1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Briefer History of Time

  1. Sep 3, 2013 #1
    Hi

    Im new here, and have begun to read this book. I dont have any science qualification but I've read a few books on evolution. I'm making notes as I go along and here are 2 difficulties I have.

    1) It says gravity is for large scale and quantum is at the small scale. Im just wondering what are the scales relative to ? I would have thought size is relative ?

    2) It says there is no absolute speed, however surely the guy on the train's speed is = speed of train + speed of Earth + speed of Milky Way + speed of Universe. His eyes tell him he's not moving (if he doesnt know he's on a train) but our eyes didnt evolve to read accurate speeds , certainly not at a planetary level so I dont agree that different observers are just as right as each other. I dont know if that make sense but if you assume the universe is moving in relation to a nothingness , then there is an absolute speed.

    Thats as far as I've got so far, just finishing off the chapter on Relativity , it is very interesting stuff.
     
  2. jcsd
  3. Sep 3, 2013 #2
    1) Quantum effects are generally only significant at the atomic level and smaller. Gravity is generally only important where one of the things is at least as big as a mountain and the other is at least big enough to see.

    2) There is nothing we can measure the speed of the Milky Way against, and the concept of the speed of the universe doesn't make sense, so the furthest you can go is the speed of the train relative to the centre of the Milky Way. As you read more about relativity you will find out why.
     
  4. Sep 3, 2013 #3
    Ofcourse you could use relativity to measure some absolute speed as against the speed of the universe but the problem here is we don't even know how big the universe is because the speed of light permits us to see further then some distance x.So if we don't know the end of the universe nor do we know what's "outside" of it if there even is such outside then talking about some absolute speed is useless all we can do is talk about speed of object x versus speed of object D etc.All we are doing actually is comparing speeds from different objects to different other objects and or backgrounds.

    And by the way be careful with all that evolution theory and everything that surrounds it as some areas of it are not as known as others , there are many unknowns etc , so you have to have a pretty good background and critical thinking to understand all the facts and all the parts which are still just a good guess. That makes a huge difference as there are things we know for sure (have measured them or made experiments) and things which we are not so sure but we have a say good guess or as science calls it level of certainty.
     
  5. Sep 3, 2013 #4
    We can measure the velocity of the Milky Way against the Cosmic Microwave Background. It turns out to be on the order of 550 km/s in a particular direction. See http://arxiv.org/abs/astro-ph/9312056 .
     
  6. Sep 3, 2013 #5
    Yes it was a bad answer (although the figure for 550km/s is not the right one for the Milky Way's motion I believe). We can measure the our speed relative to the universe, but (I assume that) the reason Hawking says there is "no such thing as absolute speed" is that someone else moving (very rapidly) relative to us would measure us as having a different speed.
     
  7. Sep 3, 2013 #6

    Nugatory

    User Avatar

    Staff: Mentor

    And what is that "speed of the universe"? How do you measure it, how do you assign a value to it, what does it mean?

    To see the problem more clearly, let's try writing out your sum without skipping the details:

    Speed = Speed of train relative to earth + speed of earth relative to milky way + speed of Milky way relative to universe + speed of universe relative to ????
     
  8. Sep 3, 2013 #7
    Yes, well it depends whats outside the universe, it could be nothingness, or the universe itself could be contained in something that is moving also, so we get into a kind of Russian doll scenario. Actually it would be an arbitrary and self-satisfying thing on our part to assume the universe is not contained in something , its akin to Aristotle putting the earth in the centre of the universe. So most likely its contained in a multi-verse thing, which brings me back to my first point. What exactly is size relative to ? Could our universe just be a part of an atom making up a much larger universe ?

    Then there is the question what exactly is the definition of universe since we cant see the end of it.
     
  9. Sep 3, 2013 #8

    Drakkith

    User Avatar

    Staff: Mentor

    Self satisfying? I think it's simply one of several possibilities.

    There is very very little to be gained from this type of discussion. It's more philosophical than anything else. To the best of our knowledge there is only one universe and the concept of something being "outside" the universe, or our universe "within" another, is not a scientific one since we have no way of knowing if it is true or false, even in principle.
     
  10. Sep 3, 2013 #9
    "What exactly is size relative to ?"

    I think this part of it is a very profound question deep in the center of physics.
    It seems very interesting that the principals of interaction change with the size of the actors.
    It lends a sense of "absolute size" to a field where absolute time and absolute space don't exist.
    Right now it seems that "size" seems to be relative to the theories with which these things are described.
     
  11. Sep 3, 2013 #10
    Well it's probably wise to scale something to the smallest size we have (Planck) rather than the biggest possible out there which we don't know of because we have no way of knowing that...
     
  12. Sep 4, 2013 #11
    well humans are good at thinking they are special, their planet is special, their universe is special etc etc, and science has proved them wrong every time , but yes we have no way at the moment of testing it.
     
  13. Sep 4, 2013 #12
    This is what Im getting at and you've put it very nicely. One thing that should not be absolute is size, just because something is "small" to us , doesnt mean it should be defined scientifically as small. If a creature evolved to see at the atomic level , then atoms wouldnt look too small to it.
     
  14. Sep 4, 2013 #13

    russ_watters

    User Avatar

    Staff: Mentor

    Which is why people tend to have trouble letting go of the concept of absolute speed. The best that can be said about it is that it is unnecessary. If it doesn't affect my ability to play table tennis on a train, it has no value and can be discarded as superfluous.
     
  15. Sep 4, 2013 #14
    There is an absolute size: Atoms. An atom of any given element (at low energy state) can only ever be one fixed size. You can put energy in and make the atoms occupy higher orbits, but these are also a fixed and limited set of sizes. Anything smaller than a hydrogen atom can't be an element.
     
  16. Sep 4, 2013 #15
    No it is YOU that is making the anthropocentric assumption by insisting that our universe exists in the same spatial dimensions as every other universe (in order for the notion of speed within a multiverse to make sense). Most people that have given this serious thought realise that there is no reason for this to be the case, and that is why we say that "speed relative to the multiverse" has no meaning, not because we are denying the possibility of the existance of other universes.

    Nobody is saying this. What changes with the size of the actors is the SIGNIFICANCE of different interactions. Within an atom, gravity is insignificant but quantum mechanics and other interactions become important. When you throw a ball in the air, nothing really matters except gravity and air resistance.
     
  17. Nov 29, 2013 #16
    and what is an atom composed of ?
     
  18. Nov 29, 2013 #17
    I have just finished reading the book and have made a few notes along the way. An interesting incident happened as I
    was reading the final chapter.

    I had been cooking bacon on a pan, and after eating I poured boiling hot water into the pan. I then came back later
    to clean up and noticed that the grease had coalesced into shapes. Not just any old shapes, but perfect circles
    of various sizes. The circles had grouped together, without touching, into clusters.

    I realised that as the hot water had cooled down, matter on the pan had started to break up and form into circular shapes.
    Some circles were huge, 10 or 12 times the size of other ones. But the most common size was about the size of the head
    of a needle. Around the edges of the pan was the greatest concentration of circles.


    We think that the early conditions of the universe were much the same, very hot and then a cooling down period. (which is still happening?) Matter that was scattered during the big bang formed into circles, which then formed clusters called galaxies. I cant replicate the effects
    of gravity although I did notice some movement in the pan early on due to currents caused by the change in temperature in the water.

    So in my frying pan I replicated more or less the early conditions of the universe !
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A Briefer History of Time
  1. History of Physics (Replies: 2)

  2. History of energy (Replies: 1)

  3. History of physics (Replies: 7)

  4. History of Physics (Replies: 2)

Loading...