# The Naked Scientists Forum

### Author Topic: why can nothing go faster than light  (Read 13565 times)

#### realmswalker

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##### why can nothing go faster than light
« on: 13/10/2006 03:50:05 »
its such a basic tenet of modern physics...but i dont get why it is this way...???!?!
why cant any thing go faster than light? why did einstein set that as the constant and what not?
i mean its illogical to think that if you have an engine with infinite output and you kept pushing on something, that it couldnt overcome the speed of light eventually....
idk

#### jysk

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##### Re: why can nothing go faster than light
« Reply #1 on: 13/10/2006 07:22:38 »
An old friend answered this for me a long time ago. He nailed it using just a few words. It made me laugh too. He said; "You can't go faster then the speed of light. If you could, you'd get where you're going before you left where you were. And thats against the rules."

Pretty elegant piece of logic, I think.

On the other hand, isn't the influence of gravity immediate? Immediate is faster then 299 792 458 m / s. (I'm only assuming that immediacy of influence is a property of gravity. It can't really be measured directly can it?)

Mike

#### lightarrow

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##### Re: why can nothing go faster than light
« Reply #2 on: 13/10/2006 07:50:17 »
quote:
Originally posted by lightarrow

The fact is that in practice the speed of light is infinite. What I mean: when a body's speed is not much, our definition of speed: v = space/time is a good definition, but when the body's speed is very high (that is, near the speed of light) our definition is not good anylonger, because space and time are not (enough) independent each other anylonger.

If you were inside a space-ship moving faster and faster, you would see planets, stars, approaching you in greater and greater amount, without any limit, that is, the number of stars you would see passing by you in one second, e.g., would approach infinity.

Of course we are assuming the average number of stars in a volume of space is constant, but this doesn't change the essence of the concept.

If we defined the speed of a body in a more appropriate way, that is exactly in the way we define lenght, mass, time: using a sample of it and adding n equal samples to make a sample n-times bigger, it's possible to show, mathematically, that the speed of light would become infinite.

This is the concept: you take a train, which holds railtracks attached on top of it. You can impose a certain speed v1 = v to this first train (measured in the usual way, for example);

then you put another train on the first, and impose this second train's speed is still v respect the first one. You can define a new concept of speed, saying that this second train has speed 2*v relative to the ground.

But, using Lorentz rule of speed addition, you have, according to the usual definition of speed, that the second train moves at a speed:
v2 = (v+v1)/(1+v*v1/c^2) = 2v/(1+beta^2). beta is v/c. Repeating the same for a third train on the second and so on, you have v3,...vn.

At the limit for n-->infinite, the speed is:

1.infinite, according to the new definition
2. c, according to the usual definition (you have to compute the limit mathematically).
« Last Edit: 13/10/2006 08:14:22 by lightarrow »

#### syhprum

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##### Re: why can nothing go faster than light
« Reply #3 on: 13/10/2006 08:12:22 »
There was research published about a year ago which claimed to measure the velocity of gravitational waves by examining an occultation at radio frequencies and found it was in fact 'c'.
I will hunt thru the literature and see if I can find it

syhprum

#### another_someone

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##### Re: why can nothing go faster than light
« Reply #4 on: 13/10/2006 16:09:32 »
quote:
Originally posted by realmswalker

its such a basic tenet of modern physics...but i dont get why it is this way...???!?!
why cant any thing go faster than light? why did einstein set that as the constant and what not?
i mean its illogical to think that if you have an engine with infinite output and you kept pushing on something, that it couldnt overcome the speed of light eventually....
idk

There are three [theoretical] classes of objects.

There are massless objects that can only travel at the speed of light, no slower, and no faster.  The photon is such.

There is ordinary matter, than can only ever travel at a speed slower than the speed of light (it is not a matter of increasing speed until you get to the speed of light, and then pushing a little harder; because you will never reach the speed of light in the first place – you will get ever closer, but the closer you get to it, the harder you have to work to get closer yet, so you never quite reach it).

Then there is a theoretical class of objects known as tachyons that can only ever travel faster than the speed of light, and can never slow down to being as slow as the speed of light (if such objects exist, then they will be very strange objects indeed, and whereas with ordinary matter, you need to add energy to make it go faster, with tachyons you need to add energy to slow it down, and you could never add enough energy to get it to slow down to the speed of light).

