Speed of an electromagnetic wave and gravity

  • #1
PhanthomJay
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Suppose a strong electromagnet is attracting a metal object and accelerating it towards the magnet. Suddenly, the power to the magnet is shut off. I presume that the object continues to accelerate for a very very brief period before moving at constant speed, because the electromagnetic wave can only travel at light speed, and thus, the influence of the magnet on the object cannot be instantaneous. Now maybe the definition of simultaneity makes this assumption invalid, but my real question is, supposing I reword the question to read 'a strong gravity field is attracting the object, and suddenly, the gravity field is shut off...". Are the results of its influence on the object the same as the electromagnetic force, in terms of the time delay of its influence on the object?
 

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  • #2
bcrowell
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Are the results of its influence on the object the same as the electromagnetic force, in terms of the time delay of its influence on the object?

Yes, GR predicts that small-amplitude disturbances in the gravitational field propagate at c, although this century-old prediction has never been successfully subjected to an empirical test. More info here: http://www.lightandmatter.com/html_books/genrel/ch09/ch09.html [Broken]
 
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  • #3
PhanthomJay
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Yes, GR predicts that small-amplitude disturbances in the gravitational field propagate at c, although this century-old prediction has never been successfully subjected to an empirical test.
Thanks for the response, makes sense. But whatever happened to that LIGO unit that was supposed to detect gravity waves? I thought we'd have some answers by now, wasn't it several years ago when it went on-line? Also, what if there is no such thing as a gravity wave...then how can it propagate at any speed? And then , how could you turn off a gravity field anyway, since that would require the destruction of mass, which is impossible? And while I'm on a roll with questions, what about that Hadron collider that was supposed to detect particles smaller than quarks, gravitons, maybe even create mini black holes (?)..what's that thing doing for my money??
 
  • #4
bcrowell
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LIGO wasn't really expected to get a positive result. The next generation might. There is lots of indirect evidence for gravitational waves, as described at the link in #2. There just hasn't been a direct test of propagation at c.
 
  • #5
PhanthomJay
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LIGO wasn't really expected to get a positive result. The next generation might.
So i've waited for what is it 10 years now,for it to tell me something, with all its hype, and now I'll probably be long gone before it does. What a pity.
There is lots of indirect evidence for gravitational waves, as described at the link in #2. There just hasn't been a direct test of propagation at c.
Thanks. Now what about that LHC collider. What's it doing these days??
 
  • #6
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Thanks. Now what about that LHC collider. What's it doing these days??

Wiki says the following happend on 28 Feb 2010:

The LHC continues operations ramping energies to run at 3.5 TeV for 18 months to two years, after which it will be shut down to prepare for the 14 TeV collisions (7 TeV per beam).

Unfortunately the glitch in the system has been a great disappointment to all the physics community because it's postponed everything to almost one and a half year later so the operations are still behind the schedule!

AB
 
  • #7
PhanthomJay
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Wiki says the following happend on 28 Feb 2010:

The LHC continues operations ramping energies to run at 3.5 TeV for 18 months to two years, after which it will be shut down to prepare for the 14 TeV collisions (7 TeV per beam).

Unfortunately the glitch in the system has been a great disappointment to all the physics community because it's postponed everything to almost one and a half year later so the operations are still behind the schedule!

AB
Thanks, nothing happens overnight, that's for sure. See you in 2012..2013...2014.........2xxx?????. I guess we should have expected that something would go wrong; it's not easy unlocking the Secret of the Nothingness, within which all answers lie. Pure conjecture upon my part, but until something or someone tells me differently, I firmly believe that all that is, and all that is not, are one and the same......
 
  • #8
George Jones
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So i've waited for what is it 10 years now,for it to tell me something, with all its hype, and now I'll probably be long gone before it does. What a pity.

Why so soon? :confused:

Read the abstract of

https://dcc.ligo.org/public/0001/P080127/001/P080127-02.pdf.

From the abstract: "Advanced LIGO is expected to make several gravitational wave detections per year."

From the body of the article: "Construction of Advanced LIGO subsystems has already begun and installation and commissioning will commence at the LIGO sites after S6, in 2011, with the goal of having first data collection as soon as 2014."

These dates are probably somewhat optimistic, but, even allowing for delays, it seems the next decade will be an interesting time.
 
  • #9
PhanthomJay
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Why so soon? :confused:

..From the abstract: "Advanced LIGO is expected to make several gravitational wave detections per year."

From the body of the article: "Construction of Advanced LIGO subsystems has already begun and installation and commissioning will commence at the LIGO sites after S6, in 2011, with the goal of having first data collection as soon as 2014."

These dates are probably somewhat optimistic, but, even allowing for delays, it seems the next decade will be an interesting time.
Well that's good news, thanks, I hope I have 10 good years left...don't we all.... and thanks for the article.
 
  • #10
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Well that's good news, thanks, I hope I have 10 good years left...don't we all.... and thanks for the article.

Well, if you believe in an afterlife, you can still learn. If you believe in no afterlife, then you won't care. Either way you're not losing.
 
  • #11
PhanthomJay
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Well, if you believe in an afterlife, you can still learn. If you believe in no afterlife, then you won't care. Either way you're not losing.
Ahh, the 'win-win" scenario....I feel better already,thanks!
 
  • #12
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Ahh, the 'win-win" scenario....I feel better already,thanks!

Yeah, that's how my father tried to comfort me when I first confronted a fear of death as a kid. As I remember I simply became more hysterical. :wink: It's good to see the magic still endures.

All kidding aside, I think LIGO stands to produce real data, and if an 'event' occurs which produces LARGE g-waves... who knows? Maybe we'll turn on LISA and *BAM*, lousy with gravitational waves. :smile:
 

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