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14 Jan 2019 21:26

A-L-E-X wrote:
Source of the post I heard that it was felt on seismographs as far away as London?

It was certainly measured by infrasound detectors several 1000 km away.  It was also recorded by seismometers, which detected the blast wave hitting the ground.  This was a pretty weak signal, but still detectable by the most sensitive seimometers up to a few 1000 km.
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14 Jan 2019 23:26

A-L-E-X wrote:
Source of the post  If I were in free fall inside the black hole does that mean I could actually be moving faster than the photons?

No.  You will never outpace a photon moving in the same direction as you.  And everyone always measures the same speed of light, even if they are falling inside of a black hole. :)

Another example: you can still see your feet if you fall feet-first into a black hole.  Those photons are not rising upward -- you are instead hitting them on the way down.  The photons that you see from your feet were emitted at a larger distance from the singularity than where your head is now.  At what speed do those photons reach your eyes?  Exactly c.  Your view of your feet is also exactly the same as if you were not inside a black hole at all.  (Because of the equivalence principle).
 
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16 Jan 2019 03:49

Watsisname wrote:
A-L-E-X wrote:
Source of the post  If I were in free fall inside the black hole does that mean I could actually be moving faster than the photons?

No.  You will never outpace a photon moving in the same direction as you.  And everyone always measures the same speed of light, even if they are falling inside of a black hole. :)

Another example: you can still see your feet if you fall feet-first into a black hole.  Those photons are not rising upward -- you are instead hitting them on the way down.  The photons that you see from your feet were emitted at a larger distance from the singularity than where your head is now.  At what speed do those photons reach your eyes?  Exactly c.  Your view of your feet is also exactly the same as if you were not inside a black hole at all.  (Because of the equivalence principle).

Thanks!  So the photons I see from my feet are "ghosts" of the past, where my feet were before I looked down there (I wonder if we can calculate how long ago they were emitted from my feet?)
Does the equivalency principle also apply to why if were to invent a time machine that we'd be in the same spot (that is, our location relative to the earth would not change) if we were able to change our position in time?

Stellator- Is this from the faraway galaxy that seems to have a lot of these?
 
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16 Jan 2019 04:52

A-L-E-X wrote:
Source of the post I looked down there (I wonder if we can calculate how long ago they were emitted from my feet?)

About 6 nanoseconds.  Or, the same time ago as if you were not inside a black hole. :)

A-L-E-X wrote:
Source of the post Does the equivalency principle also apply to why if were to invent a time machine that we'd be in the same spot (that is, our location relative to the earth would not change) if we were able to change our position in time?

The equivalence principle says that for sufficiently localized experiments (confined to small enough regions of space), freefall in a gravitational field is indistinguishable from floating in empty space with no gravity, and also that being in an accelerated frame of reference (like a rocket) is indistinguishable from being fixed in place in a gravitational field (like on the surface of a planet).

There are any number of (faster than light, not allowed) paths in space-time that lead to the past.  But staying in the same position in space and going back in time has no physical sense.
 
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17 Jan 2019 02:22

Thanks Wat, so if we were to invent a hypothetical time machine that could go to either the past or the future, and let's say we went 6 months in either direction, after emerging from the time machine would we end up directly opposite to where the earth was in its orbit around the sun?
 
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17 Jan 2019 04:16

A-L-E-X wrote:
Source of the post after emerging from the time machine would we end up directly opposite to where the earth was in its orbit around the sun?

No. The sun has meanwhile also traveled about 3.5 billion kilometers in its orbit around the Milky Way forwards or backwards.
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17 Jan 2019 04:38

JackDole wrote:
A-L-E-X wrote:
Source of the post after emerging from the time machine would we end up directly opposite to where the earth was in its orbit around the sun?

No. The sun has meanwhile also traveled about 3.5 billion kilometers in its orbit around the Milky Way forwards or backwards.

Ah so there is no way to predict where we would end up because everything is moving?  The sun would've moved, plus the galaxy itself is moving and the universe is expanding.
 
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17 Jan 2019 05:09

A-L-E-X, the problem with saying anything about what time travel into the past would do is that depends on how you assume time travel into the past would work.  In this case, "what is your choice of path?"

If you could travel faster than the speed of light, then certain observers will say that you were moving backward in time.  Different observers have different perspectives on what sets of events are simultaneous, and if moving faster than light then your arrival at your destination may be seen to occur before your departure in some reference frames.

If you follow a closed path and return to where you started while going faster than light, then everyone agrees you moved backward in time, including you.  You would return to your own past.  How much into the past depends on the path taken and the speed.

If we imagine you going into the past without moving (keeping the time machine fixed to the Earth for example), then that's a particular choice of path.  Basically you would be following Earth's world line through space-time backwards.  This is no more possible than going faster than the speed of light is, so I would say this is not real physics.


