# Speed of Light Question.
**URL:** https://boards.straightdope.com/t/speed-of-light-question/114119
**Category:** Factual Questions
**Created:** [June 14, 2002, 11:05pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119 "2002-06-14T23:05:20Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Nobody](https://boards.straightdope.com/u/Nobody)
#### Post date: [June 14, 2002, 11:05pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/1 "2002-06-14T23:05:20Z")
Ok, it’s stated that any object moving faster and faster gains mass and needs more energy to accelerate it. So an object moving at the speed of light has infinite mass and needs infinite energy to move it.
Does that mean that light has infinite mass and needs infinite energy to move it ?
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [June 14, 2002, 11:09pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/2 "2002-06-14T23:09:29Z")
I thought it was the other way around; that it has no mass, which is why it can’t go slower.
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [June 14, 2002, 11:10pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/3 "2002-06-14T23:10:33Z")
(but I should add that this is only what I _thought_)
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### Author:  [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)
#### Post date: [June 14, 2002, 11:22pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/4 "2002-06-14T23:22:42Z")
What **Mangetout** said is the basic gist. There are some semantics we could go through about what qualifies as “mass”, but simply put, light has zero mass, yet has momentum. Anything with zero mass and non-zero momentum must travel at the speed of light.
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### Author:  [@kgriffey79](https://boards.straightdope.com/u/kgriffey79)
#### Post date: [June 14, 2002, 11:41pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/5 "2002-06-14T23:41:52Z")
I don’t want to hijack this thread, but a quick question for our physics experts. What about gravitrons? How fast do they move? What is their size? Just wondering 🙂 Sorry 🙂
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### Author:  [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)
#### Post date: [June 15, 2002, 12:05am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/6 "2002-06-15T00:05:40Z")
Of course gravitons are very theoretical, but the clear assumption seems to be that they shouldn’t have mass since they support interactions over long distances and because, well, it doesn’t seem right that a graviton should produce its own gravitation. 🙂
So gravitions theoretically should travel at the speed of light.
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### Author:  [@Nobody](https://boards.straightdope.com/u/Nobody)
#### Post date: [June 15, 2002, 12:14am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/7 "2002-06-15T00:14:10Z")
Ok, to make sure I ask this question right, I looked up mass on
> **[Dictionary.com Is The World’s Favorite Online Dictionary](https://www.dictionary.com/)**
>
> Dictionary.com is the world’s leading online source for English definitions, pronunciations, word origins, idioms, Word of the Day, and more.
And, for the definition for this thread, got:
A property of matter equal to the measure of an object’s resistance to
changes in either the speed or direction of its motion. The mass of an object is not dependent
on gravity and therefore is different from but proportional to its weight.
So, if mass is 0, then, weight is 0 too? Or how does that work?
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### Author:  [@g8rguy](https://boards.straightdope.com/u/g8rguy)
#### Post date: [June 15, 2002, 12:27am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/8 "2002-06-15T00:27:14Z")
Right. Weight is really a force, and force isn’t an intrinisic property. Mass, however, is. Any massless particle must move at the speed of light. The old saw about objects gaining mass as they move faster is, although a matter of semantics, basically out of favor with physicists these days. An objects mass is always the same; it’s what’s called a relativistic invariant. It’s the energy that blows up for massive particles that move close to c, not the mass.
Gravitons, if they exist (and I’d be shocked if they didn’t), as they carry a force that behaves classically just like the electromagnetic force that photons carry, would be massless and hence move at the speed of light. They’d basically be a whole lot like photons, with two important exceptions:
1. They’d be spin 2, not spin 1
2. They’d interact with everything, not just with charged particles
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### Author:  [@g8rguy](https://boards.straightdope.com/u/g8rguy)
#### Post date: [June 15, 2002, 12:28am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/9 "2002-06-15T00:28:38Z")
Oh, and I guess I should add that weight is just mg, where g is the local acceleration due to gravity and m is of course the mass.
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### Author:  [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)
#### Post date: [June 15, 2002, 12:28am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/10 "2002-06-15T00:28:42Z")
In classical “Newtonian” mechanics yes. In that model, weight is a “force” and that force is zero on any object that has a zero mass. But it is more complicated when you view gravitation as space-time curvature ala General Relativity. In that model, gravitation is not a force, and light is still effected by it despite it having zero mass.
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### Author:  [@Nobody](https://boards.straightdope.com/u/Nobody)
#### Post date: [June 15, 2002, 2:34am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/11 "2002-06-15T02:34:17Z")
Yeah, I’ve read a little about gravitation as a space-time curvature.
Anyway, my question has been answered. Thanks.
