6 ms·

Physicist here (by degree, not profession). What you state is not true: by currently accepted physical theories, time travel to the past IS very much possible.

Physicist here (by degree, not profession). What you state is not true: by currently accepted physical theories, time travel to the past IS very much possible. It's known as a "closed, time-like curve" in the literature. Now, most mechanisms explored for doing so wouldn't allow travel back further than the initial creation of the time travel device, so you could say that time travel to our past is not possible since the construction of such a thing is necessarily in the future. But as a matter of physics, general relativity allows it. Now that said, most physicists are convinced that time travel cannot be possible, because if it were then paradoxical situations could be constructed--e.g. going back in time and killing yourself (the earlier you) before you entered the machine, or killing your grandfather before your father was born, etc. There are as many theorized rules preventing or limiting time travel as there are physicists that have looked at it, but critically they are all ADDITIONAL rules, postulated not to explain observable phenomenon but purely to think up a way time travel paradoxes might be avoided. Physics, as currently understood from available experimental evidence, 100% allows time travel to the past, paradoxes included.

trhway 7y ago
> then paradoxical situations could be constructed--e.g. going back in time and killing yourself (the earlier you) before you entered the machine, or killing your grandfather before your father was born Can't the paradox be resolved by considering it as a fork/branching of a new reality/timeline where the grandfather is killed, kind of similar to many world interpretation in QM. I'd speculate that using model of lazy fork it may be possible to get some ballpark estimates on energy requirements/etc. below the typical "all the energy in the Universe". Such forks/perturbations possibly also dissipate, like a small wave in the pond, and the forked reality gets naturally merged into the mainline ( i think this is how QM's many world while possibly true is still compatible with "realistic" situation of limited number, may be even just 1, Universe mainlines)
pdonis 7y ago
> by currently accepted physical theories, time travel to the past IS very much possible Only if you make use of exotic matter, which nobody has ever observed and which there are good reasons to think cannot exist.
pdonis 7y ago
> Physics, as currently understood from available experimental evidence, 100% allows time travel to the past, paradoxes included. General Relativity, which is where the term "closed timelike curve" is used, does not allow paradoxes. A spacetime with closed timelike curves in it (for example, the Godel universe) still has only one thing happening at each event. The real problem conceptually with such solutions is that they appear to contradict free will: since only one thing can happen at each event, if you experience visiting that event multiple times, you cannot freely choose what to do at that event.
- adrianN 7y ago
  Luckily "free will" as would fit a layman's interpretation of the term is already incompatible with physics, so the existence of CTCs doesn't pose additional problems.
  - pdonis 7y ago
    > "free will" as would fit a layman's interpretation of the term is already incompatible with physics That depends on which layman you talk to and which theory of physics you are using.
  - ralusek 7y ago
    I think most people would interpret free will to mean something that is perfectly compatible with any definition of determinism. If I decide to raise my right hand, I raise it. Don't get pedantic in regards to disabled people. It doesn't matter if the universe is completely deterministic, that doesn't mean that I'm not the entity that decided to raise my right hand. The deterministic universe may have produced me, but the software I'm running is me. The reason I raised my hand is because of me. Sure there are an infinite number of factors that have fed into the circumstances of my birth, and everything that's happened to me, but everything that I do is specifically as a consequence of my free will. I am the software, deterministic or not.
    - gpderetta 7y ago
      That's an interpretation of free will that I completely subscribe to, but I do not think it is a majority view.
cartoonworld 7y ago
Regarding the time paradoxes, the second part of your comment reminded me of a very interesting (and good) 1977 Czech film called "Tomorrow I'll Wake Up and Scald Myself with Tea"[0], in which some Nazis survive WWII. They get ahold of some anti-aging pills, and after centuries, hijack a time machine back to World War II in order to give Hitler the bomb, reversing the course of the war. However, their efforts are repeatedly stymied by comical time paradoxes. [0] https://en.wikipedia.org/wiki/Tomorrow_I'll_Wake_Up_and_Scald_Myself_with_Tea https://en.wikipedia.org/wiki/Tomorrow_I'll_Wake_Up_and_Scal...
jbattle 7y ago
I've never understood why "paradoxes make cause-and-effect-reasoning difficult" concerns are a valid argument that something could not be possible. Does physics care whether the workings of the universe are accessible to our logic?
- gpderetta 7y ago
