travelling at the spead of light

[GoHeelsWeeeehoooo]

i have some confusions regarding the whole travelling at or close to the speed of light....i've you are travelling to a distant object...as you get closer to the speed of light the distance between you and the object shrinks...is that correct? so if you are travelling to an object 10 light years away the time it takes to get there can be less than 10 years?

physics always messed up with my mind

[Le Chacal]

If you're the passenger of such a space ship, the answer is yes, for you, the distance between you and the star seems to shrink when you approach the speed of light, so the journey seems to be shorter.

If my memory is not confused, for exemple, at 87% of speed of light, distance seemed halved and time for the journey too, but for an hypothetical observer "outside" the space ship and with a velocity non-relativistic (that's very hypothetical no ? ) the spaceship needs (with a journey of 10 LY ) 11.49 years to reach the star, although passengers seemed to be aged only 5 or 6 years more...

Is that clear ? I'm not sure to explain it correctly

[GoHeelsWeeeehoooo]

so hypotetically if you could travel at the speed of light would the trip to a star 10 light years away seem instantanous to you?

[Spaceman Spiff]

Yes, if you could get up to the speed of light, the universe in front of you (and behind, but not to the sides) would appear to contract to zero depth, and you'd cross it instantly. At least, this is according to the Special Theory of Relativity, Albert Einstein's first Relativity theory published in 1905.

However, since energy has mass*, it turns out that you could never get to the speed of light, because you also become more energetic (kinetic energy) and heavier the faster you go, until you would be infinitely heavy at the speed of light. You'd have to use more force as you became heavier to accelerate, so nothing massive can ever quite reach the speed of light.

These experiences for you are different to those people would have back on Earth. Let's assume you can accelerate very quickly, because acceleration is dealt with by Einstein's other Relativity theory, General Relativity, published in 1915. You set off to that star 10 light years away on 1st Jan 2010 at 87% the speed of light. Obviously, you shoudn't arrive at this star for 10 light years / 0.87 c = 11.494 years, or about 1st Jul 2021. However, you think the star now looks only 5 light years away, so you think you arrive in 5 light years / 0.87c = 5.747 years, or about 1st Oct 2005. You tick off the days on your calendar, and that's the date you seem to arrive. The same applies on the way back, and when you get back to Earth, your calendar will show the date to be 1st July 2021, but everyone else on Earth will show you the date on their calendars says 1st Jan 2033.

Particles of light, photons, do travel at the speed of light. They have no kinetic energy because they have no inertial^H^H^H^H^H^H^H^H^H rest mass, which is why they can do this. For them, everything does seem to be in one place and everything seems to happens at the same time. That's why they can interfere with other photons that haven't been created yet, or know when a quantum entangled photon halfway across the laboratory has had its polarisation changed. Or so it seems.

Spiff.

* E = mc??.

[GoHeelsWeeeehoooo]

so how fast would you have to go for the time difference between the person in the ship and someone observing from otside to become noticable?

[Cham]

Particles of light, photons, do travel at the speed of light. They have no kinetic energy because they have no inertial mass, which is why they can do this. For them, everything does seem to be in one place and everything seems to happens at the same time. That's why they can interfere with other photons that haven't been created yet, or know when a quantum entangled photon halfway across the laboratory has had its polarisation changed. Or so it seems.

Spiff.

* E = mc??.

Sorry, there's some confusion here. Photons DO have kinetic energy : K = h f, where "f" is the frequency and "h" is Planck's constant. However, The relativistic Newton equation do not apply to them as photons do not have an intrinsic mass.

Also, the explanation about photon interferences isn't valid, because electrons can interfere too (because of quantum mechanics), and yet they aren't "everywhere at the same time because of their absence of mass". The photon interference is a pure quantum phenomenon and has nothing to do with special relativity (to which photons are just particles).

[d.m.falk]

I think alot of the misconception about the status of photonic mass is due to the scientific reluctance to say that Einstein's Theory of Special Relativity is incorrect on the matter.

But photons do have mass, with its observable effects- Recoil, decay, gravitationally lensing or bending, and resistance to medium density. (That is, light slows down in an atmosphere, and moreso in fluids and solids.)

If photons have no mass, they cannot have any energy, however, it takes energy to create and expel, and its energy is detected when received, either by eyes or by radios. Some (most?) will argue otherwise, and try to explain that without violating same-said Theory of Special Relativity.

The speed of light is also not infinite, but actually rather slow; however, our present understanding of physics has difficulties grasping that it isn't, and may, with much reluctance, require revision of standard theories to account for observed anomalies that appear to defy long-held theory.

Not saying everyone's wrong, just we don't know everything-- We're really only just beginning to understand how our own universe works, and it will be many generations from now when we truly are close enough to truly grasp both the beauty, complexity and simplicity our Universe beholds to us.

