• Lucid Dreaming - Dream Views




    Page 1 of 3 1 2 3 LastLast
    Results 1 to 25 of 68
    Like Tree11Likes

    Thread: Special Relativity misconceptions

    1. #1
      Banned
      Join Date
      Dec 2010
      Gender
      Posts
      1,590
      Likes
      521

      Special Relativity misconceptions

      This is a work in progress.

      DISCLAIMER: No mathematical proof or cited sources are given here, mostly because a) mathematical proofs are too advanced for the audience and b) finding articles that talk about 100 year old well-established physics is nigh impossible.

      If you hold any of the following misconceptions, I strongly recommend finding an undergrad physics textbook and reading it, or just do us a favour and don't spread your wrong ideas.

      Here's my list of what I think are the most prevalent and/or annoying public misconceptions about special relativity.

      Most of these are, of course, the result of dumbed-down pop science programs on tv and the failure of our education system.

      4) Scientists used to say that there was a "sound barrier" that could never be exceeded, but they were obviously wrong about that. Maybe they're wrong about the "light barrier".

      Bullets exceeded the sound barrier in the 19th century. No actual scientist has ever, in the history of Enlightenment, ever said that nothing can go faster than sound.

      Furthermore, Einstein didn't say the speed of light was a "barrier". He said it was the same for all observers. Everything else in SR follows from that postulate (among a couple of others). More on this later.


      3) Accelerating to the speed of light makes your mass infinite.

      Not mass. Momentum. Momentum approaches infinity from the perspective of a stationary observer.

      Why not mass? Because in SR, mass is an invariant. An invariant is a quantity that never changes for any observer. The best known invariant is the speed of light, c.

      3b) Accelerating to the speed of light requires infinite energy.

      Nope. From the perspective of the stationary observer, your expended energy appears to increase asymptotically. However, ship-board energy consumption is linear.

      2) E=mc^2 --> atomic bombs.

      No. In reality, nuclear chemistry --> atomic bombs. Although it was Einstein that persuaded President FDR to create a nuclear program, and although in his letter to the president he probably did reference the equation E=mc^2, it was only meant as a sort of ex-post-facto explanation for how so much energy can come from such a small bomb. In fact, E=mc^2 would not be at all helpful for anyone trying to build a bomb.


      1) You can't go faster than the speed of light.

      Actually, you can. There are just a few provisos.

      First, no one will ever observe you going faster than light. To all observers, even the
      stationary ones, you will only appear to be going, at best, slightly less than c.

      Second, for the person actually in the spaceship, they must be content to measure their speed by counting landmarks as they pass by, like stars. If you see 3 stars go by in a year of travel, and you know that stars are (sans length contraction) about 5 light years apart, then you're EFFECTIVELY* going 15c.

      *I say effectively because you're not actually surpassing c, technically speaking. But if you're willing to ignore time dilation, there isn't a difference as far as you're concerned.

      Third, time dilation. If you go home, everyone you ever knew is long since dead. Of course, if your goal was to travel into the distant future, then this is a plus!

      Fourth, more time dilation. Whatever destination you're trying to reach will ALSO appear to pass into the future as you approach it. So, for example, if you want to go to Alpha Centauri because in your super powerful telescope you see a naked Navi waving at you, too bad. By the time you arrive, she will have moved on.

      But, all that being said, suppose you have a ship that can accelerate at 1g (relative to
      itself) for many years at a time. At 1g acceleration, you can go anywhere in the universe in a single human lifetime, shipboard.

    2. #2
      Xei
      UnitedKingdom Xei is offline
      Banned
      Join Date
      Aug 2005
      Posts
      9,984
      Likes
      3082
      Good thread. I knew all of that basically, except I wasn't sure about 2. I'd always been confused by 2 and I actually made a thread in this forum about it a while back asking what E = mc^2 has to do with anything when really it's just the interplay of nuclear forces that releases the energy in the same way that the interplay of electrical forces releases chemical energy. Most people said I was wrong and stupid so it's nice to know that it was actually them accepting things without understanding them.

