Are photons massless?

[Wictorian]

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?

 

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?

[ki8aras]

bro this is the whole deal with photons, they have the properties of an object and a wave, meaning you will come to 2 positive conclusions if you do 2 experiments

[Sakkura]

Photons have no rest mass. But they do carry energy, which contributes to the stress-energy tensor, so they do exert a (very tiny) gravitational effect. They also travel through spacetime and are thus of course affected by gravitational distortion of spacetime.

[Wictorian]

4 hours ago, James Evens said:

You wanted to go down this rabbit hole? 

ask for book recommendations. This is far to complicated for a Thread on LTT.

I didn't realise. I would be happy if you recommended some books. I had downloaded some books from springer and they are very complicated for a beginner.

[harryk]

4 minutes ago, Wictorian said:

I didn't realise. I would be happy if you recommended some books. I had downloaded some books from springer and they are very complicated for a beginner.

QED: The Strange Theory of Light and Matter by Richard Feynman is the seminal text for this topic short of dense scientific papers and textbooks.

[Printmaher]

23 hours ago, harryk said:

QED: The Strange Theory of Light and Matter by Richard Feynman is the seminal text for this topic short of dense scientific papers and textbooks.

Thank you for suggesting a book. I found also a lecture on youtube 

[tikker]

On 9/11/2021 at 10:48 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?

A nice thought experiment, but unfortunately you can't do that.

On 9/11/2021 at 10:48 AM, Wictorian said:

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

Yes they are massless. They have to be, because they travel at the speed of light and only massless things can do that.

On 9/11/2021 at 10:48 AM, Wictorian said:

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

Because in the theory of general relativity gravity isn't just some force that acts on mass. Energy bends spacetime. Everything travels along paths in this space time, hence if a photon travels past some mass it will follow this curve in spacetime. Think of the of balls rolling on a trampoline with a bowling ball in the centre as a lower dimensional example.

On 9/11/2021 at 10:48 AM, Wictorian said:

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

Yes because radio waves are also light, just not at a frequency our eyes are sensitive to.

[Falkentyne]

15 hours ago, Printmaher said:

Thank you for suggesting a book. I found also a lecture on youtube 

You can read the book here if you're interested.

 

http://libgen.is/search.php?req=The+Strange+Theory+of+Light+and+Matter&open=0&res=25&view=simple&phrase=1&column=def

[gudvinr]

On 9/11/2021 at 12:10 PM, ki8aras said:

bro this is the whole deal with photons, they have the properties of an object and a wave, meaning you will come to 2 positive conclusions if you do 2 experiments

No, they do not. Photon is a particle. It doesn't have properties of a wave but rather its wave function has well, properties of wave.

Difference is that wave function exists only as mathematical abstraction and only used to conveniently describe probabilities and not behaviour of single photon.

[gudvinr]

On 9/11/2021 at 11:48 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?

If photon would have any mass, in terms of gravitational interactions it should behave like any other massive object with corresponding mass. It doesn't really matter whether it Planck mass or any other value.

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

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

It is assumed that they do not have mass or have extraordinarily small mass. Is is experimentally proven that photon mass is less than 10^-53 kg. This limit is 10²³ times lower than measured electron mass. Simple implication of photon being massless that if it isn't then it wouldn't travel at so-called speed of light c. Since c is theoretical speed limit that only achievable by massless particles, it is safe to imply that it is true. You can read about Lorentz factor and relativistic energy-momentum equation to understand why it's impossible for objects with mass to achieve such speed. Otherwise photon's frequency would depend on its speed (don't mistake with relativistic Doppler effect).

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

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

It is the existence of gravitational forces caused by mass. Photons affected by gravity not because they have mass but because something else has and thus creates such forces.

Newtonian mechanics has its limits and simply doesn't work on such scales so you can't describe how gravitational forces affect photons based purely in a sense of massive objects attraction. So that's why misconception about requiring a mass to be affected by gravity even exists.

It isn't clear why gravity works how it works, but general relativity theory tries to explain how it might affect photons in virtual 4-dimensional space where 4th dimension proportional to a time.

This should be carefully noted, that it doesn't mean that light travels in some 4-dimensional world, it is just mathematical concept that connects time passage with gravitational forces.

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

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

Light and radio waves are just named ranges of electromagnetic waves carried by photons. So, basically yes.

[Wictorian]

55 minutes ago, gudvinr said:

If photon would have any mass, in terms of gravitational interactions it should behave like any other massive object with corresponding mass. It doesn't really matter whether it Planck mass or any other value.

It is assumed that they do not have mass or have extraordinarily small mass. Is is experimentally proven that photon mass is less than 10^-53 kg. This limit is 10²³ times lower than measured electron mass. Simple implication of photon being massless that if it isn't then it wouldn't travel at so-called speed of light c. Since c is theoretical speed limit that only achievable by massless particles, it is safe to imply that it is true. You can read about Lorentz factor and relativistic energy-momentum equation to understand why it's impossible for objects with mass to achieve such speed. Otherwise photon's frequency would depend on its speed (don't mistake with relativistic Doppler effect).

