Nothing can go faster than twice the speed of light.

[Aphrel]

Do you have a reference for that?

As far as I understand the "size of observable universe" means the current distance to the furthest object we can currently observe, and it would seem that acceleration would move the objects further away from us (while there are fewer and things we can see). And: can the size of the observable universe in an expanding universe be smaller than the age of the universe (multiplied by c)?

I dont have a reference and i could be completly wrong. But as i understand it the expansion of space is accelerating, so the distance needed for the expansion of space to exceed the speed of light should be shrinking.

Today we need X distance to have an expansion of space large enough to surpass the speed of light (i think its 26 billion lightyears, anything beyond that we cannot see anymore). But if the expansion is accelerated to say the double. We whould only need 0.5X distance for light not to reach us anymore.
So not only will galaxies move away from us at an accelerated pace, the range we can see will also decrease.

[Deleted]

If movement is relative, then if you have one thing moving the speed of light one way, and another thing moving the speed of light the exact opposite way, than the truth is relativity means that nothing can go faster than TWICE the speed of light, since the gap between those two would be created at twice the speed of light.

Nope, not true.. if you shoot a beam of light from a moving car.. it wont be as fast as the speed of light plus the speed of car, its still the speed of light, just gets blue shifted a bit.

[May90]

That doesn't make sense to me. Do you have an article you are referencing?

Since a tachyon travels at speeds faster than c, the relativistic equations mean that it would have imaginary energy. In order to remedy this, the tachyon's mass must then be imaginary, forcing the tachyon to have a negative non-imaginary energy instead. Note that this imaginary mass squared is a negative mass. This is in fact why tachyons go faster than light: their negative energy implies an increase in velocity. This means it is impossible for tachyons to go slower than c.

The 'Higgs Field derivation' is directly related to tachyonic fields--not really tachyons. The tachyonic field to the Higgs field is like the imaginary numbers to the real numbers.

I don't have any articles in mind really, these are just things I've heard back when Higgs was found, I still worked in experimental high energy physics and some people talked about it. I know about Tachyons and their imaginary mass, but I've also heard about purely negative mass assumption making it possible to go faster than c. Perhaps I'm mistaken.

[gobblejobble]

except a fat chick shoppin in a donut shop HIIIYOOOOOOOOO! lol

[Forogil]

I don't have any articles in mind really, these are just things I've heard back when Higgs was found, I still worked in experimental high energy physics and some people talked about it. I know about Tachyons and their imaginary mass, but I've also heard about purely negative mass assumption making it possible to go faster than c. Perhaps I'm mistaken.

I thought the negative mass was needed for warp drives and stabilizing worm-holes, which both are ways of traveling faster than c (in some sense).

Could that be it, or is it something completely different?

[HumbleDuck]

From the static observer's point of view with two massless particles moving away from them, the simple summation is applicable.

Nah, to calculate the relative speed of one particle in another's frame, you still have to use Lorentz transformation rather than simple summation.

[jdbond592]

Ahem...Einstein never said that nothing could travel faster than light. Light speed itself isn't constant. Whatever flies.!

[HumbleDuck]

I have no idea about a purely negative mass tachyon. It would not surprise me--any particle that travels faster than c is generally called a tachyon as opposed to a tardyon. Someone somewhere probably cooked one up under the "class" of tachyons, but generally a tachyon should have a negative square mass.

Tachyons are a part of physics that aren't taken seriously but make a good headline in magazines. ;-)

Edit: Eh, that's kind of harsh. They are studied but only for completeness, in my opinion. Tachyon existence is very dubious.

Yes, that was harsh. :-<

[Forogil]

Nah, to calculate the relative speed of one particle in another's frame, you still have to use Lorentz transformation rather than simple summation.

Yes, but if I look at two particles moving away from me (in opposite directions) at 0.9c, I can still say that the distance between them (as perceived in my reference frame) increases like time multiplied by 1.8c, and thus their "relative speed" is 1.8c in my reference frame - according to the simple formula that distance is time multiplied by velocity.

It doesn't contradict relativity - it's just that this "relative velocity" is not used in relativity (since it just confuses things), and instead relativity only discusses speed of one object in the specific reference frames.

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Edit: Eh, that's kind of harsh. They are studied but only for completeness, in my opinion. Tachyon existence is very dubious.

I assume they are also studied so that we can search for them, and experimentally verify that they don't exist (similarly as some other strange ideas).
If we didn't know how they behave we wouldn't know how to look for them.

[Deleted]

We can't tell for sure because our laws stop working the moment you take v >= c for any particle with a non-zero rest mass. Hypothetically, using time dilation, an object moving faster and faster will appear to have a slower passing of time the closer it gets to c. Once it hits c (physically impossible), its clock should stop running. If it goes over c (impossible again), its clock should run with an imaginary time considering you get a negative in the root, for which we have no clue what it implies -- http://www.physlink.com/education/askexperts/ae283.cfm

Thank you for explaining that and for the link, will check it out later.