    3. #3
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      I may be wrong here but I'll see what people say.

      Quote Originally Posted by cmind View Post
      3) Accelerating to the speed of light makes your mass infinite.

      Not mass. Momentum. Momentum approaches infinity from the perspective of a stationary observer.

      Why not mass? Because in SR, mass is an invariant. An invariant is a quantity that never changes for any observer. The best known invariant is the speed of light, c.
      This is a little misleading. There are two types of mass in SR. One of them is the "rest mass" which is the mass measured in a reference frame where the object is at rest. This is an invariant.

      The second is the "relativistic mass" which is the one used in the equation P=mv for momentum and that does increase without bound as v approaches c. In a reference frame where the particle is at rest, relativistic mass agrees with the rest mass.

      This is why there's a fundamental distinction between particles with mass and particles without mass with respect to being able to travel at the speed of light.

      3b) Accelerating to the speed of light requires infinite energy.

      Nope. From the perspective of the stationary observer, your expended energy appears to increase asymptotically. However, ship-board energy consumption is linear.
      Right, but as you say later on, you never measure yourself moving at the speed of light so your energy consumption increases linearly without bound...

      1) You can't go faster than the speed of light.

      Actually, you can. There are just a few provisos.
      The net effect of all of these provisos is that, effectively, you can't go faster than the speed of light. If you start on a journey, with whatever acceleration, and a photon starts at the same time, you will never catch it. That photon will reach the destination first.

      Maybe this is like the case of one racer taking longer to get up to speed but being able to run faster? Nope. In that case, if the race is long enough, the faster racer will eventually win. That's not the case here. No matter how fast you accelerate and how long the race is, that photon will always win.

      And the clock attached to that photon will measure zero time.

      But, all that being said, suppose you have a ship that can accelerate at 1g (relative to
      itself) for many years at a time. At 1g acceleration, you can go anywhere in the universe in a single human lifetime, shipboard.
      Not really. You're still restricted to points that are on the interior of your future light cone.
      Last edited by PhilosopherStoned; 03-03-2011 at 08:38 PM.
      nina likes this.
      Previously PhilosopherStoned

    4. #4
      Ad absurdum Achievements:
      1 year registered 1000 Hall Points Made lots of Friends on DV Referrer Bronze Veteran First Class
      Spartiate's Avatar
      Join Date
      Jul 2007
      Gender
      Location
      Block 4500-7000
      Posts
      4,825
      Likes
      1112
      Is the existence of the speed of light and answerable question? As in why it is what it is, why not faster or slower? It's ironic how slow the speed of light is compared to the size of the universe.

    5. #5
      Banned
      Join Date
      Dec 2010
      Gender
      Posts
      1,590
      Likes
      521
      The speed of light is directly derived from the permeability and permitivity of free space. Permitivity is measured, but permeability is actually just a variation on pi. So the speed of light is derived from the permitivity constant and pi. The permitivity itself is entirely controlled by the interactions of subatomic particles and has nothing to do with the universe at large. So in other words, the speed of light is completely unrelated to the size of the universe.

    6. #6
      Banned
      Join Date
      May 2007
      LD Count
      Loads
      Gender
      Location
      Digital Forest.
      Posts
      6,864
      Likes
      385
      Permittivity*

      Also, spart v1.0's point was that the universe is so gigantically big, yet the top speed limits travel throughout it.

    7. #7
      Banned
      Join Date
      Dec 2010
      Gender
      Posts
      1,590
      Likes
      521
      Quote Originally Posted by A Roxxor View Post
      Permittivity*

      Also, spart v1.0's point was that the universe is so gigantically big, yet the top speed limits travel throughout it.
      I originally spelled it that way, but this browser's spell checker said there was only 1 t.

