Are photons massless?

[tim0901]

  

On 9/11/2021 at 9:48 AM, Wictorian said:

2- Are photons massless or do they have so little mass?

 

     3- If they are massless why are they effected by gravity?

 

           4- If they are massless and effected by gravity, if for instance radio waves are massless are they effected by gravity too?

 

 

2) Yes, photons are massless. Photons do not have mass because they do not interact with the Higgs field. (Specifically, the vacuum expectation value of the Higgs field doublet returns a mass of zero for one of the four bosons - the photon. The other three however (W⁺, W^- and Z) do have mass.) This means that - by definition - the mass of a photon is zero. Experimentally, while trying to prove this theory, we have deduced a minimum maximum value of mass that they may have, but this is because proving that something doesn't exist is practically impossible. As far as we are concerned, our experiments back up our theory here.

 

Their mass is not zero due to E=mc², rather E=mc² returns zero because they have no mass.

 

3) Photons are affected by gravity because massive objects bend spacetime.

 

Photons in free space travel in straight lines. However they appear to follow curves in space due to the curvature of spacetime caused by massive objects like a black hole. This video gives a pretty good explanation of this. The objects are pushed onto the sheet in a straight line - it is merely the curvature of the sheet that causes them to move in a curve.

 

4) Yes. Radio waves etc are all affected by gravity too. They just don't appear to be in day-to-day usage on Earth.

 

Source: my degree in theoretical physics.

[tikker]

15 minutes ago, tim0901 said:

Experimentally, while trying to prove this theory, we have deduced a minimum value of mass that they may have, but this is because proving that something doesn't exist is practically impossible. As far as we are concerned, our experiments back up our theory here.

Don't you mean a maximum value (i.e. a mass less than <insert whatever small number it currently is>)? Setting a minimum value would clash with being massless.

16 minutes ago, tim0901 said:

Their mass is not zero due to E=mc², rather E=mc² returns zero because they have no mass.

Plus there's the whole "+ p^2 * c^4" bit so yeah, being massless has no problem with having energy.

[tim0901]

2 minutes ago, tikker said:

Don't you mean a maximum value (i.e. a mass less than <insert whatever small number it currently is>)? Setting a minimum value would clash with being massless.

 

Whoops ty for that - yes I do mean a maximum. It's too early for this xD

[tim0901]

On 9/11/2021 at 9:48 AM, Wictorian said:

1- I calculated a photons energy assuming it's mass is equal to planck mass and speed of light is equal to 300 million. The result is 6.54 joules. If a photon had 6.54 joules of energy would it effect the surroundings?

This is incorrect due to the fact that the Planck mass is not the lowest possible mass an object can have. It is merely the energy scale at which the quantum effects of gravity are strongest.

 

For reference, the Planck mass is 1.22 x10¹⁹ GeV. The mass of an electron is 0.51 MeV, or 0.51 x10^-3 GeV. The energy you have given your photon is actually incredibly high for a single particle, far more than is achievable in any particle accelerator on Earth, which only reach the range of 10^3 GeV (TeV scale).

 

A photon with this level of energy (known as a gamma ray) would most definitely make an impact on its surroundings, although a single photon wouldn't do much. It would likely undergo pair production almost immediately, creating particles completely unknown to us today due to its insane energy.

 

A beam of them, however, would be the strongest laser in existence and would quickly burn a hole though anything you put in its path. The protons circulating around the LHC have the combined energy equal to that of a TGV travelling at 150Km/h - a beam of photons with 10¹⁶ times as much energy would be much worse. You want a death ray? This will give you a death ray.

[gudvinr]

16 hours ago, Wictorian said:

Now, is it impossible for a photon to attain that kind of energy?

Usually energy in particle physics measured using electron-volts (eV). You can get value by dividing joules by 1.6e-19.

Which means 6.54 J ≈ 40'000'000'000'000 MeV. You can compare that with energy of photons in gamma-rays that somewhat tops at 8 MeV.

 

I wouldn't say that it's completely impossible, but it's definitely not usual amounts of energy.

[Quackers101]

what does one mean by saying mass, and how do you want to use it or define it?

a weight in energy? will it have more of a mass/weight the deeper we go or like into the quantum world?

from electrons to electricity to other energies or forces. if you find a way to keep photons from going anywhere.