A-L-E-X wrote:
Source of the post so if we were to invent a hypothetical time machine that could go to either the past or the future, and let's say we went 6 months in either direction,

Going 6 months into the future (compared to everyone else) is allowed in physics, but not if you stay on Earth. :)
 
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17 Jan 2019 23:54

Not to mention the abrupt air pressure-change surrounding you if you use the classical 'teleporter' technique that sci-fi loves so much within a planetary atmosphere. Even if you could determine with all certainty where you would emerge on Earth (without ending up in space, high in the atmosphere or horribly fused with something) the re-arrangement of air molecules around your body as triggered by its sudden presence/absence will compress or decompress to such an extent that fusion reactions occur, making the surrounding atmosphere explode with an equivalent nuclear force equal to the mass missing and/or being added. Nasty.

When you really think about it, time-travel is one of those awful things that no-one with any ounce of sense would consider doing if it were possible and probably make sure they wouldn't accidentally either, simply due to the colossal s***tstorm that can occur if even the slightest thing goes awry or they forget something.  
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18 Jan 2019 03:52

Watsisname wrote:
A-L-E-X, the problem with saying anything about what time travel into the past would do is that depends on how you assume time travel into the past would work.  In this case, "what is your choice of path?"

If you could travel faster than the speed of light, then certain observers will say that you were moving backward in time.  Different observers have different perspectives on what sets of events are simultaneous, and if moving faster than light then your arrival at your destination may be seen to occur before your departure in some reference frames.

If you follow a closed path and return to where you started while going faster than light, then everyone agrees you moved backward in time, including you.  You would return to your own past.  How much into the past depends on the path taken and the speed.

If we you imagine going into the past without moving (keeping the time machine fixed to the Earth for example), then that's a particular choice of path.  Basically you would be following Earth's world line through space-time backwards.  This is no more possible than going faster than the speed of light is, so I would say this is not real physics.


A-L-E-X wrote:
Source of the post so if we were to invent a hypothetical time machine that could go to either the past or the future, and let's say we went 6 months in either direction,

Going 6 months into the future (compared to everyone else) is allowed in physics, but not if you stay on Earth. :)

Wat, I was thinking of a closed timelike curve (CTC) which I take it would be option 2.  Since there is no known physics for going faster than light is there a way to do it without going faster than light?  I've seen some research papers on it, but can't access them right now.  And once in the past could you do something differently to change your future?
For going into the future, the "easy" way to do it would be to accelerate close to the speed of light so our rate of time would be much slower than everyone else's on planet Earth, but that would take us away from the planet.  You could go 3 light months away and 3 light months back though couldn't you?  Then you could end up back on Earth with 6 months having gone by on Earth but you would have experienced barely any time?

Stellator- even just the idea of quantum teleportation makes me nervous lol, what if it doesn't turn out right?  And you're basically destroying yourself to create a clone.  And have to do so again to come back home.  Remember during the early years of Star Trek when they were doing it they fused that poor dog with a rock?

I like the idea of uploading one's consciousness into a body with an unlimited lifespan to make the journey far better.
 
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18 Jan 2019 04:02

A-L-E-X wrote:
Source of the post Stellator- even just the idea of quantum teleportation makes me nervous lol, what if it doesn't turn out right?

Yes, Star Trek teleportation is pretty dreadful, but I was talking about a more traditional teleportation wherein your entire body is transported to another reference without decon/reconstruction. Hence the mass displacement. That specific mode of teleporting is rather unscientific as it relies on time travel principles that violate several laws of thermodynamics and relativity.
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18 Jan 2019 04:08

A-L-E-X wrote:
Source of the post Wat, I was thinking of a closed timelike curve (CTC) which I take it would be option 2.  Since there is no known physics for going faster than light is there a way to do it without going faster than light?  I've seen some research papers on it, but can't access them right now.

Only by assuming some weird distortion to the geometry of the space-time (such as in the Alcubierre metric, or the region near the ring singularity of a rotating black hole).  But just because general relativity allows us to create a metric containing CTCs does not mean nature allows us to.  The very presence of CTCs in a metric is a good signal that the metric is not physically valid there.

A-L-E-X wrote:
Source of the post For going into the future, the "easy" way to do it would be to accelerate close to the speed of light so our rate of time would be much slower than everyone else's on planet Earth, but that would take us away from the planet.  You could go 3 light months away and 3 light months back though couldn't you?  Then you could end up back on Earth with 6 months having gone by on Earth but you would have experienced barely any time?

Sure, this is allowed. :)  You can travel some distance in space slower than light, and then come back again slower than light, and people on Earth will have experienced more time than you.  This is not actually due to your great speed, but rather a shift in reference frames somewhere along your journey.  You must accelerate to turn around and come back, while people on Earth do not.  (Also contrary to common claims, this does not necessarily require general relativity, but can be understood with just special relativity.)

