Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: JMLCarter on 30/03/2011 00:47:37
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Some people seem to be searching for gravitons with particle accelerators and others for gravity waves with, basically, "great big set squares".
I'm familiar with this established theory that photons are electro-magnetic waves.
But it must be wrong to confuse gravitons and gravity waves because their scale is so different?
Could any-one explain the situation?
Is it possible for both searches to be successful?
[MOD EDIT - PLEASE FORMAT YOUR POST TITLES AS QUESTIONS, IN LINE WITH OUR FORUM POLICY. THANKS. CHRIS] [JMLCarter = apologies there - retitled again to clarify question].
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I know of no search which is looking for gravitons, the hypothetical quantum particle that is related to gravity waves. The search is on for the Higgs boson which gives particles their mass. This is very different from a graviton.
The energy of electromagnetic quanta is related to their frequency by Planck's constant and as the frequency gets lower the quanta get less energetic and more difficult to detect individually. I would expect that the energy of gravitons was related to planck's constant in a similar way. now the lowest frequency electromagnetic quanta that can be detected individually are infra red with a wavelength of micrometers and a frequency in excess of terahertz. Below this frequency the energy can only be detected classically. like radio waves
The highest frequencies of gravity waves that are expected to be around now are way below even normal radio waves in frequency that is normal audio frequencies of hundreds or thousands of cycles per second.
The conditions for generating high frequency gravity waves can only have existed deep in the origins of the universe. It is interesting to think that maybe there is a sort of cosmic microwave background of gravity waves throughout the universe it would introduce a sort of structural uncertainty throughout the universe. Now I wonder?
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Gravitons are too weakly interacting to be detectable so far as I know. Gravity waves, on the other hand, can in theory be detected, and there are searches for them. The basic idea is that since gravity distorts space, you can design a system with two very long arms of a precise length. You can send light down both arms and by having it interfere when it comes back, you can very accurately measure any slight length changes in the arms. If a gravity wave comes by, it will stretch one arm differently from the other and you should be able to detect that.
See, for example: http://www.ligo.caltech.edu/
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There is/was? a idea with using three triangulated spacecrafts. placed out in space, at some Langranian point I think, not sure if that was needed? Using gyros and lasers to keep a exact distance relative each other in space. If a gravity wave came their relative position should change and the lasers would measure their distance relative each other. But I think it became to expensive? Otherwise it sounded very clever.
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Thanks. Gravitons are more speculative than I had understood. I remember reading about LISA some years ago.
What's perhaps misleading me (and what I am trying to sort out) is that
1) in electromagnetism a photon (the force carrier) can be understood as an is EM wave.
2) why, then, in gravity is a (theoretical) graviton (the force carrier) NOT viewed/understandable as a single gravity wave.
If they exist, would there be a very large number of gravitons responsible for a single gravity wave?
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If they exist, would there be a very large number of gravitons responsible for a single gravity wave?
You're exactly right. If they exist, a lot of gravitons acting together in the right way would make up a gravitational wave.
In the same way, you need extremely sensitive detectors to catch a single photon, while a very basic camera can detect light. A light wave is made up of many photons acting together in the right way, just as a gravity wave is thought to be made of many gravitons.
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A light wave is made up of many photons?? I thought it was just one.
One photon has wave behaviour as shown by Young's Slits.
When an electron transitions to a lower state it emits a single photon, which must deposit all it's energy in a detector in one interaction.
I am not so sure my question is answered??
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There is an important extra property to consider and that is coherence. Conventional light sources like flames and normal electrical discharges are incoherent and the photons add up randomly and individually. Lasers and radio signal generators are coherent and all the waves add up in phase so the many photons are all in sync.
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A light wave is made up of many photons?? I thought it was just one.
Classical waves are made up of many photons added up in the appropriate way. I suppose a good description would be that it's made up of N photons per second, added up in the right way, where N is proportional to the classical intensity. If you turn the intensity down far enough you can make it so that you see approximately 1 photon per second. Quantum statistics mean that you might sometimes see none, or sometimes see 2, though. Light which consists of exactly one photon has no classical counterpart. I'm not sure if that answers your question or confuses it more.
One other point: the classical sine-wave which is used to represent light is not a photon. This sine wave results from the appropriate way of adding up many photons as well as ignoring quantum noise. Appropriately accounting for the photons means there will be a quantum uncertainty around the sine wave, giving it a width. In other words, since you never know exactly how many photons there are hitting your detector, the height of the sine wave (which is proportional to the number of photons) has some uncertainty, and therefore some width to it.
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Ok, this being the case then, if a gravity wave is detected, will it not also provide evidence in support of the graviton?
To put it another way, is there any theory for having gravity waves but no gravitons?
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Sure :)
General relativity.
That's pure geometry as I see it. Not so if you want it to come down to something 'touch able'. Then you start from the geometry and then search for the 'particles' creating it.
Take a look H e r e. (http://www.mathpages.com/rr/s6-08/6-08.htm) for a description of both. Einstein seems to have considered particles as a step towards 'fields'. Which makes it palatable for me too :) I think he's right, there are so many ways to describe this universe it seems. The real problem becoming how to 'fit them together.'
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Would I be right in thinking that we are looking at two different "types" of wave?
QT says that a particle (eg a photon) can also be considered as a wave, but here we are not talking about the classical "light wave", which, as already pointed out, is composed of a multitude of photons. It seems reasonable to suppose that if the graviton is ever discovered, it, too, will have a particle/wave duality, but that would not be the same thing as the gravity waves currently being sought.
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Thanks Yor_on for this great article!!!
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Two questions:
Now that the Higgs boson has been found, would it be safe to bet that Graviton could potentially exist, maybe as a messenger particle?
Second question: putting aside Graviton, the actual gravitational wave generated by our planet movement or even a binary system, would it be theoretically possible to "ride" the wave to far reaches of space close to speed of light? Similar to a surfer catching a wave to the shore? It would be a different question than using "warp drives" which produces wrinkles in space and shifts from one point of space to another.
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Your wave of hypothetical particles (gravitons) would drop in density inverse squarely so any actual force attributable to the wave will also decrease inverse squarely so you wouldn't get very far and certainly get no where near light speed. That is unless gravitational waves can be focused as with laser light.
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understood. Thank you Jeffrey.
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Now that the Higgs boson has been found, would it be safe to bet that Graviton could potentially exist, maybe as a messenger particle?
No. The graviton remains hypothetical, and "messenger particles" aren't actual particles zipping back and forth. Hydrogen atoms don't twinkle, magnets don't shine.
Second question: putting aside Graviton, the actual gravitational wave generated by our planet movement or even a binary system, would it be theoretically possible to "ride" the wave to far reaches of space close to speed of light? Similar to a surfer catching a wave to the shore? It would be a different question than using "warp drives" which produces wrinkles in space and shifts from one point of space to another.
No, sorry. A surfer rides a wave because he's falling down the front face of it, due to gravity. There isn't anything like that for a gravitational wave in space.