  > Does physics care whether the workings of the universe are accessible to our logic? the universe itself doesn't care, but physics, that is, our math based theories, certainly they do, in fact they require it.
  - garmaine 7y ago
    Not in the way the post you are replying to means. Our “math based theories” of physics do not require a world without cause-effect paradoxes. Indeed they seem to explicitly permit them.
    - deleted 7y ago
      [deleted]
- cartoonworld 7y ago
  If I look at a random numeric oracle of 1-10, and consider the result as N, and apply to it some kind of mathematical formalism of: If ( N + 10 = 20 ) then eat(last_bagel) My wife comes home and is cross with me for eating the last_bagel. So, I hop in my handy time machine, travel back to before the time where I access the oracle, and alter it to only produce a random number from 1-9. If I succeed, my wife is no longer cross with me. But, if she is no longer cross with me, then why would I alter past events? Likely, I would have no cause to travel back in time in this scenario, so the oracle remains programmed to produce up to 10. Now my wife is cross with me again. Which one happens?
  - vokep 7y ago
    Of course there is no answer to this question yet, but thats the point. Just because a thought experiment doesn't solve it, doesn't mean it cannot be physically produced. For all we know in actual implementation you would simply eat the bagel, time travel back and bias the oracle, but your conscious experience only ever consists of finding that your oracle was modified and has your fingerprints on it somehow.
raattgift 7y ago
You can't get CTCs in Minkowski space. Hawking 1982 https://doi.org/10.1103/PhysRevD.46.603 https://doi.org/10.1103/PhysRevD.46.603 has an excellent argument based on gravitational backreaction about the implausibility of CTCs (and closed null curves) in any finite spacetime, including expanding spacetimes in which there are cosmic horizons at finite distances. In our standard cosmology you'd need extremely exotic matter. > general relativity allows it General Relativity requires neither the WEC nor the Lorentzian (1,3,0) metric signature (e.g. one can use a signature discontinuity on some hypersurface or atypical values for (virt,phys,rank)). You can describe low-entropy "objects" with all sorts of unaligned directions for their arrows-of-time that way, but good luck explaining the observations of the cosmic microwave background or recovering Special Relativity in a lab test. AWEC is there to give us large scale structure at all, not "purely to think up a way time travel paradoxes might be avoided". > paradoxical situations could be constructed--e.g. going back in time and killing yourself Start much more easily. Set up a worldtube containing the timelike and null paths for para-positronium in an anti-de Sitter_{3} space in 1+3 Minkowski space by tracing the worldtubes of the leptons and the photons as they loop around for some small multiple of the lifetime of para-positronium. You'll want the block universe view: at any given point on each closed there is a frozen configuration. Coarsely, at each point (t,x,y,z) on the loop of each CTC there is an electron, a positron, or neither; more technically the conjugate variables at each point on a closed curve are fixed. So you need a way to recover the para-positronium from the photons. There seems to be a lot fewer degrees of freedom than in the absence of the CTC! https://en.wikipedia.org/wiki/Positronium#States https://en.wikipedia.org/wiki/Positronium#States https://en.wikipedia.org/wiki/Anti-de_Sitter_space#Closed_timelike_curves_and_the_universal_cover https://en.wikipedia.org/wiki/Anti-de_Sitter_space#Closed_ti... You'll quickly find that thinking of a human being (radiating in the IR while metabolizing, shedding skin and hair, perspiring, and so on) "lucky" enough to be near a supersymmetric charged rotating black hole in AdS_{5} with microscopic parts on a large set of worldtubes some of which are closed (some closed and timelike, some closed and null) and some of which might not be, is so intractable that you can only make extremely wild guesses about interactions with a grandparent whose microscopic components are almost entirely not on closed curves. > Physics, as currently understood from available experimental evidence, 100% allows time travel to the past No time travellers showed up at Hawking's champagne party afaict, but maybe they were sneakily stealing socks out of his washing machine rather than trying to make small talk. > paradoxes Most of these vanish in a block universe view where everything is encoded in a nontrivial set of spacetime-filling fields. Lots of apparent paradoxes arise from incomplete values surfaces in initial values formulations. Lots of others relate to something similar to pareidolia where given a block universe people will insist that they see faces and cats and planets without admitting that they're imposing a particular time axis (and orientation) and using that as a basis for segmenting a huge bunch of worldtubes (that collectively they take as being a single object).
Gustomaximus 7y ago
