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  • #16
    Originally posted by marlon View Post
    Cool thread. I have A LOT of questions lol. I'll do one at a time.

    The theory that the universe is always expanding... why, and where can it be expanding into?

    I have taken some advanced undergraduate (yes- oxymoron) calc courses so feel free to throw around math terms. I love that stuff.
    * you want this one? i still have to finish gravity.
    "Your not gonna find a bang maid cause theres no such thing."
    "I already did...your mom....good bye.".

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    • #17
      Originally posted by TommyKav View Post
      * you want this one? i still have to finish gravity.
      Yeah, I'll take it. I can segue nicely from what I am typing into that.
      Ph.D., Theoretical Physics '16
      kind of a douche

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      • #18
        Originally posted by TommyKav View Post
        * you want this one? i still have to finish gravity.
        "I just have one question for you. That's Bob right? That's Johnnie Jackson right? I'm Ben White, right? Who are you?"

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        • #19
          more on gravity........now i was talking about these 2 fields working together in certain fashions. well fields arent the only snobs on the block, some particles are as well. lets take the photon. the Photon does not interract with the higgs field but due to the nature of how other particles interract with the gravitational field they interract with gravity. or at least they move along the paths gravity specifies for them.

          but TOM i thought matter and energy are interchangeable? then why cant energy, like light, create gravity? It can and it cant. Photons are more accurately tightly packed groups of light waves of the same frequency and amplitude.....ahhh back to work...more to come soon
          "Your not gonna find a bang maid cause theres no such thing."
          "I already did...your mom....good bye.".

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          • #20
            Originally posted by fatslob View Post
            All right... Is it sad that I saw this and the very first thing I was going to ask you was "Where does gravity come from?"

            Okay... Lessee... What is time? And how does gravity affect it?
            Ok I will interpret "where does gravity come from?" as "what is gravity?" Right now the best handle that physics has on gravity is given by Einstein's theory of general relativity. This theory tell us that what we perceive as "gravity" is actually the warping of space and time. Gravity is caused by the presence of mass or energy. To be a little more technical, gravity is the curvature of space-time that is caused by the presence of mass and/or energy.

            Now let's explain what we mean by "curvature". The best way I know of to describe this without using some formidable mathematics is by analogy. Say you have an ant walking around on the surface of an apple. Now all the ant can see is the surface of the apple; he can't see that there's an up or down, all he can see is left/right, forwards/backwards. We say that the ant lives in two dimensions, not three. Let's say our ant has an ant friend and they are walking side-by-side, parallel with each other. They are both walking in straight lines. Since they are walking in the same direction, the distance between them never changes.

            Now the ants are approaching the top of the apple where the stem is. Let's imagine that the stem will be coming between the ants on their paths. If the ants continue to walk straight on the apple near the stem, all of a sudden they will find that their paths cross! We can see that they walked into the dimple of the apple, where the surface of the apple is CURVED. This "curvature" in the apple deflected the paths of the ants even though they continued to walk in a straight line.

            Now they ants will say "DUDE WHAT THE FUCK JUST HAPPENED?!" Since they cannot see into the 3rd dimension (up & down), they cannot perceive the curve of the apple. We say that the apple curves into the third dimension.

            This is completely analogous to the effect of mass and energy creating gravity. In the same way that the stem created a dimple on the surface of the apple, making it curve into another dimension, the presence of matter or energy in our universe makes space curve into another dimension, which we know as “time”. If we travel in a straight line very near a star (which has a large mass), we will find our path has been deflected by the curvature of space. This curvature is produced by the mass of the star. We say that the star has exerted a gravitational force on us that caused us to deviate from our initial straight line. However, if we took the fourth dimension into account, we would see that we actually travelled in a straight line IN FOUR DIMENSIONS! This 4-D straight line is called a “geodesic”.

            So according to the general theory of relativity, gravity is the curvature of space (and time) that is caused by the presence of mass. Now remember the good ol’ E=mc^2? That tells us that matter and energy are the same thing. Thus, energy will create gravity as well as matter.