It is the existence of gravitational forces caused by mass. Photons affected by gravity not because they have mass but because something else has and thus creates such forces.

Newtonian mechanics has its limits and simply doesn't work on such scales so you can't describe how gravitational forces affect photons based purely in a sense of massive objects attraction. So that's why misconception about requiring a mass to be affected by gravity even exists.

It isn't clear why gravity works how it works, but general relativity theory tries to explain how it might affect photons in virtual 4-dimensional space where 4th dimension proportional to a time.

This should be carefully noted, that it doesn't mean that light travels in some 4-dimensional world, it is just mathematical concept that connects time passage with gravitational forces.

Light and radio waves are just named ranges of electromagnetic waves carried by photons. So, basically yes.

What I mean with the first question is, wouldn't that much energy floating around hurt us? So is it possible that they are lighter than planck mass?

[tikker]

46 minutes ago, Wictorian said:

What I mean with the first question is, wouldn't that much energy floating around hurt us? So is it possible that they are lighter than planck mass?

Sunlight can cause skin cancer, X-rays and gamma rays are quite harmful to us, so yes the energy can hurt us. It's not just that energy floating around that hurts us though. Just think of the amount of energy the Earth has if you'd plug that into E=mc^2; quite a bit, yet we're not harmed by it.

 

Your line or reasoning is overall sound. Assuming the Planck mass for a photon would result in incredibly powerful photons, just an order of magnitude lower than the oh-my-god particle. Your conclusion then also makes sense: getting hit with that incredible amount of energy from thousands or millions of photons would be bad, so their mass has to be lower. Your starting point is ultimately wrong though. In our current understanding the photon has to be massless, because it travels at c. They don't accelerate, they don't slow down, they simply always travel at the speed of light.

 

You have good thinking though. Being able to think about and come up with these kind of thought experiments are very valuable in the scientific world. In essence you've done a sort of proof by contradiction.

[Kadzo]

 

[gudvinr]

On 9/13/2021 at 1:33 PM, Wictorian said:

So is it possible that they are lighter than planck mass?

It isn't only possible. As I mentioned, it was experimentally proven that their mass less than 10^-53 kg. Given that Planck mass in SI units is ~10^-8 kg you can be pretty sure that they indeed are lighter.

[Wictorian]

1 hour ago, gudvinr said:

It isn't only possible. As I mentioned, it was experimentally proven that their mass less than 10^-53 kg. Given that Planck mass in SI units is ~10^-8 kg you can be pretty sure that they indeed are lighter.

Then planck mass isn't really planck mass¿ Also still it doesn't mean it is possible if they are massless.

[porina]

Random thinking: I recall the energy of a photon is planck's constant multiplied by the frequency of light. If we apply that to E=mc^2 and rearrange for mass... Take a 550nm wavelength photon (green-ish), that works out around 4*10^-36 kg. Not zero but pretty close to it for practical purposes.

[Sakkura]

1 hour ago, porina said:

Random thinking: I recall the energy of a photon is planck's constant multiplied by the frequency of light. If we apply that to E=mc^2 and rearrange for mass... Take a 550nm wavelength photon (green-ish), that works out around 4*10^-36 kg. Not zero but pretty close to it for practical purposes.

Yes, though that is separate from rest mass. It has (as far as we know) a rest mass of zero. Its energy is still equivalent to mass and contributes to the stress-energy tensor (and thus a photon does exert gravity, just incredibly weakly).

[tikker]

3 hours ago, Wictorian said:

Then planck mass isn't really planck mass¿

Well at this point you have to consider what the Planck units actually are. They aren't defined as the smallest or lowest thing possible, they are defined such a way that in natural units they are 1. Because they are made up of the Planck constant and other fundamental constants, they take on a certain value in the SI system. IIRC they "just" denote the regime where you need to start using QM to describe things rather than our other theories.

3 hours ago, Wictorian said:

Also still it doesn't mean it is possible if they are massless.

While we of course will and should still try to measure a mass experimentally or set an upper limit at least to see if observations and theory are consistent, as said they have to be massless in the framework of Special Relativity if they travel at the speed of light. Special Relativity has two assumptions (inertial being a very important keyword):

1. The laws of physics take the same form in all inertial frames of reference.
2. the speed of light in free space has the same value c in all inertial frames of reference.

For a massive particle you have the following:

Quote

Total energy E is defined to be E = γmc², where m is mass, c is the speed of light,

γ=1/√1−v2/c2

 

and v is the velocity of the mass relative to an observer.
https://courses.lumenlearning.com/physics/chapter/28-6-relativistic-energy/

As you can see if a massive particle would attain light speed with v = c, it will result in γ, and hence E, being infinite. Ergo, a massive particle cannot travel at light speed. This implies that if photons travel at the speed of light, they have to be massless. If they turn out to have mass, they don't travel at the speed of light.

3 hours ago, porina said:

Random thinking: I recall the energy of a photon is planck's constant multiplied by the frequency of light. If we apply that to E=mc^2 and rearrange for mass... Take a 550nm wavelength photon (green-ish), that works out around 4*10^-36 kg. Not zero but pretty close to it for practical purposes.