Another way to get time dilation to take you into the future is to hover in a gravitational field.  Time passes more slowly in stronger gravity.  In fact I age about 6 microseconds per year more slowly here in Bellingham than my friend in Denver due to my being at sea level.
 
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18 Jan 2019 04:11

I think the only reasonably scientific way for us to travel great distances like that is to upload our minds into some sort of mechanical unlimited lifespan nonorganic form (can this be done)? or if that cant be done, we need some sort of stasis chamber / suspended animation to keep us in our current state until we arrive at our destination, however far into the future that might be.

Of course if we could travel close to the speed of light we wouldn't experience the same rate of passage of time, but although physically possible, it'll be extremely difficult to get a ship to accelerate to, let's say 90-95% of the speed of light and also avoid collisions with space dust, which at those velocities would tear holes into the hull of any ship (unless we had some sort of magnetic field to repel them.)  If we did then we could decelerate outside the solar system of our destination world and accelerate to those speeds again outside of it when we wanted to leave.

Wat- how fast would we have to accelerate to complete this 6 month "trip" in let's say one hour?  95% of the speed of light?

The gravitational field idea is even better, Wat!  How strong does the field have to be to reduce the 6 month trip to 1 hour?  I've seen maps of gravitiatonal sinks on Earth but I dont think any of them are strong enough for that :P  We'd probably need to create a black hole for that lol.

So a CTC wouldn't be a physical valid solution even with an artificially created singularity would it, Wat?  And even if we could create it, would it allow us to change our future - or would we be doomed to do the same things over again even if we knew we did it in the "past" (this is assuming we would still retain those memories, which we might not, it could just be a hard reset for everything in the path of the CTC.)

Besides a CTC is there any other physically valid way to go to the past?  (Either just to view it or to change it.)
 
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18 Jan 2019 04:33

Watsisname wrote:
A-L-E-X wrote:
Source of the post For going into the future, the "easy" way to do it would be to accelerate close to the speed of light so our rate of time would be much slower than everyone else's on planet Earth, but that would take us away from the planet.  You could go 3 light months away and 3 light months back though couldn't you?  Then you could end up back on Earth with 6 months having gone by on Earth but you would have experienced barely any time?

Sure, this is allowed. :).

I see marketability here: "Time-travel for cheaters or the lazy" :lol:.

A-L-E-X wrote:
Source of the post upload our minds into some sort of mechanical unlimited lifespan nonorganic form (can this be done)?

Of course it can be done. If our current advances in medicine, nanotechnology and robotics are anything to go by, once ships start accelerating to 'light-speed' - heavily modified cyborgs and uploaded minds will probably be the cultural norm. At those speeds it would be unpleasant to be fleshy anyway.  

A-L-E-X wrote:
Source of the post we need some sort of stasis chamber / suspended animation to keep us in our current state until we arrive at our destination, however far into the future that might be.

Well, hopefully not too far into the future, because most stasis designs breakdown very quickly at astronomical timescales. Usually simply mechanical failure would be culprit, but with traditional 'cryofreezing' there is a hard limit to how long you can 'sleep' if you are flesh and blood without sustaining PERMANENT damage to all organs, not to mention mental abilities. The exact time is dependent on freezing tech you use (actually frozen on ice or replacing all you fluids with a special ultra-cool preservative juice?). Something like two overlapping gravity-fields would be an ultra-advanced way of doing as you are essentially trying to duplicate general relativity time-warping to such a degree that time stands still within the fields. As with luminal travel time warping, this would be illusory of course. With that technology your worst enemy would be your own body, as the thousands of years pass, the tiny amounts of radioactive elements in your system like Potassium will decay and will slowly turn you into a human Chernobyl reactor...

Really, taking your meatsack into space is a bad idea, I'd much rather upload a copy of my mind and send it off to colonize the galaxy in a cyborg or to inhabit a vat-grown organic body supplied once my destination is reached. It's the Transhumanist dream!
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18 Jan 2019 05:12

A-L-E-X wrote:
Source of the post Wat- how fast would we have to accelerate to complete this 6 month "trip" in let's say one hour?  95% of the speed of light?

*chuckles*
 
6 months in 1 hour is a time dilation factor of 4380.  To achieve this by special relativity, we must travel at a speed of 0.9999999739c, or 7.81 m/s slower than light.

To achieve it by instead hovering in a gravitational field, we must hover at 1.0000000521 times the event horizon radius of a black hole.  If we choose the supermassive black hole SgrA*, then this is 22 centimeters above the event horizon.  The acceleration required to hover there is 4.61x1013 m/s2.


A-L-E-X wrote:
Source of the post Besides a CTC is there any other physically valid way to go to the past?  (Either just to view it or to change it.)

To change the past, no.  To view the past, look at something far away. :)

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