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brianthewhitie7
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What happens to photons when they collide?
Does light have garvity? And all other particles?
Does light have garvity? And all other particles?
If there is enough energy, a particle-antiparticle pair may form.What happens to photons when they collide?
Light is subject to gravity, like anything else. I am not sure what your second question means?Does light have garvity? And all other particles?
StatMechGuy said:Photons are massless, and as such they don't bend space-time themselves. As for "photons colliding", the simplest process I can think of that matches what you're talking about would be this:
Photon A and Photon B pair produce into two electron-positron pairs. Positron A and Electron B annihilate, as do Positron B and Electron A, and this results in two photons coming out. This is an extremely high-order effect, though, and light generally doesn't scatter off of itself.
Photon A and Photon B pair produce into two electron-positron pairs. Positron A and Electron B annihilate, as do Positron B and Electron A, and this results in two photons coming out. This is an extremely high-order effect, though, and light generally doesn't scatter off of itself.
Ok, the stress-energy tensor also contains energy, momentum and pressure. But if a single flying photon bent space-time, how could it be that the amount of this bending depends on the frame of reference in which you observe the photon?jambaugh said:Photons have energy and momentum and thus they do bend space-time. The source of gravitation in Einstein's theory is not just rest-mass but rather the whole of the stress-energy tensor. Assuming a quantum theory of gravity is developed wherein we can actually say what a graviton is or if they exist, then in principle photons could scatter off each other by exchanging gravitons.
lightarrow said:Ok, the stress-energy tensor also contains energy, momentum and pressure. But if a single flying photon bent space-time, how could it be that the amount of this bending depends on the frame of reference in which you observe the photon?
(Since a photon's energy and momentum depends on that, see Doppler effect).
Are photon-photon collisions without the presence of matter possible? If yes, have they been ever detected?mathman said:Photon-photon collision would produce one particle-antiparticle pair. Electron-positron is the easiest, but others are possible if the photon energies are high enough. These processes are extremely rare nowadays, but right after the big bang they were very common.
Ok, but does it implies that the space-time curvature caused from a single photon depends on the frame? So, in some frames, we see almost no curvature; in others we see a black hole ? This can't be true.masudr said:Because the components of the "bending of spacetime" a.k.a. curvature tensor also depend on the frame, as do the components of the energy-momentum 4-vector of the photon (and in general, the components of the stress-energy tensor of the EM field).
lightarrow said:Are photon-photon collisions without the presence of matter possible? If yes, have they been ever detected?
lightarrow said:Are photon-photon collisions without the presence of matter possible? If yes, have they been ever detected?
lightarrow said:Ok, the stress-energy tensor also contains energy, momentum and pressure. But if a single flying photon bent space-time, how could it be that the amount of this bending depends on the frame of reference in which you observe the photon?
(Since a photon's energy and momentum depends on that, see Doppler effect).
Curvature is a frame independent and a coordinate independent property of spacetime.lightarrow said:Ok, but does it implies that the space-time curvature caused from a single photon depends on the frame? So, in some frames, we see almost no curvature; in others we see a black hole ? This can't be true.
Yes. I wonder how could we be so sure about the photon generating curvature without such a quantum theory of gravitation.jambaugh said:Let me add to the other reply in saying this. First since we're talking General Relativity and there's as yet no quantum theory of gravitation let's stick to the classical realm and talk about classical light bending space.
Say a laser source fixed in the origin of a coordinate system emits, in his frame, 1 photon every minute with wavelenght 500 nm in the +x direction. How will those photons bend spacetime as seen from that frame? Is it also necessary to specify the coherences of the source? Make all the simplifications you need.To address your question about Lorentz transformations keep in mind that once space-time gets bent you must consider General Covariance and not just Lorentz covariance. It is true that in SR you can transform a monochromatic plane wave to arbitrarily low frequencies but remember that a similar transformation on non-plane-waves will decrease the frequencies in one direction for one part of the wave while increasing the frequencies elsewhere. (Imagine in the extreme case the Lorentz transformation of spherical waves).
