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Special Relativity Simplified:

January 17, 2014 Astrophysics 2015
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Albert Einstein

Albert Einstein

Einstein put forth special relativity, which explains motion at near-light speeds. Although there are many consequences of Special Relativity, this complex theory consists of only two postulates, both of which are fairly easy to understand. However, to understand these postulates, we must first understand relativity. As an aside, it is a common misconception that relativity came from Einstein, but relativity is an old concept, dating back to Galileo (way back in 1632). Einstein’s Special Relativity, on the other hand, wasn’t published until 1905.

Now then, from his experiments, Galileo deduced that two observers moving at a constant velocity will get the same results from all mechanical experiments. To make it easier to understand, think of it like this: being inside of a vehicle that is traveling at a constant velocity is the same as being inside a vehicle that is at rest (velocity = 0). To prove this, look at where you are right now. You may think that you are at rest, but the Earth is actually moving at an approximately constant velocity through space around the Sun. You don’t feel a thing while sitting there and reading this, and there is no experiment that you can do here on Earth to determine the Earth’s speed.

Hence, one phrase that is important in relativity is the “reference frame.” In relativity, velocity is not absolute, but rather, velocity depends on where you are making the observation. The point at which you are making your observation is where your reference frame is. A reference frame that is moving at a uniform velocity is called an “inertial reference frame.” Ultimately, Galilean relativity can be summarized by saying that all mechanical laws of physics are valid in inertial reference frames. Technically, the Earth is a non-inertial reference frame because of two reasons: 1.) the Earth’s spin on its own axis and 2.) the Earth’s revolution around the Sun. However, the effects of these two are small and, for most purposes, can be neglected.

Another concept of Galilean relativity is the addition of velocities. It can be best understood with this example: you are on a vehicle travelling at 10m/s, then you throw a ball forward at 10m/s. An outside observer will then see the ball to have a velocity of 20m/s. If you threw the ball backwards, the outside observer will then see the ball to have a velocity of 0 i.e., it will appear to stop.

Here’s where Einstein comes in. The first postulate of Einstein generalizes Galilean relativity to include all the laws of physics. Simply stated: “All the laws of physics are valid in all inertial reference frames”. The concept of inertial reference frames is quite easy to understand, but what is a non-inertial reference frame? A non-inertial reference frame is simply an accelerating reference frame. There are “fictitious forces” present in a non-inertial reference frame, the same “force” that pushes us forward when the bus driver suddenly hits the brakes. You may be a bit bored by now, but don’t worry, the more interesting part of Special Relativity is in the second postulate.

Jets of subatomic particles moving at nearly the speed of light via NASA

Jets of subatomic particles moving at nearly the speed of light via NASA

The second postulate states that: “the speed of light in a vacuum is constant in all inertial reference frames.” It simply means that, wherever you look from, you will measure the same speed of light in a vacuum (as long as it moves at a constant velocity). At first, this doesn’t sound so amazing, but when you think of it, it is. For example, if you are in a spaceship travelling at the speed of light and you fire a laser up front, Galilean relativity’s addition of velocities tells us that an observer outside the spaceship will see the laser at twice the speed of light, but Einstein tells us that the observer will still see light travel at light speed. How is that possible? The constancy of the speed of light has two consequences that make it possible, namely, length contraction & time dilation.

Length contraction is the shortening of length of an object travelling at relativistic speeds (speeds that are near the speed of light) relative to an observer outside the object. It is described in a limerick by George Gamow:

There once was a young man named Fisk,
Whose fencing was extremely brisk,
So fast was his action,
The Lorentz contraction,
Foreshortened his foil to a disk.

Time dilation, on the other hand, is the slowing down of time at an object moving at relativistic speeds. This is described in the twin paradox. The paradox starts with two twins who are (of course) the same age. One of the twins (twin A) rode a spaceship that travels at relativistic speeds to some distant place. After traveling for some time, twin A returns again at the same speed. Because twin A moved at relativistic speeds, he would be younger than twin B when he comes home (twin A would have experienced a slowing down of time).

To get an idea of the magnitude of the effects of both time dilation and length contraction, we solve for a magic number, called the Lorentz factor. This number ranges from 0 to infinity depending on the velocity of the moving object. This tells us how much slower time moves or how much shorter the object is relative to an outside observer when moving at velocity v.