[Sakkura]

20 hours ago, StDragon said:

Not entirely an oxymoron. Yes, photons are massless. Mass also warps the fabric of spacetime. However, a massless object (photon), can still be influenced by the warping of spacetime caused by other objects with mass.
 

The Universe is what it is. Just don't ask me to explain why  

It's not just mass that warps spacetime. It's any contribution to the stress-energy tensor, which means even the energy of massless photons contributes.

 

If you somehow manage to pack enough photons together into a small enough volume, you can generate a black hole. That's called a Kugelblitz.

[tikker]

16 hours ago, Quackers101 said:

what does one mean by saying mass, and how do you want to use it or define it?

a weight in energy? will it have more of a mass/weight the deeper we go or like into the quantum world?

from electrons to electricity to other energies or forces. if you find a way to keep photons from going anywhere.

Weight isn't energy. Weight is the force of gravity pulling on an object and hence it is expressed in Newtons. It's the m*a bit in F=m*a in the case that a is the acceleration due to gravity, where m is the mass.

[Quackers101]

1 hour ago, tikker said:

Weight isn't energy.

just don't care about what I'm saying, I'm just ranting nonsense.

but is energy weightless? does it have other forces or systems on it that we can't see?

if there to be a balance between energy put into things, and put out of things. is there a balance of total energy of everything, if there is or was a limit to the universe and all energies in it?

[tikker]

28 minutes ago, Quackers101 said:

but is energy weightless? does it have other forces or systems on it that we can't see?

Gravitational forces act on masses, so no energy is not weightless nor does it have weight in the usual sense. Since energy does distort spacetime like mass, you can argue that a box of light/energy "weighs more" than an empty box, but not in a "weighs X grams more" sense.

28 minutes ago, Quackers101 said:

if there to be a balance between energy put into things, and put out of things. is there a balance of total energy of everything, if there is or was a limit to the universe and all energies in it?

I believe conservation of energy doesn't hold for the Universe if I remember my studies correctly. Locally it holds, but since the Universe is expanding light loses energy over time, for example, because the wavelength stretches and thus energy is lost.

[tim0901]

On 9/16/2021 at 3:17 AM, Quackers101 said:

what does one mean by saying mass, and how do you want to use it or define it?

a weight in energy? will it have more of a mass/weight the deeper we go or like into the quantum world?

from electrons to electricity to other energies or forces. if you find a way to keep photons from going anywhere.

In simpler terms one would say that mass is weight divided by gravitational acceleration (9.81m/s²). This is because, in physics, 'weight' is defined as the force exerted on an object by a gravitational body such as the Earth.

 

So to find the mass of an object, we find its weight (in Newtons, the unit of force) and divide it by 9.81. This is what your set of scales does - or rather is what it is calibrated around - as the result of this calculation is the mass of your object in Kilograms. So a person of mass 100kg would have a weight of 981 Newtons. Day-to-day English uses mass and weight interchangeably as synonyms for each other, but the distinction between them is very important in physics.

 

Mass as a concept is important because it doesn't change as your acceleration due to gravity changes. In free space you are weightless, but not massless - our person's mass is still 100kg - and as such it provides a much better representation of how much "stuff" you have.

 

But mass has another definition, given by the most famous equation in the world: E=mc². This tells us quite simply that mass is energy and, conversely, that energy is mass. The 100kg mass of our person from before is, according to this equation, equal to 9x10¹⁸ Joules of energy, or 9 billion gigajoules. And yes, this does mean that, when you move something and give it more energy, it does - in effect - gain more mass. The amount is tiny at the speeds we encounter in day-to-day life, but when you're talking about speeds encountered inside particle accellerators or nuclear reactors, it starts to get much more significant - potentially far beyond the original "rest" mass of the object. And conversely, you can take mass and turn it back into energy - this is what happens in the centre of the sun during nuclear fusion.

 

The mass of objects doesn't change as you look more closely, although it is easier to identify where this mass is coming from (just like when you take something apart). We know that most of the mass of an atom is found in the protons and neutrons at the centre, whilst most of the mass of a proton or neutron is found in the form of potential energy due to strong sub-atomic forces.

