Musings on Time Travel

[Kev M]

The undisputed fact is that if time travel were actually possible then we would be getting inundated with calls. texts, emails saying..

" have you been affected by time travel ? .... please call xxxx solicitors...no win no fee "

[Stu]

53 minutes ago, Kev M said:

The undisputed fact is that if time travel were actually possible then we would be getting inundated with calls. texts, emails saying..

" have you been affected by time travel ? .... please call xxxx solicitors...no win no fee "

Or even “you are going to be affected by time travel in the future, make your claim before it happens…..” 😉

[saac]

3 hours ago, Kev M said:

The undisputed fact is that if time travel were actually possible then we would be getting inundated with calls. texts, emails saying..

" have you been affected by time travel ? .... please call xxxx solicitors...no win no fee "

How would you know your timeline had been affected - you would have no reference !  I'm not investing in these time travel ambulance chasers .

Jim 

[andrew s]

On 27/03/2022 at 20:47, iantaylor2uk said:

In my earlier comment I was careful to say that fundamental particles sometimes act as if they are point particles (e.g as in the photoelectric effect) and sometimes as waves (e.g. as in diffraction). We don't really understand what a particle is. They must be some sort of stable state of space-time ultimately.

Well within Quantum field theory (QFT), the best theory we have, the field is king. Particles are localised excitations of the field. In most cases there is not a number operator so asking how many particles (e.g. photons) is meaningless. The idea of photon traveling from a star to your eye while appealing is mistaken. In a very  real sense (the maths) photons only exist when created, at the star, and destroyed,  by your eye. The rest of the time you just have an energised quantum field with little or no localisation in to particles.

Regards Andrew 

[andrew s]

On 27/03/2022 at 22:33, saac said:

Not sure energy is a function of time.  But photons certainly do travel in time as said earlier it is a matter of relative frames of reference. Photons will take approximately 11 minutes in our frame of reference to travel from the surface of the sun to Earth.  I'm not sure why you think they do not travel with a passage of time. 

Jim 

I think the issue here is not differentiating between coordinate time and Proper time.

In our frame of reference we set up a Minkowski coordinate system based on SR (neglecting gravity for now). We use this to calculate speeds and in SR the speed of light is fixed and is the same in all inertial frames.

So yes in this frame a light pulse takes time to get from there to here. It is in essence a coordinate time as to do the measurement you need a coordinate system and to synchronise clocks using Einstein's synchronisation convention (which makes the speed of light the same in both directions - not the only option but the one physics chooses).

In doing this we use a clock and ruler. As Einstein said a good local clock measures Proper time and a good local measuring stick measures proper distance. As @vlaiv said before your local clock always measures one second per second  by definition.

So what about proper time for the light pulse well no material clock can travel at the seed of light so Proper time is formally undefined. If you apply the normal SR transformations you get a proper time of zero. However, this is wrong as a light pulse is not in an inertial frame which is what the equations relate. You can't ride along on a light beam.

On time travel if you add in gravity for separated (I.e. non local) events you can't consistently say if two events happen at the same time so all bets are off. 

Personally time travel happens to me all the time - one second per second but seems to get faster as I get older 😊 

Regards Andrew 

 

Edited March 29, 2022 by andrew s

[andrew s]

On 27/03/2022 at 22:43, vlaiv said:

Only if you are away from that mass and looking at the clock next to that mass.

If you were standing on surface of neutron star - your clock would tick rather normally to you - it would be the rest of universe that is "sped up".

By the way - there is very interesting thing that this leads to - and that is:

falling into black hole will make all universe until its end, "play out" in front of your eyes. Similarly - for observer looking at you falling into the black hole - you'll sort of stay frozen close to event horizon slowly fading out of view (red shift).

 This gives a good description of falling into a black hole. Here is the relevant section on what you see as you fall in.

"Will you see the universe end?

