Time Travel possible or not?

[Famine]

I run into the end of my understanding pretty quickly.

So does everyone - that's why there's a number of bets in the community about certain properties of black holes

 

[DolHaus]

Did you just try to educate a spacecraft flight engineer in stellar phenomena?

I feel that being a theoretical subject his guess is as good as mine and I've known talented engineers who can't put together flat pack furniture so assuming ones abilities or knowledge based on their job title is never entirely wise.


Somebody please explain the zero surface area and infinite mass thing? Even if it is highly condensed then it condenses into something whether that be the size of a moon, a car or an electron, it doesn't just disappear. Where does it get the extra mass that takes it from a sun with finite mass to an impossible object that somehow doesn't destroy everything else by simply existing? Its relative weight may change due to gravity but its mass surely remains the same?

Not quite.

The gravitational focus is enough to stretch spacetime to a degree that it holds no meaning relative to the remainder of the universe. If we could see into a black hole we'd see that it has just formed, and everything that has fallen into it is still falling, and will never appear to reach it, slowing down as it gets closer to the center - asymptotically approaching zero speed from our perspective. They are like stopped time across infinite space (again, from our perspective). Where they exist in our universe and when is a meaningless question. The place where that star collapsed is no longer well defined.

Edit:

@Famine, admittedly as a spacecraft engineer (navigator) I know more about this than they lay person, but I'm no astrophysicist unfortunately. I run into the end of my understanding pretty quickly.

Sounds like a technicality, because we can't define them they aren't worth discussing?
Back to the original subject that was time travel, my point is that there was a time before the black hole existed so although we (theoretically) may still be able to see the black hole forming we can't see beyond this point. Its not time travel but instead its like watching a video of a deceased person, you can see them but that doesn't mean they still exist. Just because we can't observe the time passing doesn't mean its not still passing, it's always moving forward at a relative speed, never stops, never goes backwards much like gravity always pulls and never pushes

 

[Danoff]

Sounds like a technicality, because we can't define them they aren't worth discussing?
Back to the original subject that was time travel, my point is that there was a time before the black hole existed so although we (theoretically) may still be able to see the black hole forming we can't see beyond this point. Its not time travel but instead its like watching a video of a deceased person, you can see them but that doesn't mean they still exist. Just because we can't observe the time passing doesn't mean its not still passing,

It's not like watching a deceased person. It's like watching a feather fall. Time is literally passing at a slower rate inside the black hole than outside. It's not a trick of light. It's not that we'd just get the information slowly, it's that the time progresses at different speeds. This is why we have to make relativistic corrections on spacecraft clocks.

When I say an object that first got trapped into the black hole (assuming it was headed right for the center) hasn't hit the bottom yet - I mean it hasn't hit the bottom yet, not that the light from it hitting bottom just hasn't reached us yet. It won't ever hit the bottom of that black hole as far as we're concerned (we being outside the black hole) because time for the object is moving much slower and the space it is traveling along is so stretched that our universe will end before it hits.

 

[Liquid]

Not to go all "using Sci-Fi to explain real space phenomena" here but isn't 'time dilation' the reason for the 600 year gap in Rimmerworld? Is that in any way similar to the passage of time inside or outside of a black hole?

Space Corps directive 557284/B orders @Famine to explain.

 

[DolHaus]

It's not like watching a deceased person. It's like watching a feather fall. Time is literally passing at a slower rate inside the black hole than outside. It's not a trick of light. It's not that we'd just get the information slowly, it's that the time progresses at different speeds. This is why we have to make relativistic corrections on spacecraft clocks.

When I say an object that first got trapped into the black hole (assuming it was headed right for the center) hasn't hit the bottom yet - I mean it hasn't hit the bottom yet, not that the light from it hitting bottom just hasn't reached us yet. It won't ever hit the bottom of that black hole as far as we're concerned (we being outside the black hole) because time for the object is moving much slower and the space it is traveling along is so stretched that our universe will end before it hits.