 

Fun fact: you can't actually have a stationary photon. Because it has no mass, it cannot travel at any speed other than at the speed of light (for the medium it is in).

 

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I believe conservation of energy doesn't hold for the Universe if I remember my studies correctly. Locally it holds, but since the Universe is expanding light loses energy over time, for example, because the wavelength stretches and thus energy is lost.

Other way around. Locally, conservation of energy may appear to be broken (your redshift-affected photon) but overall it is conserved. The universe is a thermodynamically closed system and as such the amount of energy it contains remains constant - nothing goes in, nothing comes out.

 

Your photon appears to lose energy as its wavelength stretches (gravitational redshift) but this energy isn't wiped from existence. Under Newtonian mechanics (F=ma etc.) we would say that the energy lost by the photon is converted into potential energy - the same way a ball gains potential energy as you lift it from the ground. Under General Relativity it gets more complicated than this but the end result is the same - energy is still conserved.

 

Regarding the expansion of the universe, I don't think there's an exact consensus on this due to our limited understanding of the mechanisms involved - in particular dark energy, but also surrounding a quantum theory of gravity. One explanation I've heard is that energy is conserved because the energy lost to redshift is balanced out by the increase in dark energy - that the energy goes into the expansion of the universe.

[Wictorian]

3 hours ago, tim0901 said:

Other way around. Locally, conservation of energy may appear to be broken (your redshift-affected photon) but overall it is conserved. The universe is a thermodynamically closed system and as such the amount of energy it contains remains constant - nothing goes in, nothing comes out.

 

Your photon appears to lose energy as its wavelength stretches (gravitational redshift) but this energy isn't wiped from existence. Under Newtonian mechanics (F=ma etc.) we would say that the energy lost by the photon is converted into potential energy - the same way a ball gains potential energy as you lift it from the ground. Under General Relativity it gets more complicated than this but the end result is the same - energy is still conserved.

 

Regarding the expansion of the universe, I don't think there's an exact consensus on this due to our limited understanding of the mechanisms involved - in particular dark energy, but also surrounding a quantum theory of gravity. One explanation I've heard is that energy is conserved because the energy lost to redshift is balanced out by the increase in dark energy - that the energy goes into the expansion of the universe

Bullshit.

[tikker]

6 hours ago, tim0901 said:

Other way around. Locally, conservation of energy may appear to be broken (your redshift-affected photon) but overall it is conserved. The universe is a thermodynamically closed system and as such the amount of energy it contains remains constant - nothing goes in, nothing comes out.

Is an infinitely large (to our current understanding) system such as the Universe really a closed system though?

6 hours ago, tim0901 said:

Your photon appears to lose energy as its wavelength stretches (gravitational redshift) but this energy isn't wiped from existence. Under Newtonian mechanics (F=ma etc.) we would say that the energy lost by the photon is converted into potential energy - the same way a ball gains potential energy as you lift it from the ground. Under General Relativity it gets more complicated than this but the end result is the same - energy is still conserved.

Cosmological redshift, not gravitational redshift. The former is the expansion of the Universe. The latter is caused by massive things such as clusters. In this case the conservation of energy makes sense as the photon energy decreases while the potential energy from climbing out of the potential well increases.

6 hours ago, tim0901 said:

Regarding the expansion of the universe, I don't think there's an exact consensus on this due to our limited understanding of the mechanisms involved - in particular dark energy, but also surrounding a quantum theory of gravity. One explanation I've heard is that energy is conserved because the energy lost to redshift is balanced out by the increase in dark energy - that the energy goes into the expansion of the universe.

I also think there is indeed no exact consensus yet, because of the remaining uncertainties such as dark energy etc. as you mention. The reason could potentially not be conserved is because the amount of energy in matter seems to be fixed, we don't see matter being created anymore so the matter energy density will go down as (1 + z)^3, with z being the redshift. Photons follow that same "spatial" reduction plus an additional "spectral" factor due to cosmological redshift, making their energy density go down as (1 + z)^4. The dark energy or vacuum energy density remains constant in our current theories, however, so as the Universe expands it will create vacuum energy as (1 + z)^3. It's that constantness of the dark energy density that breaks the conservation of energy I believe.

3 hours ago, Wictorian said:

Bullshit.

