Questions Show: Relativity, Relativity and More Relativity

Everyone loves a theme. And this week we've collected together some of your questions about relativity. More light speed spacecraft, twin paradoxes, and the mixing up of gravity, time and mass. If you've got a question for the Astronomy Cast team, please email it in to info@astronomycast.com and we'll try to tackle it for a future show. Please include your location and a way to pronounce your name.

Relativity, Relativity and More Relativity

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Relativity Tutorial – UCLA

General Relativity – Internet Encyclopedia of Science

What would happen if you're traveling close to light speed through the solar system — would you mess it up?

• Traveling at light speed – Time magazine
• The Relativistic Mass Increase — U of Virginia
• The Physics of Flying High and Fast – Daily Camera

If I watched my friend accelerate close to speed of light, will I see him get younger or age slower?

• Arrow of time – Wiki
• Discussion of aging and light speed at BAUT Forum

Can parts of the Universe be traveling back in time?

• How Can Galaxies Recede Faster Than the Speed of Light? – Universe Today
• Paper:  Varying Speeds of Light — arXiv

What form does relativistic mass increase take?

• What is relativistic mass?
• Problems with variable mass

Why don't photons have mass?

• Mass of photons — USA Today
• How gravity affects photons – Goddard Space Flight Center

Twin paradox:  Why is the "stay at home" twin's frame of reference preferred?

• NOVA's Time Traveler page — PBS
• Twin Paradox – Wiki
• Professor Solves Einstein's Twin Paradox — PhysOrg
• Discussion of Twin Paradox — BAUT Forum
• Time Dilation — Wiki

Does gravity equal time?

• Why does gravity slow time? –Perimeter Institute
• Discussion of ball on a rubber sheet model of gravity – BAUT Forum

How can the state of an electromagnetic wave change when the wave exists outside of time?

• Electromagnetic waves – NASA
• Quantum Mechanics — Wiki

How do we know the speed of light constant? Could it be different in different part of the universe?

• Why do we assume that the Universe for all observers is homogeneous and istropic? – Cornell University
• Homogeneity and Isotropy — UCLA

1. Sci-Fi Si Says:
   October 26th, 2008 at 1:09 pm

   If time were reversed, I like the Universe around me would get younger. If I were to 'time travel' I would have to isolate myself from the effects of time and re-wind the entire Universe and everything in it (apart from myself) around me!

   I, like many others do not accept the 'infinate number of Universes theory' so if I were to travel backward through time why would there be a Universe of matter there to greet me? There wouldn't.

   If we think of the matter in the Universe as a surfboard moving on the 'sea of time' by jumping off the surfboard, I would remain in a quantum state able to 'be' in all times spontaneously – just like any quantum particle.

   It would not be possible for me to travel back to 'kill my grandfather' nor forward. 'Information' can not be sent either forward, nor backwards through time.

   There are NO paradoxes.

   The wave-particle duality is not a problem, in a quantum state particles are in a state of 'all times' but they can only realise themselves in the absolute here and now. Thus the photonic version of the Youngs slits experiment is explained, this not only alignes itself with Feynman's Q.E.D, it show that probability is exactly what happens.

   This is what I think anyway…

   

   P.S. LOVE you show, thank you so much Fraser and Pamela

2. Sci-Fi Si Says:
   October 26th, 2008 at 1:11 pm

   P.P.S. Sorry about the typo's and spelling miskoooots – lol

3. Dosey Says:
   October 26th, 2008 at 1:28 pm

   If E=M C^2 and a photon has Zero mass then

   E = 0 * C^2 = 0

   Therefore light can not have any energy

   Doh!

4. Simon Says:
   October 26th, 2008 at 1:38 pm

   I love this eposode, science breaks all over the place, Dr. Pam is fantastic, but there clearly aren't any sensible explanations for some of the questions asked.

   Love it.

