# Does an object achieve ∞ mass or 0 mass at the speed of light?

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1. Nov 27, 2013

### Remon

I'm debating whether this should go in the GR & SR section, but anyways, I heard both claims and both of them are supported by calculations and observations. The ∞ mass is proved by equations from SR (and the fact that a certain amount of energy pushing an object to c will always be converted to mass to add on to the existing mass, thus, requiring even more energy to push it and so on...) and the zero rest-mass is proved because a photon is the only thing in the universe that has zero rest-mass (I think), which is why light (aka photons) is fastest thing in the universe, but how can both results be possible? Wouldn't it be either zero mass or ∞ mass? Or is it really both? Thanks in advance.

2. Nov 27, 2013

### phinds

You are quoting oranges and asking about apples. Photons travel at c because they ARE massless. Matter cannot travel at c because to do so it would have to be given infinite mass-energy. You can't conflate the two as you seem to be trying to do.

3. Nov 27, 2013

### Remon

Yeah, I think I understand you're saying, but one thing still confuses me: Wouldn't an object reaching c achieve 0 mass or the mass equivalent to light/a photon? I mean I know this is wrong, but it seems to make more sense that a lighter object (with no mass) would travel at the highest possible speed (and an object with ∞ mass to not travel at all...ever), because as proven by the mass (or the 'massless') of a photon, it travels at the highest possible speed since it has no mass, and on the other hand, an object with ∞ mass can never travel anywhere at all. Once again, I'm aware that all of this is wrong, but why does it seem to make more sense? This also implies (if its true) that the less mass an object has, the faster it can travel, is this true?

4. Nov 27, 2013

### Staff: Mentor

It is clearly true that less massive objects can travel faster with the same energy input, but that doesn't have anything to do with the issue.

What you are missing is that infinity is not a number. So no matter how much energy you add, you will never reach "infinity energy", and thus no object with mass will ever reach the speed of light.

5. Nov 27, 2013

### 94JZA80

no, this is not true...but it is true that a smaller mass can be accelerated up to a specific velocity quicker than a larger mass accelerated to the same specific velocity. likewise, if two different masses are accelerated up to the same specific velocity in the same amount of time, the smaller mass requires a smaller force to get there than the larger mass does. it is all about the proportionality between the three variables force, mass, and acceleration...

6. Nov 27, 2013

### Remon

I'm aware of that, but on the contrary, any object with no/zero mass (which I know is not possible, except for photons) can easily travel at c, right? So how can the same hold for ∞ mass? Wouldn't ∞ mass react in the complete opposite way of zero mass? Because here, they both seem to travel at the same speed, maybe its because ∞ mass comes from the ∞ energy that is being applied... now I'm just confusing myself

7. Nov 27, 2013

### Staff: Mentor

Because infinity is not a number, there are no meaningful statements about the behavior of an object with infinite mass. Stop trying to attach meaning to meaningless statements and you'll stop confusing yourself.

8. Nov 27, 2013

### Staff: Mentor

It doesn't.

You are mixing up cause and effect.

9. Nov 27, 2013

### Remon

Ok, so I guess shouldn't ever talk about ∞ mass or energy since it simply doesn't exist, at least the bit about zero rest-mass objects moving the fastest is true

10. Nov 27, 2013

### phinds

Yes, it's true but I still think you are trying to conflate objects with no mass (photons) with objects WITH mass as though zero and non-zero are just difference in quantity. In this case, they are not. They are differences in quality/kind. You cannot extrapolate from one to the other as you could if it was only a difference in quantity.

11. Nov 27, 2013

### Remon

Yes, but isn't that the only factor that determines an object's speed/velocity relative to another object (assuming the same exact amount of force being applied)? That factor being the simple presence or absence of mass? I don't know, maybe I just don't fully understand it, but thank you all for trying to help.

12. Nov 27, 2013

### phinds

No, it is not. When photons are created the are immediately traveling at c. Matter doesn't work that way. What determines the speed of matter is how much force is applied to it and what you are talking about as a reference frame. ALL reference frames see photons as traveling at c. It's completely different.

EDIT: in case you are not aware of it you, right now as you are reading this, are traveling at .99999c from some frame of reference and by some other frame of reference you are traveling at .8221c and from your frame of reference you are motionless. ALL of those frames of reference see photons traveling at c in a vacuum.

Last edited: Nov 27, 2013
13. Nov 27, 2013

### PAllen

It might also be worth noting that no matter how much energy you add to body with mass, its (rest) mass doesn't change. This gets at how misleading the old concept of relativistic mass is. In modern terminology:

Adding more and more kinetic energy to a body increases its momentum and does nothing to its mass (which means rest mass). The higher the kinetic energy of a given body, the smaller the fraction of its total energy is due to rest mass.

Now, in this more correct terminology, the limit is natural: to reach c, you must have all energy be kinetic energy, and none be rest mass. This is the case of a photon.

14. Dec 12, 2013

### Ahmed_iAM

One question that is slightly off topic, if something with mass cannot travel at or faster than the speed of light, then how do scientists expect to receive nuetrinos from a supernova before its light reaches the earth? I dont know if it has to do with the collapse of its core prior to the explosion but an explanation would be greatly apreciated.

15. Dec 12, 2013

### HallsofIvy

Staff Emeritus
Essentially, yes. The neutrinos from supernova, that arrive before the light, must have left before the light left.

16. Dec 12, 2013

### ZapperZ

Staff Emeritus
Which scientists are expecting to receive such neutrinos?

Zz.

17. Dec 12, 2013

### phinds

A different situation, but one that might help you understand things better is this: A photon created in the core of our sun takes about 100,000 years to reach the surface and then another 8 minutes to get to earth. This is because they are absorbed and then new ones are emitted when they travel through the core.

Neutrinos on the other had don't GET absorbed and emitted, they just pass through everything and take 8 minutes to get here.

Similarly, but to a much lesser degree (see the next paragraph), the photons leaving the surface of a nova or supernova are slowed down (but not nearly as much) by absorption / re-emission whereas neutrino are not.

I can't give a citation, so this is just anecdotal information and may be wrong but I seem to recall that in one case it was expected that the neutrinos from a particular supernova would get to Earth about 5 hours earlier than the photons but as Hallofivy said, that's just because they left sooner, not because they traveled faster.

18. Dec 12, 2013

### PAllen

All of them, I assume. See:

http://profmattstrassler.com/2011/09/20/supernovas-and-neutrinos/

"Meanwhile, a look back at older photographs revealed one showing visible light from the supernova that was taken only 3 hours after the neutrinos had arrived on earth. Since the shock wave from the supernova blast had to make its way out of the exploding star before the debris could begin to shine, whereas the neutrinos from the explosion could sail right through the star unimpeded, a delay of a few hours between the arrival of the neutrinos and the arrival of the light was expected."

19. Dec 12, 2013

### Bill_K

But if the photon is absorbed and reemitted, then it is not the same photon. Photons can't be slowed down, they always travel at c, so something else (energy?) takes 100,000 years to reach the surface, but not the photon.