If an external observer sees me slow down asymptotically as I fall, it might seem reasonable that I'd see the universe speed up asymptotically—that I'd see the universe end in a spectacular flash as I went through the horizon.  This isn't the case, though.  What an external observer sees depends on what light does after I emit it.  What I see, however, depends on what light does before it gets to me.  And there's no way that light from future events far away can get to me.  Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time."

The whole is well worth a read and gives a full context for the quote.

Regards Andrew 

Edited March 29, 2022 by andrew s

[Elp]

It may seem a far off, impossible idea to travel in time, but like a lot of things in science and science fiction, eg flight, space travel seemed an impossibility too. There's a lot we like to know we understand but fail to do so including our own biology.

[vlaiv]

55 minutes ago, andrew s said:

 This gives a good description of falling into a black hole. Here is the relevant section on what you see as you fall in.

"Will you see the universe end?

If an external observer sees me slow down asymptotically as I fall, it might seem reasonable that I'd see the universe speed up asymptotically—that I'd see the universe end in a spectacular flash as I went through the horizon.  This isn't the case, though.  What an external observer sees depends on what light does after I emit it.  What I see, however, depends on what light does before it gets to me.  And there's no way that light from future events far away can get to me.  Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time."

The whole is well worth a read and gives a full context for the quote.

Regards Andrew 

I was basing my assumption on the fact that light gets blue shifted when it falls into gravity well - and hence all clocks are seen to tick faster (same reasoning as light escaping gravity well is red shifted and clocks are seen to slow down).

You are quite right - we won't be able to see the light that has not reached us yet, but this poses another question:

Will person falling into black hole still see the end of the universe in a "flash" - the same one person floating in empty space would see in huge amount of time?

If "universe clocks" seem to speed up as you descend into a black hole, and light that can't reach us while falling into a black hole - won't reach us - is that equivalent to a "big rip" that will happen due to accelerated expansion of universe?

I'm just wildly guessing here, but what if falling observer does indeed see the end of universe as it will be as to preserve the idea of time speeding up for rest of the universe - only picture that they will see is not what we think it is - but rather what will be visible (or in fact not visible) due to galaxies moving away from us and big rip happening in the end?

[powerlord]

1 hour ago, andrew s said:

I think the issue here is not differentiating between coordinate time and Proper time.

In our frame of reference we set up a Minkowski coordinate system based on SR (neglecting gravity for now). We use this to calculate speeds and in SR the speed of light is fixed and is the same in all inertial frames.

So yes in this frame a light pulse takes time to get from there to here. It is in essence a coordinate time as to do the measurement you need a coordinate system and to synchronise clocks using Einstein's synchronisation convention (which makes the speed of light the same in both directions - not the only option but the one physics chooses).

In doing this we use a clock and ruler. As Einstein said a good local clock measures Proper time and a good local measuring stick measures proper distance. As @vlaiv said before your local clock always measures one second per second  by definition.

So what about proper time for the light pulse well no material clock can travel at the seed of light so Proper time is formally undefined. If you apply the normal SR transformations you get a proper time of zero. However, this is wrong as a light pulse is not in an inertial frame which is what the equations relate. You can't ride along on a light beam.

On time travel if you add in gravity for separated (I.e. non local) events you can't consistently say if two events happen at the same time so all bets are off. 

Personally time travel happens to me all the time - one second per second but seems to get faster as I get older 😊 

Regards Andrew 

 

And as I said earlier, the faster you run.

Plus, I might not have a DeLorean, but my current car lets me travel faster into the future than most folk too 😛

[andrew s]

11 minutes ago, vlaiv said:

I was basing my assumption on the fact that light gets blue shifted when it falls into gravity well - and hence all clocks are seen to tick faster (same reasoning as light escaping gravity well is red shifted and clocks are seen to slow down).

You are quite right - we won't be able to see the light that has not reached us yet, but this poses another question:

Will person falling into black hole still see the end of the universe in a "flash" - the same one person floating in empty space would see in huge amount of time?