The subtlety is that this does not imply that the photon therefore has that mass. It merely states that the energy that photon has is equivalent in terms of energy to an object of rest mass m.

[porina]

19 minutes ago, tikker said:

The subtlety is that this does not imply that the photon therefore has that mass. It merely states that the energy that photon has is equivalent in terms of energy to an object of rest mass m.

Good point. Following that thought, if a photon were to hit something that doesn't do anything particularly special with it, and does not get reflected, does it just get converted to heat? With thinking like this, maybe it was for the better I didn't try for a physics degree while younger. Went into engineering instead where you get away with "close enough" a lot more  

[tikker]

39 minutes ago, porina said:

Good point. Following that thought, if a photon were to hit something that doesn't do anything particularly special with it, and does not get reflected, does it just get converted to heat? With thinking like this, maybe it was for the better I didn't try for a physics degree while younger. Went into engineering instead where you get away with "close enough" a lot more  

Haha I'm an astronomer myself, so for us anything "within a factor 2", or sometimes even "within an order of magnitude", is considered fine as a sort of running joke. We're probably even worse than engineers in that regard. Unless we're talking sensitive instruments of course.

 

Yeah temperature of objects is related to how excited (no pun intended :P) the molecules are. They move, rotate and vibrate, so when photons hit matter and their energy is sufficient to bump it into the next state, they can get absorbed and excite the matter more, heating it.

[gudvinr]

17 hours ago, Wictorian said:

Then planck mass isn't really planck mass¿

Dude, what? Planck mass is just a part of Planck scale. It is unit system that defined in some specific way that makes it convenient to use in particular physics area.

It's just an agreement about multiplication of given numbers. Like Imperial but for quantum physics.

17 hours ago, Wictorian said:

Also still it doesn't mean it is possible if they are massless.

Because Planck mass doesn't have any meaning that might be relevant to photon being massless or not.

What really important is that experimental data confirm photons having constant speed that equals c in vacuum. If they weren't massless it wouldn't be possible.

Since experiments are well aligned with modern theories, it is widely assumed that they either completely massless or they have abnormally low mass.

 

Upper limit on photon mass just means that theories that we have today are good enough™ to describe nature while implying that even if photon has some mass, it is low enough to consider photon massless within boundaries of these theories.

[Wictorian]

14 hours ago, tikker said:

Well at this point you have to consider what the Planck units actually are. They aren't defined as the smallest or lowest thing possible, they are defined such a way that in natural units they are 1. Because they are made up of the Planck constant and other fundamental constants, they take on a certain value in the SI system. IIRC they "just" denote the regime where you need to start using QM to describe things rather than our other theories.

While we of course will and should still try to measure a mass experimentally or set an upper limit at least to see if observations and theory are consistent, as said they have to be massless in the framework of Special Relativity if they travel at the speed of light. Special Relativity has two assumptions (inertial being a very important keyword):

1. The laws of physics take the same form in all inertial frames of reference.
2. the speed of light in free space has the same value c in all inertial frames of reference.

For a massive particle you have the following:

As you can see if a massive particle would attain light speed with v = c, it will result in γ, and hence E, being infinite. Ergo, a massive particle cannot travel at light speed. This implies that if photons travel at the speed of light, they have to be massless. If they turn out to have mass, they don't travel at the speed of light.

The subtlety is that this does not imply that the photon therefore has that mass. It merely states that the energy that photon has is equivalent in terms of energy to an object of rest mass m.

I thought planck mass was the lowest possible mass.

 

Anyways, if we assume the mass of a photon is equal to planck mass, it turns out it jas an energy of 6.54 joules (if I did it corrctly). Now, is it impossible for a photon to attain that kind of energy? (I would guess lights would use up a lot of electricity then though)

[tikker]

2 hours ago, Wictorian said:

I thought planck mass was the lowest possible mass.

The electron mass is 9*10^-31 and the proton mass is 1.7*10^-27, much lower than the Planck mass.

2 hours ago, Wictorian said:

Anyways, if we assume the mass of a photon is equal to planck mass, it turns out it jas an energy of 6.54 joules (if I did it corrctly). Now, is it impossible for a photon to attain that kind of energy? (I would guess lights would use up a lot of electricity then though)

Not impossible I guess, but since E = hv gives a frequency of ~10^34 Hz it'd be extremely high energy gamma rays. You'd need a tremendous amount of energy to create such a photon. This is why the "oh my god particle" was so interesting and strange. That was a cosmic ray (possibly a proton) with an energy of 50 J.

 

Furthermore, since you just plug in the Planck mass in E=mc^2 that simply means the photon would have a restmass of that energy, i.e. not moving. Moving photons would have even higher energies.

[Master Disaster]

The entire question is a giant oxymoron.

 

Mass is the measure of the amount of matter in an object, weight is the measure of how much gravity effects an object.

 

Photons are pure energy which E=MC2 tells us must be massless, no mass means no weight by default.

[StDragon]

19 minutes ago, Master Disaster said:

The entire question is a giant oxymoron.

 

Photons are pure energy which E=MC2 tells us must be massless, no mass means no weight by default.

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