# How can I see something that has no mass ?

1. Jun 12, 2014

### ArtsDegreeGuy

Basically just started reading up on the theory of relativity and physics in general in the last few weeks, so I am a self confessed novice in all things science.

I have a few questions about light which I have so far been unable to find any direct answers to.

1.) If light has no mass or weight, how can I see it ?

2.) What force makes light travel forward ?

3.) Why does it always travel at a set speed ? Why is this set speed 299 792 458 m / s and not some other random number ?

I know I sound like a 3 year old asking why is the sky blue, but that is where I am at. Cheers.

2. Jun 12, 2014

### ZapperZ

Staff Emeritus
There's a disconnect between "no mass" and "see". You have somehow made an a priori assumption that only objects with mass can be "seen", presumably by your naked eye. Is this true? If this is your assumption, what made you think so? What science are you using that causes you to think that only objects with mass can be detected by the human optical system?

Zz.

3. Jun 12, 2014

### ArtsDegreeGuy

I did make that assumption. For something to exist in the physical world, for something to be 3-dimensional, it must have a mass. I am not using any scientific theory at all to justify my question, just a 'logic' assessment in physically seeing something that apparently has not mass or weight.

In the case of light, something must 'be there' to in order for light to exist. ( Photons I believe) How can something 'be' and not have a mass ? Perhaps I am also troubled be the definition of mass.

4. Jun 12, 2014

### dauto

Light has no mass but it has energy. When a photon comes to your eyes its energy is absorbed by the electrons in the photo-sensor molecules of your eye changing its structure briefly which stimulates the optical nerve allowing you to see it.

5. Jun 12, 2014

### ArtsDegreeGuy

That makes sense to me. One question to this however, how does that work with energy= mass times C squared ? ( nevermind, there is another thread on this exact question I found)

6. Jun 12, 2014

### phinds

NEVER do that in cosmology or quantum mechanics. We are animals that evolved in what turns out to be a very narrow range of physical existence and our "logic" and "intuition" and "common sense" are all based within that narrow range. There are lots of things in cosmology (the very large) and quantum mechanics (the very small) that are totally counter-intuitive to what we think it "natural" or "logical" or whatever and to assume otherwise is to walk down the road of ignorance.

7. Jun 12, 2014

### dauto

E= mc2 provides you the rest energy of the particle which is indeed zero. The rest energy is NOT the total energy of the particle which is given by the more interesting equation E = [(mc2)2 + (cp)2]1/2, where p is the momentum. For a massless particle like the photon that simplifies to E = cp.

8. Jun 12, 2014

### ArtsDegreeGuy

That is a very good way of putting it. My 'logical' view of the universe may as well be the same as my dog's, considering the 'wide range' that exists as you put it. It is hard not to see the universe this way though, especially when you don't have the maths and science behind you.

9. Jun 12, 2014

### ZapperZ

Staff Emeritus
That assumption, if you noticed, is not based on any science. It is merely based on your personal preference, not logic, for whatever the reason. So it can't be answered here, because you are asking us based on some other criteria than established physics.

In physics, there is zero such requirement.

Zz.

10. Jun 12, 2014

### ArtsDegreeGuy

Okay, that makes sense. One small question on the e=cp equation. If p = momentum, and to find the momentum of something the equation is velocity x mass, how does that work with light, if light has has no mass at rest ?

Also, ( this question is related but going off on another path) if a blackhole's can pull light into it, why doesn't earth's gravity have any impact ? I assume it just isn't strong enough or something.

11. Jun 12, 2014

### ZapperZ

Staff Emeritus
But this is an excellent opportunity for you to sit back and ask yourself "now why would I require that, and where did that requirement came from?"

As scientists, we have seen many instances where our prejudices have been revealed and destroyed, because they were based on the wrong perception on the properties of the universe. In you case, it is based on faulty logic and understanding. This is fine as long as you can identify the fault and realize that there is a "more general" rule here that is at work. Knowing and realizing that what you have accepted to be true is really isn't is one of the first steps in learning.

