Questions Show – Running Out of Gravitons and Hitting the Brakes at Light Speed

Another week, another roundup of your questions. This week listeners asked: if forces are communicated through particles, can we run out? If you were traveling at light speed, when would you know to stop? And there's even more. 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.

Running Out of Gravitons and Hitting the Brakes at Light Speed

If forces are communicated through particles, can we run out?

• Four Fundamental Forces of Nature
• Gravitons – from Cornell University
• Feynmann Diagrams and forces between particles
• Book:  The Elegant Universe by Brian Greene

If you were traveling at the speed of light, how would you know when to stop to get to the desired destination?

• Does time stop at light speed? –Discussion on PhysicsForum
• Book:  Einstein's Universe by Nigel Calder

If the Andromeda Galaxy is moving toward us really fast, would the galaxy look farther away than it really is?

• Spectrum of the Andromeda Galaxy -- from Suite 101
• Paper:  The Collision Between the Milky Way and Andromeda

Does the weak force hold anything together or push things apart?

• Weak Force
• Fundamental Forces – from Fact Guru
• Grand Unification Theory — Wiki

If Earth's magnetic field acts on iron, would a sphere of iron have a different wieght than something else?

• Mass vs. weight
• Determining the net force
• Earth's Inconsistent Magnetic Field — from Science@NASA

1. Joe Bernacki Says:
   October 3rd, 2008 at 10:24 am

   Hi Fraser and Pamela,

   Eek! I think you guys might have implied that the iron in our bodies is magnetic. This is pseudoscience … the iron in blood comes as individual iron atoms (in heme groups), so you can't have a bunch of iron atoms coming together and all getting colloquially "magnetized." This idea opens the door to dangerous quackery (magnet therapy), so I think it would be good for you guys to clarify this point.

   Also, I sent in some question a long time ago. Should I resubmit them for a shot at making the new weekly questions shows, or do you have a big long queue of questions that you're working through?

   Thanks!

2. Christian S Says:
   October 3rd, 2008 at 8:14 pm

   @ joe Bernacki. yeah, I was thinking about that too. The iron in the blood is not ferromagnetic – If it was, MRI scans would be pretty dangerous

   Anyway. Thanks for a great show guys!

3. David Madison Says:
   October 4th, 2008 at 6:59 am

   If one object emits a graviton that impacts another and causes the two to move toward each other, then the graviton must have negative momentum. Then this runs afoul of Einstein’s lessons in at least two ways. One is that gravity is not a force, but a change to the shape of distance and time. We should not expect to be able to explain gravity on the basis of a force.

   The other Einstein lesson that does not work with gravitons is energy-mass conservation. If this graviton is capable moving two objects, then it must have energy. If an object emits the graviton, then it must emit energy. If it emits energy, then it must lose the mass which is the same as energy. We are capable of measuring this mass loss should it exist, but we do not see it when look. This therefore disproves the hypothesis that gravitons convey the force of gravity.

4. Anders Says:
   October 5th, 2008 at 1:20 am

   David, two things:

   a) Einstein could be wrong. His General Theory of Relativity could be one way to look at it, but not necessarily the correct way. It has happened before.

   b) IIUC, these are not real particles but virtual particles, which allows for all kinds of weird stuff, including breaking the conservation of energy over short periods of times.

   Interesting question about iron and magnetism. Would this mean that a long steel wire would experience magnetic tidal forces?

5. Andrew Says:
   October 7th, 2008 at 10:02 am

   One nagging little question that i keep on thinking of is…. you cannot go faster than the speed of light, ok… so if you are in a spaceship, going close to the speed of light, and you throw a ball toward the front of the ship. The ball would be travelling at current speed + thrown speed. What if the ball exceeded the speed of light?! Would it blink out of existance?! Or would your arm blink out of existance since your arm had to exceed the speed of light to throw the ball!? I keep thinking of the old thought that the speed of sound was the limit, then we broke that…. will we some day find out that there is such a thing as a light boom? like a sonic boom but with light?

6. Jasper Says:
   October 9th, 2008 at 6:48 am

   Andrew, the flaw in your reasoning is that you simply add velocities as if you were in a moving car on Earth. Since you are so close to the speed of light you have to take the relativistic effects into consideration.

   Remember that as you speed up – time slows down. This is where you and a stationary observer are different. Imagine your spaceship being capable of traveling at 99% of the speed of light and that you are on your way from Earth to Alpha Centauri, 4.36 lightyears away. Now, for someone on Earth, it would seem like your travel time takes slightly longer than 4.36 years, but for you it would only take a few minutes!

   So now you are on your way to Alpha Centauri, but you still have to entertain yourself for several minutes until you arrive, so you decide to have some fun with the ball you brought. You get your ball and you throw it to the front of the space ship (incredibly fast) at 6 million metres per second. (2% the speed of light). (You're throwing it so unrealisticly fast because you are trying to exceed the speed of light.) At the same time an astronomer on Earth, with an incredible telescope, that can see your every move, is looking at you. As you throw the ball you see it moving away from you at 6 million metres per second – really fast, but no where near the speed of light. It doesn't disappear or create a singularity. You are disappointed. The astronomer on Earth sees you throwing a ball too, but he sees you in slow motion. In the time it takes you to pick up and throw your ball the astronomer has lived a whole year. For him, the ball doesn't break the speed of light either. Though the cumulative speed of the ball and the space ship is very high, it is still below the speed of light.

   You can calculate the speed of the ball like this:

   w = u + v / ( 1 + uv / c² )
   Where w is the speed of the ball as seen by the astronomer on Earth, u the speed of the space ship, v the speed of the ball as seen by you *on* the space ship and c the speed of light.

   Notice that for whatever speeds u and v (provided they are below the speed of light, as they should be, because at or above the speed of light is not possible) the cumulative speed as seen by a stationary observer is always less than the speed of light.

   Just remember – special relativity says nothing can go faster than the speed of light, ever. If you think something can go faster than the speed of light, time will be distorted to make it impossible.

7. Mike Corfield Says:
   October 14th, 2008 at 7:18 am

   Andrew, light booms exist – it is called Cerenkov radiation caused by particles travelling faster than the local speed of light (Nb light slows down when it travels through a medium). it is often seen in cooling ponds around nuclear reactors and is also produced by cosmic rays

   Mike

8. themondo Says:
   October 15th, 2008 at 10:23 pm

   I have a question for all of you smarts out there. Does a black hole represent all 4 fundamental forces of nature, in the universe, unified into one 'super unified' force because of it's density and its capability of stopping light. This capability must be faster than the speed of light, however small. So does this not say that faster than light speed is capable, only if a black hole can cause it? What say you all?

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