# Mass of light?

1. Nov 26, 2003

### decibel

today in my physics class, teacher told me that light doesent have mass....i thought it did....wuts the final answeer to this question, does it have mass or not?

2. Nov 26, 2003

### futz

Light is made of photons, which have no mass (rest mass, anyway).

3. Nov 27, 2003

### Staff: Mentor

[?] [?] You're not insulting him for asking a question, are you?

4. Nov 27, 2003

### decibel

lol...am in my final year of high school grade 12, and there has always been some debating in this forum about the mass of light, just asking a question u know!

5. Nov 27, 2003

### Zlex

Newton assumed in his particle theory that the mass of a light particle is approaching zero, meaning that it is so close to zero it might as well be zero.

6. Nov 27, 2003

### mathman

Photons have a rest mass of zero (special relativity). However they have energy and are affected by gravity (general relativity).

7. Nov 28, 2003

### StarkyDee

It's very hard to understand why it would have no mass..If light is distributed through 3rd/4th dimension, wouldn't it have to be mass for it to exist physically? and what about time, does it have a mass?

8. Nov 28, 2003

### Ambitwistor

I don't know what it means to be "distributed through 3rd/4th dimension", nor what that has to do with having mass. (If it was only distributed through two dimensions, could it be massless?)

What physical law are you thinking of, that says that something has to have mass to exist physically? Having nonzero mass simply is not a requirement for particles to exist.

Time is not an object which we can say either has or does not have mass.

9. Nov 28, 2003

### Sniper__1

time is just inyour imagination so no it has no mass and light has a mass because photons have mass this mass however is to small to be noticable so just say for all purposes besides super accuracy light has no mass.

10. Nov 28, 2003

### jcsd

This is not the first time this has been discussed, though it has not been proved experimentally that light has no mass (as it is impossible to prove such a thing and experiments can only place bounds on the mass of light), it is pretty safe to say that light (and photons) have no mass.

11. Nov 28, 2003

### mathman

Experimentally it has been observed that the speed of light is independent of any inertial reference frame. Therefore light cannot have a rest mass. (Things with rest mass have an inertial frame with speed equal zero.)

12. Nov 28, 2003

### decibel

ok so if light has no mass, how can it get sucked in by black holes?

13. Nov 28, 2003

### Ambitwistor

You could equally well ask, how can its trajectory be deflected by the gravity of any body. Light (as well as massive bodies) travel in straight lines through spacetime, but if the geometry of spacetime is curved, then "straight lines in spacetime" can be curved trajectories in space.

14. Nov 28, 2003

### StarkyDee

because black holes are sucking in matter at the speed of light, but the black hole has mass-a lot of mass, therefore light can't escape.

15. Nov 28, 2003

### LaserFloyd

Wasn't there an experiment some time ago that showed light can exert force? I can't put my finger on the guy's name. Anyhow he did an epxeriment where light beams of some sort exerted a pressure on some sort of metal in a vacuum. (I'm sure someone could elaborate more on that)

Since mass is part of the force equation, wouldn't light have mass even if it is so small it's almost non-existent?

[?]

16. Nov 28, 2003

### Ambitwistor

Yes, light exerts a force. It can do that because force is a change in momentum (F=dp/dt), and light does have momentum. (F=ma only in non-relativistic mechanics; it is interesting to note that F=dp/dt was Newton's original definition, which still holds in relativity.)

17. Nov 29, 2003

### cepheid

Staff Emeritus
I think astronaut Ed Lu recently tried this experiment ("pushing" something with a flashlight) onboard the ISS (just for fun) and reported a success of sorts. Sorry, no source (you might want to check that...he wrote "letters from space" that should be available online somewhere at NASA or something).

To futz...hello to a fellow Edmontonian.

18. Nov 29, 2003

### futz

19. Nov 29, 2003

### Scooby

First post...
Anyways, yes light does exert force. It is being looked at as a possible form of space travel. Essentially it would work the same as a sail boat using the sun's light as it's "wind."
Assuming this works as planned you could potentially reach any speed imagianble since there is no friction in space and each particle of light would bounce off causing you to move forward. It's being looked at as a form of long distance space travel.

Um. First post though and i registered just to say what most already know im sure.
http://www.techtv.com/news/culture/story/0,24195,3337579,00.html
http://www.popsci.com/popsci/aviation/article/0,12543,337873,00.html [Broken]

Last edited by a moderator: May 1, 2017
20. Nov 29, 2003

### Staff: Mentor

Just a clarification here - Ambitwisor, I think you missed his wording and the misconception continued: The phrase "sucked in" is misleading. NOTHING gets "sucked in by a black hole" any more than the earth's gravitational field "sucks" anything in. The gravitational field of a black hole is only special as a result of its magnitude. So a black hole with the mass of the earth would act the same as the earth toward an object in orbit. And a photon of light will only enter the black hole if it is headed on a trajectory that crosses the event horizon.

21. Nov 29, 2003

### Ambitwistor

As far as I can see, the question refers to how a massless particle can be influenced by gravity (such a question usually based on Newtonian conceptions of how a gravitational field exerts a force proportional to mass). Whether we choose to say that light (or matter, or whatever) is "sucked in" or not is not relevant to that question.

22. Nov 30, 2003

### Staff: Mentor

Maybe he can answer that, but I think that is the crux if his question.

23. Nov 30, 2003

### decibel

yes it is russ

24. Nov 30, 2003

### Ambitwistor

Just to be clear: you said that the crux of your question involves specifically light being "sucked into" black holes, and not with whether light is influenced by the gravitational interaction of another body.

If that's the case, then you accept that gravity will attract massless particles, but you don't know how gravity can suck massless particles into black holes? Why do you accept one, but don't understand the other?

25. Nov 30, 2003

### Staff: Mentor

He probably didn't understand either - its just that the way he asked the question focused on one particular misconception.

I've said this before, I tend to think that when people understand a topic REALLY well (like you), they tend to miss some of the more basic misconceptions people have about the topics. With my admittedly limited understanding, I think I'm more able to recognize basic questions like that.

The misconception of a black hole being some sort of cosmic vacuum cleaner is a very common one though.

In any case, do you get it now, decebel? Gravity is a result of the curvature of space-time by objects with mass. If two objects with mass interact they will have a resulting force between them. Light doesn't have mass, so there is no gravitational force - it simply follows a straight path through space. This path looks curved to us though because of the curvature of space around a massive object.

None of that is any different for the earth or a black hole of the same mass. A photon of light traveling at a specific distance from the center of either will act exactly the same - follow the same curved looking path. And an object with mass at a certain distance from either will experience the same gravitational attraction.

The difference is simply that in a black hole, the mass is so concentrated that you can get close enough to it that there is a threshold where the curvature is so great that light will not escape.

An interesting biproduct of matter falling into a black hole is that as it gets closer to that threshold (the event horizon) it accelerates, gaining energy, and as a result radiates energy. Detecting these emissions (they are x-rays) is one of the ways we can detect a black hole.

Last edited: Nov 30, 2003
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