I think we're arguing the same thing from different ends, I may have misinterpreted your earlier mention of black holes to be in terms of using them as a means of time travel rather than just an explanation of relativity

For bonus points, if you were piloting a spacecraft at the speed of light (Impossible, I know) what would you see if you looked in the direction of travel and what would you see in the opposite direction?

 

[GTsail]

For bonus points, if you were piloting a spacecraft at the speed of light (Impossible, I know) what would you see if you looked in the direction of travel and what would you see in the opposite direction?

I have no idea what you would see looking forward, but looking backwards, wouldn't you see whatever you could see out the backwindow of your Delorean?

 

[Famine]

Somebody please explain the zero surface area and infinite mass thing?

Zero size, infinite density.

If a star collapses into a neutron star, it has the same mass in far less space and becomes considerably denser. If it collapses further into a quark star, it has the same mass in far less space and becomes considerably denser. If it collapses into a black hole, it has the same mass in no space at all and becomes infinitely dense.

Fun fact: Neutron stars are so dense that you can see more than 75% of their surface area when you look at them, due to gravitational lensing.

Not to go all "using Sci-Fi to explain real space phenomena" here but isn't 'time dilation' the reason for the 600 year gap in Rimmerworld? Is that in any way similar to the passage of time inside or outside of a black hole?

Space Corps directive 557284/B orders @Famine to explain.

I think that was just an orange swirly thing in space. But yes, relativistic time dilation.

For a properly excellent 'sci-fi' treatment of black holes, try the Stargate SG-1 episode "A Matter of Time".

 

[DolHaus]

Zero size, infinite density.

If a star collapses into a neutron star, it has the same mass in far less space and becomes considerably denser. If it collapses further into a quark star, it has the same mass in far less space and becomes considerably denser. If it collapses into a black hole, it has the same mass in no space at all and becomes infinitely dense.

Fun fact: Neutron stars are so dense that you can see more than 75% of their surface area when you look at them, due to gravitational lensing.

I think that was just an orange swirly thing in space. But yes, relativistic time dilation.

For a properly excellent 'sci-fi' treatment of black holes, try the Stargate SG-1 episode "A Matter of Time".

I understand how you reached the wording of your conclusion but I'm still not all that sure it can occupy zero space and maintain mass, that doesn't make sense. Anything being infinite is impossible as there is a finite amount of mass in the universe

 

[Danoff]

For bonus points, if you were piloting a spacecraft at the speed of light (Impossible, I know) what would you see if you looked in the direction of travel and what would you see in the opposite direction?

You couldn't look in either direction because time would not have moved for you while you were traveling at that speed. If you were accelerated from rest to the speed of light (without any of the adverse side effects like obtaining infinite mass or becoming a fine red mist), you would not notice time passing until you slowed down from the speed of light. It would seem continuous from your perspective, but from the perspective of observers outside you would appear to not have aged or noticed however much time you had been traveling.

 

[DolHaus]

You couldn't look in either direction because time would not have moved for you while you were traveling at that speed. If you were accelerated from rest to the speed of light (without any of the adverse side effects like obtaining infinite mass or becoming a fine red mist), you would not notice time passing until you slowed down from the speed of light. It would seem continuous from your perspective, but from the perspective of observers outside you would appear to not have aged or noticed however much time you had been traveling.

I meant from a visible light perspective, what would you see?

 

[Obelisk]

I meant from a visible light perspective, what would you see?

My educated guess would be you'd see pitch black looking into the black hole, but looking out at the Universe, it'd be heavily distorted.

 

[Famine]

I understand how you reached the wording of your conclusion but I'm still not all that sure it can occupy zero space and maintain mass, that doesn't make sense.

The singularity at the centre of a black hole has the mass of the star that collapsed to form it (plus the mass of anything it absorbs, less the gradual decay effect of Hawking Radiation).

It also occupies zero space. As I said earlier, the conditions past the event horizon are such that space-time is curved so much that the traditional three dimensions of space are no longer at right angles to one another but parallel. As such it cannot be expressed in terms of size because the concept of size ceases to have any meaning.

Density is mass/volume. The singularity at the centre of a black hole has mass but it has no volume. Even if that mass were that of a grain of sand, dividing it by zero leads to an infinite result.