We're still learning.

And, oh yes- Before I forget:
http://www.phys.uni.torun.pl/~jkob/physnews/node35.html

d.m.f.

[t00fri]

I think alot of the misconception about the status of photonic mass is due to the scientific reluctance to say that Einstein's Theory of Special Relativity is incorrect on the matter.

But photons do have mass, with its observable effects- Recoil, decay, gravitationally lensing or bending, and resistance to medium density. (That is, light slows down in an atmosphere, and moreso in fluids and solids.)

If photons have no mass, they cannot have any energy, however, it takes energy to create and expel, and its energy is detected when received, either by eyes or by radios. Some (most?) will argue otherwise, and try to explain that without violating same-said Theory of Special Relativity.

The speed of light is also not infinite, but actually rather slow; however, our present understanding of physics has difficulties grasping that it isn't, and may, with much reluctance, require revision of standard theories to account for observed anomalies that appear to defy long-held theory.

Not saying everyone's wrong, just we don't know everything-- We're really only just beginning to understand how our own universe works, and it will be many generations from now when we truly are close enough to truly grasp both the beauty, complexity and simplicity our Universe beholds to us.

We're still learning.

And, oh yes- Before I forget:
http://www.phys.uni.torun.pl/~jkob/physnews/node35.html

d.m.f.

You are not serious about all this are you??
I wouldn't know were to start responding ...


Bye Fridger

[Cham]

But photons do have mass, with its observable effects- Recoil, decay, gravitationally lensing or bending, and resistance to medium density. (That is, light slows down in an atmosphere, and moreso in fluids and solids.)

If photons have no mass, they cannot have any energy, however, it takes energy to create and expel, and its energy is detected when received, either by eyes or by radios. Some (most?) will argue otherwise, and try to explain that without violating same-said Theory of Special Relativity.
d.m.f.

Hmm, there's some confusion here too, sorry.

Photon don't have a mass. But it has energy and momentum ! We CAN have energy without mass !

What is mass, really ? m = E/c^2 ? That's the source of confusion. In physics, what we mean by mass is "proper mass", or "intrinsic mass", which enters evolution equations as a scalar parameter. There is none in the Maxwell equations. Mass isn't m = E/c^2, which is an abuse of language.

Another source of historical confusion is the notion of "relativistic mass", defined by m = gamma m_0, where m_0 is the "proper mass" and "gamma" is the relativistic factor. This kind of mass isn't observable and doesn't have an operationnal meaning.

[Fightspit]

Another source of historical confusion is the notion of "relativistic mass", defined by m = gamma m_0, where m_0 is the "proper mass" and "gamma" is the relativistic factor. This kind of mass isn't observable and doesn't have an operationnal meaning.

M = M0 / ( 1 - (V?? / C??) )^1/2.

With M0: Mass initial of something
V: speed of something
C: speed constant of light.

and gamma = 1 / ( 1 - (V?? / C??) )^1/2

[GoHeelsWeeeehoooo]

i didn't mean for this thread to start any arguments...i was just curious what is the fastest you could go without having to deal with really bad time distortions between those travelling at the high speed and those observing from somewhere else

[Fightspit]

i didn't mean for this thread to start any arguments...i was just curious what is the fastest you could go without having to deal with really bad time distortions between those travelling at the high speed and those observing from somewhere else

Replace M and M0 By T and T0:

T = T0 / ( 1 - (V?? / C??) )^1/2.

With T0: Mass initial of something
V: speed of something
C: speed constant of light.

and gamma = 1 / ( 1 - (V?? / C??) )^1/2

:Wink:

[Cham]

Okay, I think some clarifications are necessary about what "mass" really is about. Here are some basic, small and easy formulas :



Equation (1) DEFINES what is "relativistic mass" of a particle. This is just a stupid definition WITHOUT ANY PHYSICAL CONTENT (sorry for the uppercase). The only reason for this definition is to have some relativistic equations which LOOKS LIKE old classical equations, and nothing more. The proper mass of the particle is m_0 and is what can be obtained by an experiment, not "m". This formula doesn't apply to a photon.

Equation (2) is exact and comes from special relativity. The "m" which is entering this equation is exactly the same as in (1), WHEN APPLIED TO A MASSIVE PARTICLE such an electron. The formula still apply to a photon, but then, the "m" doesn't mean anything. It may be called "mass of the photon", but this is really arbitrary and doesn't have any experimental meaning without gravity.

Equation (3) is exact for any massive particle. It doesnt apply to a photon, because of equation (1) ! This is the reason why (1) was used. Because (3) looks like an old classical formula.

Equation (4) is the definition of "Kinetic Energy" of a particle. It applies to any kind of particles. For photons, we set m_0 = 0.

