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Thread: The density of the universe.

  1. #1 The density of the universe. 
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    What is it calculated to be now?

    Has it always been the same (in the time from "now" backwards to just after the BB ?

    What units are used?

    If you wind back the universe to its earliest reckonable time do those units change at all? (do we start to talk about energy instead of mass and is how do measure plasma ? -if that is supposed to be the irreducible form of energy/matter that the universe started out as )

    Are they really saying that at the "start" of inflation the amount of matter presently observed (and unobserved?) is an exact (?) match to that obtaining then ?

    And (although it would be beyond my appreciation no doubt) are all these calculations done with due regard to spacetime or can they be shown more simply with space and time being addressed separately?


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    According to this page it is about 9.910−30 g/cm3:
    WMAP- Content of the Universe

    This is not constant. After all, a good popular description of the big bang theory is that the universe has evolved from a hot dense state.

    And (although it would be beyond my appreciation no doubt) are all these calculations done with due regard to spacetime or can they be shown more simply with space and time being addressed separately?
    Not sure what you are asking here. It is only space that is expanding (over time).


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    That was a stupid mistake on my part (sorry) .I should have asked "is (= has it been) the sum total of the mass(or mass-energy) constant?"

    I was finding it a bit hard to imagine the total mass of the universe being concentrated to a very small volume (in and around the BB) and was trying to track down some of the calculations involved
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    Quote Originally Posted by geordief View Post
    That was a stupid mistake on my part (sorry) .I should have asked "is (= has it been) the sum total of the mass(or mass-energy) constant?"
    There is no simple answer to this. There has been some discussion of this here: Is the whole Energy (+mass) of the universe constant?

    I was finding it a bit hard to imagine the total mass of the universe being concentrated to a very small volume (in and around the BB) and was trying to track down some of the calculations involved
    My (limited) understanding is that going back to the earliest times requires a pretty good understanding of quantum theory.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by geordief View Post
    That was a stupid mistake on my part (sorry) .I should have asked "is (= has it been) the sum total of the mass(or mass-energy) constant?"
    There is no simple answer to this. There has been some discussion of this here: Is the whole Energy (+mass) of the universe constant?

    Yes I saw that thread after I posted.

    I was finding it a bit hard to imagine the total mass of the universe being concentrated to a very small volume (in and around the BB) and was trying to track down some of the calculations involved
    My (limited) understanding is that going back to the earliest times requires a pretty good understanding of quantum theory.


    What about the period before (I mean after I suppose) quantum effects become important. Would the mass/energy total then ( say 1000 years after the BB) be an exactish match for what obtains now?
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    Quote Originally Posted by geordief View Post
    What about the period before (I mean after I suppose) quantum effects become important. Would the mass/energy total then ( say 1000 years after the BB) be an exactish match for what obtains now?
    The problem is that there is no good definition of energy on cosmological scales, so it isn't really possible to answer that.
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    Speed and heat allow those particles to ignore gravity to a certain extent, so the density can be much higher since heat was much higher in the early days of the universe. Since the heat of the universe is much lower, gravity is more effective. (Even though a particle going faster has greater gravity.) Anyway, if the density of the universe was the same now as it was in the early days of the universe, gravity would have stopped its expansion and it would crunch back down. Since we are still expanding, and since the average temperature still goes down, the density must be going down, too.

    That's an oversimplified explanation. It gets a lot more complicated and far above what I am capable of so that people can calculate approximations of the density. It looks like Strange posted one. But that's at least one way to think about 'why' the universal density is decreasing.

    The energy thing gets complicated, especially when you factor in spontaneously appearing virtual particles and infinities that cancel each other out and such. I can't really explain it because I don't really understand it myself. My guess would be yes, conservation of energy would say that the total energy of the universe has always been the same, but the nature of that energy and various infinities and such may make the question itself academic. But that is purely my guess. It could easily be false.
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    Speed and heat allow those particles to ignore gravity to a certain extent
    No, not at all.
    (Even though a particle going faster has greater gravity.)
    See above.
    Its the way nature is!
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    Quote Originally Posted by AlexG View Post
    Speed and heat allow those particles to ignore gravity to a certain extent
    No, not at all.
    (Even though a particle going faster has greater gravity.)
    See above.
    A particle has greater gravity if it is sped up to incredible speeds, it has more energy overall if it is sped up and so greater gravity.

    A particle that is going faster is less likely to be trapped by another objects gravity. It is less affected by another objects gravity in that respect.