5. Bob Says:
   October 26th, 2008 at 1:53 pm

   If gravity affects everything, including light, but gravity only travels AT the speed of light, how come gravity isn't sucked in by a black hole. We know 'Hawking' radiation emits a stream of particles – but they manage to eacape and so does gravity – err…

   Maybe light does have mass, just really, really small. Solar sails work (in theory) by photons hitting it, so if a photon has NO mass then how come they manage to move the spaceship??

6. sridevi Says:
   October 26th, 2008 at 5:59 pm

   You guys are amazing.luv this show.Pls continue to do this THANK YOU

7. andyc Says:
   October 27th, 2008 at 6:39 am

   Has anyone heard of Expansion Theory. Mark McCutcheon seems like a bit of a 'crank' but some of the things he suggests to seem to make sense.

   thanks for the show – love it!

8. Andrew Says:
   October 29th, 2008 at 12:28 am

   There's a more general formula for energy that explains the solar sail situation:

   E^2 = p^2*c^2 + m^2*c^4

   where c is the speed of light and p is momentum, and m is the mass of the particle.

   Although light has no mass, each photon (which is both a particle and a wave) does have momentum, as explained by the more general definition of energy. So if you aim a laser beam at a solar sail long enough, you can transfer a huge amount of momentum to it, and accelerate it to pretty large speeds.

9. Matthias Says:
   October 30th, 2008 at 2:37 pm

   I think you got the twin paradox quite wrong.

   As long as both observers are moving inertially, they will both see the other one age slower.

   But for them to meet again, one of them has to turn around. This makes his a non-inertial frame of reference, thus invalidating the equivalence of their points of view.

10. Daniel Says:
    November 4th, 2008 at 3:52 pm

    Matthias is correct with the Twin Paradox. Since there is no preferred reference frame, both twins will see the other twin age slower. This gives rise to the apparent paradox. So long as the spaceship twin never speeds up, slows down, or turns around this paradoxical state continues.

    The clarification to the paradox comes from the spaceship twin needing to change his direction to meet the other twin. Then the spaceship twin enters a non-inertial reference frame and leaves the realm of special relativity. It is still curious that if the spaceship can change direction instantaneously, the accelerating twin will age all in that instant.

11. reevesAstronomy Says:
    November 9th, 2008 at 8:25 pm

    Gotta love topics that rip your brain apart. I'm still thinking through this episode and working it out in my mind. Changing time and choosing a moving frame of reference is hard to imagine being figured out by a human mind.

12. AJenbo Says:
    November 30th, 2008 at 9:22 am

    Best explanation of quantum mechanics ever! Thank you Fraser Cain.

13. Adrian Morgan Says:
    February 20th, 2009 at 6:19 am

    I was disappointed by the mishandling of the twin paradox. According to what I was taught when I was a science undergraduate, the question from Russell Dorward isn't a question about the Twin Paradox; it is the Twin Paradox.

    There is widespread confusion here: many people use the term "twin paradox" to mean merely the result that one twin would be older than the other, while others insist that the term properly refers to the (erroneous, but the point is to find the flaw) result that both twins would be older than the other depending on whose frame of reference is used. I am assured that the latter definition is correct, and therefore did not expect the former definition to be perpetrated on Astronomy Cast.

14. Samuel Baulch Says:
    May 21st, 2009 at 10:45 pm

    I'm not as trained or anywhere near as intelligent as most of the people on this website, and this question may have been answered before now. So I'm sorry if it has. My question has to do with the twin paradox, I understand that information would stop this from being possible because it can't travel faster than light, but lets say for one moment it could. If both twins had a video conference going while one was traveling, my question is. What would the other see? I know one would seem to age slower and one faster. However how would they interact, would one be acting super fast looking like a VHS stuck in fast forward, and the other really slow with a deep voice? I'm sorry if this seems basic to everyone else, but I'm struggling to get my head around it.

    Could someone please clarify

    – Samuel Baulch

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