Source: Relativity Calculator

Source: Relativity Calculator

At 95% the speed of light, the Lorentz factor will be 3.2. Therefore, for an object moving at 95% the speed of light, time will slow down 3.2 times and the length of the object will be shortened 3.2 times. Moving at relativistic speeds can be seen as time travelling towards the future. If you could move faster than light (although we know that that is impossible), you can move backwards in time instead. An interesting part of Lorentz factor is that it approaches infinity as you approach the speed of light, which means that the effects of time dilation and length contraction increase the faster you move

The reason why we can’t make a spaceship move at the speed of light, or even close to it, can also be seen in the Lorentz factor. In high school physics, we are told that the momentum of an object p is equal to the mass times the velocity, but according to special relativity, the momentum is equal to mass times the velocity times the Lorentz factor. At speeds approaching that of light, the momentum increases. Because of this, it would take infinite energy to move something at the speed of light. Experimental physicists have been able to accelerate particles in large particle accelerators only to very close the speed of light. Don’t worry if what you have learned in high school physics is wrong, the formula for momentum reduces back to p = mv since the speeds that we normally use is much less than the speed of light. This formula would still be correct.

Special relativity is a beautiful theory with stunning implications. Although we can’t observe its effects directly in our everyday lives, by understanding it we appreciate the beauty of the universe more, and this fuels our desire to further explore the secrets of the cosmos.

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100 Comments

  1. Mike Fusco January 17, 2014 at 9:05 am -

    It is like recovering from Stockholm Syndrome, but still admitting the bad guys had your interest in mind.

    Reply
  2. Nick Sifniotis January 17, 2014 at 9:06 am -

    Special relativity is pretty simple, just apply the Lorentz transformation to all your equations and you’re halfway there. Teach us some General Relativity – that shit is interesting.

    Reply
  3. Xevi Fernández January 17, 2014 at 9:07 am -

    to understand the world where we live, more than a brilliant mind, we need a wide imagination :D

    Reply
  4. Mark Jacobson January 17, 2014 at 9:11 am -

    Can’t see it’s effects in every day life? It must be the work of the devil, just like evolution!

    Reply
  5. Torin Sarasas January 17, 2014 at 9:11 am -

    We do, without knowing, observe it in our everyday life. The GPS system (has to) respect both general and special relativity theories. The satellite’s atomic clock, set to tick 38 microsecs slower than one on Earth.

    Reply
  6. Mark Jacobson January 17, 2014 at 9:14 am -

    It’s not every day I carry my atomic clock though..

    Reply
  7. Shahnawaz Ahmad Zargar January 17, 2014 at 9:15 am -

    Genis research by Genius personalities

    Reply
  8. Cheryl Lee Gravitt January 17, 2014 at 9:22 am -

    Nope.

    Reply
  9. Kathe Hulbert January 17, 2014 at 9:28 am -

    tru

    Reply
  10. Umer Abrar January 17, 2014 at 9:35 am -

    like this page for latest developments in the field of physics and astronomy……… http://www.facebook.com/physicsbyumer

    Reply
  11. Aaron Paglinawan January 17, 2014 at 9:44 am -

    wow. :D

    Reply
  12. Matt Mackowski January 17, 2014 at 9:44 am -

    Rebecca Elizabeth

    Reply
  13. Vikas Rana January 17, 2014 at 9:45 am -

    greatest mind ever

    Reply
  14. Ianis Poje January 17, 2014 at 9:45 am -

    Every one should read “das gottesformöl” by Jose Rodrigues Dos Santos !! Amazing book !

    Reply
  15. Steven Palukaitis January 17, 2014 at 9:46 am -

    Rich Monzo

    Reply
  16. Sajida Sahouli January 17, 2014 at 10:05 am -

    Because we think differently in every moment ,relativity rules in every process or life -rule.

    Reply
  17. Abhishek Kumar January 17, 2014 at 10:13 am -

    If we think deeply we’d understand that relativity has its implications in everything.

    Reply
  18. Andy Glossop January 17, 2014 at 10:22 am -

    Excellent !! But I strongly suggest we seriously sort out our Societal problems first ! Before we start invading other planets and dictating how things should be run. !! Shouldn’t we be learning about massive mistakes still happening .. Before moving forward. ??