Since a plane-wave extending over all of space would have infinite energy it is not physically reasonable. Rather you must consider at best a beam of finite cross section. Said beam will already disperse due to diffractive effects and if you add the dispersion due to gravitational coupling it will further loose its unidirectional behavior. In the end you should find that any physically meaningful case will have a sensible and consistent interpretation in all frames.
As I see it you are improperly thinking in absolute terms while invoking relativity. Specifically you consider an absolute "bent" vs "not bent" question about space-time while Lorentz transforming a "photon". The question should be "how bent" given a "photon" of "what frequency" at a given space-time point. And the answer is of course "its all relative" but relative in a consistent way to the choice of frame.
I admit not to know very much about the stress-energy tensor, but I wonder how is it possible to construct a non-zero invariant quantity (or set of quantities) for a photon, since a photon's energy depends on the frame and there isn't any proper frame for a photon.MeJennifer said:Curvature is a frame independent and a coordinate independent property of spacetime.
A photon does, albeit extremely little, curve spacetime. All objects with a non-vanishing stress-energy tensor are sources of spacetime curvature.
lightarrow said:I admit not to know very much about the stress-energy tensor, but I wonder how is it possible to construct a non-zero invariant quantity (or set of quantities) for a photon, since a photon's energy depends on the frame and there isn't any proper frame for a photon.
Sorry if I keep asking, of course I will look for the tensor transformation rule, but is it possible to explain it in a simple way?masudr said:The stress energy tensor is not a constant: it's a covariant rank-2 tensor. That means that its components aren't constant. Instead they transform under a change of coordinates, but under a very specific fashion. See the tensor transformation rule for more details.
lightarrow said:Sorry if I keep asking, of course I will look for the tensor transformation rule, but is it possible to explain it in a simple way?
We can only be sure about what GR says should happen given we can speak meaningfully about the stress-energy of a photon.lightarrow said:First of all, thank you for your answers.
Yes. I wonder how could we be so sure about the photon generating curvature without such a quantum theory of gravitation.
Say a laser source fixed in the origin of a coordinate system emits, in his frame, 1 photon every minute with wavelenght 500 nm in the +x direction. How will those photons bend spacetime as seen from that frame? Is it also necessary to specify the coherences of the source? Make all the simplifications you need.
How will those photons bend spacetime, seen from a ref.frame moving in the -x direction at v = 299,792,457.9999999999 m/s?
And if v is in the +x direction?
Certainly these gravitational waves generated from the photon (if they exist) will be in the form of a wave packet, which will have a group velocity. This velocity should be equal to light's velocity; so will this wave packet be stationary with respect to the photon? It doesn't seem possible, but if it is, then the photon shouldn't find any curvature in its path. If it's not possible, which will arrive first to a specific distance, the gravitational wave packet or the photon? Everything seems absurd to me.jambaugh said:To answer your question:
First take the weak field approximation to GR and calculate the first order gravity waves generated by the light.
Second solve Maxwell's equation for the propagation of that light through the space-time curved by said gravity waves.
Particles are tiny units of matter that make up everything in the universe. They can be solid, liquid, or gas, and are made up of atoms and subatomic particles like protons, neutrons, and electrons.
Particles interact with each other through four fundamental forces: gravity, electromagnetism, strong nuclear force, and weak nuclear force. These forces determine how particles attract, repel, and bind together to form matter.
According to the law of conservation of mass, particles cannot be created or destroyed, but can only be transformed from one form to another. This is demonstrated through processes such as nuclear reactions, where mass can be converted into energy.
Particles are the building blocks of everything in the physical world, and their behavior and interactions are studied in the field of particle physics. This helps us understand the fundamental laws of nature and how the universe works at a microscopic level.
Particles play a crucial role in many aspects of our daily lives. They are used in technology, medicine, and energy production. For example, semiconductors made of particles are used in electronic devices, and nuclear particles are used in medical imaging and cancer treatment.