            For the second question, I’ll take the latter half first: “How does gravity affect time?” It turns out that gravity makes time go MORE SLOWLY. Let’s say we have two astronauts in orbit around a very large star. The astronauts synchronize their clocks so that they read the same time (12:00 PM) and are running at the same speed. Then one of the astronauts travels right next to the star, so that he can feel its gravity very strongly. He stays there for a while, then travels back to where the second astronaut is. When they compare their clocks, let’s say the clock of the astronaut who was stationary reads 2:00 PM. The clock of the astronaut who was near the star will read a time that is BEFORE 2:00 PM. This is because the presence of gravity SLOWS DOWN TIME!

            This happens because in the presence of gravity, time curves into the other space dimensions, just like the space dimensions curve into time! Space and time become all mushed-up together into one entity called spacetime.

            To sum up, gravity is the curvature of spacetime caused by the presence of mass or energy. This is general relativity in a nutshell.

            Now, “what is time?” Fuck if I know. One way to look at it is that it is another way to “move” (i.e. another dimension). We have 3 space dimensions: up/down, left/right, and forward/back. We also have time. However time is special in that it never seems to go backwards; we can only go forwards in time. Why is that? Nobody really knows.

            Originally posted by marlon View Post
            Cool thread. I have A LOT of questions lol. I'll do one at a time.

            The theory that the universe is always expanding... why, and where can it be expanding into?

            I have taken some advanced undergraduate (yes- oxymoron) calc courses so feel free to throw around math terms. I love that stuff.
            As near as we can tell, space is expanding. But into what? It turns out the question is wrong. Let’s think of a balloon. As we blow up this balloon, we will find the distance between two points on the balloon will increase. It’s not that there is any new rubber between the points, it’s just that the rubber between them is stretching out.

            That is analogous to the way space is expanding. No new space is getting created, it’s just that space itself is getting stretched out. A guy on the balloon wouldn’t think his balloon was expanding into something, he would just think “DUDE WTF that point is getting farther away WTF!”

            Now, why is it expanding? The current models suggests that there is something in all space called “dark energy.” We saw earlier that energy will produce gravity. And gravity is always attractive; it always pulls thing together and never pushes apart. We can think of dark energy as being repulsive (although this is technically not correct). If dark energy created gravitational repulsion instead of attraction, then space would expand instead of contract. And if this dark energy were EVERYWHERE in space, then EVERYWHERE would push out against EVERYWHERE, causing the expansion of the universe. Again, this is actually not correct. Dark energy actually has what is called “negative pressure.” This negative pressure is what drives the accelerated expansion of the universe.
            Ph.D., Theoretical Physics '16
            kind of a douche

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            • #21
              Physics blows my mind.

              Does that mean since the volume of the universe increases, the "negative pressure" decreases, and eventually the expansion of the universe will stop? Not actually stop, but slow down to almost negligible increase in volume over time.
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              • #22
                Originally posted by marlon View Post
                Physics blows my mind.

                Does that mean since the volume of the universe increases, the "negative pressure" decreases, and eventually the expansion of the universe will stop? Not actually stop, but slow down to almost negligible increase in volume over time.
                actually the rate of expansion depends on your distance form another object. the further an object is from you the faster it is moving away from you. dark energy is a term used to describe something we dont really know anything about. our space doesnt need to expand into anything else, it has all the room it needs to expand as much as it wants. it simply creates more room as it expands. that is the reason it would be totally safe to create a new universe in a lab. within a fraction of a second it would "pinch" itself off from our universe and expand in a space all its own. the negative pressure will only decrease if there is enough matter in the universe so that gravity can take over and reverse the process. this will not cause a decrease in pressure rather it will be a greater opposing force. as it stands now the universe will expand to the point where it will eventuall pull atoms and sub atomic particles appart and become a vast expance of uniform energy. there isnt enough matter in the universe to stop the expansion.
                "Your not gonna find a bang maid cause theres no such thing."
                "I already did...your mom....good bye.".