    Reply
    • Ollie Hancock January 17, 2014 at 2:17 pm -

      dont try to fight the exponential noosospheric rise bro :), it just waists energy trying to go against the grain of life, what were dealing with is evolution; involution & revolution – or involved revolution

      Reply
    • Jhe Romano January 17, 2014 at 7:36 pm -

      bro its our nature to explore! we move froward by learning from our mistakesin the past..you cant navigate the future without the lessons wee gathered in the past..

      Reply
  19. Hector Lopez January 17, 2014 at 10:38 am -

    This applies to life as well, instead of just an academic point of view.
    It really opens my mind

    Reply
  20. Stephanie Spencer January 17, 2014 at 10:57 am -

    exactly! why cant there be someone in todays society that is half as intelligent as he was?

    Reply
  21. Franziska Lieber January 17, 2014 at 11:20 am -

    <3

    Reply
  22. Jeana Begs January 17, 2014 at 11:47 am -

    OMG, just the words QUARKS & QUASARS put me in panic mode. I’m so terrible at physics! I just can’t wrap my head round that.

    Reply
  23. Dion Cook January 17, 2014 at 11:52 am -

    Amazing

    Reply
  24. Eduart Hoxha January 17, 2014 at 11:59 am -

    Life , and human kind (life s) are much like a grand scale viruses , like viruses look for a host to thrive , so will mankind , our destiny is in finding planetary systems , that can host ! It’s a matter of time until the theory of relativity is replaced , and of we go , to the distant stars and galaxies to spread the disease or blessing

    Reply
    • Corey Long January 17, 2014 at 4:16 pm -

      the theory of relativity is exactly what makes inter-stellar/galactic travel possible, by traveling at a pre-calculated velocity, you could manipulate the affect of time dilation so that it was possible to reach another star within one human life time

      Reply
  25. Cory Birnbaum January 17, 2014 at 12:09 pm -

    Real talk right there einstein good shit nigga

    Reply
  26. Austin January 17, 2014 at 12:24 pm -

    A slight technicality, the Earth doesn’t have a constant velocity, it has a constant speed, but because it’s basically circular motion about the sun (elliptical, really, but close enough). It has acceleration towards the sun. It doesn’t ruin the article, but as a physics student who’s going into engineering (and dislikes scientific illiteracy) it bugs me. That’s just me, if no one else cares/notices, don’t bother.

    Reply
    • Corey Long January 17, 2014 at 4:07 pm -

      your right about the velocity not being constant, however its the direction of the vector, not the magnitude that changes in circular motion. So as far as calculations go you can usually assume constant velocity. unless of course you’re working with more than one force

      Reply
    • Jethro Andal January 20, 2014 at 11:34 pm -

      I think I wrote: “approximately constant velocity” on the article. The Earth’s revolution around the sun really means that the Earth does not have a constant velocity but the Earth does this revolution at a very long path distance so we can assume the Earth to be moving in constant velocity. A proof of this is why can’t we feel the Earth moving? When we are in a vehicle and the vehicle suddenly turns, we feel a fictitious force. We don’t feel that here on Earth.

      Reply
  27. Pak Son January 17, 2014 at 1:12 pm -

    Think outside the box.

    Reply
  28. Matthew Wright January 17, 2014 at 1:25 pm -

    For any Epsilon greater than zero there exists a delta such that…..

    Reply
  29. Ian Smetanick January 17, 2014 at 1:26 pm -

    A persons perception of the universe should be ever changing. We all should be prepared to change our views on a regular basis, as new facts and discoveries come about. Let go of the ego, and become a radical thinker. Without radical thinking, the system become rigid and stubborn. The system must be flexible, so that it may evolve with the people.

    Reply
  30. Ollie Hancock January 17, 2014 at 2:12 pm -

    You guys should totally start a page dealing with the same subject but also educating people on the specific equations and mechanics of it all, for those not lucky enough to be studying it, I would totally dig that :D

    Id also be happy to help

    Reply
    • Bryant Baker January 17, 2014 at 2:35 pm -

      Because that would drive off the people who don’t read the articles and tend to just post things out of ignorance, ok never mind thats the best idea ever.