Equations (5) and (6) are of quantum origin. They are exact and apply to ANY particle, especially to the photon. What is "f" and "lambda" for an electron ? Well, that's another story !

Finally, equation (7) is simply Newton's equation, which is central to all classical mechanics. Well folks, THIS EQUATION IS TERRIBLY WRONG ! It applies to any massive particle ONLY if the speed is low compared to light speed. You can't correct that equation by replacing "m" with "m_0". It's a bit more complicated.

[Fightspit]

You can't correct that equation by replacing "m" with "m_0". It's a bit more complicated.

Another source of historical confusion is the notion of "relativistic mass", defined by m = gamma m_0, where m_0 is the "proper mass" and "gamma" is the relativistic factor. This kind of mass isn't observable and doesn't have an operationnal meaning.

M = M0 / ( 1 - (V?? / C??) )^1/2.

With M0: Mass initial of something
V: speed of something
C: speed constant of light.

and gamma = 1 / ( 1 - (V?? / C??) )^1/2

You think my equation is wrong

[Fightspit]

I find this:
http://semsci.u-strasbg.fr/relativ.htm

P.S.: Sorry, it is in french...

[BlindedByTheLight]

Equation (1) DEFINES what is "relativistic mass" of a particle. This is just a stupid definition WITHOUT ANY PHYSICAL CONTENT (sorry for the uppercase). The only reason for this definition is to have some relativistic equations which LOOKS LIKE old classical equations, and nothing more. The proper mass of the particle is m_0 and is what can be obtained by an experiment, not "m". This formula doesn't apply to a photon.

Equation (2) is exact and comes from special relativity. The "m" which is entering this equation is exactly the same as in (1), WHEN APPLIED TO A MASSIVE PARTICLE such an electron. The formula still apply to a photon, but then, the "m" doesn't mean anything. It may be called "mass of the photon", but this is really arbitrary and doesn't have any experimental meaning without gravity.

Just to clarify (b/c I'm a little confused)... the "m" in Equation (1) is simply defintional without any physical meaning... but then that same "m" is used in Equation (2) and suddenly DOES have physical meaning?

[Cham]

You can't correct that equation by replacing "m" with "m_0". It's a bit more complicated.

Another source of historical confusion is the notion of "relativistic mass", defined by m = gamma m_0, where m_0 is the "proper mass" and "gamma" is the relativistic factor. This kind of mass isn't observable and doesn't have an operationnal meaning.

M = M0 / ( 1 - (V?? / C??) )^1/2.

With M0: Mass initial of something
V: speed of something
C: speed constant of light.

and gamma = 1 / ( 1 - (V?? / C??) )^1/2

You think my equation is wrong

The only error is the interpretation of m_0. It isn't "Mass initial of something". It's the "proper mass", "true mass", "intrinsic mass" of the particle.

[BlindedByTheLight]

What is mass, really ? m = E/c^2 ? That's the source of confusion. In physics, what we mean by mass is "proper mass", or "intrinsic mass", which enters evolution equations as a scalar parameter. There is none in the Maxwell equations. Mass isn't m = E/c^2, which is an abuse of langu

Perhaps also, if you have the time, you might expound on the above? Why mass "isn't e=mc2" and why, exactly, it's an abuse of language?

Thanks!

[Fightspit]

The only error is the interpretation of m_0. It isn't "Mass initial of something". It's the "proper mass", "true mass", "intrinsic mass" of the particle.

Oh yes, i just want to say M0 represent a massive object not a particule,you see

[Cham]

Equation (1) DEFINES what is "relativistic mass" of a particle. This is just a stupid definition WITHOUT ANY PHYSICAL CONTENT (sorry for the uppercase). The only reason for this definition is to have some relativistic equations which LOOKS LIKE old classical equations, and nothing more. The proper mass of the particle is m_0 and is what can be obtained by an experiment, not "m". This formula doesn't apply to a photon.

Equation (2) is exact and comes from special relativity. The "m" which is entering this equation is exactly the same as in (1), WHEN APPLIED TO A MASSIVE PARTICLE such an electron. The formula still apply to a photon, but then, the "m" doesn't mean anything. It may be called "mass of the photon", but this is really arbitrary and doesn't have any experimental meaning without gravity.

Just to clarify (b/c I'm a little confused)... the "m" in Equation (1) is simply defintional without any physical meaning... but then that same "m" is used in Equation (2) and suddenly DOES have physical meaning?

"Physical meaning" is a subtle notion. What I mean is this :

Equation (1) doesn't have any physical meaning because it's simply a definition. It is not a RELATION between TWO observable things ("m" isn't observable, or measurable if you prefer). Equation (2) IS a relation between two independant entities, which can BOTH be subject to experiments. Just replace (1) into (2) so "m" vanish entirely from the equation. What remains are "E" and "m_0", which are the true observable things here.