Anything being infinite is impossible as there is a finite amount of mass in the universe

Firstly, are you sure about both of those things?

Secondly, even if you are very sure of that for some reason, remember when you said that black holes weren't all that mysterious and I pointed out that they certainly are given that they break all of our knowledge of physics? Welcome aboard.

 

[DolHaus]

The singularity at the centre of a black hole has the mass of the star that collapsed to form it (plus the mass of anything it absorbs, less the gradual decay effect of Hawking Radiation).

It also occupies zero space. As I said earlier, the conditions past the event horizon are such that space-time is curved so much that the traditional three dimensions of space are no longer at right angles to one another but parallel. As such it cannot be expressed in terms of size because the concept of size ceases to have any meaning.

Density is mass/volume. The singularity at the centre of a black hole has mass but it has no volume. Even if that mass were that of a grain of sand, dividing it by zero leads to an infinite result.
Firstly, are you sure about both of those things?

Secondly, even if you are very sure of that for some reason, remember when you said that black holes weren't all that mysterious and I pointed out that they certainly are given that they break all of our knowledge of physics? Welcome aboard.

I'm enjoying the journey 👍

Why wouldn't there be finite mass in the universe if it all came from the big bang? Even if you can turn all the energy in the universe into mass (somehow?) then you still run into the problem that energy can't be created or destroyed, only changed. Having anything being infinite in energy would surely throw everything else massively out of balance

My educated guess would be you'd see pitch black looking into the black hole, but looking out at the Universe, it'd be heavily distorted.

Forget the black hole, we're now travelling in space 👍

 

[Danoff]

I meant from a visible light perspective, what would you see?

From a visible light perspective, no time has passed. It would look to you like you had teleported.

I know you want me to say something like everything behind you would look like it had stopped and everything in front of you would look like it was moving at 2x speed, but to assess "stopped" and "2x speed" you have to be able to assess it against a passage of time, which you cannot perceive. The speed of light is independent of the speed of the observer. So light from behind you and in front of you still appear as though they're moving at the speed of light in your reference frame - its just that time in your reference frame has stopped while you're traveling at that speed.

If you were traveling ever so slightly below the speed of light and could perceive the trip it would appear that all of the light in front of you between where you started and where you ended would hit you in the amount of time you perceive for the trip. So if you were traveling slow enough to perceive a trip 100,000 light years long in... say... 1 second, you would see 100,000 years of light pass in 1 second (for the uninitiated this part is time). It would look like 100,000 light years of the universe compressed into 1 light second (for the uninitiated this part is distance). Behind you 1 second would tick by as normal.

 

[Famine]

I'm enjoying the journey 👍

Why wouldn't there be finite mass in the universe if it all came from the big bang?

Well, the thing about the Big Bang is that it erupted from a singularity (and possibly the only example of a White Hole). Like other singularities, it had infinite density, packing the entire mass of the universe into an area with no dimensions (but for opposite reasons - a black hole's singularity breaks the fabric of space-time, the Big Bang's created it).

Even if you can turn all the energy in the universe into mass (somehow?) then you still run into the problem that energy can't be created or destroyed, only changed. Having anything being infinite in energy would surely throw everything else massively out of balance.

Matter, rather than mass - energy can have mass after all. And energy and matter are broadly interchangeable from the universe's point of view, given that the immediate aftermath of the Big Bang was the conversion of energy to matter.

However, that aside you have to look at what you may mean with terms like "infinite". I have no idea how you'd even go about weighing it, but I often hear the figure of 10^50kg for all of the 'ordinary matter' (excluding energy, dark energy, dark matter and the interstellar medium) in the observable universe (the bit we can detect), but more importantly than that the universe contains all of the matter and energy that there has ever been and ever will be (which is the same thing), and it contains a lot of it. It may be an infinite amount.

It also contains all of the space-time there has ever been, though this is increasing at a rate of 2.7 x 10^25 km3s per second - which means that its own density is decreasing - and similarly this may be an infinite amount. I've no idea when it will stop expanding and some theories do not ever have it stopping.