If "universe clocks" seem to speed up as you descend into a black hole, and light that can't reach us while falling into a black hole - won't reach us - is that equivalent to a "big rip" that will happen due to accelerated expansion of universe?

I'm just wildly guessing here, but what if falling observer does indeed see the end of universe as it will be as to preserve the idea of time speeding up for rest of the universe - only picture that they will see is not what we think it is - but rather what will be visible (or in fact not visible) due to galaxies moving away from us and big rip happening in the end?

No it takes a finite "time" for the in falling person (or what's left of them) to reach the singularity. There is a calculation on Physicsform some where which gives the numbers but I need to try harder to find it. I put "time" as once inside the event horizon the coordinates become more and more space like and in effect measure the radial distance from the singularity. 

Not sure about the big rip in this context. Currently the scale factor goes to infinity as time does given the current cosmological parameters. The big rip, as currently defined, requires the scale parameter to go to infinity in a finite time.

Not good on wild speculation I find the real science confusing and fascinating enough! 😊

Regards Andrew 

 

[vlaiv]

11 minutes ago, andrew s said:

No it takes a finite "time" for the in falling person (or what's left of them) to reach the singularity. There is a calculation on Physicsform some where which gives the numbers but I need to try harder to find it. I put "time" as once inside the event horizon the coordinates become more and more space like and in effect measure the radial distance from the singularity. 

Not sure about the big rip in this context. Currently the scale factor goes to infinity as time does given the current cosmological parameters. The big rip, as currently defined, requires the scale parameter to go to infinity in a finite time.

Not good on wild speculation I find the real science confusing and fascinating enough! 😊

Regards Andrew 

 

Yes, it does take finite amount of time in reference frame of falling person.

I however wonder if falling person ever reaches singularity or even passes event horizon. All that light from the universe falling after the person (but at the speed of light) - gets blue shifted - closer to event horizon - more so, and at some point it will be seriously gamma in nature and will possibly rip anything material apart?

There are so many thing at play that it is really hard to imagine what will actually happen.

[saac]

1 hour ago, andrew s said:

So what about proper time for the light pulse well no material clock can travel at the seed of light so Proper time is formally undefined. If you apply the normal SR transformations you get a proper time of zero. However, this is wrong as a light pulse is not in an inertial frame which is what the equations relate. You can't ride along on a light beam.

 

This is why I'm frustrated by the often quoted line claiming that the  photon experiences no time when it travels.  You see it so often being claimed and stated without any qualification.  It is yet another area of "popular" physics that is misleading.   My time machine won't be riding a light bean  

Jim    

[saac]

6 minutes ago, vlaiv said:

Yes, it does take finite amount of time in reference frame of falling person.

I however wonder if falling person ever reaches singularity or even passes event horizon. All that light from the universe falling after the person (but at the speed of light) - gets blue shifted - closer to event horizon - more so, and at some point it will be seriously gamma in nature and will possibly rip anything material apart?

There are so many thing at play that it is really hard to imagine what will actually happen.

I find it hard to imagine that anything living would survive long enough to experience anything.  Nonetheless the consequences of falling across an event horizon is an exciting proposition for the imagination.  I feel cheated that there are things in our universe that we will never know or even see. These are the ultimate "what if" questions  

Jim 

Edited March 29, 2022 by saac

[andrew s]

18 minutes ago, vlaiv said:

Yes, it does take finite amount of time in reference frame of falling person.

I however wonder if falling person ever reaches singularity or even passes event horizon. All that light from the universe falling after the person (but at the speed of light) - gets blue shifted - closer to event horizon - more so, and at some point it will be seriously gamma in nature and will possibly rip anything material apart?

There are so many thing at play that it is really hard to imagine what will actually happen.

This is covered in the paper I linked to. For a large black hole you  might not even notice you have passed the event horizon. Objects that are in free fall do reach the singularity in finite "time".