    That is what I mean. Do you disagree? Is that incorrect?
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    A particle has greater gravity if it is sped up to incredible speeds,
    Speeds in relation to what? If I'm crusing along side the particle, it is stationary from my POV, and has no more overall energy. You do know that there is no relativistic mass

    A particle that is going faster is less likely to be trapped by another objects gravity. It is less affected by another objects gravity in that respect.
    It is not less affected, the outcome is different. But gravity effects fast and slow objects alike. You just end up with different trajectories.
    SpeedFreek and Bad Robot like this.
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    Quote Originally Posted by SowZ37 View Post
    Is that incorrect?
    Yes.
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    Quote Originally Posted by AlexG View Post
    A particle has greater gravity if it is sped up to incredible speeds,
    Speeds in relation to what? If I'm crusing along side the particle, it is stationary from my POV, and has no more overall energy. You do know that there is no relativistic mass

    A particle that is going faster is less likely to be trapped by another objects gravity. It is less affected by another objects gravity in that respect.
    It is not less affected, the outcome is different. But gravity effects fast and slow objects alike. You just end up with different trajectories.
    The speed of light is the same in relation to all observers.

    Speed/heat/energy, they all come from the same place. Speed is one measure of an objects energy. A buick going 30mph down the road has less energy than one going 1000mph. Multiply that enough times and you see buicks with noticeably different gravity, right? How could you not?

    And if you are going fast enough, a particle can escape just about any force from another particle that would draw them together. Strong/weak interactions, magnetism, you name it. The particle might still be effected by gravity, but it can escape the gravity. It doesn't have to collide with the other particle attracting it. My context was why a lower universal density is required today, with a lower overall universal temperature. If you took the universe an hour after the big bang, and lowered its average temperature to what we have today, the particles would be unable to escape each others gravity and recollapse.

    I sense a miscommunication here, because I am sure you know that.

    Less effected by gravity is maybe a misnomer. I just mean it can escape the force, when otherwise it would be trapped by it.

    I'm open to correction here, but it is my understanding this isn't particularly advanced stuff. My understanding of how it works could be off, the why as it were, it wouldn't be the first time I knew about the A, and the C, but was wrong about the B, (how to get there,) but I'm pretty confident that my 'what would happen' is correct.
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    Quote Originally Posted by SowZ37 View Post
    The speed of light is the same in relation to all observers.
    But you are not talking about light; you are talking about objects.

    Speed/heat/energy, they all come from the same place. Speed is one measure of an objects energy. A buick going 30mph down the road has less energy than one going 1000mph. Multiply that enough times and you see buicks with noticeably different gravity.
    But speed relative to what? There is no absolute measure of speed (or energy).

    The particle might still be effected by gravity, but it can escape the gravity.
    But it is still affected in exactly the same way.

    It doesn't have to collide with the particle. My context was why a lower universal density is required today, with a lower overall universal temperature.
    That is simply because the universe is expanding.

    If you took the universe an hour after the big bang, and lowered its average temperature to what we have today, the particles would be unable to escape each others gravity and recollapse.
    But you wouldn't be able to reduce the temperature (because there is nowhere for the heat to go). Oh, yes you can: let the universe expand and it will cool.

    You seem to be mixing cause and effect.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by SowZ37 View Post
    The speed of light is the same in relation to all observers.
    But you are not talking about light; you are talking about objects.

    Speed/heat/energy, they all come from the same place. Speed is one measure of an objects energy. A buick going 30mph down the road has less energy than one going 1000mph. Multiply that enough times and you see buicks with noticeably different gravity.
    But speed relative to what? There is no absolute measure of speed (or energy).

    The particle might still be effected by gravity, but it can escape the gravity.
    But it is still affected in exactly the same way.

    It doesn't have to collide with the particle. My context was why a lower universal density is required today, with a lower overall universal temperature.
    That is simply because the universe is expanding.

    If you took the universe an hour after the big bang, and lowered its average temperature to what we have today, the particles would be unable to escape each others gravity and recollapse.
    But you wouldn't be able to reduce the temperature (because there is nowhere for the heat to go). Oh, yes you can: let the universe expand and it will cool.

    You seem to be mixing cause and effect.
    So what are you saying?

    There are minimum energy/speed requirements necessary to escape another objects gravity. Maybe there is no absolute metric to measure them by, by if I took an object capable of escaping some planets gravity and siphoned off energy and siphoned off energy a little bit at a time, it would eventually "slow down" to the point that it falls into the planet, right?