So the universe may be of infinite size and contain infinite energy. Or it may not.

 

[homeforsummer]

a White Hole

A white hole?










(Couldn't resist...)

 

[Obelisk]

A white hole?










(Couldn't resist...)

Please don't start another time loop in here...

 

[DolHaus]

From a visible light perspective, no time has passed. It would look to you like you had teleported.

I know you want me to say something like everything behind you would look like it had stopped and everything in front of you would look like it was moving at 2x speed, but to assess "stopped" and "2x speed" you have to be able to assess it against a passage of time, which you cannot perceive. The speed of light is independent of the speed of the observer. So light from behind you and in front of you still appear as though they're moving at the speed of light in your reference frame - its just that time in your reference frame has stopped while you're traveling at that speed.

If you were traveling ever so slightly below the speed of light and could perceive the trip it would appear that all of the light in front of you between where you started and where you ended would hit you in the amount of time you perceive for the trip. So if you were traveling slow enough to perceive a trip 100,000 light years long in... say... 1 second, you would see 100,000 years of light pass in 1 second (for the uninitiated this part is time). It would look like 100,000 light years of the universe compressed into 1 light second (for the uninitiated this part is distance). Behind you 1 second would tick by as normal.

I remember reading somewhere that you would see a singular point of light directly in front you while travelling at light speed because your eyes could only detect the light you were colliding with as you passed through it. As for behind, I was hoping someone would know but alas, I guess it would just be darkness. I'm glad that all of this is practically impossible, its getting complicated

the universe contains all of the matter and energy that there has ever been and ever will be (which is the same thing), and it contains a lot of it. It may be an infinite amount.

But surely this is the very definition of finite?


Is it possible that black holes are balancing force that counters the effect of the big bang? Singularity explodes, universe develops over massive amount of time, more and more suns turn into black holes until they eventually absorb everything and each other until all the matter is in one place before becoming unstable and starting the whole thing again

 

[Danoff]

I remember reading somewhere that you would see a singular point of light directly in front you while travelling at light speed because your eyes could only detect the light you were colliding with as you passed through it. As for behind, I was hoping someone would know but alas, I guess it would just be darkness. I'm glad that all of this is practically impossible, its getting complicated

No it wouldn't be darkness. Your question is better answered by the one I posed which is that you're traveling at slightly less than the speed of light. So say you travel 100,000 light years in 1 second (from your perspective, from the perspective of other observers it takes you 100,000 years). It would be a bright flash of light over that 1 second because you would see 100,000 light years of universe compressed into a space that looks like 1 light second of universe.

If you look behind you, you're getting passed by light from behind you at a rate of 1 second over 100,000 years. So one second of light behind you moves past you in 100,000 years. But you only experience 1 second of time in that 100,000 years, so you see 1 second of light in 1 second of time (from your perspective). In otherwords, things look normal.

 

[DolHaus]

No it wouldn't be darkness. Your question is better answered by the one I posed which is that you're traveling at slightly less than the speed of light. So say you travel 100,000 light years in 1 second (from your perspective, from the perspective of other observers it takes you 100,000 years). It would be a bright flash of light over that 1 second because you would see 100,000 light years of universe compressed into a space that looks like 1 light second of universe.

If you look behind you, you're getting passed by light from behind you at a rate of 1 second over 100,000 years. So one second of light behind you moves past you in 100,000 years. But you only experience 1 second of time in that 100,000 years, so you see 1 second of light in 1 second of time (from your perspective). In otherwords, things look normal.

But you can only see those things because of light, if you are accelerating away at the same speed as light then the light can't reach your eyes therefore darkness?

 

[Famine]

But surely this is the very definition of finite?

It's finite in the sense that it's bounded by the universe, but if the universe itself is not finite then there is no upper limit. All we know about the universe's size is that the observable universe (what we can see) is finite and increasing in size at a rate of 2.7 x 10^25 km3s per second and that there are other observable universes that are behaving similarly and have some overlap with ours but we cannot ever observe their observers.

