frame for a photon cannot exist because if it did, the photon would not exist and then there is no rest frame.

What can be said is that for a photon, everything is in one point in spacetime. It probably is more correct to say that there is zero spacetime interval for a photon than time interval. What that implies is that the entire life of a photon occurs simultaneously in one place. At light speed, you die and are you are born at the same time, in the same place.

Again, that applies only if you are travelling at c. If you are an observer of light, and you are travelling at less than c, then as long as you do not accelerate or try and change reference frame, Newton's idea of time applies perfectly well. You measure a time interval. According to you light measures a time interval of zero, but you will measure some non-zero interval. There is no need to bring in Lorentz transforms in the matter as long as we are talking about what you see. Only when trying to calculate what light sees that we call on the Lorentz transforms.

Objects cannot travel at the speed of light or higher. Thus they must travel at a lower speed that that of light. A black hole cannot accelerate them beyond the speed of light thus they must gain velocity less than that of light speed. Their mass on the other hand increases gradually as they gain energy.

Naturally the above paragraph is nothing but speculation. We have absolutely no idea of what could happen in a black hole because there is no way for us to make measurements in a black hole and return them to the outside. In fact, according to us, nothing ever falls into the black hole. It simply gets closer and closer, and falls in slower and slower and then just stops. Time dilation due to gravity is so large that time stops for the black hole. Now I know what I said above for light: that we would still see light with a non-zero time. The difference is that light has a constant speed, but here the object accelerates and this breaks the symmetry of the situation above: You know who is falling in and not.

Outside a black hole, you could try and approximate the acceleration withe following equation:

Where M is the black hole's mass, r is the distance from the black hole and R is the Schwarzschild radius. It holds until you get to the Schwarzschild radius, then it stops working because we don't know how it works beyond there.

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Cheers for the replies kyx, appreciate you taking the time to explain.

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