George

#### tony6789

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##### Re: why can nothing go faster than light
« Reply #5 on: 13/10/2006 16:38:14 »
cause light is ...ummm..fast?

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#### bostjan

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##### Re: why can nothing go faster than light
« Reply #6 on: 14/10/2006 08:10:57 »
because photons have no mass.

#### science_guy

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##### Re: why can nothing go faster than light
« Reply #7 on: 14/10/2006 09:57:37 »
quote:
Then there is a theoretical class of objects known as tachyons that can only ever travel faster than the speed of light, and can never slow down to being as slow as the speed of light (if such objects exist, then they will be very strange objects indeed, and whereas with ordinary matter, you need to add energy to make it go faster, with tachyons you need to add energy to slow it down, and you could never add enough energy to get it to slow down to the speed of light).

could a tachyon possibly be an antiphoton?

quote:
If you could, you'd get where you're going before you left where you were. And thats against the rules.

thats not entirely true.  you are always where you are, and when you are.  The only thing that arrives before, is your image.  If light was not a barriar, than the next milestone would be instantanious.  Even if you take no time at all to move from point a to point b, the image would appear at point b slightly after the image at point a dissappears.  if you were to travel faster than instantanous, than you would meet up with yourself at the point, and there cannot be two of you in existance.

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#### ukmicky

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##### Re: why can nothing go faster than light
« Reply #8 on: 14/10/2006 13:47:30 »
quote:
thats not entirely true. you are always where you are, and when you are. The only thing that arrives before, is your image. If light was not a barriar, than the next milestone would be instantanious. Even if you take no time at all to move from point a to point b, the image would appear at point b slightly after the image at point a dissappears. if you were to travel faster than instantanous, than you would meet up with yourself at the point, and there cannot be two of you in existance.

But if your image arrived before you it would still go againts the rules as that image would contain information which is basically the same thing as you arriving before you left.

Michael
« Last Edit: 14/10/2006 20:47:57 by ukmicky »

#### bostjan

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##### Re: why can nothing go faster than light
« Reply #9 on: 14/10/2006 21:13:33 »
quote:
Originally posted by science_guy

could a tachyon possibly be an antiphoton?

no.  photons are like their own anti-particles, but i don't think anything should be a photon's anti-particle, because if you annihilate photons, what do you get as a result?

#### syhprum

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##### Re: why can nothing go faster than light
« Reply #10 on: 16/10/2006 16:23:22 »
I have now located the article concerning the measurement of the velocity of gravitational waves
http://arxiv.org/PS_cache/gr-qc/pdf/0206/0206022.pdf

syhprum

#### science_guy

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##### Re: why can nothing go faster than light
« Reply #11 on: 17/10/2006 15:56:55 »
quote:
But if your image arrived before you it would still go againts the rules as that image would contain information which is basically the same thing as you arriving before you left.

I already explained that arriving before you left is impossible, since man cannot achieve time travel.  Going faster than light does not mean going faster than time, although the faster you go, the slower time gets.  Your image is the local light being bounced off your ship.  If you were going faster than light, than you would arrive at your destination before the light that bounced off you at the starting point did, but you would only be invisible for as long as it takes for light from your ship to reach the observer at the location.  Your image is not somthing that contains important information about you, but how the observer sees the light bouncing off your ship.

Another reason that Going as fast as light might be impossible, is that when you are going as fast as light, than time essentialy stops, therefore the signals to stop would never reach the engines in the ship and you would be trapped in hyperspace.

quote:
no. photons are like their own anti-particles, but i don't think anything should be a photon's anti-particle, because if you annihilate photons, what do you get as a result?

if light is considered energy, than maybe the annihilation of an anti-photon and a photon is the reverse effect of the annihilation of any other matter/antimatter reaction:  Maybe it would create mass.