    The gravity thing. Sure, it would still be effected, but it would be able to escape the gravity.

    As to the energy thing and the Big Bang I was talking, yeah, it doesn't really make sense. The scenario I was trying to play out in my head is probably just nonsense. Even if I somehow was able to siphon energy out of that universe and somewhere else (?) that would just reduce the total gravity a proportionate amount, so nothing really would have been accomplished in regards to what I am talking about, right? It's a stupid example, you're right.

    But what I was trying to say, and either don't have the comprehension of the principles required to get from A to C or possible the vocabulary/communication skills necessary to say it, is that we know the universe has a lower average density now than it has at any time in the past. If that wasn't the case, it would have to contract.

    If, from some other universe somehow, I multiplied the matter in the universe by a large amount without increasing the space between the matter, would it start contracting? I would guess that it would. Or would something allow the universe to keep expanding?
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    Speed/heat/energy, they all come from the same place. Speed is one measure of an objects energy. A buick going 30mph down the road has less energy than one going 1000mph. Multiply that enough times and you see buicks with noticeably different gravity, right? How could you not?
    Speed is a relative quantity. The buick doing 30 mph rear ends me when I'm doing 10 mph. The one going 1000 mph rear ends me while I'm doing 999 mph. From my POV the buick doing 30 contains a lot more energy than the buick doing 1000.

    How you could not is because it doesn't work that way.
    Its the way nature is!
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    To another universe, where the rules are simpler
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    Quote Originally Posted by SowZ37 View Post
    So what are you saying?

    There are minimum energy/speed requirements necessary to escape another objects gravity.
    But the early universe was homogeneous (as well as dense) so there was no gravity to escape from - gravity would have been the same everywhere.

    It is only later when the universe was large enough, and cool enough, for structures to form that gravitational differences became relevant.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by SowZ37 View Post
    So what are you saying?

    There are minimum energy/speed requirements necessary to escape another objects gravity.
    But the early universe was homogeneous (as well as dense) so there was no gravity to escape from - gravity would have been the same everywhere.

    It is only later when the universe was large enough, and cool enough, for structures to form that gravitational differences became relevant.
    But the universe couldn't be completely homogenous, wouldn't there have to be at least minuscule irregularities? Otherwise there would be no galaxies and such. Shoot, wouldn't the uncertainty principle make it impossible for everything to be completely uniform at any size other than a singularity? Wouldn't some particles shift towards one side of the dense state just because of uncertainty?

    Quote Originally Posted by AlexG View Post
    Speed/heat/energy, they all come from the same place. Speed is one measure of an objects energy. A buick going 30mph down the road has less energy than one going 1000mph. Multiply that enough times and you see buicks with noticeably different gravity, right? How could you not?
    Speed is a relative quantity. The buick doing 30 mph rear ends me when I'm doing 10 mph. The one going 1000 mph rear ends me while I'm doing 999 mph. From my POV the buick doing 30 contains a lot more energy than the buick doing 1000.

    How you could not is because it doesn't work that way.
    So then how do I compare a particles energy to the energy required to escape the attraction of another particle? What metric do I use? Also, the buick going 1000 MPH still has more gravity than the one going 999 MPH, right? I mean, maybe not at that scale, but if we scaled it up?




    Be patient with me, please, all my knowledge on this stuff has been from reading. I've never taken classes or known anyone to talk to about it. Purely reading. So if I misread something, and think that is what the author is trying to communicate, I have no safety net to keep me from building knowledge off of incorrect assumptions/readings.
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    Quote Originally Posted by SowZ37 View Post
    But the universe couldn't be completely homogenous, wouldn't there have to be at least minuscule irregularities? Otherwise there would be no galaxies and such. Shoot, wouldn't the uncertainty principle make it impossible for everything to be completely uniform at any size other than a singularity? Wouldn't some particles shift towards one side of the dense state just because of uncertainty?
    Yes, there were quantum scale fluctuations (that later became the large-scale structures of the universe). Gravity is irrelevant at those scales.