[HumbleDuck]

Yes, but if I look at two particles moving away from me (in opposite directions) at 0.9c, I can still say that the distance between them (as perceived in my reference frame) increases like time multiplied by 1.8c, and thus their "relative speed" is 1.8c in my reference frame - according to the simple formula that distance is time multiplied by velocity.

It doesn't contradict relativity - it's just that this "relative velocity" is not used in relativity (since it just confuses things), and instead relativity only discusses speed of one object in the specific reference frames.

Well, that's like saying sum of two numbers can be bigger than C.

Sure it can, but that doesn't mean any of those particles can move faster than C.

[Deleted]

Kinda confused as to why exactly this thread hasn't ended with 'well, the OP is wrong, nothing goes faster than c, /thread'.

I suppose this back and forth is moving in the same way as the vaccines/autism debate, the competent vs. the uninformed masses.

[May90]

Nah, to calculate the relative speed of one particle in another's frame, you still have to use Lorentz transformation rather than simple summation.

This doesn't contradict what I said.

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I thought the negative mass was needed for warp drives and stabilizing worm-holes, which both are ways of traveling faster than c (in some sense).

Could that be it, or is it something completely different?

I don't know much about these things, honestly. In any case, neither negative nor imaginary mass hasn't been observed anywhere and, likely, is just a mathematical abstraction.

[HumbleDuck]

This doesn't contradict what I said. .

But how?
Did you mean both particles are moving along the same direction?

[May90]

But how?
Did you mean both particles moving along the same direction?

I mean, if I stay in my room and turn on two focused light emitters, emitting light in the opposite directions, then from my point of view the photons will be moving away from each other at 2c, i.e. the distance between them will be increasing at 2c (assuming my room has only vacuum in it).

[HumbleDuck]

I mean, if I stay in my room and turn on two focused light emitters, emitting light in the opposite directions, then from my point of view the photons will be moving away from each other at 2c, i.e. the distance between them will be increasing at 2c (assuming my room has only vacuum in it).

What does that have to do with speed?
You are simply stating that the distance between two events can be further than the distance light can travel in a given time.

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A pretty good point. I have a tendency to overcompensate a counterforce to an unnecessarily one-sided mainstream pressure.

That aside, I would still say tachyons are not a "serious" area of research. There was a host of experimentation done to try to find them in the 60's, but the initial inspiration for them was 'disproven', resulting in the phenomenon known as tachyon condensation. Nowadays any researcher dealing with tachyons is usually a physicist who set themselves up for tachyon condensation.

They use to be used in string theory, but stronger string theories that incorporate supersymmetry have dropped them entirely.

I know it sounds harsh, but I'm trying to ground everyone back to reality from the tachyon hype. It's not some super exciting area of research waiting to burst forth with new discoveries that will completely upend all of physics with these superluminal particles. I'm not going to dismiss the research as meaningless or unimportant, but it's another of the many solutions-that-mathematically-exist. I still think it's important for even SR students to learn about them just to have a richer understanding of how the EFE behaves, setting themselves up for success in GR.

Ah, the experimental people calling math not serious again. Even though my practises are not that much related to tachyons anymore, I do not dare to call any type of research ( specially theoretical ones) not serious.

[DesMephisto]

your logic is flawed. If I am running in one direction and Usian Bolt is running in the other direction from me then that doesn't mean I am running as fast as me + Usian Bolt combined you genius.

You don't run the same speed as Usian Bolt, however the gap at which the increment increases between two usain bolts running at the exact same speed would be equal to the distance in which they're traveling away from each other which would be relative to the speed they're traveling.

[satimy]

by taking a wormhole you can effectively travel distances faster than light.

[May90]

What does that have to do with speed?
You are simply stating that the distance between two events can be further than the distance light can travel in a given time.

What I mean is, there is a reference system in which two objects/particles can move away from each other at the speed faster than c (but no faster than 2c), meaning the distance between them increases at the rate faster than c in that system. It doesn't violate the natural limit of c, because that limit applies to one object in a given reference system.

[Deleted]

What I mean is, there is a reference system in which two objects/particles can move away from each other at the speed faster than c (but no faster than 2c), meaning the distance between them increases at the rate faster than c in that system. It doesn't violate the natural limit of c, because that limit applies to one object in a given reference system.

That's the problem. 2nd principle of special relativity doesn't simply mean "speed of light is 300000 km/s" but:

The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

This mean that for all reference system (particles included) the speed of the other particle is always c. Hypotetically if you sit on one photon and you look to the other photon you'll see it moving away from you at c and not 2c, because you're in an inertial frame with your photon and you're still.

Doesn't exist a reference system which they can move away from each other at speed faster than c because this would go against the principle. Principle is an experimental fact!
I don't know what's your physics (and LaTex) skills but this is the Velocity composition for Special Relativity, where u' is the speed of photon moving away from you (in his system) , u is the speed you are going to calculate (the speed of the photon moving away in your system) and v is the speed between both systems:
u=\frac{u'+v}{1+u'v/c^2}

assuming the photon is moving at speed of light (lol really?), you can replace u' with c. Now you're going to see that u=c (the photon is moving away from you regardless your speed, in fact v is simplified).
ps. sorry for my bad english :/