      As to the size of the universe comment, I guess I'm failing to see the point. Yes, it's a spot of bad luck that the place is so big compared to the speed we can go. But then again, would you really want to live in a "small" universe?

    8. #8
      Ad absurdum Achievements:
      1 year registered 1000 Hall Points Made lots of Friends on DV Referrer Bronze Veteran First Class
      Spartiate's Avatar
      Join Date
      Jul 2007
      Gender
      Location
      Block 4500-7000
      Posts
      4,825
      Likes
      1112
      Quote Originally Posted by cmind View Post
      would you really want to live in a "small" universe?
      No, a faster one .

    9. #9
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      A faster one would be a smaller one
      nina likes this.
      Previously PhilosopherStoned

    10. #10
      Ad absurdum Achievements:
      1 year registered 1000 Hall Points Made lots of Friends on DV Referrer Bronze Veteran First Class
      Spartiate's Avatar
      Join Date
      Jul 2007
      Gender
      Location
      Block 4500-7000
      Posts
      4,825
      Likes
      1112
      How do you figure?

    11. #11
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      If we could travel anywhere in the universe (or even to some sizable subset of it) and come back to the planet and people that we left, then it would essentially be a small universe.
      Previously PhilosopherStoned

    12. #12
      Ad absurdum Achievements:
      1 year registered 1000 Hall Points Made lots of Friends on DV Referrer Bronze Veteran First Class
      Spartiate's Avatar
      Join Date
      Jul 2007
      Gender
      Location
      Block 4500-7000
      Posts
      4,825
      Likes
      1112
      Well that's a matter of interpretation of definitions... Even if you could zip around the universe like that, the universe itself would still be the largest object that exists, so compared to everything else within the universe, it would still appear large.
      sloth likes this.

    13. #13
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      Sure but we could imagine a universe that "went on forever" which is essentially what we have now. The universe we are considering would be small in relation to that. I agree that it's a matter of definition though.
      Previously PhilosopherStoned

    14. #14
      Banned
      Join Date
      Dec 2010
      Gender
      Posts
      1,590
      Likes
      521
      A new one I just heard on the internets:

      "Magnetic fields travel faster than light; they're instantaneous. This didn't support Einstein's theory, so that's why he called them 'fields'."

      This sounds a bit too crockery to be added to the big list, but still funny. By the way, changes in magnetic fields do go at the speed of light.

    15. #15
      Banned
      Join Date
      Apr 2007
      Location
      Out Chasing Rabbits
      Posts
      15,193
      Likes
      929
      PhilosopherStoned is correct in everything he said in his first post. You seem to know relativity pretty well.

      You can not travel faster than light, you can never get to 15c. I did the math, you are correct that at a constant 1g, a human lifetime is more than enough to reach the edge of the universe, but that's for two reasons. Time dilation, and The Lorenz transformation. At high speeds, time slows down, and the universe shrinks. What stonedphilosopher said about a photon registering a trip of zero seconds is correct, at c, time ceases to exist. He forgot to mention, that it also would have registered a trip of zero meters. Because at c, space also ceases to exist. To something traveling the speed of light, the universe, is a singularity.

    16. #16
      Xei
      UnitedKingdom Xei is offline
      Banned
      Join Date
      Aug 2005
      Posts
      9,984
      Likes
      3082
      A singularity, or a plane?

    17. #17
      Banned
      Join Date
      Dec 2010
      Gender
      Posts
      1,590
      Likes
      521
      Quote Originally Posted by ninja9578 View Post
      You can not travel faster than light, you can never get to 15c. I did the math, you are correct that at a constant 1g, a human lifetime is more than enough to reach the edge of the universe, but that's for two reasons. Time dilation, and The Lorenz transformation.
      Like I clearly pointed out in the OP. Did you intentionally not read certain parts?