Is it possible that black holes are balancing force that counters the effect of the big bang? Singularity explodes, universe develops over massive amount of time, more and more suns turn into black holes until they eventually absorb everything and each other until all the matter is in one place before becoming unstable and starting the whole thing again

Yes - that's why it's posited that the Big Bang is a (or rather 'the') white hole. It matches the physical description of black hole singularities, with an extreme concentration of mass and an absence of space-time, but is a region of space that nothing can access rather than a region of space that nothing can escape. And it may be the case that white holes require black holes and vice versa to solve several of the problems we have with black holes - like the information loss paradox.

Although be wary of using terms like "in one place", as both black holes and, if they exist, white holes represent an absence of space-time (through destruction and origin respectively) so there isn't really a "place" any more. Mind you, I used the term "region", and that wouldn't apply either...

 

[dylansan]

This simulation of falling into a black hole was posted on reddit recently:


One of the criticisms is that it doesn't show the acceleration of time in the universe behind you as you plummet in, but I think it's sufficiently terrifying as is.

There's also these, which are probably more accurate:






http://jila.colorado.edu/~ajsh/insidebh/schw.html

 

[eran0004]

No it wouldn't be darkness. Your question is better answered by the one I posed which is that you're traveling at slightly less than the speed of light. So say you travel 100,000 light years in 1 second (from your perspective, from the perspective of other observers it takes you 100,000 years). It would be a bright flash of light over that 1 second because you would see 100,000 light years of universe compressed into a space that looks like 1 light second of universe.

Follow-up question: If you were exposed to the light (and other radiation) from 100,000 light years of universe over one second, would that turn you into a piece of charred coal or would you actually be able to survive? I mean, you should be absorbing a lot of energy in a short time, right?

 

[DK]

This simulation of falling into a black hole was posted on reddit recently:


One of the criticisms is that it doesn't show the acceleration of time in the universe behind you as you plummet in, but I think it's sufficiently terrifying as is.

There's also these, which are probably more accurate:






http://jila.colorado.edu/~ajsh/insidebh/schw.html

That looks like a job for VR!

 

[Danoff]

Is it possible that black holes are balancing force that counters the effect of the big bang? Singularity explodes, universe develops over massive amount of time, more and more suns turn into black holes until they eventually absorb everything and each other until all the matter is in one place before becoming unstable and starting the whole thing again

Yea, that's why I posted that in the deep thoughts thread. Black holes appear to me to be an imperfection in our particular universe. A place where our physical laws break down and matter/energy can effectively leave our universe - tearing a hole in it and leaving behind a permanent wake (which we call a black hole). Hawking theorized that a nearly infinite number of universes were created with the big bang, and that many of those universes (or all of them) would have instabilities in their physical laws that would lead to their collapse. What might that look like? It would look like singularities in the laws of physics that cause matter and energy to collapse right out of the universe. It would look like an ever increasing number of black holes.

It occurred to me just last night that the gravity wave from the center of the black hole will not ever reach beyond it because that wave has to propagate over an infinite amount of spacetime. As a result, the gravity experienced from a black hole is leftover entirely from its formation.

But you can only see those things because of light, if you are accelerating away at the same speed as light then the light can't reach your eyes therefore darkness?

If you're traveling (not accelerating) away from that light at the same speed that it is traveling toward you you do not experience time (which I've mentioned a few times earlier). That's why I said you were traveling at below the speed of light, say slow enough that you experience 1 second (instead of zero seconds) to travel 100,000 light years. At that point you would be passed by 1 second worth of light over 100,000 light years and you would experience 1 second of time over 100,000 light years of travel. So it would look normal (directly behind you).

Follow-up question: If you were exposed to the light (and other radiation) from 100,000 light years of universe over one second, would that turn you into a piece of charred coal or would you actually be able to survive? I mean, you should be absorbing a lot of energy in a short time, right?

Probably it would be the cosmic rays that would turn you into swiss cheese, or any of the tons of radiation that would bake you, or the light that would blind you. Yea, you need to be isolated from all of that. Presumably you have a ship with like... forward shields or something. Maybe boost them with auxiliary power for the trip.