# Thread: Do photons have mass?

1. It might be down below 10^-35 of a gram but could they have mass, which would explain why they are susceptible to gravity?

To put it badly, surely only nothing weighs nothing. Everything that exists must have mass.

2.

3. Originally Posted by Cyberia
It might be down below 10^-35 of a gram but could they have mass, which would explain why they are susceptible to gravity?
I assume by mass you mean rest mass

To put it badly, surely only nothing weighs nothing. Everything that exists must have mass.
Why?

4. Originally Posted by Cyberia
It might be down below 10^-35 of a gram but could they have mass, which would explain why they are susceptible to gravity?

To put it badly, surely only nothing weighs nothing. Everything that exists must have mass.
Photons do not have rest mass, but they have energy. Gravitational force treats mass and energy the same way.

5. Janus. I mean (real) mass.

If a photon were just a vibration of some kind, it would need a medium and would have no uniformity, there being the quantum equivalents of tsunamis and ripples in a puddle.

Anything that exists should have mass, no matter how small.

6. Originally Posted by mathman
Photons do not have rest mass, but they have energy. Gravitational force treats mass and energy the same way.

Space is literally nothing. It is as much nonsense to talk of it being warped by gravity as it is to talk of it being heated by infra-red radiation travelling through it.

Gravity can affect energy even if it has the smallest possible mass but how does it effect it if it has no mass?

7. Originally Posted by Cyberia
Originally Posted by mathman
Photons do not have rest mass, but they have energy. Gravitational force treats mass and energy the same way.

Space is literally nothing. It is as much nonsense to talk of it being warped by gravity as it is to talk of it being heated by infra-red radiation travelling through it.

Gravity can affect energy even if it has the smallest possible mass but how does it effect it if it has no mass?
A photon has energy given by E=hf, where h is Planck's constant and f is frequency. Also in general E=mc^2. So if you want to call it mass, then a photon has mass m=hf/(c^2).

8. What proof do you have that space is nothing and can't be bent?

9. According to the big bang, the universe started off as a point source and is presently around a hundred billion light years across and still growing. If space is nothing, no problem but if space is some kind of material that can be warped by gravity, etc then you have to explain it's seemingly infinite elasticity without any change in properties.

What proof do you have that space cannot be heated?

10. Heat is, by definition, the average kinetic energy of a group of particles. While space may be something, it's not a group of particles. On the other hand, it's probably possible to define something very similar, such as the average amplitude of gravity waves within a region. I suspect that while such a definition would be valid, the results would be much, much lower than the 3K microwave background, to the point of being practically nonexistent.

As far as bending and stretching goes, you're taking the analogy too literally. Space isn't a material like rubber, but it's still not nothing.

11. Originally Posted by MagiMaster
Heat is, by definition, the average kinetic energy of a group of particles.
Slight pedantic correction. You've just described temperature, not heat. Heat (AFAIK) is the transfer of energy across temperature differentials/gradients, not the energy itself.

12. Originally Posted by Cyberia
Everything that exists must have mass.
This is just not true, and Janus already alluded to this in the first response. When you say "mass," you are referring to "rest mass." That's what it means when it's used in the vast majority of cases (relativistic mass is much less common in the literature, and definitely not used in general parlance). However, a photon does not have rest mass, because by definition a photon can only travel at the speed of light (cannot be "at rest"). If a photon exists, it is traveling at c, and is "massless."

13. Originally Posted by inow
Originally Posted by MagiMaster
Heat is, by definition, the average kinetic energy of a group of particles.
Slight pedantic correction. You've just described temperature, not heat. Heat (AFAIK) is the transfer of energy across temperature differentials/gradients, not the energy itself.
Sorry about that. Let me add that, under that definition, you can't have heat without temperature, so heating space still wouldn't make sense.

14. It is as much nonsense to talk of it being warped by gravity
Gravity doesn't warp space, the warping of space is gravity AFAIK.

15. Originally Posted by MagiMaster
What proof do you have that space is nothing and can't be bent?
*sigh* and what proof do you have it isn't? instead of asking stupid rhetorical questions like that either think of a useful answer or don't post, it just makes you look petty and arrogant

Originally Posted by Cyberia
It might be down below 10^-35 of a gram but could they have mass, which would explain why they are susceptible to gravity?

To put it badly, surely only nothing weighs nothing. Everything that exists must have mass.
Photons are 'packets' of light energy, E=hf, E also = mc^2, Energy and Mass are related, you can't have mass without energy and likewise you cannot have massless energy.

16. Originally Posted by Booms
Originally Posted by MagiMaster
What proof do you have that space is nothing and can't be bent?
*sigh* and what proof do you have it isn't? instead of asking stupid rhetorical questions like that either think of a useful answer or don't post, it just makes you look petty and arrogant
I know the feeling. I have had to "sigh" with exasperation, myself, after reading some posts here.
It must be very difficult, for you, to tolerate those individuals with minds so inferior to your own.

17. Originally Posted by Cyberia
According to the big bang, the universe started off as a point source and is presently around a hundred billion light years across and still growing. If space is nothing, no problem but if space is some kind of material that can be warped by gravity, etc then you have to explain it's seemingly infinite elasticity without any change in properties.
No one is saying that space is any kind of material substance. Space-time curvature refers to the fact that it follows the rules of non-euclidean geometry.

18. Originally Posted by Halliday
Originally Posted by Booms
Originally Posted by MagiMaster
What proof do you have that space is nothing and can't be bent?
*sigh* and what proof do you have it isn't? instead of asking stupid rhetorical questions like that either think of a useful answer or don't post, it just makes you look petty and arrogant
I know the feeling. I have had to "sigh" with exasperation, myself, after reading some posts here.
It must be very difficult, for you, to tolerate those individuals with minds so inferior to your own.
yeah I know my response was equally petty and arrogant, I just felt the need to voice my annoyance

...and yes.. it is,veeerrry difficult, makes me wanna scream sometimes

19. Voice your discomfort however you like, but if you, or anyone else, wants to assert that the universe doesn't follow the rules that modern physics says it does, you need to provide evidence. (BTW, modern physics says that space-time can be bent and photons have no rest mass. If you want to say otherwise, show me some kind of evidence.)

20. Originally Posted by Janus
Originally Posted by Cyberia
It might be down below 10^-35 of a gram but could they have mass, which would explain why they are susceptible to gravity?
I assume by mass you mean rest mass
Since light can only move at one speed (C), doesn't it seem kind of nonsensical to draw a line between "rest mass" and "relativistic mass"?

I guess it's important to be clear that light isn't going to come to a complete stop and leave some kind of material behind, but that doesn't change the fact that it has mass in every state it is capable of assuming. (Since it's only capable of moving at one speed.)

To put it badly, surely only nothing weighs nothing. Everything that exists must have mass.
Why?
Data is an example of something that doesn't have mass. The medium that stores the data has mass, but the data itself has no mass. (And the medium's mass usually doesn't change as you add more data to it.) This is more of a philosophical question, really.

21. Another point of view from Wiki. BTW, an electron has a mass of about eV.

Also, the total energy of an object is , where m is the rest mass (aka invariant mass) and p is momentum.

22. Photons we know for sure are a form of energy. In relativity, mass and energy are two manifestations of the same phenonmanon. Therefore, photons are affected by very strong gravitation.

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