The link I am looking for is dead unfortunately.  All that light is mainly the CMB on the area of the event horizon so yes over time it will add mass. The reality is we don't know as we belive GR breaks down at the singularity. 

Regards Andrew 

[andrew s]

19 minutes ago, saac said:

I find it hard to imagine that anything living would survive long enough to experience anything.  Nonetheless the consequences of falling across an event horizon is an exciting proposition for the imagination.  I feel cheated that there are things in our universe that we will never know or even see. These are the ultimate "what if" questions  

Jim 

As I just posted for a large black hole you may not notice you have crossed the event horizon but sure as eggs us eggs you will get spaghettified. 

Regards Andrew 

R

[Elp]

The theory of a singularity even if the maths of spacetime may lean towards it still seems a little strange, wouldn't it be something if a black hole was like a wormhole to another galaxy/universe, drawing in matter to fuel another. Black holes also emit jet streams of energy out into the cosmos and it's also a little odd most if not all galaxies have one in the centre.

[Kev M]

4 minutes ago, Elp said:

it's also a little odd most if not all galaxies have one in the centre.

I think this is how all ( spiral ) galaxies are made....it would be odder if they didn't have one.

 

[andrew s]

10 minutes ago, Elp said:

 Black holes also emit jet streams of energy out into the cosmos and it's also a little odd most if not all galaxies have one in the centre.

The accretion disks around black holes, powered by gravitational potential energy, emit jets not the black holes themselves. 

Regards Andrew 

 

[Kev M]

2 hours ago, andrew s said:

For a large black hole you  might not even notice you have passed the event horizon.

If my maths is correct a black hole with a mass of about 1 Billion Billion suns may have a gravitational field at its event horizon similar to that here on earth

 

[vlaiv]

8 minutes ago, Kev M said:

If my maths is correct a black hole with a mass of about 1 Billion Billion suns may have a gravitational field at its event horizon similar to that here on earth

 

If my "math" is correct - every black hole has the same gravitational field at its event horizon - that is why we call it event horizon, it is the place where gravity is so strong that it curves light paths so much that light cannot escape black hole any more (no event beyond event horizon can be seen as all light trajectories from it curve back towards singularity).

[Sunshine]

Time travel must be a thing, I only wish my future self from the year 2045 would come back, go back, uhh  and bring me a Tak 125 fluorite doublet which I am calling will happen, guaranteed. These guys have proof that time

travel is real, I'm not convinced but, for the sake of the 125mm Tak doublet I am willing to believe anything. 

 

 

[saac]

3 hours ago, andrew s said:

As I just posted for a large black hole you may not notice you have crossed the event horizon but sure as eggs us eggs you will get spaghettified. 

Regards Andrew 

R

Would it be possible to even approach the event horizon, would the gravity tidal force on approach not be lethal ?  Assuming no accretion disc just how close could we come to the event horizon?

Jim 

[George Jones]

8 minutes ago, saac said:

Would it be possible to even approach the event horizon, would the gravity tidal force on approach not be lethal ?  Assuming no accretion disc just how close could we come to the event horizon?

Jim 

If the black hole is large (larger than about 100,000 solar masses), spaghettification occurs inside the event horizon, i.e., crossing the event horizon of a large black hole is not problematic for a human. If a black hole is "small" (smaller than about 100,000 solar masses), spaghettification occurs outside the event horizon.

[andrew s]

10 minutes ago, saac said:

Would it be possible to even approach the event horizon, would the gravity tidal force on approach not be lethal ?  Assuming no accretion disc just how close could we come to the event horizon?

Jim 

The radius of the EV horizon is proportional to the mass while the tidal forces are proportional to the qube root of the mass. For small black holes there are significant tidal forces outside the EV but for very large ones you can get well inside before you would feel them.

Regards Andrew 

[George Jones]

I have my own personal time machine - pop music. Listening to the old songs takes me back to a time decades earlier.