    So then how do I compare a particles energy to the energy required to escape the attraction of another particle? What metric do I use? Also, the buick going 1000 MPH still has more gravity than the one going 999 MPH, right? I mean, maybe not at that scale, but if we scaled it up?
    It still depends what you are measuring the speed relative to.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by SowZ37 View Post
    But the universe couldn't be completely homogenous, wouldn't there have to be at least minuscule irregularities? Otherwise there would be no galaxies and such. Shoot, wouldn't the uncertainty principle make it impossible for everything to be completely uniform at any size other than a singularity? Wouldn't some particles shift towards one side of the dense state just because of uncertainty?
    Yes, there were quantum scale fluctuations (that later became the large-scale structures of the universe). Gravity is irrelevant at those scales.

    So then how do I compare a particles energy to the energy required to escape the attraction of another particle? What metric do I use? Also, the buick going 1000 MPH still has more gravity than the one going 999 MPH, right? I mean, maybe not at that scale, but if we scaled it up?
    It still depends what you are measuring the speed relative to.
    Let's say both Buick's are trying to hit a ramp and escape the orbit of Earth. I don't really care what actual escape velocity is here, so I'm not asking for a MPH calculation, but on a more basic level. Why am I allowed to calculate/compare the speed of the cars in that context, but not certain others?

    What if both Buick's escaped Earth, and are now hurtling through space. And both are hoping to effect the trajectory of other objects via their gravity. How would they go about increasing the gravity with which they influence other objects?
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    Also, the buick going 1000 MPH still has more gravity than the one going 999 MPH, right? I mean, maybe not at that scale, but if we scaled it up?
    No. Gravity is not effected by relative velocity. And all velocity is relative. (of particles with mass)

    How would they go about increasing the gravity with which they influence other objects?
    They wouldn't. Gravity is a property of mass, so the only way of increasing their gravity would be to add mass, say by filling the gas tank.
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    Quote Originally Posted by AlexG View Post
    Also, the buick going 1000 MPH still has more gravity than the one going 999 MPH, right? I mean, maybe not at that scale, but if we scaled it up?
    No. Gravity is not effected by relative velocity. And all velocity is relative. (of particles with mass)
    But increasing energy can increase mass, right? Wouldn't the Buick at 1000 MPH have more energy than the one at 999 MPH? Why else does approaching light speed increase mass?
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    But increased energy can increase mass, right?
    No. Relativistic mass is a misnomer.
    Wouldn't the Buick at 1000 MPH have more energy than the one at 999 MPH?
    Kinetic energy is a relative quantity, and does not add to gravity.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
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    Quote Originally Posted by SowZ37 View Post
    But increased energy can increase mass, right? Wouldn't the Buick at 1000 MPH have more energy than the one at 999 MPH?
    Yes and no. If you heat an object (adding energy) then it will have more mass.

    The example of something moving relative to you is much more complicated. You have to take into account all the things that affect gravity in GR (energy, momentum, etc.). There is no easy answer to this. Even people I know who have a good understanding of the math have hesitated to give an answer. The simple answer, though, is no.
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    Quote Originally Posted by AlexG View Post
    Wouldn't the Buick at 1000 MPH have more energy than the one at 999 MPH?
    Kinetic energy is a relative quantity, and does not add to gravity.
    But if I were to run at 95 POTSOL, would that increase my mass?

    Quote Originally Posted by Strange View Post
    Quote Originally Posted by SowZ37 View Post
    But increased energy can increase mass, right? Wouldn't the Buick at 1000 MPH have more energy than the one at 999 MPH?
    Yes and no. If you heat an object (adding energy) then it will have more mass.

    The example of something moving relative to you is much more complicated. You have to take into account all the things that affect gravity in GR (energy, momentum, etc.). There is no easy answer to this. Even people I know who have a good understanding of the math have hesitated to give an answer. The simple answer, though, is no.
    Is there literature I can read on it you can point me to? And what level of math do I need to understand before I could at least grasp the basics? Does it, say, use complex numbers for example?
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    Quote Originally Posted by SowZ37 View Post
    But if I were to run at 95 POTSOL, that would increase my mass, right?
    Not really. (I had to guess what POTSOL means.)

    The idea of relativistic mass increase is generally disliked because it causes this sort of confusion.
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    95 POTSOL
    Relative to what?

    that would increase my mass, right?
    No.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by SowZ37 View Post
    But if I were to run at 95 POTSOL, that would increase my mass, right?
    Not really. (I had to guess what POTSOL means.)

    The idea of relativistic mass increase is generally disliked because it causes this sort of confusion.
    The explanation I've always heard, even from people like Stephen Hawking, is that you can't actually achieve light speed because the closer you get to it the more your mass multiplies so you get diminishing returns on the investment of more energy. Is this an oversimplification?

