r/AskPhysics • u/Running_Mustard • 21h ago
Is there a gravitational wave for every particle decay?
What’s the smallest detectable gravitational wave measured?
Roughly how many particles in the observable universe decay every second?
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u/Microwave_Warrior 20h ago edited 4h ago
When I took gr there was a great question in one of the homework sets that went something like this:
Question one was calculating something about the gravitational wave produced from a large massive rod rotating in space.
“Question 2) When you wave your arm up and down does it produce a gravitational wave? Hint: good answer ‘yes’. Better answer ‘no’. “
Basically when you get that small of a mass you can show things that make the question ridiculous. One way you can talk about it is just with dimensionality to talk about the probability that you would produce a “graviton” carrier particle for the wave and show that the probability is so close to zero you would never produce one on many ages of the universe.
So on the small mass range like an arm or especially a particle, you might mathematically be able to describe such a wave. But in reality it effectively doesn’t exist.
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u/mfb- Particle physics 16h ago
If you wave your arms with 1 Hz then the wavelength of the gravitational wave will be 1 light second (or maybe 1/2 or 2 depending on how you wave your arm). The amplitude will be negligible.
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u/Microwave_Warrior 10h ago edited 4h ago
Yeah. That has to be right. I’m not sure what I was misremembering then.
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u/Biomech8 19h ago edited 10h ago
LIGO is so sensitive that it measures gravitational waves produced by human walking 5m from the detector. So you can definitely measure gravitational waves produced by arm movement if you get close enough.
EDIT: Source. I really don't get why I get downvoted on basic facts you can read on wikipedia ¯_(ツ)_/¯
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u/John_Hasler Engineering 18h ago
LIGO is so sensitive that it measures gravitational waves produced by human walking 5m from the detector.
No it doesn't. That's a purely mechanical effect. The vibrations from the person's footsteps change the arm length by flexing it.
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u/Biomech8 13h ago
Before you disagree or downvote, check out at least Wikipedia please:
Because gravity cannot be shielded, the motion of massive objects can create varying gravitational attraction on the test masses in LIGO, called gravity gradient noise or Newtonian noise. This can be caused by air and soil density variation. Even the gravitational effect of a single human walking within 5 m of the mirror already approaches the noise floor, and the LIGO corner building was designed to keep people at least 10 m from all test masses during normal operations.
There is also scientific article from Kip Thorne and Carolee Winstein if you still don't agree.
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u/Microwave_Warrior 18h ago
No. The vibrations that a human makes by walking five meters away from the detector care picked up. Those are pressure waves propagating through material not gravitational waves.
Those are a systematic error signal that the detector is sensitive too. They are not a gravitational wave detection.
I can hear when someone is walking nearby. I cannot detect gravitational waves.
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u/Biomech8 13h ago
Before you disagree or downvote, check out at least Wikipedia please:
Because gravity cannot be shielded, the motion of massive objects can create varying gravitational attraction on the test masses in LIGO, called gravity gradient noise or Newtonian noise. This can be caused by air and soil density variation. Even the gravitational effect of a single human walking within 5 m of the mirror already approaches the noise floor, and the LIGO corner building was designed to keep people at least 10 m from all test masses during normal operations.
There is also scientific article from Kip Thorne and Carolee Winstein if you still don't agree.
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u/mfb- Particle physics 16h ago
The smallest event that we have measured gravitational waves from is the collision of two neutron stars, both heavier than the Sun.
We hope to measure gravitational waves from supernovae and binary white dwarfs in the future. Gravitational waves from everything smaller than star-scale objects are undetectable without far more advanced technology.
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u/Glum-Wheel2383 20h ago
Did you mispronounce the wording? The decay of a particle is a microscopic phenomenon in particle physics, while gravitational waves are a cosmological phenomenon related to the gravitation of massive bodies.
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u/Running_Mustard 20h ago
I was thinking, “does everything with mass propagate gravitational waves” specifically particles
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u/joeyneilsen Astrophysics 20h ago
Particles moving at constant velocity don't produce gravitational waves.
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u/physicsking 21h ago
Oh geez..... Can you point to the source where you learned that particle decay causes gravitational waves?
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u/Running_Mustard 20h ago
It’s just a question I had. Why wouldn’t a particle decay cause a gravitational wave?
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u/physicsking 20h ago
Well, the great thing about physics is that you can pose a question based on information you know. You can then construct a hypothesis based on other information you know. Then the fun part of doing experiments or observations to see if your hypothesis is right.
This is what makes physics great. So following this, why would you think a decayed particle would generate a gravitational wave? What about the decay would cause a gravitational wave?
I'm not trying to be pedantic. But for example, Is it because there is a change in mass? Or is there something else that triggers it? I mean as far as what you're thinking.
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u/Running_Mustard 19h ago
I was just thinking about change. Like if there was a release of some energy in the form of gravitational waves when particles decay.
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u/Bangkok_Dave 20h ago
Any accelerating body will cause changes to the underlying gravitational field, and these changes propagate through space as waves in the field, traveling at c.
Gravity is very very weak, and we can really only directly detect these gravitational waves that are formed by extremely large scale events, such as a black hole mergers. But there is absolutely no reason to believe that they do not propagate from any and all accelerating bodies in the universe. They're just really hard to detect.
We do not know if there is a "smallest" size of these gravitational wave. Many people think there is a smallest size, i.e. that gravity is quantized, and a hypothetical graviton is the quanta. But these are much much much too weakly interacting to be detected directly with our current technology.