Zz.

12. Jun 12, 2014

### enorbet

Greetings
Since you are signed on as "ArtsDegreeGuy" perhaps something "arty" can reinforce this.... perchance some Beatles

Code (Text):

"The Fool On The Hill"

Day after day, alone on the hill
The man with the foolish grin is keeping perfectly still
But nobody wants to know him
They can see that he's just a fool
And he never gives an answer

But the fool on the hill
Sees the sun going down
And the eyes in his head
See the world spinning around

13. Jun 12, 2014

### phinds

Hey, by the way ... now that we've beat you up, welcome to the Physics Forum

14. Jun 12, 2014

### ArtsDegreeGuy

Absolutely. I only ask questions through my faulty logic, never answer questions with it. ( At least in science.)

A quote which describes this is (paraphrasing) "It is not what you don't know that gets you into trouble, but what you think you know and simply is not true".

15. Jun 12, 2014

### Orodruin

Staff Emeritus
The relation p = mv is only valid for objects moving at velocities much smaller than that of light. In relativity, E^2 = m^2 c^4 + p^2 c^2 is the defining relation. It can further be deduced that v = pc^2/E, which for small p reproduces the nonrelativistic limit.

To try to answer your question about the actual value of c: it is merely a result of the units we have chosen - which are based on what happens to be relevant for our everyday lives.

16. Jun 12, 2014

### ArtsDegreeGuy

That is all good. I did expect some level of 'beating' before I joined up, this is the internet. Cheers for the welcome.

17. Jun 12, 2014

### ArtsDegreeGuy

To be honest, the top part went a little over my head, I will have to read it over for a while.

As for why light is that speed. Are you saying it is because that is what we have measured it at and therefor it is so ? It is a simple measurement and why the results of that measurement are this way is something we don't know yet ... ?

18. Jun 12, 2014

### Orodruin

Staff Emeritus
Today we actually do it the other way around, we are actually defining one meter as the distance light travels in a given time. The reason we do this is that it is the most accurate definition one can make of a length unit.

Of course, this is done in such a way as to have a meter being essentially as long as it was before (up to uncertainties in the definition).

The real question should be why the length scale typical to our life is related with this number to the time scale it percieves as relevant.

As a side note, physicists will often use different unit systems. In particular, particle physicists are very fond of using units where c = 1.

Last edited by a moderator: Jun 12, 2014
19. Jun 12, 2014

### CWatters

Eyes are just a type of sensor so perhaps substitute "seeing" with "detecting". Light is a form of energy and it's not hard to detect energy.

Light has momentum even though it has no mass. Light also has wave like properties.

Light only travels at 3*10^8m/s in a vacuum. In other materials it goes slower.
http://news.bbc.co.uk/1/hi/sci/tech/655518.stm

20. Jun 12, 2014

### Nanosuit

1) Photons reaching your eyes.Photon IS light!!!

2) It has no ress mass, but it does have momentum! Energy of photon=its momentum x c

3) That is the 'apparent' speed barrier of the universe.Nothing can travel faster.If light could travel faster, the universe would have 'that' speed as the so-called speed barrier.

21. Jun 12, 2014

### Drakkith

Staff Emeritus
The numerical value of C is the result of our units of time and distance, specifically the second and the meter. The fact that there is a maximum speed limit is simply the way it is.

22. Jun 12, 2014

### Deuterium1971

I was under the impression that the speed limit of light in a vacuum was a constant, but the light can move through other material at variable speeds. I remember reading somewhere that light actually went faster than the "speed limit" when traveling through Cesium gas, but that may have been an optical illusion, not certain.
As for your questions, Energy is seen, Gravity is felt and Strong Force holds it all together. In a way you can never see mass at all, and need energy for the receptor in your eye to activate. You wouldn't want to see MASS anyway, who wants anything hard and unforgiving like a splinter of wood or tungsten dust touching their retina. Food for thought.