    18. #18
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      It seems a little weird to discount a fairly major portion of relativity to get a result (faster than light travel) and then claim that the negation of that result is a misconception about special relativity.
      Previously PhilosopherStoned

    19. #19
      Lurker
      Join Date
      Mar 2011
      Posts
      1
      Likes
      0

      Stretching out of space

      Faster than light
      This explanation is of a closed universe. An open universe cannot be
      visualized because every point in space is saddle shaped.
      Einstein did not know when he penned his famous theories, that space
      was stretching.
      In fact the universe is stretching or expanding from the three
      dimensions we know, into a higher dimension.
      We can not visualize this but we can visualize two dimensions
      expanding into three.
      It would look like the skin of a balloon. The skin of the balloon is
      just like a sheet of paper "flatland", a two dimensional universe.
      If you put little dots all over it to represent the galaxies and you blow
      this balloon up, you will see the dots are moving away from each other
      just like the galaxies we observe. No matter which direction you look,
      you are looking into the past at a time when the universe was smaller.
      Now lets say this balloon has a north and south pole.
      If you are standing at the north pole and you look out across this two
      dimensional surface you will notice that the universe has no edge. If
      light were infinitely fast and you had a powerful enough telescope you
      could see the back of your head.
      But light takes time to travel. And so as we look out across space we
      don't see the surface of a sphere but a sight line spiraling in toward
      the center like the shell of a nautilus.
      Let's imagine that the universe is 5 billion light years around and
      the light leaving a star at the south pole travels two and a half
      billion years to reach us if the universe were not expanding.
      But because it is expanding, by the time the light reaches us six and
      three eighths billion light years later, the universe is now twenty
      billion light years around.
      If you graph this out on a sheet of paper, you will notice that space
      was stretching out so fast that it exceeded the speed of light.

      There is also another scenario in which space swirls faster than light.
      The space around a spinning black hole is being spun by the immense
      gravity at a rate faster than light.

      The following was taken from wikipedia

      Universal expansion

      The expansion of the universe causes distant galaxies to recede from us faster than the speed of light, if comoving distance and cosmological time are used to calculate the speeds of these galaxies. However, in general relativity, velocity is a local notion, so velocity calculated using comoving coordinates does not have any simple relation to velocity calculated locally[15] (see Comoving distance#Uses of the proper distance for a discussion of different notions of 'velocity' in cosmology). Rules that apply to relative velocities in special relativity, such as the rule that relative velocities cannot increase past the speed of light, do not apply to relative velocities in comoving coordinates, which are often described in terms of the "expansion of space" between galaxies. This expansion rate is thought to have been at its peak during the inflationary epoch thought to have occurred in a tiny fraction of the second after the Big Bang (models suggest the period would have been from around 10−36 seconds after the Big Bang to around 10−33 seconds), when the universe may have rapidly expanded by a factor of around (10 to the 20th) to (10 to the 30th) power.[16]

      There are many galaxies visible in telescopes with red shift numbers of 1.4 or higher. All of these are currently traveling away from us at greater than the speed of light. Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.[17][18] However, because the expansion of the universe is accelerating, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future,[19] because the light never reaches a point where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Comoving distance#Uses of the proper distance). The current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light years away, but the signal would never reach us if the event was more than 16 billion light years away.[18]
      Last edited by rad802; 03-06-2011 at 11:48 PM.

    20. #20
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      The more I think about it, the less I like this thread.

      Concerning mass/energy equivalence, I think that it's more instructive to say that it's responsible for all chemistry, including nuclear chemistry.

      Concerning the speed of light issue, I think that it makes more sense to point out that we are constantly moving at the speed of light. When we are "at rest", we are moving at light speed through "time". When we begin "moving" (through space), our velocity vector rotates so that we are now moving partly through space and partly through time. A massive object cannot ever rotate its velocity vector to lay on its light cone. Likewise, a massless particle can never rotate its velocity vector off of its light cone.
      Previously PhilosopherStoned

    21. #21
      Banned
      Join Date
      Apr 2007
      Location
      Out Chasing Rabbits
      Posts
      15,193
      Likes
      929
      Quote Originally Posted by cmind View Post
      DISCLAIMER: No mathematical proof or cited sources are given here, mostly because a) mathematical proofs are too advanced for the audience and b) finding articles that talk about 100 year old well-established physics is nigh impossible.
      I disagree, I think it's because you don't understand them, most is not that complicated. Most don't even require calculus.