Definitely not a physicist here but isn't forward time travel somewhat possible by approaching speed of light? I read about effects where if I left earth, travelled very fast a year to me might be many to earth. Could a person not therefore 'travel fast' somewhere and come back so a place like earth had advanced significantly in time relative to them? Not sure if that is time travel in a 'jump to a new time' sense but it effectively is. And I wonder how far you could push that with theoretical technology. 1 day for 1000 years type thing...
- garmaine 7y ago
  We are doing forward time travel right now at a rate of 1sec/sec.
  - rbanffy 7y ago
    Only from your point of view. My head and my feet travel at slightly different speeds.
    - raattgift 7y ago
      What are your head and your feet? Fluids are flowing in and out of them, outer layers are sloughing off, they're warm enough to be quite bright in infrared, potassium-40 in your head and feet are beta-decaying and so on. A decent fraction of the momentary mass in your brain may be in your feet four heartbeats later (and vice-versa). Are you implicitly considering them in some time-averaged way? (A comment similar to yours would be that your head is older than your feet because of general relativistic effects. Even if we ignore everything before you reached adulthood, there's a Ship of Theseus thing going on thanks to tissue turnover on timescales of weeks to months e.g. https://en.wikipedia.org/wiki/Bone_remodeling_period https://en.wikipedia.org/wiki/Bone_remodeling_period ).
  - raattgift 7y ago
    Whose second? You and I at different points on Earth and our seconds differ. Even a decade ago we could show that a second precisely defined in the middle of the floor of a laboratory could not apply to the whole of the laboratory. "General relativity states that time speeds up for objects as gravity weakens. To demonstrate this, Chou and his colleagues raised one optical clock 33 centimetres above another. The slightly lower gravity at that height meant that compared with the reference clock, the raised clock ticked with a fractional boost in frequency of 4 × 10^–17, equivalent to a gain of 90 billionths of a second over 79 years." Even ignoring the distance between different edges of your brain and that there are humans in orbit and in mountain communities as well as below sea level, there are a number of processes in human brains that produce movement an order of magnitude faster (over short distances) not to mention people driving or flying much than the speed in this paragraph: "To demonstrate special relativity, which says that time slows down for moving objects, the researchers jolted the single atom in their optical clock so that it oscillated at relative speeds of less than 10 metres per second, or 36 kilometres per hour. This time, the clock's ticks seemed to drop by a fractional frequency of almost 6 × 10^–16." https://www.nature.com/news/2010/100923/full/news.2010.487.html https://www.nature.com/news/2010/100923/full/news.2010.487.h... (Chou et al.'s article is available at http://spiff.rit.edu/classes/phys200/lectures/dilation/chou/1630.pdf http://spiff.rit.edu/classes/phys200/lectures/dilation/chou/... and starts on the bottom of the second page of the PDF) [and cf my Finally paragraph below] Importantly when relating an ultraprecise clock on the ground with an ultraprecise clock passing above it in a blimp at, say, a constant linear ten metres per second, a Lorentz transform will need general-relativistic corrections. These corrections seem negligibly small but it seems safe to bet that in a few years they'll matter in a rack full of high end computers. Clock precision has also gotten much better http://iopscience.iop.org/article/10.1088/1742-6596/723/1/012004/pdf http://iopscience.iop.org/article/10.1088/1742-6596/723/1/01... -- "Measurements made with the rubidium fountains (and other clocks at USNO) after the first 1.5 years of continuous operation resulted in the most precise test of the universality of the gravitational redshift and enabled constraints on the coupling of fundamental constants to gravity to be improved by on average a factor of four" Meanwhile, just to plug some of the work HN's user ISL's group is doing for an extreme setup where the dependence of second not just on position on Earth but on dynamical processes in different stratified-planet layers already matters for practical purposes: https://www.npl.washington.edu/eotwash/node/17 https://www.npl.washington.edu/eotwash/node/17 ("seismic Newtonian noise ... the gravitational attraction of an optic to high density regions of the ground caused by passing seismic waves"). Finally, and also because GPS is mentioned here and there in this thread, I recommend slides 16-19 of http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_files/I08-VanBaak-GPS_Flying_Clocks_and_Relativity.pdf http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_f... If we are to insist on the notion of the flow of time, we should be aware that it poses some serious practical problems as one increases the density of marks on a chosen time axis in a dynamical spacetime like ours and populated with sources (in the Einstein Field Equation sense) like those around us. Approaching the limit of ultradense time coordinates we have to consider how parts of ourselves are sources, some organs and regions of organs being denser than others just for starters. And that's true even without getting anywhere near metaphysics and philosophical issues.