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                • #23
                  How many theoretical dimensions are there, and how is this relative to space and time?
                  What is string theory's role?

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                  • #24
                    Thanks for the responses guys. I don't know what to think lol.

                    Originally posted by Pheedno View Post
                    How many theoretical dimensions are there, and how is this relative to space and time?
                    What is string theory's role?
                    I like.
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                    • #25
                      "This happens because in the presence of gravity, time curves into the other space dimensions, just like the space dimensions curve into time! Space and time become all mushed-up together into one entity called spacetime."

                      I wish I could "see" this graphically.
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                      • #26
                        in string theory there ar 9 physical dimensions and time...totalling 10. In M theory they added an 11th dimension to explain some of the wierd effects of gravity. m theory as it stands now is the closest we haveto a unified theory encompassing quantumn mechanics and gravity.
                        "Your not gonna find a bang maid cause theres no such thing."
                        "I already did...your mom....good bye.".

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                        • #27
                          Originally posted by marlon View Post
                          "This happens because in the presence of gravity, time curves into the other space dimensions, just like the space dimensions curve into time! Space and time become all mushed-up together into one entity called spacetime."

                          I wish I could "see" this graphically.
                          http://media.photobucket.com/image/s...0415223658.jpg

                          there ya go....its accurate in a 2-d manner
                          "Your not gonna find a bang maid cause theres no such thing."
                          "I already did...your mom....good bye.".

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                          • #28
                            Originally posted by marlon View Post
                            Physics blows my mind.

                            Does that mean since the volume of the universe increases, the "negative pressure" decreases, and eventually the expansion of the universe will stop? Not actually stop, but slow down to almost negligible increase in volume over time.
                            I think I didn't explain dark energy very well. Keep in mind all of this is HIGHLY theoretical and most likely wrong, but it's the best we understand it right now. In any given amount of space, there is a certain amount of this dark energy. Now as the universe increases, the total volume of space increases. Thus, there is more space, and more dark energy. But the total amount of matter and regular energy in the universe does NOT increase. So the gravitational attraction from regular matter/energy remains the same, but there is MORE repulsion from dark energy (because there is more space). Thus the expansion of the universe accelerates as time goes on.

                            Originally posted by Pheedno View Post
                            How many theoretical dimensions are there, and how is this relative to space and time?
                            What is string theory's role?
                            It depends on your theory. As far as we know, we just have the 3 space dimensions and time. Certain theories hypothesize other dimensions, but there is ZERO physical evidence for it. I am pretty prejudiced against string theory every since I read Roger Penrose's critique of it in The Road to Reality. (At least I think it was that book... I know it was Penrose.)

                            Originally posted by TommyKav View Post
                            in string theory there ar 9 physical dimensions and time...totalling 10. In M theory they added an 11th dimension to explain some of the wierd effects of gravity. m theory as it stands now is the closest we haveto a unified theory encompassing quantumn mechanics and gravity.
                            The only problem is that string theory has holes in it big enough to drive a Mack truck through. It also currently has zero basis in reality and is best described as a mathematical exercise.

                            That being said, some of the things it says are very promising. I think string theory has the potential to give us some remarkable insights into the universe, but I seriously doubt that it represents reality.
                            Last edited by Sammich; 09-30-2009, 09:50 PM.
                            Ph.D., Theoretical Physics '16
                            kind of a douche

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                            • #29
                              Thanks dino-bird for answering my questions.

                              Here's another:

                              So as this universe constantly expands and energy is spread out, is the fate of our universe just blackness?
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                              • #30
                                Originally posted by marlon View Post
                                Thanks dino-bird for answering my questions.

                                Here's another:

                                So as this universe constantly expands and energy is spread out, is the fate of our universe just blackness?
                                Yep. In about 100 billion years all the stars will have burnt out, all the black holes will have evaporated, and all the matter will have decayed. The universe will be cold, lifeless, black, and dead.

                                Have a good night.

                                Of course, that's assuming everything we know now is correct...
                                Ph.D., Theoretical Physics '16
                                kind of a douche

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