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### Author:  [@Ring](https://boards.straightdope.com/u/Ring)
#### Post date: [June 15, 2002, 3:27am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/12 "2002-06-15T03:27:09Z")
> [@](#):
>
> \*Originally posted by Undead Dude \*
> \*\*In classical “Newtonian” mechanics yes. In that model, weight is a “force” and that force is zero on any object that has a zero mass. But it is more complicated when you view gravitation as space-time curvature ala General Relativity. In that model, gravitation is not a force, and light is still effected by it despite it having zero mass. \*\*
Yes, but that’s because a photon follows a lightlike geodesic, which of course is curved in curved spacetime. It is true though that the gravitating mass/photon system has a higher mass than the gravitating mass alone.
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### Author:  [@andy\_fl](https://boards.straightdope.com/u/andy_fl)
#### Post date: [June 15, 2002, 4:13am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/13 "2002-06-15T04:13:30Z")
> [@](#):
>
> \*Originally posted by Joel \*
> \*\*So an object moving at the speed of light has infinite mass and needs infinite energy to move it.
> Does that mean that light has infinite mass and needs infinite energy to move it ? \*\*
You can exceed the speed of light very easily - I do it everyday while going to work :o. In fact you do so too. Light has been made to travel at a few inches per second in certain media and in certain mediums it even stops moving.
So when you ask questions like that - make sure you mean the speed of light in vacuum.
Radiation particles from nuclear reactors (submarines) exceed the speed of light commonly and produce Cerenkov radiation. This is an usual gig in Nuclear Engineering colleges to attract new students who get hooked to the blue light.
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### Author:  [@kniz](https://boards.straightdope.com/u/kniz)
#### Post date: [June 15, 2002, 4:20am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/14 "2002-06-15T04:20:43Z")
What **andy\_fl** is saying is that he doesn’t drive to work in a vacuum. 😛
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### Author:  [@Nobody](https://boards.straightdope.com/u/Nobody)
#### Post date: [June 15, 2002, 6:04am UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/15 "2002-06-15T06:04:35Z")
andy\_fl tisk, tisk, tisk, so nit picky. It’s obvious to everybody what I ment so I have no regrets of how I phrased my question 😛
Anyway, another question for anybody who wants to answer. How can anything exist with no mass? If something has substance, how can it be massless and weightless?
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### Author:  [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)
#### Post date: [June 15, 2002, 1:14pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/16 "2002-06-15T13:14:26Z")
Well, “substance” isn’t a physics term. 🙂
When you are exposed to light does it seem “substantial”?
Certainly light has properties, such as momentum, energy and frequency. But “substance”, being the vague term that it is seems more like “matter” (which is also a less than precise term).
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [June 15, 2002, 3:41pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/17 "2002-06-15T15:41:51Z")
> [@](#):
>
> and because, well, it doesn’t seem right that a graviton should produce its own gravitation.
Gravitons _do_ produce gravitation. That makes gravitational waves non-linear, and difficult to solve exactly (except in the small amplitude limit). Energy and momentum are the sources for gravitation in GR.
> [@](#):
>
> How can anything exist with no mass?
Well, they can have energy, like photons and gravitons. They can have spin, like neutrinos (although they have energy too, and probably a small mass). Could something have spin and zero energy? I don’t think so, but I’m not certain.
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### Author:  [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)
#### Post date: [June 15, 2002, 3:55pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/18 "2002-06-15T15:55:29Z")
“Gravitons _do_ produce gravitation” – **ZenBeam**
Don’t you think it would be better to say that they **transmit** gravitation rather than say that they produce gravitation?
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### Author:  [@g8rguy](https://boards.straightdope.com/u/g8rguy)
#### Post date: [June 15, 2002, 5:19pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/19 "2002-06-15T17:19:27Z")
Well, in all fairness, because gravitons also couple to other gravitons, I suppose one could argue that they both transmit and produce gravitation, in the same sense that photons and neutrinos and other really light particles produce gravitation.
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### Author:  [@vertigo](https://boards.straightdope.com/u/vertigo)
#### Post date: [June 15, 2002, 6:32pm UTC](https://boards.straightdope.com/t/speed-of-light-question/114119/20 "2002-06-15T18:32:29Z")
> [@](#):
>
> \*Originally posted by Undead Dude \*
> \*\*“Gravitons _do_ produce gravitation” – **ZenBeam**
>
> Don’t you think it would be better to say that they **transmit** gravitation rather than say that they produce gravitation? \*\*
Something that has always bugged me is how something that is seemingly projected from and object (gravitons from the earth, for example) have an effect of attracting another object that is also projecting gravitons (a satellite in orbit). Is it that gravitons are actually only affecting space/time and indirectly “pulling” other objects towards their area of effect? Whenever I hear about ‘gravitons’ I can’t help but imagine that they are invisible tentacles that grab objects and draw them in, much like a tractor beam (I know that’s stupid, but it’s what pops into my head).
Can anyone give me a link to a good explanation of the mechanics of a graviton?
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