trhway 7y ago

> then paradoxical situations could be constructed--e.g. going back in time and killing yourself (the earlier you) before you entered the machine, or killing your grandfather before your father was born Can't the paradox be resolved by considering it as a fork/branching of a new reality/timeline where the grandfather is killed, kind of similar to many world interpretation in QM. I'd speculate that using model of lazy fork it may be possible to get some ballpark estimates on energy requirements/etc. below the typical "all the energy in the Universe". Such forks/perturbations possibly also dissipate, like a small wave in the pond, and the forked reality gets naturally merged into the mainline ( i think this is how QM's many world while possibly true is still compatible with "realistic" situation of limited number, may be even just 1, Universe mainlines)

pdonis 7y ago

> by currently accepted physical theories, time travel to the past IS very much possible Only if you make use of exotic matter, which nobody has ever observed and which there are good reasons to think cannot exist.

pdonis 7y ago

> Physics, as currently understood from available experimental evidence, 100% allows time travel to the past, paradoxes included. General Relativity, which is where the term "closed timelike curve" is used, does not allow paradoxes. A spacetime with closed timelike curves in it (for example, the Godel universe) still has only one thing happening at each event. The real problem conceptually with such solutions is that they appear to contradict free will: since only one thing can happen at each event, if you experience visiting that event multiple times, you cannot freely choose what to do at that event.

adrianN 7y ago
Luckily "free will" as would fit a layman's interpretation of the term is already incompatible with physics, so the existence of CTCs doesn't pose additional problems.
- pdonis 7y ago
  > "free will" as would fit a layman's interpretation of the term is already incompatible with physics That depends on which layman you talk to and which theory of physics you are using.
- ralusek 7y ago
  I think most people would interpret free will to mean something that is perfectly compatible with any definition of determinism. If I decide to raise my right hand, I raise it. Don't get pedantic in regards to disabled people. It doesn't matter if the universe is completely deterministic, that doesn't mean that I'm not the entity that decided to raise my right hand. The deterministic universe may have produced me, but the software I'm running is me. The reason I raised my hand is because of me. Sure there are an infinite number of factors that have fed into the circumstances of my birth, and everything that's happened to me, but everything that I do is specifically as a consequence of my free will. I am the software, deterministic or not.
  - gpderetta 7y ago
    That's an interpretation of free will that I completely subscribe to, but I do not think it is a majority view.

cartoonworld 7y ago

Regarding the time paradoxes, the second part of your comment reminded me of a very interesting (and good) 1977 Czech film called "Tomorrow I'll Wake Up and Scald Myself with Tea"[0], in which some Nazis survive WWII. They get ahold of some anti-aging pills, and after centuries, hijack a time machine back to World War II in order to give Hitler the bomb, reversing the course of the war. However, their efforts are repeatedly stymied by comical time paradoxes. [0] https://en.wikipedia.org/wiki/Tomorrow_I'll_Wake_Up_and_Scald_Myself_with_Tea https://en.wikipedia.org/wiki/Tomorrow_I'll_Wake_Up_and_Scal...

jbattle 7y ago

I've never understood why "paradoxes make cause-and-effect-reasoning difficult" concerns are a valid argument that something could not be possible. Does physics care whether the workings of the universe are accessible to our logic?