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#### lightarrow

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##### Re: why can nothing go faster than light
« Reply #12 on: 17/10/2006 16:29:02 »
quote:
Originally posted by science_guy

if light is considered energy, than maybe the annihilation of an anti-photon and a photon is the reverse effect of the annihilation of any other matter/antimatter reaction:  Maybe it would create mass.
A gamma photon with high enough energy colliding with a nucleus can already create mass generating a pair electron-antielectron. Where is the anti-photon, in that case?
« Last Edit: 17/10/2006 16:29:46 by lightarrow »

#### syhprum

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##### Re: why can nothing go faster than light
« Reply #13 on: 17/10/2006 22:46:28 »
A photon as its alto ego a wave packet can either annihilate or reinforce a similar one but of course they cannot be described as photons and anti-photons

syhprum
« Last Edit: 18/10/2006 15:56:21 by syhprum »

#### DoctorBeaver

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##### Re: why can nothing go faster than light
« Reply #14 on: 18/10/2006 13:51:40 »
The concept of tachyons has been thrown out. That was only a stop-gap theory to plug a hole in an existing theory; but the theory that needed it has been modified so the need for the existence of tachyons became void.

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#### science_guy

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##### Re: why can nothing go faster than light
« Reply #15 on: 18/10/2006 15:44:35 »
wow, our topic has been graced with DoctorBeaver's presence![^][^]

What was the theory that concerned this?

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#### DoctorBeaver

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##### Re: why can nothing go faster than light
« Reply #16 on: 19/10/2006 13:21:45 »
quote:
Originally posted by science_guy

wow, our topic has been graced with DoctorBeaver's presence![^][^]

What was the theory that concerned this?

I'm not sure offhand. There was something about it a book I was readng a while back.

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#### Rapax

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##### Re: why can nothing go faster than light
« Reply #17 on: 19/10/2006 13:34:04 »
quote:
Originally posted by realmswalker

its such a basic tenet of modern physics...but i dont get why it is this way...???!?!
why cant any thing go faster than light? why did einstein set that as the constant and what not?
i mean its illogical to think that if you have an engine with infinite output and you kept pushing on something, that it couldnt overcome the speed of light eventually....
idk

Ok, I'll try and outline this briefly, but keep in mind that this explanation will be rather simplified and therefor probably imprecise in certain details. Ok? Here goes:

The basic idea that we're starting with here, is that the speed of light is apparently always the same, no matter from where you are observing it, or how fast you as an observer are moving. This may be hard to accept, but so far, all experiments done to test this idea seem to confirm it. So let's just accept that as fact for the moment (if this is actually true, and the implications of it not being true might be an interesting topic in another thread).

So now, imagine two lamps, of identical build, fastened to the roofs of two identical rooms. On the floor, opposite from the lamps, there are two light detectors. One of the rooms is stationary from the observers point of view. So now, in the stationary room, the lamp shines a beam of light to the detector on the floor below it. If the observer measurs how long it took the beam to reach the detector, he'll see that it is obviously the height of the room, divided by the speed of light.
Now the same experiment is conducted in the second room, while it is moving sideways at a constant speed relative to the observer. The beam of light now travels a longer distance, namely the diagonal of a rectangle with the height of the room, and the width being the distance travelled during the time it takes. So now, if we accept that the speed of the light was the same, and the distance was larger, and the time from our point of view was the same, the only way to explain it is if the time in the room was running slower that for the observer. Clear so far?
Now we'll keep imagining that experiment, while speeding up the moving room. You'll see that the angle at which the beam of light is travelling gets flatter and flatter, the closer we get to the speed of light. The distance the beam travels gets longer and longer, so the local time in the room gets slower and slower. If we continue this up until the room is moving at the speed of light, time will stop completely in the room. If you've followed this far, you've understood a major part of relativity.

Now, analogue to time slowing down, things moving at high speeds also tend to get heavier (actually, more massive. There are similar thought experiments that show why this is so, but they're quite a bit more complex that the time slowing one we did above). This increase of mass doesn't happen linearly. At first up to about half the speed of light, the gain in mass is very small, after that, it keeps getting quicker. For the last percent of lightspeed, the increase in mass is very very large. Now along with an increase in mass, comes an increase in inertia. You'll notice this if you try pushing a cart. The more mass is on the cart, the harder you have to push. Now if our object would actually reach the speed of light, it's mass would increase to infinite. This means that the closer we get to the speed of light, the more energy we'd need to keep speeding things up. The actually get to lightspeed, we'd need an infinite amount of energy, which in a finite universe is more than can ever be available. The conclusion of all this, is that objects without mass can move at the speed of light, because their mass stays at zero. Anything with a non-zero mass cannot, because it would take an infinite amount of energy to get it to those speeds.