      4) Scientists used to say that there was a "sound barrier" that could never be exceeded, but they were obviously wrong about that. Maybe they're wrong about the "light barrier".
      How?

      Gamma = 1 / (sqrt(1 - (v^2 / c^2))
      v > c will make gamma = 1 / x(i) where x is a scalar

      How can you do a transform with an imaginary number?


      I'll ignore the rest, some of it was right, some of it was not right. You should read up on the difference between mass and relativistic mass though.
      Last edited by ninja9578; 07-22-2011 at 02:05 PM.

    22. #22
      Haunted by entropy. Achievements:
      1 year registered Made lots of Friends on DV Populated Wall Veteran First Class 5000 Hall Points
      sloth's Avatar
      Join Date
      Apr 2006
      LD Count
      20 years worth
      Gender
      Location
      Deep in the woods
      Posts
      2,131
      Likes
      584
      Quote Originally Posted by PhilosopherStoned View Post
      Sure but we could imagine a universe that "went on forever" which is essentially what we have now. The universe we are considering would be small in relation to that. I agree that it's a matter of definition though.
      I can imagine a universe that "goes on further than forever". This universe is small in relation to that.
      ---o--- my DCs say I'm dreamy.

    23. #23
      Banned
      Join Date
      Dec 2010
      Gender
      Posts
      1,590
      Likes
      521
      Quote Originally Posted by ninja9578 View Post
      I disagree, I think it's because you don't understand them, most is not that complicated. Most don't even require calculus.


      How?

      Gamma = 1 / (sqrt(1 - (v^2 / c^2))
      v > c will make gamma = 1 / x(i) where x is a scalar

      How can you do a transform with an imaginary number?


      I'll ignore the rest, some of it was right, some of it was not right. You should read up on the difference between mass and relativistic mass though.
      You do understand that the bolded parts are the misconceptions, right? /facepalm

    24. #24
      Banned
      Join Date
      Apr 2007
      Location
      Out Chasing Rabbits
      Posts
      15,193
      Likes
      929
      Oops.

    25. #25
      Rational Spiritualist DrunkenArse's Avatar
      Join Date
      May 2009
      Gender
      Location
      Da Aina
      Posts
      2,941
      Likes
      1091
      Quote Originally Posted by sloth View Post
      I can imagine a universe that "goes on further than forever". This universe is small in relation to that.
      I could imagine a "point" that was more pointless than pointless. The post that I'm responding to is pointless in relation to that. What's your point?
      tommo likes this.
      Previously PhilosopherStoned

    Page 1 of 3 1 2 3 LastLast

    Similar Threads

    1. Hypnosis Misconceptions
      By louie54 in forum Science & Mathematics
      Replies: 22
      Last Post: 07-25-2012, 07:21 PM
    2. Warrior Tiger's Visions of Misconceptions
      By WarriorTiger in forum Dream Journal Archive
      Replies: 144
      Last Post: 06-07-2010, 09:18 PM
    3. relativity
      By slash112 in forum Science & Mathematics
      Replies: 65
      Last Post: 09-09-2009, 03:51 PM
    4. Replies: 4
      Last Post: 04-07-2009, 03:25 AM
    5. Tell me about Relativity
      By TimeStopper in forum Ask/Tell Me About
      Replies: 2
      Last Post: 08-15-2008, 09:35 PM

    Bookmarks

    Posting Permissions

    • You may not post new threads
    • You may not post replies
    • You may not post attachments
    • You may not edit your posts
    •