gpderetta 7y ago
> Does physics care whether the workings of the universe are accessible to our logic? the universe itself doesn't care, but physics, that is, our math based theories, certainly they do, in fact they require it.
- garmaine 7y ago
  Not in the way the post you are replying to means. Our “math based theories” of physics do not require a world without cause-effect paradoxes. Indeed they seem to explicitly permit them.
  - deleted 7y ago
    [deleted]
cartoonworld 7y ago
If I look at a random numeric oracle of 1-10, and consider the result as N, and apply to it some kind of mathematical formalism of: If ( N + 10 = 20 ) then eat(last_bagel) My wife comes home and is cross with me for eating the last_bagel. So, I hop in my handy time machine, travel back to before the time where I access the oracle, and alter it to only produce a random number from 1-9. If I succeed, my wife is no longer cross with me. But, if she is no longer cross with me, then why would I alter past events? Likely, I would have no cause to travel back in time in this scenario, so the oracle remains programmed to produce up to 10. Now my wife is cross with me again. Which one happens?
- vokep 7y ago
  Of course there is no answer to this question yet, but thats the point. Just because a thought experiment doesn't solve it, doesn't mean it cannot be physically produced. For all we know in actual implementation you would simply eat the bagel, time travel back and bias the oracle, but your conscious experience only ever consists of finding that your oracle was modified and has your fingerprints on it somehow.

raattgift 7y ago

You can't get CTCs in Minkowski space. Hawking 1982 https://doi.org/10.1103/PhysRevD.46.603 https://doi.org/10.1103/PhysRevD.46.603 has an excellent argument based on gravitational backreaction about the implausibility of CTCs (and closed null curves) in any finite spacetime, including expanding spacetimes in which there are cosmic horizons at finite distances. In our standard cosmology you'd need extremely exotic matter. > general relativity allows it General Relativity requires neither the WEC nor the Lorentzian (1,3,0) metric signature (e.g. one can use a signature discontinuity on some hypersurface or atypical values for (virt,phys,rank)). You can describe low-entropy "objects" with all sorts of unaligned directions for their arrows-of-time that way, but good luck explaining the observations of the cosmic microwave background or recovering Special Relativity in a lab test. AWEC is there to give us large scale structure at all, not "purely to think up a way time travel paradoxes might be avoided". > paradoxical situations could be constructed--e.g. going back in time and killing yourself Start much more easily. Set up a worldtube containing the timelike and null paths for para-positronium in an anti-de Sitter_{3} space in 1+3 Minkowski space by tracing the worldtubes of the leptons and the photons as they loop around for some small multiple of the lifetime of para-positronium. You'll want the block universe view: at any given point on each closed there is a frozen configuration. Coarsely, at each point (t,x,y,z) on the loop of each CTC there is an electron, a positron, or neither; more technically the conjugate variables at each point on a closed curve are fixed. So you need a way to recover the para-positronium from the photons. There seems to be a lot fewer degrees of freedom than in the absence of the CTC! https://en.wikipedia.org/wiki/Positronium#States https://en.wikipedia.org/wiki/Positronium#States https://en.wikipedia.org/wiki/Anti-de_Sitter_space#Closed_timelike_curves_and_the_universal_cover https://en.wikipedia.org/wiki/Anti-de_Sitter_space#Closed_ti... You'll quickly find that thinking of a human being (radiating in the IR while metabolizing, shedding skin and hair, perspiring, and so on) "lucky" enough to be near a supersymmetric charged rotating black hole in AdS_{5} with microscopic parts on a large set of worldtubes some of which are closed (some closed and timelike, some closed and null) and some of which might not be, is so intractable that you can only make extremely wild guesses about interactions with a grandparent whose microscopic components are almost entirely not on closed curves. > Physics, as currently understood from available experimental evidence, 100% allows time travel to the past No time travellers showed up at Hawking's champagne party afaict, but maybe they were sneakily stealing socks out of his washing machine rather than trying to make small talk. > paradoxes Most of these vanish in a block universe view where everything is encoded in a nontrivial set of spacetime-filling fields. Lots of apparent paradoxes arise from incomplete values surfaces in initial values formulations. Lots of others relate to something similar to pareidolia where given a block universe people will insist that they see faces and cats and planets without admitting that they're imposing a particular time axis (and orientation) and using that as a basis for segmenting a huge bunch of worldtubes (that collectively they take as being a single object).