    Quote Originally Posted by AlexG View Post
    95 POTSOL
    Relative to what?

    that would increase my mass, right?
    No.
    Light. If light was moving away from me, and I was chasing after it and running 95% as fast, so it moved away at 5% of light speed compared to it just moving away from me at 100% light speed. Could I just as easily say that I am moving away from the photon at light speed, as opposed to it moving away from me, and that is a partial explanation for why chasing it wouldn't change my mass?
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    Yes. The closer you get to light speed, the more energy is required to accelerate you. The energy needed to move a mass at light speed is infinite, which is why you can't reach it.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
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    Quote Originally Posted by AlexG View Post
    Yes.
    Okay, that makes sense. So what is the real reason I couldn't get a baseball to go the speed of light? Why do I get diminishing returns on adding more energy to the accelerator if it's mass isn't increasing?

    EDIT: Oh, wait, yes to which of my questions?

    Quote Originally Posted by AlexG View Post
    Yes. The closer you get to light speed, the more energy is required to accelerate you. The energy needed to move a mass at light speed is infinite, which is why you can't reach it.
    Okay, so you aren't actually gaining mass. It just becomes harder to accelerate you. But your mass hasn't literally changed.
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    Yes to 'is this an oversimplification'.

    Nothing with mass can travel at the speed of light.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
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    Quote Originally Posted by SowZ37 View Post
    Is this an oversimplification?
    Yes. The infinite energy thing is a result of the same thing that make it impossible to reach the speed of light. (That "thing" being the geometry of space-time.)
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    Quote Originally Posted by AlexG View Post
    Yes to 'is this an oversimplification'.

    Nothing with mass can travel at the speed of light.
    If all velocity is relative, that why do we both observe the speed of light the same but not, say, the speed of a buick? Or is my premise flawed? I am under the impression that all observers view the speed of light the same, as opposed to viewing the speed of other objects relative to their position and speed.
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  34. #33  
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    Quote Originally Posted by SowZ37 View Post
    If all velocity is relative, that why do we both observe the speed of light the same but not, say, the speed of a buick?
    All speed (except the speed of light) is relative. Better?
    ei incumbit probatio qui dicit, non qui negat
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  35. #34  
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    All velocity of objects with mass are relative. The velocity of light is invariant, and is measured to be the same by all observers, regardless of their relative motions.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    Yeah, that makes sense. But since the speed of light is the same regardless of your position, that would make it impossible to accurately compare your speed to the speed of light, right? So even, say, measuring a spaceship by POTSOL, (percentage of c,) would be a bit of a misnomer.

    Or is my logic flawed.
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  37. #36  
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    Yeah, that makes sense. But since the speed of light is the same regardless of your position, that would make it impossible to accurately compare your speed to the speed of light, right? So even, say, measuring a spaceship by POTSOL, (percentage of c,) would be a bit of a misnomer.
    You are correct.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    But aren't there theoretical speeds/energy levels at which the weak and strong interactions and electromagnetism break down? Or not break down, but stop being distinguishable? I don't really understand GUTs, but I know they exist. How does that work with the relative nature of speed?
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    Energy levels, yes. Speed no. And they don't break down, they merge into a single force. The energy levels needed for this are those which existed at about 10-33 seconds into the BB.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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  40. #39  
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    Quote Originally Posted by AlexG View Post
    Energy levels, yes. Speed no. And they don't break down, they merge into a single force. The energy levels needed for this are those which existed at about 10-33 seconds into the BB.
    But if energy levels are relative, how would we calculate that? And if it is calculated, could we compare an objects energy to the amount of energy required to unify the forces and get some sort of objective metric for its energy? Percentage of GUT or something?

    I imagine no, but I am curious why not.
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  41. #40  
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    But if energy levels are relative
    Energy levels aren't relative. The amount of energy needed for reunification of the three forces is right about at the Planck scale, 1.22 1019GeV .
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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  42. #41  
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    Quote Originally Posted by AlexG View Post
    But if energy levels are relative
    Energy levels aren't relative. The amount of energy needed for reunification of the three forces is right about at the Planck scale, 1.22 1019GeV .
    Okay, so that explains that. So to take it full scale back to gravity, if I wanted to increase my gravity, I would have to increase my energy somehow but not my kinetic/potential energy. Any other types of energy that are relative or would't effect my gravity/mass?
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