      Reply
    • Ollie Hancock January 17, 2014 at 2:36 pm -

      lets do this :D

      Reply
    • TheBorc January 19, 2014 at 12:58 am -

      I love the idea. It would be significantly more work to dive deep into a subject. Personally, I’d be happy to do more. Recently I picked up Einsteins Relativity…
      Reading a book Einstein wrote…. I … I can’t really explain how it makes me feel.
      But I like it.

      Reply
  31. William L Nelsen January 17, 2014 at 2:20 pm -

    Phillip NelsenNick Hollander.

    Reply
  32. Judith Swarens Reed January 17, 2014 at 2:48 pm -

    Nothing can be carved in granite.

    Reply
  33. Wayne Gilbert January 17, 2014 at 3:18 pm -

    Umm..you can carve granite, diamond and even a black dwarf star with the right materials.

    Reply
  34. Owen Heaney January 17, 2014 at 3:54 pm -

    Was lucky enough to get the chance to do a summer school at Cambridge University on special relativity. Truly mind-boggling but utterly fascinating subject.
    To think that all that came from a single experimental observation!

    Reply
  35. Alex Brightman January 17, 2014 at 4:09 pm -

    Rhyse Adams

    Reply
  36. Stephen Coyles January 17, 2014 at 5:39 pm -

    Cory Birnbaum einstein is white in my poster

    Reply
  37. Michael Perlatti January 17, 2014 at 5:45 pm -

    Let’s maybe prove it before we celebrate it.

    Reply
    • Jethro Andal January 20, 2014 at 11:36 pm -

      The theory of special relativity has been proven by many experiments.

      Reply
  38. Martin Wehausen Jr. January 17, 2014 at 6:46 pm -

    that means ALL our problems…now we need next generation brains to fix…….and create new problems…

    Reply
  39. Nikhil Mhaskar January 17, 2014 at 9:28 pm -

    Very beautiful simplification please tell about general relativity

    Reply
    • Jethro Andal January 20, 2014 at 11:35 pm -

      General relativity is much more complex but maybe I can find time to write about it.

      Reply
  40. Evangeline Ahad January 18, 2014 at 12:40 am -

    hit the books..

    Reply
  41. Subodh Deshpande January 18, 2014 at 12:43 am -

    i have a question tell me nature of time at which speed time flows

    Reply
  42. Subodh Deshpande January 18, 2014 at 12:43 am -

    i have a question tell me nature of time at which speed time flows

    Reply
  43. Lyes Demri January 18, 2014 at 3:43 am -

    the kind of quotes that sound very wise but that you really don’t know how to apply in real life.

    Reply
  44. Lyes Demri January 18, 2014 at 3:43 am -

    the kind of quotes that sound very wise but that you really don’t know how to apply in real life.

    Reply
  45. Kevin Chomyshyn January 18, 2014 at 5:05 am -

    It takes Integrity to Know X

    Reply
  46. Kevin Chomyshyn January 18, 2014 at 5:08 am -

    Evolution: the present moment, and choices made…

    Reply
  47. Nion Sib January 18, 2014 at 6:02 am -

    That is the best BS (bad science) I`v seen in a long time. LMAO

    Reply
  48. Besart Lutfiu January 18, 2014 at 9:55 am -

    Erblina Purellku xD

    Reply
  49. Paul Green January 18, 2014 at 10:31 am -

    Before you have the correct answer you must ask the correct question..

    Reply
  50. Lesley Schaefers January 19, 2014 at 5:47 am -

    Just the the test site in Nv. & we are now being hurt by the WiFi~~

    Reply
  51. YouPurr January 22, 2014 at 11:49 am -

    And here is a cat moving at relativistic speeds
    http://www.youpurr.com/cat-2244

    Reply
  52. Diogo March 12, 2014 at 10:08 am -

    I think the postulate “the speed of light in a vacuum is constant in all inertial reference frames” was established by Maxwell. Simply put, Maxwell obtained the speed of EM waves by calculating the strength of the magnetic and electric fields. The result was that speed was close to the speed of light, which had been previously determined by other researchers through other experiments. Therefore, the speed of light could be calculated, let’s say, without resorting to classical mechanics way to obtain velocity.
    However, Einstein observed that there was an inconsistency between the Galileo’s relativity (or classical mechanics) and Maxwell’s speed of light invariance. Very simply put, the inconsistency was solved by concluding that not only space is relative, but time as well.
    Let me know what you think.

    Reply

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