Gustomaximus 7y ago

Definitely not a physicist here but isn't forward time travel somewhat possible by approaching speed of light? I read about effects where if I left earth, travelled very fast a year to me might be many to earth. Could a person not therefore 'travel fast' somewhere and come back so a place like earth had advanced significantly in time relative to them? Not sure if that is time travel in a 'jump to a new time' sense but it effectively is. And I wonder how far you could push that with theoretical technology. 1 day for 1000 years type thing...

garmaine 7y ago
We are doing forward time travel right now at a rate of 1sec/sec.
- rbanffy 7y ago
  Only from your point of view. My head and my feet travel at slightly different speeds.
  - raattgift 7y ago
    What are your head and your feet? Fluids are flowing in and out of them, outer layers are sloughing off, they're warm enough to be quite bright in infrared, potassium-40 in your head and feet are beta-decaying and so on. A decent fraction of the momentary mass in your brain may be in your feet four heartbeats later (and vice-versa). Are you implicitly considering them in some time-averaged way? (A comment similar to yours would be that your head is older than your feet because of general relativistic effects. Even if we ignore everything before you reached adulthood, there's a Ship of Theseus thing going on thanks to tissue turnover on timescales of weeks to months e.g. https://en.wikipedia.org/wiki/Bone_remodeling_period https://en.wikipedia.org/wiki/Bone_remodeling_period ).
- raattgift 7y ago
  Whose second? You and I at different points on Earth and our seconds differ. Even a decade ago we could show that a second precisely defined in the middle of the floor of a laboratory could not apply to the whole of the laboratory. "General relativity states that time speeds up for objects as gravity weakens. To demonstrate this, Chou and his colleagues raised one optical clock 33 centimetres above another. The slightly lower gravity at that height meant that compared with the reference clock, the raised clock ticked with a fractional boost in frequency of 4 × 10^–17, equivalent to a gain of 90 billionths of a second over 79 years." Even ignoring the distance between different edges of your brain and that there are humans in orbit and in mountain communities as well as below sea level, there are a number of processes in human brains that produce movement an order of magnitude faster (over short distances) not to mention people driving or flying much than the speed in this paragraph: "To demonstrate special relativity, which says that time slows down for moving objects, the researchers jolted the single atom in their optical clock so that it oscillated at relative speeds of less than 10 metres per second, or 36 kilometres per hour. This time, the clock's ticks seemed to drop by a fractional frequency of almost 6 × 10^–16." https://www.nature.com/news/2010/100923/full/news.2010.487.html https://www.nature.com/news/2010/100923/full/news.2010.487.h... (Chou et al.'s article is available at http://spiff.rit.edu/classes/phys200/lectures/dilation/chou/1630.pdf http://spiff.rit.edu/classes/phys200/lectures/dilation/chou/... and starts on the bottom of the second page of the PDF) [and cf my Finally paragraph below] Importantly when relating an ultraprecise clock on the ground with an ultraprecise clock passing above it in a blimp at, say, a constant linear ten metres per second, a Lorentz transform will need general-relativistic corrections. These corrections seem negligibly small but it seems safe to bet that in a few years they'll matter in a rack full of high end computers. Clock precision has also gotten much better http://iopscience.iop.org/article/10.1088/1742-6596/723/1/012004/pdf http://iopscience.iop.org/article/10.1088/1742-6596/723/1/01... -- "Measurements made with the rubidium fountains (and other clocks at USNO) after the first 1.5 years of continuous operation resulted in the most precise test of the universality of the gravitational redshift and enabled constraints on the coupling of fundamental constants to gravity to be improved by on average a factor of four" Meanwhile, just to plug some of the work HN's user ISL's group is doing for an extreme setup where the dependence of second not just on position on Earth but on dynamical processes in different stratified-planet layers already matters for practical purposes: https://www.npl.washington.edu/eotwash/node/17 https://www.npl.washington.edu/eotwash/node/17 ("seismic Newtonian noise ... the gravitational attraction of an optic to high density regions of the ground caused by passing seismic waves"). Finally, and also because GPS is mentioned here and there in this thread, I recommend slides 16-19 of http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_files/I08-VanBaak-GPS_Flying_Clocks_and_Relativity.pdf http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_f... If we are to insist on the notion of the flow of time, we should be aware that it poses some serious practical problems as one increases the density of marks on a chosen time axis in a dynamical spacetime like ours and populated with sources (in the Einstein Field Equation sense) like those around us. Approaching the limit of ultradense time coordinates we have to consider how parts of ourselves are sources, some organs and regions of organs being denser than others just for starters. And that's true even without getting anywhere near metaphysics and philosophical issues.