#### science_guy

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##### Re: why can nothing go faster than light
« Reply #18 on: 19/10/2006 15:43:18 »
Very interesting.

Can you explain why mass gets larger when you get closer to the speed of light?  I would understand why time might slow down...

could going faster be an example of converting time into space?  Whereas going slower is an example of converting space into time?

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#### DoctorBeaver

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##### Re: why can nothing go faster than light
« Reply #19 on: 20/10/2006 13:33:53 »
quote:
Originally posted by science_guy

Very interesting.

Can you explain why mass gets larger when you get closer to the speed of light?  I would understand why time might slow down...

could going faster be an example of converting time into space?  Whereas going slower is an example of converting space into time?

[xx(]

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#### another_someone

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##### Re: why can nothing go faster than light
« Reply #20 on: 20/10/2006 14:13:34 »
quote:
Originally posted by science_guy
Can you explain why mass gets larger when you get closer to the speed of light?  I would understand why time might slow down...

I suppose the starting point should be whether someone can explain what mass is.

Mass seems to be nothing but a number that tells you how responsive or unresponsive something is to external force, which is just another way of saying that the closer an object is to travelling at the speed of light, the more unresponsive it becomes to external forces that may try and alter its velocity.

George

#### science_guy

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##### Re: why can nothing go faster than light
« Reply #21 on: 20/10/2006 15:48:15 »
quote:
Originally posted by DoctorBeaver

quote:
Originally posted by science_guy

Very interesting.

Can you explain why mass gets larger when you get closer to the speed of light?  I would understand why time might slow down...

could going faster be an example of converting time into space?  Whereas going slower is an example of converting space into time?

[xx(]

Oops! did we confuse you? [^][^]

quote:
I suppose the starting point should be whether someone can explain what mass is.

Mass seems to be nothing but a number that tells you how responsive or unresponsive something is to external force, which is just another way of saying that the closer an object is to travelling at the speed of light, the more unresponsive it becomes to external forces that may try and alter its velocity.

so, because we are getting closer to the velocity of light, the measure of mass vs. external forces increases, and therefore the amount of external forces has to increase in order to go faster?

If you were observing a ship going closer to the velocity of light, would it actually appear to increase in mass, or would it just require more force to move, and therefore appear to increase in mass because of greater resistance to movement?

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#### lightarrow

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##### Re: why can nothing go faster than light
« Reply #22 on: 20/10/2006 16:19:40 »
quote:
Originally posted by science_guy

Can you explain why mass gets larger when you get closer to the speed of light?  I would understand why time might slow down...
If the equation m = E/c^2 it's not a mystery for you, then the answer to your question is:

E = m(0)*c^2/SQRT[1-(v/c)^2]

where m(0) is the "rest mass" = "invariant mass" = the mass of the body when it's at rest. Sometimes it's called just "mass", while the one we are talking now it's called "relativistic mass".

So, from those equations you can see that m = m(0)/SQRT[1-(v/c)^2] and this means that the more v approaches c, the more v/c approaches 1 and the greater is the (relativistic) mass m, approaching infinity.

Said in another way: if we want to preserve the law of momentum conservation, then the momentum p must be defined in this way:

p = m(0)*v/SQRT[1-(v/c)^2]. This comes from Lorentz transforms.

So, the term multipling v can be seen as our "mass".
« Last Edit: 20/10/2006 16:25:50 by lightarrow »

#### lightarrow

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##### Re: why can nothing go faster than light
« Reply #23 on: 20/10/2006 16:24:03 »
quote:
Originally posted by science_guy
If you were observing a ship going closer to the velocity of light, would it actually appear to increase in mass, or would it just require more force to move, and therefore appear to increase in mass because of greater resistance to movement?
Both. The increase in mass is effective in any sense, even for what concerns gravitational mass.

#### science_guy

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##### Re: why can nothing go faster than light
« Reply #24 on: 20/10/2006 16:25:49 »
and thats why we can never reach the speed of light, becuase we cant reach infinite engine output!  Makes perfect sense.

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#### The Naked Scientists Forum

##### Re: why can nothing go faster than light
« Reply #24 on: 20/10/2006 16:25:49 »