I wouldn't call something outright bullshit without a counter argument

[tim0901]

3 hours ago, tikker said:

Is an infinitely large (to our current understanding) system such as the Universe really a closed system though?

There are multiple ways you can look at it.

 

First you can consider that the term 'universe' refers to a closed system by definition. It is defined by modern physics as "all of space and time and its contents" - nothing can enter or leave it, because an "outside" of the universe does not exist.

 

You can also consider that time symmetry holds for the universe under Noether's theorem - the source of our conservation laws. For every (differentiable) symmetry there is a conservation law and the symmetry with respect to time results in the conservation of energy. As such, the universe must be a closed system.

 

Quote

I also think there is indeed no exact consensus yet, because of the remaining uncertainties such as dark energy etc. as you mention. The reason could potentially not be conserved is because the amount of energy in matter seems to be fixed, we don't see matter being created anymore so the matter energy density will go down as (1 + z)^3, with z being the redshift. Photons follow that same "spatial" reduction plus an additional "spectral" factor due to cosmological redshift, making their energy density go down as (1 + z)^4. The dark energy or vacuum energy density remains constant in our current theories, however, so as the Universe expands it will create vacuum energy as (1 + z)^3. It's that constantness of the dark energy density that breaks the conservation of energy I believe.

The amount of dark energy in the universe doesn't remain constant in our current theories, rather its density does. This observation comes from the Friedmann equations, which show that the density of dark energy does not scale with the scale factor of the universe (ρ ∝ a⁰), unlike ordinary matter or radiation which scale according to the inverse cube (ρ ∝ a^-3) and inverse fourth (ρ ∝ a^-4) of the scale factor respectively. For this reason some theories consider dark energy to be a property of space itself.

 

As such, the amount of dark energy in the universe increases as the volume of the universe increases - if the density remains constant but the volume increases then this must be the case. This is - we believe - why the expansion of the universe is accelerating: as the universe expands and brings more and more space into existence, more of this energy-of-space appears and the faster the expansion occurs. This increase in dark energy also therefore balances out the reduction in other forms of energy in the universe, allowing energy to be conserved.

 

7 hours ago, Wictorian said:

Bullshit.

Do not blindly dismiss that which you do not understand. I can provide sources and reading material for everything I've written here.

[Quackers101]

10 hours ago, tim0901 said:

Fun fact: you can't actually have a stationary photon. Because it has no mass, it cannot travel at any speed other than at the speed of light (for the medium it is in).

thats why I want to capture light, swallow it in my "endless" black hole and eat it.

[tikker]

1 hour ago, tim0901 said:

First you can consider that the term 'universe' refers to a closed system by definition. It is defined by modern physics as "all of space and time and its contents" - nothing can enter or leave it, because an "outside" of the universe does not exist.

I don't think you can define something to be a closed system. You can prove or disprove it, but you can't just define it as such. The Universe being a closed system makes sense though from criterion for being unaffected by the outside. As you say an infinite system has not "outside" after all.

1 hour ago, tim0901 said:

The amount of dark energy in the universe doesn't remain constant in our current theories, rather its density does. This observation comes from the Friedmann equations, which show that the density of dark energy does not scale with the scale factor of the universe (ρ ∝ a⁰), unlike ordinary matter or radiation which scale according to the inverse cube (ρ ∝ a^-3) and inverse fourth (ρ ∝ a^-4) of the scale factor respectively. For this reason some theories consider dark energy to be a property of space itself.

a is just 1 / (1+z), so we're saying the same thing here

1 hour ago, tim0901 said:

As such, the amount of dark energy in the universe increases as the volume of the universe increases - if the density remains constant but the volume increases then this must be the case. This is - we believe - why the expansion of the universe is accelerating: as the universe expands and brings more and more space into existence, more of this energy-of-space appears and the faster the expansion occurs. This increase in dark energy also therefore balances out the reduction in other forms of energy in the universe, allowing energy to be conserved.

Yep, you're right. I refreshed my cosmology a bit and it does seem to cancel out with the other energies indeed. I deal with galaxies, which are "slighty" smaller scale my cosmology has faded over the years.

[sub68]

On 9/11/2021 at 4:48 AM, Wictorian said:

if for instance radio waves are massless are they effected by gravity too?

Radio waves are technically radiation.